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Laser desorption - laser photoionization ion trap mass spectrometry for the direct analysis of solid… Specht, August Anders 2003

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Laser Desorption - Laser Photoionization Ion Trap Mass Spectrometry for the Direct Analysis of Solid Samples  By A u g u s t A n d e r s Specht B . S c . H . , Q u e e n ' s U n i v e r s i t y , 1998  A THESIS SUBMITED IN P A R T I A L F U L F I L L M E N T OFT H E REQUIREMENTS FORTHE DEGREE OF DOCTOR OF PHILOSOPHY  In THE F A C U L T Y OF G R A D U A T E  STUDIES  Department o f C h e m i s t r y  W e accept this thesis as c o n f o r m i n g to the required standard  THE UNIVERSITY OF BRITISH C O L U M B I A D e c e m b e r 2003 © A u g u s t A n d e r s Specht, 2 0 0 3  ABSTRACT T h i s thesis describes w o r k associated w i t h the c r e a t i o n o f a n o v e l a n a l y t i c a l instrument, a two-laser i o n trap mass spectrometer. T h i s d e v i c e c o m b i n e s one o f the m o s t sensitive a n d selective direct s o l i d s a m p l i n g techniques, two-laser s o l i d s a m p l i n g , w i t h a v e r y versatile mass spectrometer, the i o n trap. T h i s w o r k demonstrates s o m e o f the p o t e n t i a l advantages associated w i t h c o u p l i n g these t w o techniques. T h e results f r o m this w o r k c a n be d i v i d e d into three sections. S e c t i o n one s h o w s data associated w i t h the d e v e l o p m e n t and characterization o f this n e w instrument w h i c h c o u p l e s I R laser d e s o r p t i o n f o l l o w e d b y U V laser p h o t o i o n i z a t i o n and a n a l y s i s u s i n g a n i o n trap mass spectrometer. F o r c a l i b r a t i o n , a n e w type o f s o l i d sample preparation m e t h o d i n v o l v i n g activated c h a r c o a l as the s o l i d substrate w a s used. It w a s f o u n d that b o t h the I R and U V intensity, and the d e l a y between t h e m , p l a y a n important r o l e i n b o t h the m a g n i t u d e and type o f signals observed. A m e t h o d o f gas phase i o n a c c u m u l a t i o n w a s also e x a m i n e d . F i n a l l y , this section demonstrates the t e c h n i q u e ' s a b i l i t y to p r o v i d e direct qualitative i n f o r m a t i o n for P A H content for N . I . S T . S R M 1944 r i v e r sediment w i t h n o s a m p l e pre-treatment. T h e s e c o n d s e c t i o n o f this thesis s h o w s data relating to the detection o f the p h a r m a c e u t i c a l agent S p i p e r o n e d i r e c t l y o n a s o l i d b i o l o g i c a l l y relevant tissue m a t r i x . T h i s data s h o w s that the two-laser i o n trap m e t h o d is suitable for a p p l i c a t i o n s w i t h c o m p l i c a t e d matrices w i t h no need for sample pre-treatment. F i n a l l y , the t h i r d section o f this thesis describes the a d d i t i o n o f a t h i r d , tunable laser to the system. T h i s laser a l l o w s for o p t i c a l p r o b i n g o f the trapped i o n c l o u d . V i s i b l e a b s o r p t i o n spectra for the gas phase P A H cations i s o m e r s phenanthrene and anthracene are s h o w n . T h e t h i r d laser also a l l o w s for the p o s s i b i l i t y o f w a v e l e n g t h selective p h o t o d i s s o c i a t i o n o f P A H i s o m e r s for r e s o l v i n g c o m p l i c a t e d i s o m e r m i x t u r e s .  ii  TABLE OF CONTENTS  ABSTRACT  II  T A B L E OF CONTENTS  Ill  LIST OF T A B L E S  VII  LIST OF FIGURES  VIII  LIST OF ABBREVIATIONS  XV  ACKNOWLEDGMENTS 1  INTRODUCTION  1  1.1  OVERVIEW  1  1.2  M A S S SPECTROMETRY  2  1.2.1  Theory of the Quadrupole  Ion Trap  4  1.2.2  Mathematical  of the Ion Trap  7  1.2.3  The 3D Quadrupole  1.3  Description  Ion Trap as a Mass Spectrometer  S A M P L I N G A N D IONIZATION FOR M A S S S P E C T R O M E T R Y  1.3.1  Two-Laser  1.3.2  Laser - Solid Interactions  1.3.3  Laser Photoionization  1.4 1.4.1  2  XIX  Solid Sampling  (L2MS)  24  29  :  ...  33 .-. 36  EXPERIMENTAL 2.1  .....22  ..26  C O U P L I N G T w o - L A S E R S O L I D S A M P L I N G WITH A N I O N T R A P This Work  15  38  INTRODUCTION  38 iii  2.2  T H E ION T R A P E L E C T R O D E S A N D V A C U U M M A N I F O L D  40  2.3  ELECTRONICS A N D TIMING  47  2.3.1  Ion Trap Electronics  47  2.3.2  The Laser Timing  49  2.4  S O F T W A R E FOR ION T R A P O P E R A T I O N  53  2.5  O V E R A L L SYSTEM A N D TIMING  59  T W O L A S E R ION TRAP MASS S P E C T R O M E T R Y F O R T H E A N A L Y S I S OF E N V I R O N M E N T A L SAMPLES  62  3.1  INTRODUCTION  62  3.2  EXPERIMENTAL  66  3.3  R E S U L T S A N D DISCUSSION  67  3.3.1  Test Of Instrument Effectiveness  3.3.2  Sample Preparation  3.3.3  Two Laser Ion Trap Mass Spectra  3.3.4  Effect of IR Power on Observed Signal  81  3.3.5  Effect of UV power on Observed Signal  85  3.3.6  Semi-Quantitative Analysis  3.3.7  Selective Ion Accumulation  3.3.8  SRM1994 Analysis  3.4  67 74 78  88 90 94  CONCLUSIONS  97  DESORPTION PROFILES OF PAHS F R O M A C T I V A T E D C H A R C O A L 98 4.1  INTRODUCTION  98 iv  4.2  EXPERIMENTAL  100  4.3  RESULTS A N D DISCUSSION  100  CONCLUSIONS  109  4.4  5  .  DETECTION OF THE DRUG SPIPERONE ON BIOLOGICAL MATRICES  6  5.1  INTRODUCTION  5.2  EXPERIMENTAL  5.3  R E S U L T S A N D DISCUSSION  115  5.4  CONCLUSIONS  124  Ill ,  OPTICAL SPECTROSCOPY IN A N ION TRAP  114  .  ..  ..'  ".  ....132  6.1  INTRODUCTION  6.2  EXPERIMENTAL  136  6.3  R E S U L T S A N D DISCUSSION  141  .132  6.3.1  Photodissociation of Trapped Cations  6.3.2  Fragmentation Pathways in Anthracene and Phenanthrene  150  6.3.3  Visible Spectra of Phenanthrene and Anthracene cations....  .....153  6.4  7  H I  CONCLUSIONS  141  159  SEMI-QUANTITATIVE DETERMINATION OF P A H ISOMERS DIRECTLY  F R O M SOLID MATRICES  162  7.1  INTRODUCTION  162  7.2  EXPERIMENTAL  167  7.3  R E S U L T S A N D DISCUSSION  : 168  v  7.4  8  CONCLUSION  178  CONCLUSIONS  182  8.1  GENERALITIES  182  8.2  ENVIRONMENTAL ANALYSIS  183  8.3  BIOLOGICAL ANALYSIS  186  8.4  SPECTROSCOPY OF T R A P P E D IONS  188  REFERENCES  191  vi  LIST O F T A B L E S  T a b l e 3.1 P A H c o m p o u n d s certified to be c o n t a i n e d i n S R M 1944a T a b l e 4.1 E x p e r i m e n t a l l y determined i d e a l delay times b e t w e e n laser events  96 103  T a b l e 4.2 T a b l e o f P A H mass and e x p e r i m e n t a l l y d e t e r m i n e d M a x w e l l - B o l t z m a n n temperature for laser d e s o r p t i o n f r o m activated c h a r c o a l  108  T a b l e 7.1 S u m m a r y o f s y m b o l s and reference slopes used i n this chapter  172  T a b l e 7.2 S u m m a r y o f o b s e r v e d data for the " u n k n o w n " s a m p l e  179  vn  LIST OF FIGURES  F i g u r e 1.1 P i c t o r i a l d i a g r a m o f the i o n trap mass spectrometer, (a) A cross s e c t i o n t h r o u g h the l o n g i t u d i n a l axis, (b) A t h r e e - d i m e n s i o n a l representation  6  F i g u r e 1.2 A g r a p h i c a l representation o f the stable solutions to the M a t h i e u e q u a t i o n i n b o t h the r a d i a l (r) and a x i a l (z) directions i n terms o f a and q u  u  [13]  12  F i g u r e 1.3 A h enlarged v i e w o f the e x p e r i m e n t a l l y important r e g i o n o f the M a t h i e u stability d i a g r a m [13]  13  F i g u r e 1.4 A n enlarged v i e w o f the stability d i a g r a m s h o w i n g the p o s s i b l e p o s i t i o n s o f several i o n s o f different mass o n the stability d i a g r a m under v a r i o u s R F v o l t a g e conditions  16  F i g u r e 1.5 S c h e m a t i c c a r t o o n o f two-laser m a s s spectrometry as it has been p r e v i o u s l y applied  ...  25  F i g u r e 1.6 S i m p l i f i e d J a b l o n s k i d i a g r a m for a n u m b e r o f p o s s i b l e p h o t o n / m o l e c u l e interactions, (a) S i n g l e p h o t o n i o n i z a t i o n (b) N o i o n i z a t i o n (c) R e s o n a n t t w o p h o t o n i o n i z a t i o n (d) N o n R e s o n a n t m u l t i p h o t o n i o n i z a t i o n (e) T w o p h o t o n resonant ionization  31  F i g u r e 2.1 A c o n c e p t u a l representation o f the two-laser i o n trap s y s t e m at U B C  39  F i g u r e 2.2 V a c u u m m a n i f o l d for the two-laser i o n trap system at U B C  41  F i g u r e 2.3 S o l i d s a m p l e probe for the t w o - l a s e r i o n trap s y s t e m at U B C  43  F i g u r e 2.4 C l o s e u p v i e w o f the I R and U V lasers as they pass t h r o u g h the i o n trap  45  F i g u r e 2.5 T i m i n g d i a g r a m for the two-laser system used at U B C  52  F i g u r e 2.6 P r i m a r y operating w i n d o w for I o n T r a p c o n t r o l software. W r i t t e n i n the L a b V i e w environment  54 viii  F i g u r e 2.7 F l o w chart for the I o n T r a p software.......  ".  .'  56  F i g u r e 2.8 L o g i c a l b l o c k d i a g r a m s h o w i n g the flow o f energy and i n f o r m a t i o n for the two-laser i o n trap system at U B C  60  F i g u r e 3.1 T h e s i x p o l y c y c l i c aromatic h y d r o c a r b o n s ( P A H s ) p r i m a r i l y u s e d i n this w o r k . ;  63  F i g u r e 3.2 M a s s s p e c t r u m o f C C L i o n i z e d b y electron i m p a c t i o n i z a t i o n , r e s u l t i n g i n the f o r m a t i o n o f C C L * ions  68  F i g u r e 3.3 M a s s s p e c t r u m o f C C I 4 i o n i z e d b y electron i m p a c t i o n i z a t i o n and ejected f r o m the i o n trap b y the resonance ejection m o d e o f operation  70  F i g u r e 3.4 M a s s s p e c t r u m o f CCI4 i o n i z e d b y electron i m p a c t i o n i z a t i o n w i t h a single N B B W c y c l e after i o n i z a t i o n . T h e N B B W c y c l e caused the r e m o v a l o f a l l m a s s c o m p o n e n t s b e l o w 119 T h and above 120 T h  71  F i g u r e 3.5 M a s s s p e c t r u m o f CCI4 i o n i z e d b y electron i m p a c t i o n i z a t i o n a n d w i t h r e m o v a l o f 119 T h b y a single frequency ejection after i o n i z a t i o n  73  F i g u r e 3.6 M e c h a n i c a l press and sample cup u s e d i n the creation o f s o l i d samples for the t w o - l a s e r system at U B C  77  F i g u r e 3.7 T y p i c a l mass spectrum c o l l e c t e d w i t h the two-laser m o d e o f i o n i z a t i o n o f a s a m p l e o f c h a r c o a l s p i k e d w i t h five P A H s  ;  F i g u r e 3.8 E x p a n d e d v i e w o f F i g u r e 3.7 between mass 130-265 T h  79 80  F i g u r e 3.9 M a g n i t u d e o f the pyrene peak area as a function o f the n u m b e r o f laser c y c l e s for a c h a r c o a l sample s p i k e d w i t h pyrene  82  ix  F i g u r e 3.10 Effect o f I R p o w e r o n o b s e r v e d s i g n a l for phenanthrene as a f u n c t i o n o f the n u m b e r o f laser shots for three I R laser p o w e r s . • M e a s u r e d I R P o w e r (Left H a n d Units). •  Integrated P e a k A r e a s ( R i g h t H a n d U n i t s )  83  F i g u r e 3.11 U V p o w e r (uJ) vs. integrated peak areas for f i v e P A H s . T h e x - a x i s is i n the units o f u J o f U V energy i n the trap v o l u m e and the y - a x i s is i n terms o f integrated peak areas  86  F i g u r e 3.12 C o n c e n t r a t i o n vs. measured s i g n a l for a series o f p y r e n e / c h a r c o a l standards. ;  .89  F i g u r e 3.13 M a s s s p e c t r u m f r o m a sample c o n t a i n i n g 6 P A H s (chrysene depleated) [black l i n e ] . A f t e r chrysene gas phase pre c o n c e n t r a t i o n [gray l i n e ] .  92  F i g u r e 3.14 A v e r a g e chrysene peak height n o r m a l i z e d relative to a single laser c y c l e as a f u n c t i o n o f the n u m b e r o f laser c y c l e s  93  F i g u r e 3.15 T w o laser mass spectrum o b s e r v e d f r o m a sample o f standard reference m a t e r i a l 1944a  95  F i g u r e 4.1 Integrated peak areas vs. I R - U V delay t i m e for A c e n a p h t h e n e , Phenanthrene, P y r e n e , C h r y s e n e , and B e n z o [ a ] p y r e n e  102  F i g u r e 4.2 C h a r t o f e x p e r i m e n t a l l y determined M a x w e l l - B o l t z m a n n temperature v s . P A H mass  108  F i g u r e 5.1 P h o t o o f b r a i n tissue extracted f r o m a m a l e Sprague D a w l e y rat u s e d i n this work  116  F i g u r e 5.2 P h o t o o f l i v e r tissue extracted f r o m a m a l e Sprague D a w l e y rat u s e d i n this work F i g u r e 5.3 T w o laser mass spectrum o f S p i p e r o n e o n a c h a r c o a l m a t r i x  116 118  F i g u r e 5.4 T w o laser mass spectrum o f S p i p e r o n e o n a c h a r c o a l m a t r i x w i t h 1,5, a n d 10 laser c y c l e s f o l l o w e d b y a s i n g l e N B B W pulse  119  F i g u r e 5.5 T w o laser mass spectrum o f S p i p e r o n e o n a c h a r c o a l m a t r i x w i t h f i v e laser c y c l e s f o l l o w e d b y a single N B B W pulse and a c o l l i s i o n i n d u c e d d i s s o c i a t i o n ( C I D ) waveform  121  F i g u r e 5.6 T w o laser mass spectrum o f a slice o f b r a i n tissue f r o m a m a l e SpragueD a w l e y rat  122  F i g u r e 5.7 T w o laser mass s p e c t r u m o f a slice o f l i v e r tissue f r o m a m a l e SpragueD a w l e y rat  123  F i g u r e 5.8 T w o laser mass spectrum o f a slice o f b r a i n tissue f r o m a m a l e S p r a g u e D a w l e y rat w h i c h had been s p i k e d w i t h a S p i p e r o n e s o l u t i o n  125  F i g u r e 5.9 T w o laser mass spectrum w i t h 5 laser c y c l e s f o l l o w e d b y a s i n g l e N B B W p u l s e o f a s l i c e o f b r a i n tissue f r o m a m a l e S p r a g u e - D a w l e y rat that h a d b e e n s p i k e d w i t h a Spiperone solution  126  F i g u r e 5.10 T w o laser mass spectrum w i t h 5 laser c y c l e s f o l l o w e d b y a s i n g l e N B B W p u l s e a n d a s i n g l e C I D w a v e f o r m o f a slice o f b r a i n tissue f r o m a m a l e S p r a g u e D a w l e y rat w h i c h h a d been s p i k e d w i t h a S p i p e r o n e s o l u t i o n  127  F i g u r e 5.11 T w o laser mass spectrum o f a slice o f l i v e r tissue f r o m a m a l e S p r a g u e D a w l e y rat w h i c h h a d been s p i k e d w i t h a S p i p e r o n e s o l u t i o n  128  F i g u r e 5.12 T w o laser mass spectrum w i t h 5 laser c y c l e s f o l l o w e d b y a s i n g l e N B B W p u l s e o f a s l i c e o f l i v e r tissue froiri a m a l e S p r a g u e - D a w l e y rat that h a d b e e n s p i k e d w i t h a Spiperone solution  129  xi  F i g u r e 5.13 T w o laser mass spectrum w i t h 5 laser c y c l e s f o l l o w e d b y a s i n g l e N B B W pulse a n d a single C I D w a v e f o r m o f a slice o f l i v e r tissue f r o m a m a l e S p r a g u e D a w l e y rat w h i c h h a d been s p i k e d w i t h a S p i p e r o n e s o l u t i o n  130  F i g u r e 6.1 D i a g r a m o f the three-laser set-up at U B C  138  F i g u r e 6.2 E n h a n c e d p h o t o o f the three-laser set-up at U B C  139  F i g u r e 6.3 D i a g r a m o f the I R desorption, U V p h o t o i o n i z a t i o n , a n d v i s i b l e photo fragmentation lasers interacting w i t h the interior o f the i o n trap  140  F i g u r e 6.4 T h e P A H isomers at 178 T h phenanthrene and anthracene  142  F i g u r e 6.5 T w o laser mass spectrum o f a sample o f phenanthrene o n activated c h a r c o a l . 143  v  F i g u r e 6.6 T w o laser mass spectrum o f a sample o f phenanthrene o n activated c h a r c o a l w i t h the a d d i t i o n o f a N B B W i s o l a t i o n pulse  145  F i g u r e 6.7 T w o laser mass spectrum o f phenanthrene o n activated c h a r c o a l w i t h the a d d i t i o n o f a N B B W i s o l a t i o n pulse f o l l o w e d b y 5. p h o t o d i s s o c i a t i o n laser shots (892 nm)  .,_  146  F i g u r e 6.8 T w o laser mass spectrum o f a sample o f anthracene o n activated c h a r c o a l w i t h the a d d i t i o n o f a N B B W i s o l a t i o n pulse f o l l o w e d b y 5 p h o t o d i s s o c i a t i o n laser shots (682 n m )  147  F i g u r e 6.9 D a u g h t e r i o n p o p u l a t i o n o b s e r v e d o v e r 0-4 p h o t o d i s s o c i a t i o n laser shots ( f o c u s i n g a r o u n d 152 T h ) o n a sample o f anthracene  148  F i g u r e 6.10 D a u g h t e r i o n p o p u l a t i o n observed o v e r 0-4 p h o t o d i s s o c i a t i o n laser shots ( f o c u s i n g a r o u n d 178 T h ) o n a sample o f anthracene  xn  148  F i g u r e 6.11 N o r m a l i z e d areas for the ratio o f 152/178 v s . the n u m b e r o f p h o t o d i s s o c i a t i o n laser shots o n a sample o f anthracene  149  F i g u r e 6.12 F r a g m e n t a t i o n efficiency (ratio o f 152/178 T h ) v s . energy at t w o different p h o t o d i s s o c i a t i o n laser wavelengths (720 n m and 740 n m ) for a s a m p l e o f anthracene  155  F i g u r e 6.13 T o p s p e c t r u m - P h o t o d i s s o c i a t i o n spectra o f the anthracene c a t i o n (ratio o f 152/178 T h vs. w a v e l e n g t h ) . B o t t o m spectrum - A n t h r a c e n e c a t i o n s p e c t r u m a c q u i r e d i n a f r o z e n a r g o n m a t r i x at 12 K [196]  156  F i g u r e 6.14 T o p spectrum- P h o t o d i s s o c i a t i o n spectrum o f the phenanthrene c a t i o n (ratio o f 152/178 T h v s . w a v e l e n g t h ) . B o t t o m spectra- Phenanthrene c a t i o n s p e c t r u m a c q u i r e d i n a f r o z e n n e o n m a t r i x at 4.2 K [165]  158  F i g u r e 7.1 A n t h r a c e n e a n d phenanthrene photofragmentation spectra a c q u i r e d i n a n i o n trap b y the R E M P D m e t h o d  166  F i g u r e 7.2 T w o laser mass s p e c t r u m o f a sample o f phenanthrene a n d p y r e n e o n activated c h a r c o a l w i t h the a d d i t i o n o f a N B B W laser pulse. T h e gray l i n e is w i t h the a d d i t i o n o f 2 0 photofragmentation laser shots at 892 n m  174  F i g u r e 7.3 T w o laser s p e c t r u m o f a sample o f anthracene and pyrene o n activated c h a r c o a l w i t h the a d d i t i o n o f a swift laser pulse. T h e gray l i n e is w i t h the a d d i t i o n o f 2 0 photofragmentation laser shots at 892 n m  175  F i g u r e 7.4 C a l i b r a t i o n c u r v e for anthracene and phenanthrene m e a s u r e d relative to p y r e n e w i t h a n d w i t h o u t the a d d i t i o n o f the photofragmentation laser  176  F i g u r e 7.5 T w o laser mass spectrum o f a n " u n k n o w n " sample o f anthracene, phenanthrene, and pyrene o n activated c h a r c o a l w i t h the a d d i t i o n o f a N B B W laser  xiii  p u l s e . T h e gray l i n e is w i t h the: a d d i t i o n o f 2 0 photofragmentation laser shots at 892 nm.  :  :  •  F i g u r e 8.1 T w o laser mass spectrum o f B u c k m i n s t e r f u l l e r e n e  xiv  177 190  LIST OF ABBREVIATIONS  [A]  A n t h r a c e n e concentration  [P]  phenanthrene c o n c e n t r a t i o n  [Py]  pyrene concentration  0-p  zero-to-peak  a  acceleration (m-s" )  AC  A l t e r n a t i n g Current  ADC  Analog-to-Digital Conversion  amu  a t o m i c mass unit  a  M a t h i e u parameter (dimensionless)  u  C  Constant u s e d i n M - B equation  CI  Chemical Ionization  CID  C o l l i s i o n Induced Dissociation  cm  Centimeter  d  distance between d e s o r p t i o n and i o n i z a t i o n events i n M - B equation  Da  Dalton  DAC  Digital-to-Analog Conversion  DC  Direct Current  DIB  D i f f u s e Interstellar B a n d s  E  Electric field (V-m" )  e  elementary charge ( 1 . 6 0 2 1 - 1 0 " C )  EI  E l e c t r o n Impact I o n i z a t i o n  ESI  Electrospray Ionization  eV  electron V o l t ( 1 . 6 0 2 1 8 9 x 1 0 - J )  f  frequency (cycles/second) .  F  Force (kg-nrs")  FAB  Fast A t o m B o m b a r d m e n t  fragA  fragmentation e f f i c i e n c y o f anthracene i n d u c e d b y v i s i b l e laser  fragP  fragmentation e f f i c i e n c y o f phenanthrene i n d u c e d b y v i s i b l e  1  19  |9  xv  laser fragPy FT-ICR  fragmentation e f f i c i e n c y o f pyrene i n d u c e d b y v i s i b l e laser •  Fourier Transform Ion Cyclotron Resonance  FWHM  F u l l W i d t h at H a l f M a x i m u m  g(t)  the observed s i g n a l i n the M - B as a f u n c t i o n o f t i m e  GC  Gas Chromatography  GPIB  G e n e r a l P u r p o s e Interface B u s  I/O  Input/Output  I  o b s e r v e d peak intensity o f anthracene  A  IEEE  Institute o f E l e c t r i c a l and E l e c t r o n i c E n g i n e e r s  I  observed peak intensity o f phenanthrene  P  Ip  observed peak intensity o f pyrene  y  IR  Infrared  K  Kelvin  k  B o l t z m a n n C o n s t a n t (1.3807 x 1 0 " J / K )  L2MS  laser desorption laser p h o t o i o n i z a t i o n mass spectrometry  LC  L i q u i d Chromatography  LITD  Laser Induced Thermal Desorption  m  mass (kg)  23  m/z  mass/charge  MALDI  Matrix Assisted Laser Desorption Ionization  M-B  Maxwell-Boltzmann  MHz  mega H z ( 1 0 cycles/second)  M-n  mass peak at parent mass ( M ) m i n u s a w h o l e n u m b e r (n)  MPI  M u l t i Photon Ionization  MS  M a s s Spectrometry  MS  N  6  T a n d e m mass spectrometry to the N - l l e v e l  MW  Molecular Weight  NBBW  Notched Broad Band Waveform  Nd:YAG  N e o d y m i u m : Y t t r i u m A l u m i n u m Garnet L a s e r  PAH  Polycyclic Aromatic Hydrocarbon  xvi  PCI  P e r s o n a l C o m p u t e r Interface B o a r d  PD  Photo Dissociation  PI  Photo Ionization  q  M a t h i e u parameter ( d i m e n s i o n l e s s )  u  r  radial coordinate i n the trap v o l u m e  REMPD  Resonance Enhanced Multiphoton Dissociation  REMPI  Resonance Enhanced M u l t i p h o t o n Ionization  RF  Radio Frequency  RLO  R a t i o ( w i t h the ) L a s e r O N  RNL  R a t i o (with) N o ( v i s i b l e ) L a s e r  r  internal radius o f r i n g electrode  0  s  second  SIMS  S e c o n d a r y I o n M a s s Spectrometry  t  t i m e (seconds) - .  TOF  T i m e o f F l i g h t mass spectrometry  U  D C c o m p o n e n t o f <P  UBC  University o f British Columbia  US E P A  U n i t e d States E n v i r o n m e n t a l P r o t e c t i o n A g e n c y  UV  Ultraviolet  V  A C component o f O  v  stream v e l o c i t y  .  0  s  :  ^  0  vs.  versus  VUV  Vacuum Ultra Violet  W  Watt  z  charge  z  a x i a l coordinate i n the trap v o l u m e  z  •  :.„•• ] . '  A o f the endcap electrode spacing  X 0  CXA  constant for desorption/ionization/trapping  efficiencies for  anthracene ap  constant for desorption/ionization/trapping phenantherene  xvii  efficiencies for  constant for d e s o r p t i o n / i o n i z a t i o n / t r a p p i n g efficiencies for pyrene p  i o n trajectory stability v a l u e ( d i m e n s i o n l e s s )  y  w e i g h t i n g constant for z  X  w e i g h t i n g constant for x  um  micrometers  $  M a t h i e u equation parameter  a  w e i g h t i n g constant for y angular frequency o f the A C potential (radians/second)  CO ,n  n  o  a p p l i e d electric potential  u  0  t h  order frequency o f a stable i o n trajectory  XVlll  ACKNOWLEDGMENTS  T h e c o m p l e t i o n o f a d o c t o r a l thesis represents a t u r n i n g p o i n t i n ones life. H o w e v e r , this is i n no w a y an achievement o f a single person. Instead, it is the result o f the interactions o f m a n y people w h o have contributed their t i m e and energy to m a k i n g this p o s s i b l e . I have been fortunate throughout m y life that these c o n t r i b u t i o n s h a v e a l w a y s a r r i v e d i n a p o s i t i v e and t i m e l y manner. A t e c h n i c a l thesis o f this type c a n o n l y be a c c o m p l i s h e d w i t h the d e d i c a t i o n , s k i l l s , and talents o f a n entire team o f w o r k e r s . I w o u l d l i k e to thank the g e n t l e m e n o f the M e c h a n i c a l and E l e c t r o n i c s shops at U B C for their a d v i c e and q u a l i t y w o r k m a n s h i p . Especially; D a v e Bains, M a r t i n Carlisle, M i l a n Coschizza, B r i a n D i t c h b u r n , Jason G o z j o l k o , B r i a n G r e e n e , K e n L o v e , R o n M a r w i c k , B r i a n Snapkauskas, and D a v e T o n k i n . I w o u l d also l i k e to thank m y m a n y a c a d e m i c colleagues and mentors w h o m p r o v i d e d m u c h support and encouragement o v e r the years. T h a n k s are e s p e c i a l l y due to m y s u p e r v i s o r M i k e B l a d e s , for a l l o w i n g a m p l e i n t e l l e c t u a l freedom w h i l e p r o v i d i n g ready support and encouragement w h e n needed. I w o u l d also l i k e to thank J o h n H e p b u r n a n d D o n D o u g l a s for a l l o w i n g m e to be a guest i n their respective groups. T h a n k s are also extended to the m a n y other a n a l y t i c a l m e m b e r s o f the department for there h e l p a n d a d v i c e , i n c l u d i n g D a v i d C h e n , A l a n B e r t r a m , B r u c e T o d d , the D o u g l a s G r o u p , the C h e n G r o u p , the H e p b u r n G r o u p , and o f course m y f e l l o w B l a d e s group m e m b e r s . I w o u l d e s p e c i a l l y l i k e to thank C h r i s B a r b o s a , D a v e M c L a r e n , D e n i s R o l l a n d , a n d K e n W r i g h t for m a n y useful discussions, and e v e n m o r e not so useful ones. I also w o u l d l i k e to thank m y f a m i l y and friends b o t h i n C a n a d a , the U S , a n d abroad. I feel t r u l y blessed to have t h e m i n m y life. E s p e c i a l l y to m y mother and father w h o never d o u b t e d m y d e c i s i o n s or m y methods. S p e c i a l thanks are also extended to J o a n n a K i r k e m y friend a n d c o m p a n i o n . I w o u l d also l i k e to a c k n o w l e d g e the f i n a n c i a l support I r e c e i v e d f r o m the U n i v e r s i t y o f B r i t i s h C o l u m b i a and the N a t u r a l S c i e n c e and E n g i n e e r i n g R e s e a r c h C o u n c i l o f Canada. F i n a l l y , I w o u l d l i k e to dedicate this thesis to G l e n N a g a n o , G r a d e 7 teacher at L o r d Strathcona E l e m e n t a r y . I often w o n d e r h o w m y life w o u l d have turned out i f I w e r e not so fortunate as to have been i n y o u r class.  xix  CHAPTER 1 INTRODUCTION 1.1 Overview T h i s thesis describes the c o n s t r u c t i o n a n d c h a r a c t e r i z a t i o n o f a n e w a n a l y t i c a l instrument that c o m b i n e s one o f the most useful and versatile mass spectrometers, the q u a d r u p o l e i o n trap, w i t h a v e r y selective and sensitive m e t h o d for direct s o l i d s a m p l e a n a l y s i s , t w o laser s a m p l i n g and i o n i z a t i o n . T h i s instrument w i l l be s h o w n to have m a n y advantages o v e r the t r a d i t i o n a l means o f p e r f o r m i n g these techniques. T o appreciate the advantages o f c o m b i n i n g these t w o methods, the r e m a i n d e r o f this chapter w i l l describe the i o n trap mass spectrometer and the t y p i c a l m o d e s o f its operation. It w i l l also p r o v i d e a n i n t r o d u c t i o n to laser s a m p l i n g theory a n d the c o m b i n a t i o n o f laser d e s o r p t i o n - laser p h o t o i o n i z a t i o n as a m e t h o d o l o g y f o r c h e m i c a l analysis. F i n a l l y , this chapter w i l l c o n c l u d e w i t h a d e s c r i p t i o n o f the advantages and disadvantages that result f r o m c o m b i n i n g this m o d e o f s a m p l i n g and i o n i z a t i o n w i t h a n i o n trap mass spectrometer.  C H A P T E R 2 w i l l describe the d e s i g n , d e v e l o p m e n t , c o n s t r u c t i o n , and i n i t i a l c h a r a c t e r i z a t i o n o f the instrument. It w i l l p r o v i d e s p e c i f i c details o f the c o m p o n e n t s a n d parts o f the instrument a n d general details o n its methods o f operation.  The remaining  chapters w i l l describe experiments that were p e r f o r m e d to characterize the potential uses o f this instrument. A l l o f these r e m a i n i n g "data" chapters w i l l have their o w n i n t r o d u c t i o n and e x p e r i m e n t a l sections i n order to place the w o r k i n context relative to  1  the literature a n d describe the specific operating c o n d i t i o n s under w h i c h the e x p e r i m e n t s were performed. C H A P T E R 3 w i l l s h o w t y p i c a l results obtained i n the routine o p e r a t i o n o f this instrument w i t h a v a r i e t y o f analytes a n d samples o f e n v i r o n m e n t a l interest. It w i l l also demonstrate some t y p i c a l m o d e s o f operation o f the instrument. C H A P T E R 4 describes the laser d e s o r p t i o n process for a particular sample type w h i c h w a s e x a m i n e d i n d e t a i l . I n C H A P T E R 5, the results o f experiments c o n c e r n i n g b i o l o g i c a l samples are presented. S p e c i f i c a l l y , these experiments were c o n c e r n e d w i t h the e x a m i n a t i o n o f d r u g m o l e c u l e s d i r e c t l y o n b i o l o g i c a l matrices. C H A P T E R S 6 a n d 7 describe the a d d i t i o n o f a t h i r d (tunable) laser to the apparatus. T h i s laser p r o v i d e s a means o f p r o b i n g i o n s that have b e e n c o n t a i n e d i n the i o n trap. C H A P T E R 6 s h o w s results that demonstrate this m e t h o d ' s a b i l i t y to c o l l e c t gas phase v i s i b l e spectra o f trapped i o n s b y the m e t h o d o f R e s o n a n c e E n h a n c e d M u l t i p h o t o n D i s s o c i a t i o n ( R E M D ) . C H A P T E R 7 demonstrates a p r a c t i c a l a p p l i c a t i o n o f the R E M P D technique to d i s c r i m i n a t e b e t w e e n isomers o f P o l y c y c l i c A r o m a t i c Hydrocarbons ( P A H s ) .  F i n a l l y , C H A P T E R 8, the " c o n c l u s i o n s " chapter w i l l  s u m m a r i z e the k e y results f r o m this course o f w o r k a n d suggest methods to i m p r o v e this instrument i n the future.  1.2 Mass Spectrometry A l l mass spectrometry ( M S ) devices have three distinct features i n c o m m o n . S a m p l e atoms or m o l e c u l e s are first c o n v e r t e d to gas phase i o n s - the i o n i z a t i o n step. The i o n s are then separated or a n a l y z e d based o n their mass to charge ratio ( m / z ) a n d then the i o n s are detected i n a systematic w a y . T h e results are a l m o s t a l w a y s c o n v e r t e d 2  into a p l o t (the mass spectrum) o f i o n count (intensity) vs. the mass to charge ratio ( m / z ) . B y careful a n a l y s i s o f the mass spectrum, one c a n g a i n a w e a l t h o f i n f o r m a t i o n (quantitative a n d qualitative) about the m o l e c u l e s o r atoms c o n t a i n e d i n the o r i g i n a l sample. A l l f o r m s o f mass spectrometry c a n be traced b a c k to the w o r k o f S i r J . J . T h o m s o n o f C a v e n d i s h L a b o r a t o r y at the U n i v e r s i t y o f C a m b r i d g e [1]. H i s w o r k , c a r r i e d out i n the latter part o f the 19th century, focused o n e x a m i n i n g e l e c t r i c a l discharges a n d l e d to the d i s c o v e r y o f the electron i n 1897. D u r i n g the next decade o f research, T h o m s o n r e a l i z e d that a b e a m o f ions c o u l d be caused to change their trajectories i n the presence o f e l e c t r i c o r m a g n e t i c fields. B y 1910 T h o m s o n h a d created the essential c o m p o n e n t s o f a mass spectrometer [2]. T h i s r u d i m e n t a r y d e v i c e c o n s i s t e d o f a s i m p l e i o n source (a d i s c h a r g e tube), a d i s p e r s i o n element ( t w o m a g n e t i c p o l e s ) , a n d a detector ( p h o t o g r a p h i c plates).  W h i l e the f i e l d o f mass spectrometry has g r o w n i m m e n s e l y since  this t i m e , the u n d e r l y i n g p r i n c i p l e s have r e m a i n e d the same. D u r i n g the last 100 years o f research, the f i e l d o f mass spectrometry has e v o l v e d f r o m the d e v e l o p m e n t phase, w h e r e the instrument w a s the experiment, to the d i v e r s i f i c a t i o n phase w h e r e n o w mass spectrometers are u s e d r o u t i n e l y i n c o m m e r c i a l labs a l l o v e r the w o r l d for sample analysis a n d w o r k l i k e that presented i n this thesis is done to f i n d w a y s o f m a k i n g the instrument e v e n m o r e useful for the case o f specific samples.  T h e f i e l d o f mass spectrometry has e v o l v e d to a p o i n t n o w w h e r e there are  currently f i v e d i s t i n c t l y different types o f mass spectrometer c o m m e r c i a l l y a v a i l a b l e . T h e s e are the: m a g n e t i c / e l e c t r i c sector mass spectrometer [3], R F linear q u a d r u p o l e mass spectrometer [4], 3 D q u a d r u p o l e i o n trap mass spectrometer [5], F o u r i e r t r a n s f o r m I o n  3  C y c l o t r o n R e s o n a n c e ( F T - I C R ) mass spectrometer [6, 7 ] , a n d the T i m e o f F l i g h t ( T O F ) mass spectrometer [8]. A l l totaled, mass spectrometry is a b i l l i o n d o l l a r business w i t h a n a l y t i c a l a p p l i c a t i o n s i n s u c h diverse fields as g e o l o g y , b i o l o g y , c h e m i s t r y , p h y s i c s , a n d e n v i r o n m e n t a l science. T h e m o s t recent a d d i t i o n to the f a m i l y o f c o m m e r c i a l l y a v a i l a b l e mass spectrometers o c c u r r e d i n 1983 w h e n the F i n n i g a n M A T C o r p o r a t i o n o f S a n Jose f o u n d a m e t h o d o f t u r n i n g the i o n trap into a useful mass spectrometer [9, 10]. B e f o r e this, the i o n trap h a d b e e n p r i m a r i l y u s e d b y p h y s i c i s t s as a means o f c o n t a i n i n g gas phase i o n s o r as an i o n source for other types o f mass analyzers [11]. S i n c e this t i m e , i o n trap mass spectrometers h a v e b e c o m e one o f the m o s t c o m m o n mass analyzers i n use because they have v e r y h i g h scan speeds, are sensitive, a n d are inherently capable o f p e r f o r m i n g M S  N  (tandem mass spectrometry) experiments [12].  1.2.1 Theory of the Quadrupole Ion Trap F o r a c o m p r e h e n s i v e , detailed account o f the d e v e l o p m e n t , theory, a n d o p e r a t i o n o f the q u a d r u p o l e i o n trap the reader is referred to the preeminent text o n the matter: " Q u a d r u p o l e Storage M a s s S p e c t r o m e t r y " b y M a r c h a n d H u g h e s [13]. A l t e r n a t i v e l y , m o r e c o n c i s e v e r s i o n s have appeared elsewhere [5, 14].  T h e role o f this s e c t i o n is to  p r o v i d e the reader w i t h a n understanding o f i o n trap operation sufficient to appreciate its a p p l i c a t i o n to the w o r k d e s c r i b e d i n this thesis. T h e quadrupole i o n trap h a d its first p u b l i c d i s c l o s u r e i n a patent f i l e d i n 1953 b y P a u l a n d S t e i n w e d e l o f the U n i v e r s i t y o f B o n n [15]. A t this t i m e , it w a s listed as " s t i l l another electrode arrangement" m e n t i o n e d a l o n g w i t h a n u m b e r o f other geometries that w e r e b e i n g d e s c r i b e d to guide and mass select i o n s . Indeed, it w a s P a u l ' s g r o u p that 4  realized that strongly focusing fields could be utilized to provide mass analysis, and the first detailed account o f the operation o f the ion trap appeared in the thesis o f Berkling i n 1956 [16]. The ion trap is, i n principle, a simple device consisting o f three electrodes: two parabolic end cap electrodes and a single central ring electrode. A schematic diagram o f the ion trap is given in Figure 1.1. The geometry o f the electrodes in an idealized mathematical state can be described by the following equations.  r  End Caps:  .2  2  _±_  2  r ~ — _£_ 2  Ring:  2  2  r 0  \  Equ. 1.1  =  _i  E  q ° - 1-  2  z o  Where, r  =  2  o  is the radius o f the ring electrode and z  0  is the closest distance from the  center o f the trap to an end cap with r and z denoting the surface o f the electrodes. For the quadrupole ion trap used in this thesis, r  Q  and z  a  are related by:  l = l  r  Equ. 1.3  2z  Therefore, to define a specific ion trap, one only needs to know the r value; 0  typically, ions traps are built with a r value between l-25mm. The ion trap used i n this 0  thesis has an internal radius r = 10.0 mm. 0  The ion trap is a dynamic and complicated device, however, to a first approximation, it can be described simply. Consider a single positive charge placed i n the center o f the ion trap. If a direct current (DC) potential were applied between the end caps and the ring electrode, so that the ring electrode was negatively charged and the end caps appeared positive, then the ion would find itself confined in the z direction (between  Figure 1.1 Pictorial diagram of the ion trap m a s s spectrometer, (a) A c r o s s section through the longitudinal axis, (b) A three-dimensional representation.  e n d caps). I f the charge made any m o v e m e n t towards one o f the e n d caps it w o u l d i t s e l f e x p e r i e n c e a restoring force p r o p o r t i o n a l to the d i s p l a c e m e n t distance. C o n v e r s e l y , i n the r a d i a l d i r e c t i o n , any s m a l l m o v e m e n t a w a y f r o m the center o f the trap w o u l d result i n the charge b e i n g accelerated towards the r i n g electrode, as the charge w o u l d e x p e r i e n c e a n attractive potential. S i m i l a r l y , i f the r i n g were set p o s i t i v e and end caps negative, then the i o n w o u l d be stable i n the r a d i a l (r) d i r e c t i o n and unstable i n the a x i a l (z) d i r e c t i o n . C o n s e q u e n t l y , a n i o n cannot be c o n f i n e d i n a static electric f i e l d (this is a restatement o f the E a r n s h a w ' s T h e o r e m ) . A s a result a l l quadrupolar t r a p p i n g d e v i c e s must w o r k o n the p r i n c i p l e o f d y n a m i c a l l y c h a n g i n g electric fields. S i m p l y , the potential o n the e n d caps and r i n g m u s t be constantly c h a n g i n g so that the charged particle w i l l never have a chance to i m p a c t one p a r t i c u l a r electrode. T h e forces a n i o n experiences at one instant w i t h i n the i o n trap c a n be d e s c r i b e d simply by:  F = ma = Ee  E q u . 1.4  W h e r e F is the force, m is the mass, a is the acceleration, E is the electric f i e l d and e is the charge o n a particle. W h i l e the forces a n i o n experiences m a y be s i m p l y stated, the m o t i o n that results f r o m the c o n t i n u o u s l y c h a n g i n g electric potentials is v e r y complex.  1.2.2 Mathematical Description of the Ion Trap A s i n g l e i o n i n an i d e a l quadrupolar field, w i l l experience an e l e c t r i c p o t e n t i a l , ()>, at any p o i n t (r, z) w i t h i n the field expressed b y the equation [4, 13]:  7  «* = - % ( r - 2 z ) + ^ 2  Equ.  2  2r  2  n  1.5  Where: <p  a p p l i e d potential  0  radial dimension z  axial dimension  r  radius o f the d e v i c e  0  I f the a p p l i e d potential  at any t i m e (t), consists o f a direct current (U)  c o m p o n e n t a n d a n alternating current (V) component, then: <p„=U-VcosQt  E  q  u  1  -  6  Where: <p  a p p l i e d potential  U  amplitude o f D C  V  a m p l i t u d e o f A C ( m e a s u r e d 0-peak)  Q  angular frequency o f the A C potential (radians/second)  t  time (seconds)  0  T h e a p p l i e d A C potential is listed i n radians/second and is equal to 27$ w h e r e / i s the frequency i n H z . M o s t a n a l y t i c a l l y useful devices operate w i t h an A C frequency i n the 1 M H z range (since 1 M H z is i n the radio-frequency range, the A C p o t e n t i a l w i l l s o m e t i m e s be referred to as the R F potential). Note on the applied potential: for this work, the end caps are earthed and the potential applied to the ring electrode only. The RF potential used here is measured peak to ground.  8  W e m a y n o w rewrite Equ.1.4 i n a differential f o r m , r e c o g n i z i n g that force FN i n the N d i r e c t i o n i s a f u n c t i o n o f acceleration, a, ( w h i c h is equal to the s e c o n d d e r i v a t i v e o f p o s i t i o n w i t h respect to t i m e ) , a n d o f the electric f i e l d E ( w h i c h is equal to the negative o f the first derivative o f the electric potential w i t h respect to p o s i t i o n ) .  r,=ma=m—d z = -e— dd> F  1 7 cEau y u . i.#  2  7  T  Equ. 1.8  l> dr  dr dt  r  d(  2  F=ma=m—^- = - e — 2  M  Substituting the potential ^ f r o m Equ.1.6 into Equ.1.5 y i e l d s :  (u - v cos at)  (u - v cos at)  _  2 z 2  2  2rl  E  q  u  .  1  -  9  2  D i f f e r e n t i a t i n g Equ.1.9 w i t h respect to r a n d z y i e l d s the potential gradients:  ^ _^ dz r =  (  f  7  _  F  c  o  s  Q  Equ. 1.10  0  .  n  ^ d  Equ. 1.11  =4(t/-FcosQr) r  <>.  .  r  •  .  S u b s t i t u t i n g equations Equ.1.10 and Equ. 1.11 into Equ. 1.7 a n d Equ.1.8 respectively gives:  F =ma = m~^^(U-VcosClt) dt rl z  *-  E(  2  u  1  R e a r r a n g i n g Equ.1.12 a n d Equ.1.13 gives the equation o f m o t i o n o f a single c h a r g e d i o n i n a quadrupolar time v a r y i n g electric f i e l d :  9  -  1  2  2e  dz 2  dt  rm  1  (U-VcosQt)z  =0  Equ. 1.14  = 0  Equ. 1.15  2  + -^-(U-VcosQt)r  I n order for a n i o n w i t h mass, m, and charge, e, to be trapped i n a d e v i c e w i t h a radius r , the values o f U, V, and i 2 m u s t be chosen i n a s p e c i f i c w a y so that the trajectory 0  o f the i o n does not increase b e y o n d the d i m e n s i o n s o f the trap as t i m e increases. T h e s o l u t i o n to these s e c o n d order differential equations is not t r i v i a l ; fortunately, there exists i n the literature solutions to p r o b l e m s o f this f o r m . It turns out that a s e c o n d order l i n e a r differential equation, k n o w n as the M a t h i e u E q u a t i o n , w h i c h w a s d e v e l o p e d o v e r 120 years ago to describe b o u n d v i b r a t i n g membranes has a f o r m w h i c h is adaptable to the problems i n  Equ.1.14 and Equ.1.15 [17, 18]. T h e c a n o n i c a l f o r m o f the M a t h i e u  E q u a t i o n c a n be w r i t t e n as: du 2  7 +K d%  -29„cos2c> = 0  Equ. 1.16  2  W h e r e , u is the coordinate a x i s . S o l u t i o n s to i n t r o d u c i n g a f e w parameters,  Equ.1.16 are w e l l k n o w n , so b y  Equ.1.14 and Equ.1.15 c a n be s o l v e d . T h e three  parameters that must be defined are:  Equ. 1.17  a.  -%eU  Equ. 1.18  mr Q 2  2  -AeV  Equ. 1.19  mr Q 2  2  10  T h e solutions to the M a t h i e u equation are determined i n terms o f the parameters a and q . u  u  F o r every p a i r o f values a and q there w i l l be a s o l u t i o n where the p o s i t i o n u  u  v a l u e , u, as a f u n c t i o n o f £ is either stable o r unstable. Stable solutions are those for w h i c h the p o s i t i o n , u, o f the i o n is p e r i o d i c and finite as t i m e increases. U n d e r these c o n d i t i o n s , w e say the i o n is trapped. C o n v e r s e l y , unstable solutions are those w h e r e the i o n p o s i t i o n increases w i t h o u t l i m i t s o v e r t i m e . Ions under these c o n d i t i o n s are not trapped.  S o l u t i o n s to the M a t h i e u . e q u a t i o n a r e b e s t understood w h e n v i e w e d g r a p h i c a l l y  on a so-called "stability diagram".  Figure 1.2 shows a g r a p h i c a l representation o f the stable solutions to the M a t h i e u equations plotted i n terms o f a and q [13]. T h i s figure is s y m m e t r i c about the a a x i s , u  u  u  so o n l y one h a l f o f the d i a g r a m is s h o w n . A s m e n t i o n e d earlier, for a n i o n to be effectively trapped, it must be stable s i m u l t a n e o u s l y i n both the a x i a l (z) and r a d i a l (r) d i m e n s i o n s . B o t h o f these stability c o n d i t i o n s are plotted s i m u l t a n e o u s l y i n this figure. This diagram  (Figure 1.2) s h o w s regions w h i c h have stable s o l u t i o n s o n l y i n (z)  or (r). A r e a s w h e r e the stability regions o v e r l a p define c o n d i t i o n s w h e r e s p e c i f i c i o n s m a y be effectively trapped i n a l l three d i m e n s i o n s . A n enlarged v i e w o f the m o s t i m p o r t a n t o v e r l a p r e g i o n (at l o w p o s i t i v e  a  u  and  q  u  Figure 1.3 [13].  values) i s s h o w n i n  F o r p r a c t i c a l reasons, v i r t u a l l y a l l i o n traps operate i n this stability r e g i o n . N o t e the iso-/3 lines o n  Figure 1.3. These /? values are parameters s p e c i f i e d b y a  u  a n d q a n d define the frequency o f m o t i o n o f the trapped ions. W h e n q < 0.4, the u  u  fundamental or " s e c u l a r " frequency (a\) o f a trapped i o n o s c i l l a t i n g w i t h i n the v o l u m e o f a q u a d r u p o l a r f i e l d c a n be f o u n d f r o m :  11  Figure 1.2 A graphical representation of the stable solutions to the Mathieu equation in both the radial (r) and axial (z) directions in terms of a and q [13]. u  u  Figure 1.3 An enlarged view of the experimentally Important region Mathieu stability diagram [13].  -i 0.2  1  0.4  f—-i  0.6  0.8  1 — r 1.0  13  1.2  i  1.4  2  "  H  u  2  Equ. 1.20  Q 2  (Note: At q values greater than 0.4, higher order terms become important a u  the ion motion becomes more complex. For this simplified analysis, only the case q <0.4 will be considered). u  F o r a specified a and q value, the i o n trajectory i n the r a d i a l plane r e s e m b l e s a u  u  L i s s a j o u s figure c o m p o s e d o f t w o frequency  components:  Equ. 1.21  Equ. 1.22 T h e analysis d i s c u s s e d above has been under the l i m i t i n g c o n d i t i o n s o f a single i o n b e i n g c o n t a i n e d i n a d e v i c e w i t h o u t c o l l i s i o n s w i t h neutral m o l e c u l e s or atoms. I n practice, these instruments are almost a l w a y s used w i t h hundreds o f thousands o f i o n s b e i n g trapped s i m u l t a n e o u s l y and w i t h b a c k g r o u n d gas pressures o f 1 m T o r r . U n d e r these c o n d i t i o n s , the ions experience C o u l o m b i c r e p u l s i o n , w h i c h c o m p l i c a t e s any a n a l y s i s . A d d i t i o n a l l y , c o l l i s i o n s o f the neutral b a c k g r o u n d gas w i t h the trapped i o n s also affect i o n m o t i o n . W h i l e a m o r e detailed account o f these processes is b e y o n d the scope o f this chapter, n u m e r i c a l methods are a v a i l a b l e to quantitate these effects [5, 19-24]. T h e net results o f neutral c o l l i s i o n s and C o u l o m b i c r e p u l s i o n p l a y a v e r y i m p o r t a n t r o l e i n i o n trap operation and w i l l be discussed later. N e v e r t h e l e s s , the analysis d e s c r i b e d above is useful and is sufficient to predict most i o n trap properties.  14  1.2.3 The 3D Quadrupole Ion Trap as a Mass Spectrometer T h e theory d e s c r i b e d i n the p r e v i o u s section shows o n l y the c o n d i t i o n s for t r a p p i n g i o n s effectively but makes no m e n t i o n as to h o w this d e v i c e m y be used as a p r a c t i c a l m a s s spectrometer.  Indeed, it is this very p r o b l e m : h o w to m a k e a useful m a s s  a n a l y z e r , w h i c h v e x e d researchers for almost 30 years. It w a s not u n t i l 1983 w h e n G e o r g e C . Stafford and h i s team at F i n n i g a n M A T d e v e l o p e d the "mass selective i n s t a b i l i t y " m o d e o f operation that the m o d e r n age o f i o n trap m a s s spectrometry r e a l l y b e g a n [9, 10]. There have been a n u m b e r o f a d d i t i o n a l methods suggested for operating the i o n trap [25], h o w e v e r , the majority o f c o m m e r c i a l a n d research d e v i c e s ( i n c l u d i n g this one) s t i l l operate under this " i n s t a b i l i t y m o d e " . T h e m e t h o d o f mass selective i n s t a b i l i t y c a n be c l e a r l y understood b y e x a m i n i n g the m o d i f i e d stability d i a g r a m i n F i g u r e 1.4. T h i s d i a g r a m is a p l o t o f the " p o s i t i o n s " o f several i o n s o f different mass o n the a , q graph for a s p e c i f i e d R F voltage. I n practice, z  z  for m o s t i o n trap operation, there is no D C potential a p p l i e d to the t r a p p i n g field, so the a v a l u e is r e d u c e d to zero (this w i l l a l w a y s be the case for the w o r k d e s c r i b e d i n this z  thesis). T h u s the r e g i o n o f stability w h i c h is most important is that w h i c h lies o n the q a x i s o f the stability d i a g r a m between q = 0 and q = 0.908. These points m a r k the z  z  intersection o f the q a x i s w i t h B =0 a n d fi = 1 respectively. A s m e n t i o n e d earlier, a n i o n z  z  z  w i l l be stable i f its a and q values are s u c h that they l i e w i t h i n the stability d i a g r a m . z  z  C o n v e r s e l y , i f the a and q values are s u c h that the p o s i t i o n o f the i o n lies outside the z  z  stability d i a g r a m , then the i o n w i l l not be contained. Therefore, because o f the nature o f the a and q equations, for a p a r t i c u l a r R F z  z  voltage and i o n trap radius there is a l o w e r l i m i t to the m / z ratio that m a y be trapped.  15  Figure 1.4 An enlarged view of the stability diagram showing the possible positions of several ions of different mass on the stability diagram under various RF voltage conditions.  1500 T h  q = 0.908  1000 T h  unstable  <— - stable  •  500 T h  w  3000 V  w  -> w  fe w  i — f — _  •1  0  i  i  i  i  i  i  i  i  I  i  0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9  16  1  I  I  S00OW  200 V  I  1.1 1.2 1.3 1.4 1.5  T h i s l o w e r mass l i m i t represents the cut o f f p o i n t where i o n s have q v a l u e s greater t h a n z  the q s t a b i l i t y point. A l l i o n s o f mass h i g h e r than this w o u l d h a v e 0 < q < 0.908 o n the z  z  stability d i a g r a m a n d w o u l d therefore be contained. B y s y s t e m a t i c a l l y i n c r e a s i n g the R P " t r a p p i n g v o l t a g e " , ions o f greater a n d greater mass b e c o m e s e q u e n t i a l l y unstable i n the z d i r e c t i o n as their q v a l u e s a p p r o a c h q = 0.908. z  z  W h e n u s e d as mass spectrometers, i o n traps are manufactured w i t h a n u m b e r o f s m a l l h o l e s d r i l l e d i n one o f the end caps. T h i s a l l o w s the ions that have g a i n e d unstable trajectories i n the z d i r e c t i o n to escape the i o n trap a n d strike a detector ( t y p i c a l l y a n e l e c t r o n m u l t i p l i e r ) . B y i n c r e a s i n g the R F voltage a n d m o n i t o r i n g the detector output, one c a n , i n a straightforward w a y , o b t a i n a n entire mass spectrum as i o n s are ejected starting at the l o w e s t mass to the highest mass. I n the p r e v i o u s paragraph, a l o w mass l i m i t w a s described. T h e r e are also s o m e f u n d a m e n t a l l i m i t a t i o n s to the heaviest i o n that m a y be effectively trapped a n d detected b y a n i o n trap mass spectrometer. T h e p r o b l e m o f t r a p p i n g h e a v y i o n s c a n be u n d e r s t o o d u s i n g the D e h m e l t potential w e l l m o d e l . T h i s m o d e l suggests that the process o f a n i o n b e i n g trapped i s analogous to the process that o c c u r s w h e n a b a l l falls d o w n into a v a l l e y . T h i s pseudo potential w e l l can be d e s c r i b e d as h a v i n g a depth:  D  1  Equ. 1.23  D  (Note: Again, this equation applies for q <0.4) u  T h e pseudo potential w e l l c a n be thought o f as d e s c r i b i n g the m a x i m u m k i n e t i c energy a n i o n m a y have a n d s t i l l be contained i n the trap (i.e. an object w i l l be trapped i f it has less k i n e t i c energy than the height o f the potential w e l l c o n t a i n i n g it). Therefore, i n an instrument w i t h fixed R F frequency, il, a n d size, z , at a g i v e n m a x i m u m R F v o l t a g e , 0  17  V , as the m a s s o f a n object increases, the size o f the potential w e l l c o n t a i n i n g that i o n decreases. T h e r e f o r e the ultimate h i g h mass-trapping l i m i t w i l l o c c u r w h e n the t h e r m a l k i n e t i c energy o f a n i o n i s greater than the pseudo p o t e n t i a l w e l l depth. O f c o u r s e , i f the instrument w e r e d e s i g n e d s p e c i f i c a l l y for t r a p p i n g v e r y h e a v y i o n s , then o n e w o u l d s i m p l y c h o o s e a l o w e r t r a p p i n g frequency. Indeed, m a c r o s c o p i c objects, s u c h as a l u m i n u m particles have b e e n trapped w i t h d e v i c e s operating at 148 H z [26]. T h i s e x a m p l e b r i n g s to the f o r e g r o u n d one o f the c l a s s i c p r o b l e m s o f the i o n trap: i f one w e r e d e t e r m i n e d to trap v e r y h e a v y i o n s (even o n a m o l e c u l a r scale - i.e. proteins) t h e n the t r a p p i n g v o l t a g e a p p l i e d w o u l d have to be large e n o u g h so that the p o t e n t i a l w e l l d e p t h w a s c a p a b l e o f c o n t a i n i n g the i o n . U n f o r t u n a t e l y , at these large t r a p p i n g v o l t a g e s , the p o t e n t i a l i s r a i s e d so h i g h that m a n y l o w mass i o n s have q v a l u e s h i g h e r t h a n q = 0.908 z  z  a n d are n o t trapped. T h i s is one o f the fundamental d o w n f a l l s o f the i o n trap; the m a s s range o f i o n s that m a y be trapped s i m u l t a n e o u s l y w i l l be l i m i t e d . W h i l e h i g h mass t r a p p i n g i s a p r o b l e m w i t h i o n traps, it turns out that i o n ejection a n d d e t e c t i o n i s m o r e often the l i m i t i n g factor i n o b s e r v i n g h i g h m a s s i o n s . T h i s o c c u r s because there exists a fundamental l i m i t for the R F voltage w h i c h m a y b y a p p l i e d b e t w e e n t w o m e t a l objects under m e d i u m v a c u u m c o n d i t i o n s before an e l e c t r i c a l discharge results. I n a n i o n trap w i t h the d i m e n s i o n s a n d pressures t y p i c a l l y u s e d this v a l u e i s about 7 0 0 0 Vo- ak- F o r instruments u s i n g the mass selective i n s t a b i l i t y m o d e o f pe  o p e r a t i o n , t h i s i m p l i e s that the heaviest i o n w h i c h m a y be ejected (i.e. h a v i n g its q v a l u e z  r a i s e d to 0.908) is t y p i c a l l y about 6 5 0 T h . Note: this thesis will use the new convention for expressing the mass to charge ratio, m/z, as a Thomson, Th.  18  T h i s l i m i t was o r i g i n a l l y not a c o n c e r n to instrument manufactures as i o n traps were i n i t i a l l y created as detectors for gas chromatographs ( G C ) , w h i c h c a n t y p i c a l l y o n l y elute m o l e c u l e s w i t h masses b e l o w this l i m i t . H o w e v e r , w i t h the recent d e v e l o p m e n t and p o p u l a r i z a t i o n o f methods for creating h i g h mass ions, electrospray i o n i z a t i o n ( E S I ) a n d m a t r i x assisted laser d e s o r p t i o n i o n i z a t i o n ( M A L D I ) for e x a m p l e , a need has arisen to adapt the i o n trap for the analysis o f h i g h mass ions. T h e m o s t p o p u l a r m e t h o d that is c o m m e r c i a l l y used today (and w a s u s e d i n this thesis) to enhance the mass range o f the i o n trap is k n o w n as resonant ejection [27]. T h i s m e t h o d i n v o l v e s a p p l y i n g a supplemental alternating potential to one or b o t h o f the end caps. T h e frequency o f this secondary potential is c h o s e n to m a t c h the frequency a n i o n w o u l d have as it oscillates i n the i o n trap w i t h a q l o w e r than 0.908. T h i s s m a l l A C z  v o l t a g e ( t y p i c a l l y a f e w v o l t s ) is e n o u g h to excite and eject a n i o n because i f the a p p l i e d frequency is i n resonance w i t h the secular frequency, energy w i l l be deposited into the i o n m o t i o n and its orbit w i l l b e c o m e larger and larger u n t i l the i o n is e v e n t u a l l y ejected. I f this supplementary A C voltage is a p p l i e d as the R F voltage is r a m p e d , a n d i f the frequency is c h o s e n to be i n resonance w i t h i o n s o f a l o w e r q v a l u e , say q = 0.303, then z  z  as the R F voltage is increased, ions w i l l c o m e into resonance w i t h this a p p l i e d voltage, as their q values approach q = 0.303, and w i l l be ejected at this p o i n t rather than at q = z  z  z  0.908. S i n c e the ions are b e i n g ejected at a q r o u g h l y a t h i r d l o w e r , w e w o u l d observe a m a s s range increase o f a p p r o x i m a t e l y 3 times. T h i s m e t h o d c o m b i n e d w i t h r e d u c i n g the R F d r i v e frequency has been used to extend the mass range substantially, f r o m 6 5 0 T h to n o w o v e r 7 0 , 0 0 0 T h [28, 2 9 ] .  19  T h e a b i l i t y to a d d a n a u x i l i a r y frequency or w a v e f o r m b e t w e e n the e n d caps turns out to h a v e m a n y a d d i t i o n a l uses besides j u s t mass range e x t e n s i o n . O n e o f the m o s t i m p o r t a n t o f these is the a b i l i t y to s e l e c t i v e l y excite specific i o n s i n the trap. T h i s a b i l i t y is l e v e r a g e d to m a x i m u m effect d u r i n g the operation o f p e r f o r m i n g t a n d e m m a s s spectrometry [30]. T a n d e m mass spectrometry ( M S / M S ) is the process w h e r e b y a specific i o n is fragmented and the resulting "daughter" ions detected.  Since n o r m a l mass  spectrometry t y p i c a l l y o n l y p r o v i d e s i n f o r m a t i o n o n the total mass o f a m o l e c u l e a n d little about its structure, M S / M S c a n p r o v i d e the analyst w i t h a w e a l t h o f i n f o r m a t i o n about the s a m p l e . T h e m o s t efficient means o f p e r f o r m i n g tandem mass spectrometry i n the i o n trap is b y a p p l y i n g a N o t c h e d B r o a d B a n d W a v e f o r m ( N B B W ) b e t w e e n the end caps. T h e m e t h o d operates o n the p r i n c i p l e that at one particular trapping v o l t a g e every m a s s w i l l have a s p e c i f i c q v a l u e , and that every q v a l u e w i l l produce i o n s o s c i l l a t i n g at u n i q u e z  z  secular frequencies. S i n c e a l l ions o f one particular mass oscillate at one frequency, i f that frequency is a p p l i e d between the end caps, then that s p e c i f i c i o n w o u l d be ejected f r o m the d e v i c e . T o p e r f o r m M S / M S , w e want the reverse o f this; w e want to eject a l l other species f r o m the trap and leave o n l y the "parent" i o n o f interest. T o a c h i e v e this, a c o m p l e x w a v e f o r m is a p p l i e d to the end caps that c o n t a i n a l l frequencies except that o f the parent i o n . T h i s effectively cleans the i o n trap o f all material but the i o n o f interest. P r a c t i c a l l y , this c o m p l e x w a v e f o r m c a n be obtained b y either t a k i n g the inverse F o u r i e r transform o f a n o t c h e d frequency range (as i n the S t o r e d W a v e f o r m Inverse F o u r i e r T r a n s f o r m ( S W I F T ) m e t h o d [31, 32]), or m o r e t y p i c a l l y , b y s i m p l y a d d i n g a n u m b e r o f  20  discrete s i n g l e frequency c o m p o n e n t s i n the t i m e d o m a i n . T h e m e t h o d o f t i m e d o m a i n w a v e f o r m s u m m a t i o n w a s u s e d to create the N B B W w a v e f o r m i n this w o r k . O n c e this parent i o n is mass selected, an a d d i t i o n a l s i n g l e frequency A C v o l t a g e is a p p l i e d . T h i s t i m e , h o w e v e r , the voltage u s e d is m u c h l o w e r than before, a n d instead o f the i o n b e i n g ejected f r o m the trap, the resonant energy a p p l i e d is o n l y e n o u g h to cause c o l l i s i o n i n d u c e d d i s s o c i a t i o n ( C I D ) [33]. T h i s d i s s o c i a t i o n is caused b y the m o l e c u l a r i o n s c o l l i d i n g w i t h b a c k g r o u n d gas neutral atoms or m o l e c u l e s a n d p r o d u c e s d i a g n o s t i c daughter i o n s . It is this a b i l i t y to r o u t i n e l y a n d s i m p l y p e r f o r m M S / M S that t r u l y sets the i o n trap apart f r o m other mass spectrometers. W h i l e other instruments, n a m e l y l i n e a r triple q u a d r u p o l a r m a s s spectrometers (triple quads) a n d F T - I C R d e v i c e s are also capable o f p e r f o r m i n g M S / M S , the i o n trap, b y its nature, is superior for this task. I n fact, the N B B W / C I D methods d e s c r i b e d p r e v i o u s l y c o u l d just as e a s i l y be a p p l i e d to one o f the daughter i o n s r e s u l t i n g f r o m a p r e v i o u s M S / M S c y c l e . T h i s m u l t i p l e t a n d e m mass spectrometry a b i l i t y ( M S ) has b e e n p r e v i o u s l y demonstrated u p to M S N  1 1  [34].  O n e final c o n s i d e r a t i o n i n the p r a c t i c a l operation o f an i o n trap is that o f b a c k g r o u n d " b u f f e r " gas. It has b e e n f o u n d e m p i r i c a l l y that the presence o f h e l i u m at a pressure o f ~1 m T o r r substantially i m p r o v e s the performance o f the i o n trap [35]. T h i s i m p r o v e m e n t o f performance results w h e n the r e l a t i v e l y light H e atoms c o l l i d e w i t h the h e a v i e r m o l e c u l a r i o n s . T h e s e c o l l i s i o n s h a v e the effect o f transferring a p o r t i o n o f the k i n e t i c energy o f the o r i g i n a l i o n to the H e atoms. T h e effects o f this " c o l l i s i o n a l c o o l i n g " o n i o n trap performance are three f o l d . F u n d a m e n t a l l y , the existence o f this b a c k g r o u n d gas is essential to the t r a p p i n g o f e x t e r n a l l y injected i o n s . S t r i c t l y s p e a k i n g , a n externally created i o n c o u l d n e v e r be  21  trapped w i t h o u t c o l l i s i o n s w i t h buffer gas because as the i o n enters the trap, it gains a certain amount o f k i n e t i c energy. I f this energy were perfectly c o n s e r v e d , then the i o n w o u l d s i m p l y continue t r a v e l i n g right t h r o u g h the trap i n a m a n n e r analogous to a b a l l r o l l i n g into a n d out o f a s y m m e t r i c v a l l e y w i t h n o frictional forces. T h e H e buffer gas, t h r o u g h c o l l i s i o n s , c a n r e m o v e e n o u g h o f this energy to a l l o w the i o n to be trapped. O n c e trapped, c o l l i s i o n a l c o o l i n g continues to r e m o v e energy f r o m the i o n c a u s i n g the a m p l i t u d e o f the i o n m o t i o n to be d i m i n i s h e d . E s s e n t i a l l y , this m i n i m i z e s the i o n " c l o u d v o l u m e " to a s m a l l e r area at the center o f the trap. T h i s b u n c h i n g o f i o n s produces increased s e n s i t i v i t y because the ions have a s m a l l e r r a d i a l d i s t r i b u t i o n a n d thus have a h i g h e r l i k e l i h o o d o f p a s s i n g through the s m a l l apertures at the center o f the e n d cap electrode w h e n ejected. F i n a l l y , c o l l i s i o n a l c o o l i n g also i m p r o v e s r e s o l u t i o n . I d e a l l y , i o n s are ejected f r o m the i o n trap w h e n they c o m e into resonance w i t h the ejection p o i n t (either q  z  =  0.908 for n o r m a l operation or some l o w e r q v a l u e for resonance ejection). H o w e v e r , z  w i t h o u t c o l l i s i o n a l c o o l i n g , as i o n s approach this l i m i t , they w i l l start to g a i n energy, as their secular frequency approaches the e x c i t a t i o n frequency. T h i s c a n cause the i o n c l o u d to e x p a n d and, w i t h o u t c o l l i s i o n a l c o o l i n g ( w h i c h helps m i n i m i z e this energy), s o m e percentage o f i o n s m a y be ejected earlier than the strict resonance p o i n t . T h i s produces a broader peak shape and thus l o w e r s r e s o l u t i o n .  1.3 Sampling and Ionization for Mass Spectrometry A s m e n t i o n e d p r e v i o u s l y , the i o n trap was o r i g i n a l l y d e s i g n e d as a g a s - s a m p l i n g d e v i c e . W h e n used i n this w a y , analyte m o l e c u l e s were s i m p l y l e a k e d into the i o n trap and i o n i z e d b y a n electron g u n (electron i m p a c t i o n i z a t i o n - E I ) situated b e h i n d one o f 22,  ...  the e n d caps (through a s m a l l h o l e i n the end cap). T h i s c o n f i g u r a t i o n is i d e a l for the i o n trap because the i o n s are created i n the center o f the trap f r o m l o w v e l o c i t y gas species. T h i s s i t u a t i o n a l l o w s for the highest p r o b a b i l i t y o f trapping; a n d v e r y g o o d s e n s i t i v i t y results. T h e r e are, h o w e v e r , a w i d e range o f m o l e c u l e s and samples for w h i c h i n t r o d u c t i o n into the i o n trap i n this w a y is not a p p l i c a b l e . F o r e x a m p l e , h i g h m o l e c u l a r w e i g h t ( M W ) , n o n - v o l a t i l e , p o l a r and/or t h e r m a l l y l a b i l e m o l e c u l e s m a y undergo d e c o m p o s i t i o n d u r i n g the v a p o r i z a t i o n step. A s a result, direct s a m p l i n g o f gas phase m o l e c u l e s b y E I m a y not a l w a y s be p r a c t i c a l or e v e n p o s s i b l e . A s a consequence o f this l i m i t a t i o n , a w i d e range o f v o l a t i l i z a t i o n a n d i o n i z a t i o n schemes have been d e v e l o p e d or adapted to extend the c a p a b i l i t i e s o f the i o n trap. Indeed, o v e r the last 2 0 years, the i o n trap has been c o u p l e d to v i r t u a l l y e v e r y v o l a t i l i z a t i o n and i o n i z a t i o n source a v a i l a b l e . F o r l i q u i d samples this i n c l u d e s electrospray i o n i z a t i o n ( E S I ) [ 3 6 , 3 7 ] , c h e m i c a l i o n i z a t i o n ( G l ) [38], and p h o t o i o n i z a t i o n (PI)[39].  "  •  v.-  F o r s o l i d samples, a n u m b e r o f p r o t o c o l s have been attempted i n c l u d i n g ; secondary i o n mass spectrometry ( S I M S ) [40], laser desorption/laser a b l a t i o n [41], fast a t o m b o m b a r d m e n t ( F A B ) [42], and matrix-assisted laser desorption i o n i z a t i o n ( M A L D I ) [43, 4 4 ] . A l l o f these direct s o l i d s a m p l i n g techniques, h o w e v e r , either p r o d u c e c o m p l e x mass spectra r e s u l t i n g f r o m extensive m o l e c u l a r fragmentation or require extensive s a m p l e preparation. O n e alternative for direct s o l i d s a m p l i n g that has yet to be t h o r o u g h l y e x p l o r e d i n a n i o n trap is the m e t h o d o f laser desorption/resonant t w o - p h o t o n i o n i z a t i o n ( T w o laser s o l i d s a m p l i n g - L 2 M S ) . P r e v i o u s to this thesis there has been one d e m o n s t r a t i o n o f the two-laser m e t h o d i n an i o n trap, h o w e v e r , this differed f r o m the  23  w o r k presented here i n b o t h a i m and scope - it w a s d e s i g n e d and b u i l t b y surface scientists, w i t h the g o a l o f detecting adsorbates o n c h e m i c a l l y m o d i f i e d m e t a l surfaces [45,46].  •  1.3.1 Two-Laser Solid Sampling (L2MS) T h e t w o - l a s e r m e t h o d for s o l i d s a m p l i n g has been k n o w n for s o m e t i m e . It w a s first d e v e l o p e d for mass spectrometry o f i n v o l a t i l e and/or t h e r m a l l y l a b i l e o r g a n i c s i n the m i d - 1 9 8 0 ' s [47-49]. S i n c e that t i m e it has been further d e v e l o p e d and u s e d for a v a r i e t y o f s o l i d samples i n c l u d i n g : p o l y m e r s [50], soot a n d c o a l tar [51], dyes [52], b i o l o g i c a l m o l e c u l e s [53], and m o l e c u l a r adsorbates [46]. T w o - l a s e r mass spectrometry, as it has been p r e v i o u s l y a p p l i e d , is a three-step process ( F i g u r e 1.5). T h e first step i n v o l v e s u s i n g a p u l s e d I R laser ( t y p i c a l l y a CO2 laser) for t h e r m a l d e s o r p t i o n o f the s a m p l e . I f the w a v e l e n g t h and p o w e r density are c h o s e n c o r r e c t l y ( t y p i c a l l y a r o u n d 1 0 W / c m ) . 6  2  t h e r m a l d e s o r p t i o n w i l l produce a p l u m e o f intact neutrals w i t h v e r y little i o n i z a t i o n or s a m p l e d e c o m p o s i t i o n . T h e second step i n this process, i n v o l v e s the use o f a p u l s e d U V laser ( t y p i c a l l y a q u a d r u p l e d N d : Y A G laser), to p h o t o i o n i z e the analyte present i n the neutral desorbate p l u m e . A g a i n , i f the w a v e l e n g t h and p o w e r are c h o s e n c o r r e c t l y , the i o n i z a t i o n process c a n be e x t r e m e l y gentle and selective and p r o d u c e m a i n l y intact m o l e c u l a r i o n s . F i n a l l y , i n v i r t u a l l y every reported e x a m p l e o f the use o f this t e c h n i q u e , the i o n s are detected u s i n g a t i m e o f flight mass spectrometer. It is i n the t e m p o r a l and spatial separation o f the d e s o r p t i o n and i o n i z a t i o n processes w h e r e the advantages o f the technique are gained. T h e separation a l l o w s a n independent o p t i m i z a t i o n o f b o t h the laser desorption and laser p h o t o i o n i z a t i o n steps, thus m a k i n g the process b o t h e x t r e m e l y gentle a n d sensitive. T h e next t w o sections w i l l 24  Figure 1.5 Schematic cartoon of two-laser mass spectrometry as it has been previously applied.  TOF M S  © ©  Pulsed IR  o  @  o  t o •8^880 o  25  w  Pulsed U V  *8 880  describe these t w o processes independently and a t h i r d w i l l describe the advantages o f p e r f o r m i n g t w o - l a s e r m a s s spectrometry i n an i o n trap.  1.3.2 Laser - Solid Interactions T h e first step o f the two-laser m e t h o d i n v o l v e s a short I R laser p u l s e i n t e r a c t i n g w i t h a s o l i d sample. T h i s process k n o w n as laser desorption has been e x a m i n e d for s o m e t i m e . Indeed, the o b s e r v a t i o n o f the interaction o f laser l i g h t w i t h s o l i d samples w a s one o f the first experiments to be p e r f o r m e d w i t h the n e w l y i n v e n t e d laser. T h i s interest is evident b y the fact that o n l y a ten year span elapsed b e t w e e n the t i m e the r u b y laser w a s d i s c o v e r e d a n d the d e f i n i n g text o n laser s o l i d interactions w a s p u b l i s h e d b y R e a d y ( i n 1971) [54]. W h i l e the study o f laser s o l i d interactions has c o n t i n u e d o v e r the laser 30 years there s t i l l r e m a i n s a great deal w h i c h is not t h o r o u g h l y understood. T h i s is due to the fact that w h e n a h i g h - p o w e r e d short d u r a t i o n laser pulse interacts w i t h a s o l i d s a m p l e a n u m b e r o f p o s s i b l e scenarios m a y result. These results depend o n a m u l t i t u d e o f interconnected factors i n c l u d i n g : the thermal c o n d u c t i v i t y o f the s o l i d , the heat c a p a c i t y o f the s o l i d , the absorption coefficient o f the s o l i d at the laser w a v e l e n g t h , the laser p u l s e d u r a t i o n , the p h o t o n density, and w a v e l e n g t h o f the laser pulse [54]. A l l o f these factors c a n c o m b i n e i n s p e c i f i c a n d c o m p l i c a t e d w a y s to produce a variety o f consequences r a n g i n g f r o m e x p l o s i v e events a n d p l a s m a f o r m a t i o n a l l the w a y d o w n to gentle h e a t i n g o f the surface. T o s i m p l i f y the analysis i n this section, w e m a y d i v i d e the p o s s i b l e o u t c o m e s o f laser s o l i d interactions into t w o classes. T h e first, c a l l e d laser a b l a t i o n , results f r o m the a b s o r p t i o n o f a large a m o u n t o f energy b y the surface. T h i s results, t y p i c a l l y , i n a v e r y 26  energetic event. I n contrast, w h e n a r e l a t i v e l y l o w flux o f energy is used, a less energetic process results - this is termed laser desorption. L a s e r a b l a t i o n results w h e n a r e l a t i v e l y h i g h p h o t o n flux (10 W / c m ) is a b s o r b e d b y a s o l i d surface i n a short time p e r i o d [55]. T h i s energy i n i t i a l l y i n d u c e s a v e r y r a p i d rise i n temperature at the interface. T h i s intense, l o c a l i z e d heat, often leads to m e l t i n g , v a p o r i z a t i o n , p l a s m a f o r m a t i o n , and p o s s i b l y absorption o f laser energy b y the p l a s m a . T h i s h i g h - e n e r g y process t y p i c a l l y produces a range o f v e r y energetic particles that are ejected f r o m the surface. These particles m a y i n c l u d e e v e r y t h i n g f r o m a t o m i c species, to m o l e c u l a r fragments; i n neutral, r a d i c a l , or i o n i c f o r m . U n d e r these c o n d i t i o n s , the s u r v i v a l o f any p o l y a t o m i c m o l e c u l e into the gas phase is h i g h l y u n l i k e l y . A s a result, laser a b l a t i o n has o n l y f o u n d c o m m o n use for elemental analysis o f s o l i d samples. F r o m a n instrumental p o i n t o f v i e w , laser a b l a t i o n i s a p o o r t e c h n i q u e to c o u p l e to a mass spectrometer. T h i s is due to the fact that laser a b l a t i o n produces particles w i t h a b r o a d d i s t r i b u t i o n o f k i n e t i c energies centered at v e r y h i g h values (1000 k e V k i n e t i c energies are not unusual) [56]. S i n c e most mass spectrometers r e l y o n either c o n t a i n i n g i o n s o r d i r e c t i n g t h e m w i t h electric fields, a b l a t i o n products are often d i f f i c u l t to a n a l y z e . F o r e x a m p l e , i o n s w i t h v e r y h i g h energy are v i r t u a l l y i m p o s s i b l e to trap i n a n i o n trap or F T - I C R and samples w i t h v e r y b r o a d spreads o f energies w r e a k h a v o c o n T O F a n a l y z e r s . O n the other end o f the energy scale is laser desorption. L a s e r d e s o r p t i o n results w h e n the energy flux o f the laser is m u c h l o w e r than the m i n i m u m for a b l a t i o n / p l a s m a f o r m a t i o n . Instead, w h a t results c a n u s u a l l y be d e s c r i b e d s i m p l y as r a p i d heating. T h i s heating process c a n o c c u r v e r y q u i c k l y , w i t h rates o f 1 0  1 1  K / s t y p i c a l . U n d e r these u l t r a -  fast heating c o n d i t i o n s t h e r m a l l y l a b i l e m o l e c u l e s c a n a c t u a l l y be d e s o r b e d before they  27  get a chance to d e c o m p o s e [57, 58]. T h i s counter i n t u i t i v e result occurs because under these c o n d i t i o n s the temperature is q u i c k l y raised h i g h e n o u g h so that d e c o m p o s i t i o n a n d d e s o r p t i o n are b o t h energetically p o s s i b l e s i m u l t a n e o u s l y (/. e. they b o t h h a v e m o r e energy than is needed to cross the a c t i v a t i o n barrier for the respective process). A t this p o i n t , frequency factors d o m i n a t e the relative rates, and often d e s o r p t i o n is f a v o r e d o v e r decomposition. T h e laser d e s o r p t i o n process produces p r e d o m i n a n t l y intact m o l e c u l a r i o n s w h i c h are essentially " b o i l e d o f f the surface w i t h r e l a t i v e l y little k i n e t i c energy (< 10 e V ) . O b v i o u s l y , i f a n a l y s i s o f m o l e c u l a r adsorbates o n surfaces w e r e the g o a l , then finding the c o r r e c t l y laser c o n d i t i o n s for the laser desorption r e g i m e to prosper rather than laser a b l a t i o n w o u l d be c r i t i c a l . A further analysis o f the laser d e s o r p t i o n process as it s p e c i f i c a l l y a p p l i e s to the e x p e r i m e n t a l w o r k done i n this thesis is c o n t a i n e d i n CHAPTER  4.  L a s e r d e s o r p t i o n , under the d e f i n i t i o n described above w i l l p r o d u c e m a i n l y intact m o l e c u l a r species w i t h l o w k i n e t i c energies and w i t h v e r y little d e c o m p o s i t i o n or i o n i z a t i o n . T h i s non-selective desorption process is i d e a l l y suited for c o m b i n a t i o n w i t h selective laser p h o t o i o n i z a t i o n . T h e p h o t o i o n i z a t i o n process w i l l be d i s c u s s e d i n S e c t i o n 1.3.3. A t this p o i n t , h o w e v e r , it w o u l d be instructive to c o m p a r e "straight laser d e s o r p t i o n " as d e s c r i b e d i n this section w i t h a technique that has g a i n e d tremendous p o p u l a r i t y o f late - M A L D I . M a t r i x A s s i s t e d L a s e r D e s o r p t i o n I o n i z a t i o n ( M A L D I ) w a s d i s c o v e r e d i n the late 1 9 8 0 ' s b y T a n a k a and c o w o r k e r s at S h i m a d z u [59-61] and d e v e l o p e d c o n c u r r e n t l y b y H i l l e n k a m p a n d K a r a s i n G e r m a n y [62]. ( T a n a k a r e c e i v e d the N o b e l P r i z e for this w o r k  28  i n 2 0 0 2 ) . T h i s technique is based u p o n m i x i n g a s m a l l amount o f analyte w i t h a large excess o f s o l i d or l i q u i d m a t r i x that absorbs strongly at the laser w a v e l e n g t h . T h i s m e t h o d p r o d u c e s intact m o l e c u l a r i o n s d i r e c t l y f r o m the d e s o r p t i o n event. E x t r e m e l y h i g h m o l e c u l a r w e i g h t i o n s have been o b s e r v e d w i t h this m e t h o d ( M W > 100 0 0 0 D a ) [63]. A m a z i n g l y , h o w e v e r , the m e c h a n i s m o f i o n i z a t i o n is s t i l l not w e l l understood. Researchers are s t i l l debating whether i o n s are " p r e - f o r m e d " i n the m a t r i x o r s o m e w h e r e i n the desorbate p l u m e . R e g a r d l e s s , this a b i l i t y to e x a m i n e v e r y h i g h M W species has greatly accelerated a n u m b e r o f other fields o f research. N o w h e r e is this clearer t h a n i n the w o r l d o f b i o c h e m i s t r y because n o w large proteins a n d D N A m o l e c u l e s m a y be e x a m i n e d d i r e c t l y b y mass spectrometry [63]. M A L D I , h o w e v e r , is not the i d e a l t o o l for a l l f o r m s o f s o l i d a n a l y s i s .  MALDI  o n l y w o r k s o n specific classes o f m o l e c u l e s , a n d d i s c o v e r i n g the correct m a t r i x for efficient i o n i z a t i o n i s b a s i c a l l y a matter o f trial a n d error. A d d i t i o n a l l y , M A L D I i s g e n e r a l l y restricted to o n l y l o o k i n g at r e l a t i v e l y h i g h mass ions because the " m a t r i x " m a t e r i a l i t s e l f is often e a s i l y i o n i z e d a n d produces intense l o w mass peaks w h i c h are anathema to l o w mass a n a l y s i s . F i n a l l y , M A L D I falls short f r o m the perspective o f p e r f o r m i n g direct a n a l y s i s o f native samples. I n order for M A L D I to be effective the s a m p l e m u s t be co-deposited w i t h the analyte. O b v i o u s l y , i f p e r f o r m i n g direct a n a l y s i s o f untreated samples o f moderate M W were, y o u r g o a l then laser d e s o r p t i o n rather t h a n M A L D I w o u l d be preferred.  1.3.3 Laser Photoionization A f t e r m o l e c u l a r desorption, the next step i n t w o laser mass spectrometry i s p h o t o i o n i z a t i o n . P h o t o i o n i z a t i o n , a n d m u l t i p h o t o n i o n i z a t i o n ( M P I ) i n p a r t i c u l a r , is a 29  versatile i o n i z a t i o n technique w i t h u n i q u e properties w h e n c o m p a r e d to e l e c t r o n g u n i o n i z a t i o n for mass spectrometry.  S i n c e M P I depends i n t r i n s i c a l l y o n the e l e c t r o n i c  properties o f the m o l e c u l e , it a l l o w s for a n a d d i t i o n a l l e v e l o f s e l e c t i v i t y i n the a n a l y s i s . A d d i t i o n a l l y , M P I is versatile i n that it is capable o f b o t h soft (no fragmentation) a n d h a r d (extensive fragmentation) i o n i z a t i o n d e p e n d i n g o n the laser p o w e r used. T h e i o n i z a t i o n potential o f most Organic m o l e c u l e s is o n the order o f 7-13 e V . Therefore, s i n g l e p h o t o n i o n i z a t i o n w o u l d require a laser b e a m i n the v a c u u m U V energy r e g i o n . I n contrast, because m a n y o r g a n i c m o l e c u l e s have electronic transitions i n the U V / V i s i b l e r e g i o n o f the spectrum, the p o s s i b i l i t y exists o f p e r f o r m i n g m u l t i p h o t o n i o n i z a t i o n . A s i m p l i f i e d J a b l o n s k i d i a g r a m d e p i c t i n g the v a r i o u s p o s s i b l e m o d e s o f i o n i z a t i o n is p r o v i d e d i n  Figure 1.6.  M u l t i p h o t o n i o n i z a t i o n is a technique that depends o n a m o l e c u l e a b s o r b i n g t w o or m o r e photons f r o m one or m o r e intense v i s i b l e / U V lasers. T h e m o s t efficient f o r m o f this i o n i z a t i o n occurs w h e n one o f the lasers is c h o s e n to have a frequency (energy) that matches a r e a l electronic transition i n the m o l e c u l e . W h e n this o c c u r s , the m o l e c u l e c a n p o t e n t i a l l y r e m a i n i n this e x c i t e d state l o n g e n o u g h to absorb a s e c o n d p h o t o n a n d b e c o m e i o n i z e d . T h i s m e t h o d is c a l l e d resonance enhanced m u l t i p h o t o n i o n i z a t i o n (REMPI) -  Figure 1.6.c [64]. O f course, there are some specific requirements i n order  for this process to be effective. A m o l e c u l e w i l l o n l y b e c o m e i o n i z e d w h e n the s u m o f the a b s o r b i n g p h o t o n energies is greater than the i o n i z a t i o n potential. A d d i t i o n a l l y , i f the laser w a v e l e n g t h does not m a t c h a real electronic transition, then the p r o b a b i l i t y o f i o n i z a t i o n decreases d r a m a t i c a l l y -  Figure 1.6.b. Therefore, b y s i m p l y c h o o s i n g the laser  w a v e l e n g t h prudently, certain species c a n be s e l e c t i v e l y i o n i z e d over others. T h i s t r u l y  30  Figure 1.6 Simplified Jablonski diagram for a number of possible photon/molecule interactions, (a) Single photon ionization (b) No ionization (c) Resonant two photon ionization (d) Non Resonant multiphoton ionization (e) Two photon resonant ionization.  Ionization Continuum  mm wmrn  (a) Single Photon  Ionization  Ionization  UJIIglJIIIl  (c) Resonant two  (b) No  (e)Non  (f) Two photon  photon Ionization Resonant MPI  Resonant ionization  31  u n i q u e a b i l i t y o f R E M P I to p r o v i d e v e r y selective i o n i z a t i o n c a n be u s e d to preselect w h i c h i o n s are to be a n a l y z e d b y the mass spectrometer. B e s i d e s straight R E M P I , there are other means o f a c h i e v i n g m u l t i p h o t o n i o n i z a t i o n . These i n c l u d e , t w o p h o t o n resonant i o n i z a t i o n ( F i g u r e 1.6.e) a n d n o n resonant m u l t i p h o t o n i o n i z a t i o n ( F i g u r e 1.6.d). B o t h o f these scenarios, h o w e v e r , r e l y o n the c r e a t i o n o f a v e r y s h o r t - l i v e d " v i r t u a l " e x c i t e d state. I n order for i o n i z a t i o n to be a c h i e v e d a s e c o n d p h o t o n must interact w i t h this v i r t u a l state before it relaxes (lifetimes t y p i c a l l y <10"  15  seconds). T h i s makes the e f f i c i e n c y o f the process far less f a v o r a b l e than  R E M P I . R e g a r d l e s s , t o t a l l y non-resonant m u l t i p h o t o n i o n i z a t i o n is p o s s i b l e w i t h v e r y h i g h laser p o w e r s . T h i s is useful i n situations where a b r o a d a n a l y s i s is r e q u i r e d . In a d d i t i o n to b e i n g a v e r y selective i o n source for mass spectrometry, R E M P I offers other advantages.  O n e advantage is that c o n t r o l o f m o l e c u l a r fragmentation i s  r e l a t i v e l y straightforward. R E M P I is inherently a v e r y gentle technique because a l m o s t no a d d i t i o n a l energy is deposited into the m o l e c u l e d u r i n g i o n i z a t i o n . I n fact, o v e r a w i d e range o f m o l e c u l a r species R E M P I has been s h o w n to p r o v i d e v e r y soft i o n i z a t i o n at l o w laser p o w e r s (<10  W / c m ) [65]. A s a result, the observed m a s s spectra are often  greatly s i m p l i f i e d because they c o n t a i n peaks c o r r e s p o n d i n g o n l y to m o l e c u l a r ions. I n contrast, the m o r e energetic E I process u t i l i z i n g 70 e V electrons produces m o l e c u l a r i o n s and often m a n y fragment ions as w e l l . A s a result, E I mass spectra o f m i x t u r e s o f m o r e than a f e w c o m p o u n d s are often i m p o s s i b l e to decipher because o f the m u l t i t u d e o f peaks. A d d i t i o n a l l y , w i t h an electron g u n , v e r y little latitude is a v a i l a b l e i n terms o f selecting the e l e c t r o n ' s energy because the i o n i z a t i o n cross section drops d r a m a t i c a l l y at l o w e r e l e c t r o n energies. W i t h M P I o n the other hand, b y s i m p l y adjusting the laser  32  p o w e r the analyst has the o p t i o n o f o b s e r v i n g just the m o l e c u l a r parent or a n u m b e r o f fragment i o n s . T h e R E M P I process is also a v e r y efficient means o f p r o d u c i n g ions. I o n i z a t i o n efficiencies o f m o l e c u l e s i n the laser b e a m path are v e r y h i g h . It has been estimated that 10-100 % o f naphthalene m o l e c u l e s i n the laser path c a n b e c o m e i o n i z e d [66]. B y c o m p a r i s o n E I sources t y p i c a l l y i o n i z e less than 1/10,000 o f those m o l e c u l e s that enter the i o n i z a t i o n r e g i o n [ 6 5 ] . T h e ultimate l i m i t to i o n p r o d u c t i o n i n R E M P I is a matter o f fundamental constants.  These i n c l u d e the absorption cross section o f the m o l e c u l e at the  laser w a v e l e n g t h a n d the relative rate o f radiationless decay. A n u m b e r o f m o d e l s a n d theories that describe the R E M P I process and the effects o f substituent atoms o n efficiencies are a v a i l a b l e i n the literature [67, 6 8 ] .  1.4 Coupling Two-Laser Solid Sampling with an Ion Trap A s m e n t i o n e d p r e v i o u s l y , the i o n trap is a d e v i c e capable o f a w i d e range o f e x p e r i m e n t a l and operational parameters. I n a d d i t i o n , two-laser s o l i d s a m p l i n g is a v e r y effective m e a n s o f p e r f o r m i n g direct s o l i d s a m p l i n g o f m e d i u m M W m o l e c u l e s . T h i s s e c t i o n w i l l n o w describe the potential advantages to be g a i n e d b y c o m b i n i n g the t w o techniques. T w o - l a s e r s o l i d s a m p l i n g ( L 2 M S ) h a s almost e x c l u s i v e l y been p e r f o r m e d w i t h a t i m e - o f - f l i g h t ( T O F ) apparatus [64, 6 9 , 70]. P r e v i o u s w o r k e r s have s h o w n that L 2 M S w i t h a T O F is capable o f v e r y l o w detection l i m i t s ; for e x a m p l e , attomolar detection l e v e l s have b e e n p u b l i s h e d for the analysis o f a n i l i n e absorbed o n s i l i c o n [71]. F u r t h e r m o r e , the o p t i c a l s e l e c t i v i t y afforded b y the i o n i z a t i o n stage a l l o w s preferential i o n i z a t i o n o f analyte m o l e c u l e s o v e r m a t r i x species that do not have a n e l e c t r o n i c 33  t r a n s i t i o n at the laser w a v e l e n g t h . T h i s s e l e c t i v i t y has a l l o w e d L 2 M S to be a p p l i e d to a w i d e v a r i e t y o f analytes a n d samples r a n g i n g f r o m b i o l o g i c a l l y important m o l e c u l e s [69, 70] to e n v i r o n m e n t a l contaminants o n s o i l and soot [51, 72]. T h e s e w o r k e r s have also s h o w n that L 2 M S is capable o f direct analysis o f samples that have not b e e n pretreated. S i n c e , a l l o f the advantages associated w i t h this technique arise p r i m a r i l y f r o m the s a m p l i n g m e t h o d rather than the mass spectrometer, one w o u l d expect that this m e t h o d w o u l d be advantageous regardless o f the type o f mass spectrometer ( T O F v s . i o n trap). . Therefore, the q u e s t i o n o f w h y p e r f o r m this e x p e r i m e n t i n an i o n trap rather t h a n a T O F r e a l l y b e c o m e s one o f w h a t are the pros a n d cons o f i o n traps v s . T O F ' s T i m e - o f - f l i g h t mass spectrometers a n d i o n traps are b o t h amenable to o p e r a t i o n w i t h transient i o n i z a t i o n sources. T h a t is to say, for each instrument, a c o m p l e t e mass s p e c t r u m c a n be o b t a i n e d f r o m a single laser c y c l e . T h i s a b i l i t y is v e r y important for laser s a m p l i n g , a n d argues against, for p r a c t i c a l reasons, p e r f o r m i n g L 2 M S w i t h a s c a n n i n g d e v i c e l i k e a linear quadrupole since thousands o f laser shots w o u l d be r e q u i r e d to o b t a i n one mass spectrum. T i m e o f flight d e v i c e s have greater v a c u u m s y s t e m requirements a n d d e m a n d faster e l e c t r o n i c s than i o n traps. O n the other h a n d , iOn traps require h i g h v o l t a g e R F p o w e r supplies. H o w e v e r , the net cost o f a h i g h performance T O F is t y p i c a l l y at least t w i c e as e x p e n s i v e as a n i o n trap. T h e m a i n advantage o f the T O F instrument o v e r the i o n trap is its a b i l i t y to acquire a huge mass range w i t h v e r y h i g h mass a c c u r a c y (precise mass values). O n e o f these advantages, h o w e v e r , is negated w i t h this s a m p l i n g s y s t e m , b y the fact that L 2 M S  34  is o n l y useful i n s a m p l i n g species up to mass 9 0 0 0 a m u [57, 7 3 ] . T h i s m a s s range is w i t h i n r e a c h o f the i o n trap. T h e i o n trap, o n the other hand, has many, advantages over the T O F w i t h respect to L 2 M S o f real samples. T h e most important o f these is the i o n traps a b i l i t y to p e r f o r m M S " , w h i c h is e x t r e m e l y useful i n e l u c i d a t i n g the components o f m i x t u r e s . T h i s advantage is accented for L 2 M S s a m p l i n g by the nature o f the i o n source. B e c a u s e L 2 M S is s u c h a soft i o n i z a t i o n technique a n d produces m a i n l y m o l e c u l a r i o n s , the a b i l i t y to p e r f o r m M S / M S o n these ions to c o n f i r m their identity i n c o m p l e x m i x t u r e s is v e r y v a l u a b l e . T h i s is e s p e c i a l l y true because the samples are often a n a l y z e d " w h o l e " , w i t h n o p r e v i o u s c h r o m a t o g r a p h y or separation steps. Therefore, samples w i l l a l m o s t a l w a y s contain complex mixtures. T h e s e c o n d k e y advantage o f the i o n trap o v e r the T O F w i t h t w o - l a s e r s a m p l i n g is the fact that the i o n s do not have to be detected i m m e d i a t e l y after the laser event.  TOF's  d o not have a n y i o n storage a b i l i t y , so whatever ions are f o r m e d f r o m the t w o - l a s e r event must be detected i m m e d i a t e l y . I n contrast, the i o n trap is capable o f s e l e c t i v e l y s t o r i n g a n d m a n i p u l a t i n g the p r o d u c e d ions. F o r e x a m p l e , it w o u l d be p o s s i b l e to c o l l e c t the i o n s f r o m several laser shots to increase the total i o n count, and thus s e n s i t i v i t y . F u r t h e r m o r e , b y careful a d d i t i o n o f a N B B W between laser c y c l e s , l o w concentration species c a n be s e l e c t i v e l y preconcentrated i n the gas phase before detection. T h i s o f course w o u l d be impossible w i t h a time-of-flight M S . F i n a l l y , it s h o u l d be m e n t i o n e d , that two-laser s a m p l i n g e v e n addresses a n d m i n i m i z e s the t w o potential d o w n f a l l s that are inherent to the i o n trap. T h e first o f these concerns t y p i c a l l y associated w i t h i o n traps is the p r o b l e m o f space charge. I f too m a n y  35  i o n s are c o n t a i n e d s i m u l t a n e o u s l y w i t h i n the trap t h e n C o u l o m b i c r e p u l s i o n b e t w e e n i o n s w i l l cause a deleterious effect. T h i s "space c h a r g e " p r o b l e m causes b r o a d e n i n g a n d shifting o f mass peaks i n the observed spectra. T w o - l a s e r s a m p l i n g addresses this p r o b l e m , because the i o n i z a t i o n process is v e r y selective. A s a result, one s h o u l d , i n theory, be able to i o n i z e o n l y m o l e c u l e s o f interest w h i l e l e a v i n g u n w a n t e d m a t r i x m a t e r i a l i n the neutral state, thus m i n i m i z i n g the total n u m b e r o f i o n s i n the trap a n d as a consequence, the space charge. The other c o n c e r n t y p i c a l l y associated w i t h i o n traps is that o f i n j e c t i o n e f f i c i e n c y . I f a n e x t e r n a l l y created i o n is to be trapped, it must first penetrate a h i g h v o l t a g e alternating electric field. T h i s field w o u l d alternately either reject the i o n o r accelerate it t h r o u g h the trap d e p e n d i n g o n the relative phase angle. A s a result, there is o n l y a v e r y s m a l l acceptance w i n d o w w i t h the correct phase angle for t r a p p i n g . C o n s e q u e n t l y , it has b e e n estimated that less than 5 % o f e x t e r n a l l y created i o n s are efficiently trapped [74, 7 5 ] . W h i l e some researchers have used " s u d d e n onset" o r fast s w i t c h i n g p o w e r supplies to increase this efficiency [76], these s p e c i a l i z e d d e v i c e s are the e x c e p t i o n . B y c o m p a r i s o n , w i t h two-laser s a m p l i n g , it is p o s s i b l e to p o s i t i o n the U V laser so that i o n i z a t i o n occurs p r i m a r i l y i n the center o f the trap. T h i s s h o u l d p r o v i d e for the m a x i m u m p o s s i b l e t r a p p i n g efficiency.  1.4.1 This Work W i t h the above j u s t i f i c a t i o n s , m o t i v a t i o n s , a n d theory i n p l a c e , w e m a y n o w r e e x a m i n e the goals o f this w o r k . T h i s thesis w i l l describe the c r e a t i o n o f a n o v e l t w o laser s o l i d s a m p l i n g i o n trap mass spectrometer. T h i s instrument s h o u l d , i n theory, be  36  capable o f a l l the experiments p r e v i o u s l y done o n a L 2 M S - T O F d e v i c e w i t h the added a b i l i t y o f b e i n g able to p e r f o r m M S a n d other post-laser s a m p l i n g i o n m a n i p u l a t i o n s . N  T o demonstrate the effectiveness o f this d e v i c e , three m a i n e x p e r i m e n t a l paths w e r e f o l l o w e d . T h e first o f these i n v o l v e d the e x a m i n a t i o n o f e n v i r o n m e n t a l l y i m p o r t a n t contaminants; p o l y c y c l i c aromatic hydrocarbons ( P A H s ) and polychlorinated biphenyls ( P C B s ) o n s o l i d samples. T h e s e c o n d course o f research, c o n c e r n e d i n v e s t i g a t i n g the p o s s i b i l i t y o f e x a m i n i n g b i o l o g i c a l l y important c o m p o u n d s ; this w o r k demonstrated the a b i l i t y to detect a d r u g m o l e c u l e d i r e c t l y o n b i o l o g i c a l tissues w i t h n o s a m p l e pretreatment. F i n a l l y , the t h i r d course o f research i n v o l v e d the a p p l i c a t i o n o f a t h i r d p u l s e d laser to probe i o n s that h a d been f o r m e d b y L 2 M S . T h i s process a l l o w e d for the semi-quantitative i d e n t i f i c a t i o n o f t w o P A H i s o m e r s d i r e c t l y i n less than f i v e m i n u t e s w i t h o u t any c h r o m a t o g r a p h y .  37  CHAPTER 2 Experimental 2.1 Introduction T h e p r i m a r y g o a l o f this thesis w a s to d e v e l o p a n d characterize a n e w s c i e n t i f i c instrument. T h i s n e w d e v i c e , the t w o - l a s e r i o n trap m a s s spectrometer, w a s constructed over a span o f t w o years, a n d i m p r o v e m e n t s w e r e made as the experiments e v o l v e d o v e r the n e x t three years. T h i s chapter w i l l describe the general c o m p o n e n t s a n d o p e r a t i n g parameters o f this n e w l y b u i l t system. T h e o v e r a l l instrument w a s constructed u s i n g a v a r i e t y o f e l e c t r o n i c s i g n a l generators a n d c o n t r o l l e r s , m e c h a n i c a l d e v i c e s s u c h as v a c u u m p u m p s , o p t i c a l c o m p o n e n t s s u c h as lenses, p r i s m s , a n d lasers, a l l operating under the c o n t r o l o f s p e c i a l l y d e s i g n e d software. T h e d i s c u s s i o n i n this section w i l l be l i m i t e d to o n l y those c o m p o n e n t s w h i c h w e r e created o r m o d i f i e d s p e c i f i c a l l y b y , o r for, the author; so c o m m e r c i a l l y a v a i l a b l e c o m p o n e n t s w i l l be identified but not d e s c r i b e d i n d e t a i l . A l s o , the d i s c u s s i o n i n this chapter w i l l focus o n l y o n details r e g a r d i n g the o v e r a l l f u n c t i o n i n g o f the instrument. O p e r a t i n g parameters a n d c o n d i t i o n s for particular e x p e r i m e n t s w i l l be f o u n d i n the relevant chapters. A n o v e r v i e w o f some o f the m o r e important c o m p o n e n t s o f the s y s t e m is s h o w n in F i g u r e 2.1.  38  Figure 2.1 A conceptual representation of the two-laser ion trap system at UBC.  Timing Electronics  39  T h e successful operation o f this d e v i c e requires m a n y c o m p o n e n t s to p e r f o r m i n s y n c h r o n i z a t i o n w i t h precise t i m i n g . T h e s e i n c l u d e : •  T h e h i g h voltage radio frequency electronics that w e r e u s e d for i o n t r a p p i n g a n d ejection.  • •  T h e laser systems, w h i c h i n c l u d e b o t h e l e c t r o n i c a n d o p t i c a l c o m p o n e n t s . T h e f u n c t i o n generators that produce the a u x i l i a r y w a v e f o r m s for i o n manipulation.  •  T h e t i m i n g electronics that i n c l u d e s a n u m b e r o f d i g i t a l d e l a y generators that p r o v i d e precise t i m i n g for the entire system.  •  T h e software to operate the entire s y s t e m a n d c o l l e c t a n d process the r a w data.  W i t h each o f these systems, the author w a s r e q u i r e d , w i t h the h e l p o f the U B C m e c h a n i c a l a n d e l e c t r i c a l shops, to either m o d i f y or synthesize n e w c o m p o n e n t s . I n order to c l a r i f y the d i s c u s s i o n , a d e s c r i p t i o n o f the v a r i o u s c o m p o n e n t s w i l l be b r o k e n d o w n into sub headings.  2.2 The lon Trap Electrodes and Vacuum Manifold T h e p h y s i c a l v a c u u m m a n i f o l d a n d the i o n trap electrodes are i n their t h i r d generation o f use, a n d w e r e m a c h i n e d b y the m e m b e r s o f the M e c h a n i c a l S e r v i c e s S h o p (Department o f C h e m i s t r y , U B C ) several years ago [77, 7 8 ] . A s c h e m a t i c d i a g r a m o f the v a c u u m m a n i f o l d i n c l u d i n g the i o n trap electrodes is s h o w n i n F i g u r e 2 . 2 . T h e v a c u u m c h a m b e r a n d i o n trap electrodes w e r e b o t h m a c h i n e d f r o m stainless steel a n d d e s i g n e d w i t h o p t i c a l experiments i n m i n d . T h e i o n trap electrodes used i n these experiments w e r e o f the " i d e a l " q u a d r u p o l a r geometry. T h i s geometry is described as " i d e a l " w h e n c o m p a r e d to the "stretched"  40  Figure 2 . 2 Vacuum manifold for the two-laser ion trap system at UBC.  41  g e o m e t r y , w h i c h is t y p i c a l o f c o m m e r c i a l d e v i c e s . I n the i d e a l case, the e n d caps are separated b y the distance specified for q u a d r u p o l a r fields (see S e c t i o n 1.2.1 i n the Introduction) whereas the "stretched" geometry has end caps that have b e e n separated b y a further 1 0 % . It has b e e n e m p i r i c a l l y determined b y some groups that this stretching i m p r o v e s the performance (by m i n i m i z i n g h i g h e r order fields) o f the instrument under s o m e c i r c u m s t a n c e s , h o w e v e r , it h a d been f o u n d i n earlier w o r k , that stretching these s p e c i f i c electrodes d i d not i m p r o v e the performance o f this p a r t i c u l a r d e v i c e [77]. T h e i o n trap electrodes are h e l d i n p o s i t i o n b y M a c o r ® m a c h i n a b l e glass c e r a m i c spacers, w h i c h p r o v i d e b o t h r i g i d i t y and electrical i s o l a t i o n for the electrodes. T h e central r i n g electrode has a radius o f 10.00 m m a n d extended h y p e r b o l i c surfaces to p r o v i d e a m o r e h o m o g e n e o u s quadrupolar f i e l d . T h i s center r i n g electrode also h a d f o u r 2.5 m m diameter holes d r i l l e d t h r o u g h it. T h e s e h o l e s , t w o o f w h i c h are p e r p e n d i c u l a r a n d t w o o f w h i c h are p a r a l l e l to the h o r i z o n t a l (i.e. a l l are 9 0 ° f r o m e a c h other) are d r i l l e d t h r o u g h the center o f the electrode a l l p o i n t i n g t o w a r d the geometric center o f the t r a p p i n g v o l u m e . T h e use o f these holes w i l l be d e s c r i b e d shortly. T h e v a c u u m m a n i f o l d w a s designed to a l l o w a s a m p l e probe to be inserted into the i o n trap f l u s h w i t h the i n s i d e o f the r i n g electrode w i t h o u t the need to break the vacuum.  T o a c h i e v e this, a differentially p u m p e d v a c u u m i n t e r l o c k w a s c u s t o m b u i l t to  a l l o w the probe to pass t h r o u g h one o f the 2 . 5 m m holes a n d into the i n t e r i o r o f the trap. T h e probe w a s c u s t o m b u i l t w i t h a M a c o r ® spacer between the probe tip a n d the base o f the p r o b e ( F i g u r e 2.3). T h i s a l l o w e d the probe t i p to float at the same potential as the central r i n g electrode w h i l e p r o v i d i n g e l e c t r i c a l i s o l a t i o n to the v a c u u m c h a m b e r as a whole.  42  Figure 2.3 Solid sample probe for the two-laser ion trap system at UBC.  30 cm  11;  1i  • Macor Spacer 1.5 m m  1  1 mm  T 4  Replaceable Probe T i p  T J  43  Spring Guide  T h e s a m p l e probe w a s inserted into the i o n trap t h r o u g h a differentially p u m p e d v a c u u m c h a m b e r . A V a r i a n m o d e l S D - 9 0 rotary vane p u m p ( V a r i a n V a c u u m ; T o r i n o , Italy) w a s u s e d to r o u g h out the s m a l l v o l u m e o f a i r before it w a s o p e n e d to the h i g h v a c u u m c h a m b e r . T h i s a l l o w e d for the direct a n d easy c h a n g i n g o f samples w i t h o u t the n e e d to break the h i g h v a c u u m . S i n c e these experiments i n t r i n s i c a l l y d e m a n d e d that the l i g h t output f r o m several lasers be d i r e c t e d to the interior o f the i o n trapV the v a c u u m h o u s i n g w a s d e s i g n e d w i t h several o p t i c a l ports. T h e h o u s i n g has four o p t i c a l ports located i n l i n e w i t h the asymptotes o f the i o n trap. T h i s a l l o w e d t w o direct lines o f sight d i a g o n a l l y t h r o u g h the i o n trap, b e t w e e n the end caps a n d the r i n g electrodes. A fifth o p t i c a l port w a s p l a c e d i n l i n e w i t h the center o f the r i n g electrode opposite the sample probe. T h i s a l l o w e d o p t i c a l access t h r o u g h the t w o 2.5 m m holes d r i l l e d i n the r i n g electrode to the s a m p l e probe. T h e s i x t h o p t i c a l port w a s located 9 0 ° f r o m the fifth a n d w a s i n l i n e w i t h the t w o h o l e s d r i l l e d t h r o u g h the r i n g p e r p e n d i c u l a r to the h o r i z o n t a l . F i g u r e 2.4 s h o w s the path o f the d e s o r b i n g a n d i o n i z i n g lasers t h r o u g h the trap. T h e detector for the i o n trap, a c h a n n e l electron m u l t i p l i e r , w a s p l a c e d into a recess b e h i n d one o f the end caps a n d located i n l i n e w i t h the center o f the trap. A n electron i m p a c t ( E I ) i o n i z a t i o n source w a s m o u n t e d i n the other end cap. T h i s a l l o w e d e l e c t r o n i m p a c t to be u s e d as a n alternative i o n i z a t i o n m e t h o d , w h i c h p r o v e d useful for calibration. T h e v a c u u m m a n i f o l d w a s evacuated u s i n g a V a r i a n " N a v i g a t o r " t u r b o m o l e c u l a r p u m p ( M o d e l T V 3 0 1 , V a r i a n V a c u u m ) w h i c h w a s b a c k e d b y a m o d e l S D - 4 0 rotary v a n e p u m p ( V a r i a n V a c u u m ) . T h e turbo m o l e c u l a r p u m p w a s c o n t r o l l e d b y software l o c a t e d  44  Figure 2 . 4 Close up view of the IR and UV lasers as they pass through the ion trap.  45  o n C o m p u t e r 2 t h r o u g h a serial c o n n e c t i o n  {there were two personal computers used in  this work, which will be designated as Computer 1, a 350 MHz Pentium II, and Comp 2, a 133 MHz Pentium I).  A " v a c u u m safety u n i t " c o n s i s t i n g o f a relay that prevented  any h i g h voltage discharges i n the event that the v a c u u m w a s lost w a s added b y the e l e c t r i c a l services shop (Department o f C h e m i s t r y , U B C ) . Pressure i n the m a n i f o l d w a s measured using a Balzers (Pfeiffer-Balzers; Nashua, N H , U S A ) m o d e l I K R 020 Piranic o l d cathode gauge head that w a s read u s i n g a B a l z e r s m o d e l P K G - 0 2 0 meter. W i t h this v a c u u m system, the lowest uncorrected pressure that c o u l d be a c h i e v e d w a s a p p r o x i m a t e l y 10" torr. S t r i c t l y speaking, a P i r a n i - c o l d cathode gauge is sensitive to 7  gas c o m p o s i t i o n , h o w e v e r , since i n a l l o f this w o r k , the b a c k g r o u n d gas c o n s i s t e d p r i m a r i l y o f h e l i u m , an appropriate c o n v e r s i o n factor c o u l d be used to find the exact pressure. I n a l l o f the experiments p e r f o r m e d i n this thesis, the b a c k g r o u n d h e l i u m pressure w a s h e l d constant at a p p r o x i m a t e l y 1 m T o r r . F i n a l l y , it s h o u l d be noted, that this v a c u u m m a n i f o l d w a s also e q u i p p e d w i t h t w o needle leak v a l v e s . T h e first v a l v e a l l o w e d for a specified pressure o f h e l i u m ( P r a x a i r , h i g h p u r i t y H e (99.995 % ) , E d m o n t o n , Canada) to fill the m a n i f o l d v o l u m e . T h e s e c o n d v a l v e p r o v i d e d for a means o f a d d i n g calibrant gas into the chamber w h e n required. T h i s gas w a s t y p i c a l l y CCI4 (carbon tetrachloride, O m n i s S o l v e n t G r a d e , B D H C h e m i c a l s , T o r o n t o , O n t . C a n a d a ) , w h i c h has k n o w n peak p o s i t i o n s w h e n i o n i z e d b y the electron g u n . T h u s a l l o w i n g the spectrum to be mass calibrated.  46  2.3 Electronics and Timing W i t h respect to the t i m i n g and electronics, w e m a y d i v i d e the instrument into t w o parts. T h e first o f these w a s the electronics for the i o n trap operation, the s e c o n d w a s for the laser operation. I n practice, these t w o systems were c o m p l e t e l y integrated, but for the d i s c u s s i o n here, it is instructive to describe t h e m separately.  2.3.1 Ion Trap Electronics A s m e n t i o n e d i n the i n t r o d u c t i o n , mass spectral data are c o l l e c t e d f r o m the i o n trap b y r a m p i n g the R P voltage (either w i t h or w i t h o u t a secondary w a v e f o r m , d e p e n d i n g o n the m o d e ) a n d s i m u l t a n e o u s l y detecting the ejected ions u s i n g the electron m u l t i p l i e r . T o p r o d u c e r e l i a b l e m a s s spectral data, it is essential that this r a m p i n g process be hardware t i m e d w i t h the data c o l l e c t i o n . T o achieve this, in-house w r i t t e n p r o g r a m s operating o n a 350 M H z P e n t i u m II computer ( C o m p u t e r 1) were used. Instrument c o n t r o l a n d data a c q u i s i t i o n software were w r i t t e n i n the L a b V i e w ( N a t i o n a l Instruments, A u s t i n , T X , U S A ) p r o g r a m m i n g e n v i r o n m e n t b y the author. ( A m o r e detailed d e s c r i p t i o n o f the software appears i n S e c t i o n 2.4) T h e digital-to-analog ( D A C ) a n d analog-tod i g i t a l ( A D C ) c o n v e r s i o n s for the m a i n instrument c o n t r o l t o o k place o n a s i n g l e P C I b o a r d , w h i c h w a s also capable o f accepting and p r o d u c i n g a variety o f d i g i t a l pulses ( P C I - M I O - 1 6 X E , N a t i o n a l Instruments). T h e D A C b o a r d c o n t r o l l e d the R F voltage u s i n g a 0-5 V c o n t r o l s i g n a l . T h e R F p o w e r s u p p l y w a s a n E x t r a n u c l e a r L a b o r a t o r i e s Inc. (Pittsburgh, P A , U S A ) q u a d r u p o l e p o w e r s u p p l y ( M o d e l 001-1) that was m o d i f i e d for use w i t h a n i o n trap.  Capacitance  m a t c h i n g w a s a c h i e v e d u s i n g a H i g h - Q - H e a d ( m o d e l 012-16), also f r o m E x t r a n u c l e a r  47  L a b o r a t o r i e s . T h e m a x i m u m R F output achievable w i t h this s y s t e m w a s a p p r o x i m a t e l y - 3 0 0 0 Vo-p (zero-to-peak) a n d the frequency used w a s f i x e d at 0.967 M H z . In the s i m p l e s t m o d e o f operation, the R F p o w e r s u p p l y w a s set to an appropriate " t r a p p i n g v o l t a g e " . A f t e r the R F source reached this l e v e l (less than 1ms w a s need to s t a b i l i z e at this voltage, but often 10ms was used) the i o n i z a t i o n event o c c u r r e d . T h i s w a s either i o n i z a t i o n b y an electron g u n ( E I ) or b y the two-laser process. T h e n e w l y created ions-were then a l l o w e d a " c o o l i n g p e r i o d " o f b e t w e e n 5 0 - 1 0 0 0 m s (user set d e l a y ) where the c o l l e c t e d ions c o l l i d e w i t h the h e l i u m buffer gas at a constant R F voltage. A f t e r the c o o l i n g p e r i o d , the R F potential w a s r a m p e d to h i g h e r v a l u e s to generate a mass spectrum (mass selective i n s t a b i l i t y m o d e ) . T h i s r a m p w a s often a c c o m p a n i e d b y a s i n g l e frequency secondary w a v e f o r m that a l l o w e d for m a s s range e x t e n s i o n (resonance ejection m o d e ) . R e s o n a n c e ejection was a c h i e v e d u s i n g a Stanford R e s e a r c h S y s t e m s ( P a l o A l t o , C A , U S A ) m o d e l D S 3 4 5 f u n c t i o n generator. T h e ejected ions were detected u s i n g an E T P ( S y d n e y , A u s t r a l i a ) m o d e l A F 1 3 8 electron m u l t i p l i e r h e l d at - 1 . 7 k V . T h e detector was h e l d at this v o l t a g e b y a m o d e l 205a-03r (Bertan H i g h Voltage, Valhalla, N Y , U S A ) D C power supply. The signal from the detector w a s a m p l i f i e d w i t h a g a i n o f 1 0 V / A u s i n g a K e i t h l e y ( C l e v e l a n d , O H , 7  U S A ) M o d e l 4 2 8 current a m p l i f i e r . T h e A D C o n the P C I b o a r d then s a m p l e d this s i g n a l a n d reported it to the software. W h e n r e q u i r e d , a secondary, c o m p l e x w a v e f o r m , c o u l d also be a p p l i e d to a n e n d cap, after i o n i z a t i o n , but before detection. T h i s w a v e f o r m w a s used to isolate the m a s s to charge ratio o f interest i n the trap. T h i s technique used the a N o t c h e d B r o a d B a n d W a v e f o r m ( N B B W ) m e t h o d o f i o n i s o l a t i o n . I n practice, this w a v e f o r m w a s f o r m e d b y  48  s u m m i n g hundreds o f discrete single frequency c o m p o n e n t s i n the t i m e d o m a i n into a s i n g l e c o m p o s i t e w a v e f o r m . T h i s w a v e f o r m w a s then l o a d e d onto a P C I - 3 2 1 arbitrary f u n c t i o n generator ( P C I Instruments Inc, A k r o n , O H , U S A ) P C I card. T h e software u s e d to create this w a v e f o r m w a s w r i t t e n b y S c i e x ( P C I 3 W A V E , S c i e x , C o n c o r d , O n t , C a n a d a ) i n the B a s i c p r o g r a m m i n g language. T h e w a v e f o r m c a l c u l a t i o n s w e r e p e r f o r m e d o n a secondary, c o m p u t e r ( C o m p u t e r .2) because the p r o c e s s i n g o v e r h e a d r e q u i r e d w a s so large as to s l o w the f u n c t i o n o f a s i n g l e computer. O c c a s i o n a l l y after a p p l i c a t i o n o f the N B B W w a v e f o r m , a t h i r d f u n c t i o n generator w a s used. T h i s f u n c t i o n generator (a second Stanford D S 3 4 5 ) w a s u s e d to a p p l y a s i n g l e frequency w a v e f o r m burst at the secular frequency o f the iori o f interest. T h i s burst w a s t y p i c a l l y l o w e r i n v o l t a g e than the N B B W voltages, and w a s u s e d to i n d u c e C I D ( c o l l i s i o n i n d u c e d d i s s o c i a t i o n ) o f the isolated ions.  2.3.2 The Laser Timing I n i t i a l l y , t w o N d : Y A G lasers were u s e d d u r i n g the course o f this w o r k . T h e first o f these w a s u s e d as the d e s o r p t i o n laser and was operated at the N d : Y A G fundamental w a v e l e n g t h o f 1064 n m w i t h a 10 ns pulse w i d t h . T h i s laser, a D C R - 2 A (Spectra P h y s i c s L a s e r s , M o u n t a i n V i e w , C A , U S A ) was directed, unfocussed, into the i o n trap u s i n g t w o quartz p r i s m s ( M e l l e s G r i o t , Rochester, N Y , U S A ) . A series o f quartz slides w e r e p l a c e d i n the l i n e o f the laser p a t h i n order to significantly reduce the laser p o w e r . T h i s w a s necessary because the m i n i m u m energy for r e l i a b l e l a s i n g was far greater than that needed for desorption. T h e average laser p o w e r measured i n the trap at the probe surface w a s t y p i c a l l y 1 0 W / c m . S i n c e the b e a m diameter was m u c h larger than" the 2 . 5 m m 5  2  apertures i n the electrodes, the aperture size w a s u s e d to calculate the p o w e r density. 49  T h e s e c o n d laser used i n this w o r k , the U V p h o t o - i o n i z a t i o n laser, w a s a N d : Y A G laser f r o m L u m o n i c s C o r p o r a t i o n ( H Y 4 0 0 , L u m o n i c s , 10 ns pulse). T h e laser energy w a s frequency q u a d r u p l e d (internal to the laser h o u s i n g ) to p r o v i d e output at 2 6 6 n m . T h i s l i g h t w a s directed b y t w o s i l v e r e d m i r r o r s ( M e l l e s G r i o t ) t h r o u g h a 1000 m m b i c o n v e x lens ( M e l l e s G r i o t ) . T h i s l o o s e l y focused l i g h t w a s directed a l o n g the d i a g o n a l b e t w e e n the end caps and the r i n g electrode (See F i g u r e 2.4). T y p i c a l energies for the U V laser w e r e o n the order o f 4 0 pJ/pulse. It is important to have a basic understanding o f the solid-state lasers u s e d i n this w o r k i n order to appreciate the e l e c t r i c a l requirements o f the system. N d : Y A G lasers are large s o l i d state d e v i c e s . T h i s means, i n this case, that the l a s i n g m e d i u m - n e o d y m i u m atoms ( N d ) , are c o n t a i n e d i n a s o l i d m a t r i x - a Y t t r i u m A l u m i n u m G a r n e t ( Y A G ) .  I n the  case o f the lasers u s e d here, the N d atoms are e x c i t e d b y a p u l s e d flash l a m p . O n c e e x c i t e d , the N d atoms w i l l b e g i n to r a d i a t i v e l y relax i n a l l directions but n o l a s i n g w i l l o c c u r . T h i s is because the laser c a v i t y is i n i t i a l l y s p o i l e d b y an o p t i c a l shutter. T h i s shutter, a Q - s w i t c h , is o n l y opened to a l l o w l a s i n g once the N d a t o m i c energy p o p u l a t i o n i n v e r s i o n is at its m a x i m u m . O n c e opened, the Q - s w i t c h a l l o w s a v e r y short, h i g h intensity - 1 0 ns laser pulse to exit the laser. " T h e r e are a c o u p l e o f k e y factors that must a l w a y s be taken into account w h e n c o n s i d e r i n g these types o f lasers. T h e flashlamps, w h i c h are u s e d to excite the atoms, p r o d u c e a tremendous a m o u n t o f heat. T h i s heat is constantly r e m o v e d f r o m the laser r o d by water c o o l i n g . T h i s b e i n g said, d u r i n g n o r m a l operation, the N d : Y A G r o d heats u p a p p r e c i a b l y . T h i s heating causes the laser r o d to change its o p t i c a l properties. L a s e r designers take this fact into account, and as a result the o p t i c a l c o n f i g u r a t i o n a n d  50  a l i g n m e n t are based o n the b e h a v i o r o f the r o d at h i g h temperatures. Therefore, these lasers o n l y r u n e f f i c i e n t l y at a specific r o d temperature. T h i s w a s an important c o n s i d e r a t i o n w h e n d e s i g n i n g the system, because the lasers w e r e d e s i g n e d to operate at 2 0 H z (based o n the laser rods r e c e i v i n g a c e r t a i n a m o u n t o f energy f r o m the flashlamps a n d thus operating at a certain temperature) whereas the i o n trap c o u l d o n l y operate at a m a x i m u m frequency o f 1 H z . Therefore, the lasers c o u l d not s i m p l y j u s t be fired w h e n r e q u i r e d b y the i o n trap because the laser energy a n d b e a m shape w o u l d be unstable (for thermal reasons). T o account for this frequency m i s m a t c h , a user b u i l t p r o t o c o l w a s r e q u i r e d . I n the s i m p l e s t case, the f l a s h l a m p s for b o t h N d : Y A G lasers w e r e l i n k e d a n d a l l o w e d to fire at the D C R - 2 A internal c l o c k rate o f 2 0 H z . O f course, because the Q - s w i t c h e s w e r e not b e i n g fired, n o laser l i g h t is emitted. T h e f l a s h l a m p f i r i n g w a s u s e d s i m p l y to keep the laser rods at a n appropriate temperature. T h e p u l s e t r a i n f r o m the D C R - 2 A internal c l o c k w a s m o n i t o r e d w i t h a h o m e b u i l t "trigger m o n i t o r " ( E l e c t r o n i c s shop, C h e m i s t r y Department, U B C ) . W h e n laser pulses w e r e r e q u i r e d , a n " e n a b l e " p u l s e w a s sent to. the "trigger m o n i t o r " , w h i c h w h e n e n a b l e d , a l l o w e d , o n receipt o f the next flash l a m p pulse, a pulse to be fed into a d i g i t a l d e l a y generator ( M o d e l 4 0 0 , B e r k l e y N u c l e o n i c s C o r p o r a t i o n , S a n R a f a e l , C A , U S A ) w h i c h triggered the Q - s w i t c h e s for the t w o lasers at t w o separate p r o g r a m m a b l e d e l a y s . B y t r i g g e r i n g the Q - S w i t c h e s independently i n this w a y , a v e r y stable a n d r e l i a b l e d e l a y b e t w e e n the t w o laser shots w a s a c h i e v e d . A t i m i n g d i a g r a m for the entire s y s t e m i n c l u d i n g the lasers is s h o w n i n F i g u r e 2 . 5 .  51  ure 2.5 Timing diagram for the two-laser system used at UBC.  D G D 535 A  RF Software controlled Waveform  Jr  D C R 2A Flashlamp linked to HY-400 Flashlamp  j  Trigger Monitor Input 1  Enable Pulse From D G D 535 B Output pulse From Trigger Monitor 142 us  B N C Output 1 To DCR-2A  -i  Q-Switch  I  L  n  •30 us  B N C Output 2 T o HY-400 Q-Switch  D G D 535 D  DS345 Function Generator  i  1  0  50  1  1  100  150  1  200  1  250  1  300  Time (ms) riming Cartoon  IR  l ' \  it 52  i  i  i  350  400  450  i  500  T h e t y p i c a l d e l a y between the I R and U V laser pulses w a s user set to 30 p s (jitter < lps).  H o w e v e r , w i t h s o m e s m a l l m o d i f i c a t i o n s , the laser d e l a y for the U V c o u l d be  e a s i l y v a r i e d b e t w e e n - 1 0 to 4 0 0 p s w i t h respect to the I R laser pulse. T h e o p t i m u m d e l a y t i m e o f 3 0 p s w a s d e t e r m i n e d e x p e r i m e n t a l l y ; h o w this t i m e w a s c h o s e n i s d e s c r i b e d i n C H A P T E R 4. W h i l e the t y p i c a l p o w e r densities a n d t i m e delays f o r the lasers are l i s t e d a b o v e , it s h o u l d be m e n t i o n e d that the exact p o w e r outputs and the exact t i m e delays w e r e accurately m e a s u r e d for each mass scan. T h i s w a s a c c o m p l i s h e d b y i n s t a l l i n g t w o fast p h o t o d i o d e s ( M e l l e s G r i o t ) each o f w h i c h was used to detect the a r r i v a l o f one o f the laser pulses. T h e p h o t o d i o d e s were connected to a 4 0 0 M H z d i g i t a l o s c i l l o s c o p e ( M o d e l T D S 3 8 0 , T e k t r o n i x , W i l s o n v i l l e , O R , U S A ) that recorded b o t h the intensity o f the lasers and the t i m e d e l a y b e t w e e n shots. T h e o s c i l l o s c o p e then reported these results t h r o u g h a G P I B interface to the computer. I n this w a y , i n f o r m a t i o n about the laser intensity and the laser delays c o u l d be b u n d l e d w i t h the mass spectral data for e a c h mass scan for subsequent s i g n a l p r o c e s s i n g . T h e G P I B interface b o a r d was a N a t i o n a l Instruments A T G P I B / T N T , a n d w a s c o n t r o l l e d b y user w r i t t e n software o n C o m p u t e r 1.  2.4 Software for lon Trap Operation T h i s u n i q u e instrument required the c r e a t i o n o f c u s t o m o r i g i n a l software.  The  software was w r i t t e n b y the author i n the g r a p h i c a l based language " L a b V i e w " . T h i s type o f p r o g r a m m i n g lends i t s e l f v e r y w e l l to data a c q u i s i t i o n a p p l i c a t i o n s a n d is e x t r e m e l y f l e x i b l e . H o w e v e r , because the r e s u l t i n g " c o d e " is i n a g r a p h i c a l format it cannot e a s i l y be transposed a n d p r i n t e d i n an a p p e n d i x for a thesis. Instead, this s e c t i o n w i l l s i m p l y attempt to s h o w the m a i n operating w i n d o w ( F i g u r e 2.6) and describe s o m e o f the 53  Figure 2.6 Primary operating window for Ion Trap control software. Written in the Lab View environment.  54  specific requirements a n d routines o f the p r o g r a m . A s i m p l i f i e d i n f o r m a t i o n a l f l o w chart d e p i c t i n g h o w s o m e o f the m a i n sub-routines are related is s h o w n i n F i g u r e 2.7. T h e software was r e q u i r e d to meet several operating c o n d i t i o n s . It w a s r e q u i r e d to be e x t r e m e l y f l e x i b l e and easily adaptable, w h i l e at the same t i m e be stable e n o u g h to p r o v i d e c o n t i n u o u s error free operation. T h i s software was d e s i g n e d w i t h a n " i n s i d e - o u t " a p p r o a c h , w h e r e the core f u n c t i o n o f the p r o g r a m - data a c q u i s i t i o n w a s w r i t t e n first, f o l l o w e d b y several higher order layers s u c h as the v i s u a l interface, file input/output, a n d inter-instrument c o n t r o l . T h e core requirement o f any software d e s i g n e d to r u n an i o n trap is the a b i l i t y to c o n t r o l the v o l t a g e o f the R T and s y n c h r o n o u s l y c o l l e c t the data f r o m the current a m p l i f i e r . I n L a b V i e w , this was done b y l o g i c a l l y l i n k i n g a n u m b e r o f sub-routines, w h i c h w o u l d sequentially l o a d a w a v e f o r m onto a hardware buffer, w a i t for a trigger, a n d then p e r f o r m a s i m u l t a n e o u s buffered w r i t e w i t h a buffered input read; b o t h d r i v e n b y a hardware c l o c k . T h e a b i l i t y to r a p i d l y and s y n c h r o n o u s l y w r i t e and read this data at a rate o f 5 0 , 0 0 0 s a m p l e s / s e c o n d w a s essential to r e c o r d i n g a stable mass spectrum. I f the read a n d w r i t e functions w e r e n ' t perfectly s y n c h e d then the mass peaks w o u l d a r r i v e at different t i m e s (positions) o n the m a s s spectra. T h e s e c o n d k e y c o n s i d e r a t i o n to this "core f u n c t i o n " w a s the a b i l i t y to e a s i l y change the R F w a v e f o r m that was b e i n g a p p l i e d . F o r instance, the t r a p p i n g v o l t a g e , the d u r a t i o n o f the c o o l i n g , a n d the slope o f the R F r a m p were a l l r e q u i r e d to be user adjustable. T o m a k e this p o s s i b l e , a r a p i d " w a v e f o r m c a l c u l a t i n g " routine w a s a d d e d before the scan f u n c t i o n . T o a first a p p r o x i m a t i o n , the rate at w h i c h the R F v o l t a g e w a s to be r a m p e d w a s r e q u i r e d to meet certain criteria.  55  gure 2.7 Flow chart for the lon Trap software.  Wait for Trigger on Software  User Controlled Trigger on Interface ^  Initialize Oscilloscope  o  Initialize Function Generator  Output to Oscilloscope through GPIB Output to Function Generator through GPIB  Calculate R F waveform  Load Waveform to Buffer  . O Wait for Hardware Trigger  Trigger from DGD 535 Digital Delay Generator Input from Current Amplifier through ADC  Synchronous Buffered  Output/Input  Output to RF generator through DAC  Convert Input Data to String and Display  Poll Oscilloscope to see if Laser Data Received  Data from Oscilloscope through GPIB  •  Download Laser Data From Oscilloscope  Bundle Mass Spectral String with Laser Data and Header Info  Save Data  Repeat from A  56  Output to Oscilloscope through GPIB  F o r instance, it has been e m p i r i c a l l y f o u n d that a suitable scan rate for the R F v o l t a g e is a c h i e v e d for a n i o n trap w h e n the ions are ejected at a rate o f 5 0 0 0 a m u / s e c o n d . S i n c e the R F p o w e r s u p p l y used i n this w o r k was capable o f a p p r o x i m a t e l y a 3 0 0 0 Vo-p m a x i m u m , this l i m i t s the m a x i m u m mass w h i c h m a y be ejected to a p p r o x i m a t e l y 3 0 0 a m u ( w i t h o u t resonance ejection). Therefore, the length o f the full mass range scan s h o u l d take a p p r o x i m a t e l y :  300amu*  Equ.  ° = 0.060seconds 5000 amu 1  S  £  C  n  d  2.1  S i n c e this corresponds to s c a n n i n g the full scale i n a t i m e o f a p p r o x i m a t e l y 0.060 seconds, a n d the R F w a s d r i v e n b y a c o n t r o l voltage o f 0-5 V , the c o n t r o l v o l t a g e s h o u l d be r a m p e d at a p p r o x i m a t e l y : 5 V / 0 . 0 6 0 seconds = 83.33 V / s e c o n d . A l s o , because w e w a n t to s a m p l e data at a rate sufficient to p r o d u c e 10 data points/peak, i f w e are s c a n n i n g 5 0 0 0 a m u / s e c o n d , w e w o u l d want to acquire 5 0 , 0 0 0 points/second. T h i s i m p l i e s that the buffered w a v e f o r m s h o u l d be l o a d e d such that e a c h step w o u l d increase the c o n t r o l voltage by: 83.33 V  t  1 second  — = 0.00166 V / s t e p « 1 . 6 6 mv/step 5 0 , 0 0 0 points :  1 second  Equ.  2.2  In a d d i t i o n to d e f i n i n g the R F r a m p , the c o n t r o l voltage also h a d to be constructed s u c h that the base " t r a p p i n g " l e v e l c o u l d be e a s i l y set. A l s o , it is o c c a s i o n a l l y u s e f u l , w h e n ejecting h e a v y i o n s , to first eject a l l o f the l o w mass components. T h i s " c l e a n i n g " p u l s e w a s also set to be user configurable. I n a l l , the entire w a v e f o r m that is c a l c u l a t e d and l o a d e d onto the output buffer is between 5 0 0 0 - 5 0 , 0 0 0 points. T h e user c o n t r o l s for this f u n c t i o n are o n the y e l l o w b o x o n the left h a n d side o f the operating w i n d o w . T h e  57  c o l l e c t e d data r e s u l t i n g f r o m the R F r a m p , the mass spectrum is d i s p l a y e d i n the large b l a c k area i n the top left o f the w i n d o w ( F i g u r e 2.6). B e s i d e s the " c o r e " operations, the i o n trap software w a s also r e q u i r e d to p r o v i d e m a n y secondary functions. These c a n be g r o u p e d into t w o categories: c o m m u n i c a t i o n w i t h the hard d r i v e (file I / O ) , and c o m m u n i c a t i o n w i t h other instruments t h r o u g h the G P I B interface. T h e file input/output p r o t o c o l used i n this software w a s r e q u i r e d to be user c o n f i g u r a b l e . Therefore, routines were w r i t t e n into the code to a l l o w the user to either operate under the " s i n g l e scan and n a m e " m o d e (a p o p - u p w i n d o w asks for a file n a m e ) , or under the " a u t o n a m e " m o d e (where a n a m i n g p r e f i x m a y be added, and then the c o m p u t e r s e q u e n t i a l l y n u m b e r s and names a l l the rest o f the files). T h e first o f these functions is useful w h e n discrete, "one t r y " experiments were preferred, whereas the autoname f u n c t i o n w a s useful for c o l l e c t i n g a large a m o u n t o f data a u t o m a t i c a l l y o v e r t i m e . T h e file I / O functions are d i s p l a y e d i n the b r o w n r e g i o n at the b o t t o m o f the operating w i n d o w . T h e software was also w r i t t e n to a u t o m a t i c a l l y create a n e w d i r e c t o r y for e v e r y day o f experiments (labeled w i t h the current date) and then store a l l data f r o m that days w o r k into this file. T h e software w a s also required to c o m m u n i c a t e w i t h t w o external d e v i c e s . T h i s c o m m u n i c a t i o n was a c h i e v e d t h r o u g h a G P I B , I E E E 4 8 8 . 2 interface. T h i s m o d e o f c o m m u n i c a t i o n w a s h a r d w i r e d into b o t h the o s c i l l o s c o p e ( T e k t r o n i x , T K D S 3 8 0 ) and a f u n c t i o n generator (Stanford R e s e a r c h , D S 345). T h e read, w r i t e , a n d o p e r a t i o n a l functions o f these d e v i c e s were hard set b y the manufacturers. Therefore, the software w a s s i m p l y r e q u i r e d to c a l l the functions t h r o u g h the G P I B interface. T h e I E E E 4 8 8 . 2  58  standard requires that e a c h data transfer packet be g i v e n a s p e c i f i c " l o c a t o r " header. Therefore, it is p o s s i b l e to string b o t h the o s c i l l o s c o p e and f u n c t i o n generator to the same G P I B c a r d because the data is o n l y read b y the appropriate d e v i c e . T h e o s c i l l o s c o p e for e x a m p l e , was i n i t i a l l y set w i t h a reset string f o l l o w e d b y a c o n f i g u r a t i o n string-; w h i c h i d e n t i f i e d the operational v o l t a g e range, t i m e d o m a i n , a n d t r i g g e r parameters. A f t e r the o s c i l l o s c o p e r e c e i v e d the p h o t o d i o d e data, a s e c o n d c a l l f u n c t i o n i n i t i a t e d a transfer o f the data to C o m p u t e r 1 for p r o c e s s i n g . T h e c o n t r o l o p t i o n s for the o s c i l l o s c o p e are s h o w n i n the blue b o x o n the right h a n d side o f the software window  (Figure 2.6). R o u t i n e s were w r i t t e n into the software to calculate the p o w e r o f  the laser shots a n d the d e l a y times between them. S i m i l a r l y , frequency a n d v o l t a g e v a l u e s w e r e sent to the S D S 345 f u n c t i o n generator v i a the G P I B . F i n a l l y , the c o l l e c t e d data from a l l sources, i n c l u d i n g the mass spectral data, the laser p o w e r s and delays, as w e l l as header i n f o r m a t i o n ( w h i c h listed a l l the user set e x p e r i m e n t a l c o n d i t i o n s ) are b u n d l e d and saved i n a spreadsheet format w h i c h is e a s i l y read b y a c o m m e r c i a l spreadsheet p r o g r a m , E x c e l ( M i c r o s o f t C o r p . R e d m a n , W A , U S A ) .  2.5 Overall System and Timing N o w that the subcomponents have been d e s c r i b e d , the o v e r a l l data f l o w a n d operation c a n be appreciated.  Figure 2.5 s h o w e d the r e q u i r e d t i m i n g for the s y s t e m w h i l e  Figure 2.8 s h o w s the b l o c k d i a g r a m o f the flow o f i n f o r m a t i o n a n d energy. T h e t i m i n g system is p r i m a r i l y d r i v e n b y the internal c l o c k o f a D G D 535 (Stanford R e s e a r c h S y s t e m s ) d i g i t a l delay generator.  T h i s 4-channel delay generator sends the i n i t i a t i o n  signals to the software, the laser c o n t r o l system, and the s y n t h e s i z e d f u n c t i o n generators.  59  Figure 2.8 Logical block diagram showing the flow of energy and information for the two-laser ion trap system at UBC.  IR LASER  UV LASER  Photodiode 1  Oscilloscope  Photodiode 2  ION TRAP Isolation Amplifier  Current Amplifier  R F Power Supply  t T  DAC  Computer 1  Function Generator  DAC  ADC  Timing Electronics  Computer 2 Legend: IR Laser UV Laser RF Voltage Digital Signal Analogue Low Voltage GPIB Interface  60  T h e t i m i n g d i a g r a m i n Figure 2.5 s h o w s the order o f operations for the " s i m p l e s t " case, i.e. there are n o secondary w a v e f o r m s a n d o n l y t w o laser shots are used. T h i s case is d i s p l a y e d as a " T i m i n g C a r t o o n " at the b o t t o m o f  Figure 2.5. C a r t o o n s , o f this type  w i l l be used throughout the rest o f the thesis, so the reader m y q u i c k l y observe the general order o f operations w i t h o u t the need for a p a g e - l o n g t i m i n g d i a g r a m . A s it stands, the m a j o r i t y o f the experiments d i s c u s s e d i n this thesis are based o n this core t i m i n g f u n c t i o n , w i t h a d d i t i o n a l features a n d components b e i n g added w h e r e needed. S i m i l a r l y , Figure 2.8 s h o w s a s i m p l i f i e d b l o c k d i a g r a m d i s p l a y i n g the g e n e r a l f l o w o f i n f o r m a t i o n a n d energy t h r o u g h the system. F o r m a n y o f the experiments d e s c r i b e d i n the f o l l o w i n g chapters, extra components were added as d e m a n d e d b y v a r i o u s e x p e r i m e n t a l p r o t o c o l s . H o w e v e r , w h i l e s o m e extra d e v i c e s w e r e added as r e q u i r e d , for the m o s t part, the core system d e s c r i b e d above w a s used for a l l the experiments described herein.  61  Chapter 3 Two Laser lon Trap Mass Spectrometry for the Analysis of Environmental Samples 3.1 Introduction T h i s t w o - l a s e r i o n trap s y s t e m w a s d e s i g n e d o r i g i n a l l y w i t h the g o a l o f e x a m i n i n g e n v i r o n m e n t a l l y important contaminants d i r e c t l y i n s o l i d matrices. S p e c i f i c a l l y , one i m p o r t a n t f a m i l y o f pollutants w a s c o n s i d e r e d : the p o l y c y c l i c aromatic h y d r o c a r b o n s (PAHs). P A H s represent a large f a m i l y o f m o l e c u l e s that are p r o d u c e d via b o t h natural a n d anthropogenic processes and are f o u n d u b i q u i t o u s l y throughout the e n v i r o n m e n t [79, 8 0 ] . Figure 3.1 s h o w s the structures o f the 6 P A H s p r i n c i p a l l y used i n this w o r k . P A H s are p r i m a r i l y f o r m e d via the i n c o m p l e t e c o m b u s t i o n o f h y d r o c a r b o n s i n c l u d i n g w o o d , c o a l , o i l , or gas u s e d i n the generation o f heat or e l e c t r i c i t y , or for m o t o r i z e d v e h i c l e s , and d u r i n g p e t r o l e u m c r a c k i n g [81]. O n l y a s m a l l fraction o f the total P A H l o a d c o m e s f r o m natural sources; p r i n c i p a l l y from forest fires or the d e c o m p o s i t i o n o f b i o m a s s i n swaps and bogs [82]. P A H s are capable o f t r a v e l i n g either attached to aerosols (for e x a m p l e soot particles) or deposited i n sludge and wastewater. T h e aerosol materials are capable o f t r a v e l i n g l o n g distances and have been f o u n d throughout the atmosphere i n remote o c e a n i c [83] and p o l a r atmospheres [84]. P A H s have l o n g been k n o w n to have potential m u t a g e n i c and c a r c i n o g e n i c properties. A s early as the 1 9 3 0 ' s , some P A H s were n a m e d as potential c a r c i n o g e n s . I n fact, the first k n o w n c h e m i c a l c a r c i n o g e n w a s the P A H dibenz(a,h)anthracene w h i c h w a s  . 6 2  Figure 3.1 The six polycyclic aromatic hydrocarbons (PAHs) primarily used in this work.  Acenaphthene 154 amu  Phenanthrene 178 amu  Pyrene 202 amu  Chrysene 228 amu  Benzo(a)pyrene 252 amu  Coronene 300 amu  63  i s o l a t e d f r o m a synthetic tar c o m p o u n d [85]. Subsequent w o r k has i d e n t i f i e d m a n y other P A H s to be potential a n i m a l carcinogens [80, 86]. These c o m p o u n d s c a n be h a r m f u l l y a d m i n i s t e r e d either t o p i c a l l y (via direct contact) or as fine particulates deposited i n the lungs [87]. A d d i t i o n a l l y , P A H s are also capable o f entering the b o d y i n d r i n k i n g water or i n s o l i d matrices s u c h as f o o d or s o i l [82]. O n c e ingested, P A H s t y p i c a l l y migrate to the fatty tissues and have b e e n s h o w n to affect l i v e r a n d k i d n e y f u n c t i o n . A s a result o f the demonstrated c a r c i n o g e n i c , m u t a g e n i c , and i m m u n o t o x i c effects o n a n i m a l s a n d h u m a n s , the U S E n v i r o n m e n t a l P r o t e c t i o n A g e n c y ( U S - E P A ) has l a b e l e d a n u m b e r o f P A H s as " P r i o r t y - 1 " pollutants [88]. T h i s d e s i g n a t i o n requires that the c o n c e n t r a t i o n o f these c o m p o u n d s be c l o s e l y m o n i t o r e d and l i m i t s are set for h u m a n exposure.  U n f o r t u n a t e l y , h o w e v e r , the task o f d e t e r m i n i n g P A H contaminant l e v e l s ,  p a r t i c u l a r l y i n s o l i d matrices, is not a t r i v i a l one. C u r r e n t l y there exist several methods a v a i l a b l e for the a n a l y s i s o f P A H s i n s o l i d s . T h e u s u a l m e t h o d for P A H analysis i n v o l v e s some f o r m o f e x t r a c t i o n f r o m the m a t r i x , t y p i c a l l y u s i n g a S o x h l e t , f o l l o w e d b y separation via c h r o m a t o g r a p h y , u s u a l l y gas c h r o m a t o g r a p h y ( G C ) , w i t h detection u s i n g mass spectrometry ( M S ) [89, 9 0 ] . T h i s m e t h o d , h o w e v e r , is l i m i t e d ; due to the l o w v o l a t i l i t y o f P A H s , the detectable m a s s range u s i n g m o s t G C / M S instruments is l i m i t e d to around 300 a m u . F u r t h e r m o r e , the l o w c o n c e n t r a t i o n o f P A H s often requires that a pre-concentration step be e m p l o y e d i n a d d i t i o n to sample c l e a n up p r i o r to analysis. A l o n g w i t h s o l u b i l i t y p r o b l e m s , the c o m b i n a t i o n o f a l l o f these pre-treatment procedures means that a f u l l a n a l y s i s c a n take days to c o m p l e t e . W h i l e other separation a n d detection methods have been suggested, none are u s e d r o u t i n e l y for analysis [91-93].  64  A n o t h e r important c o n s i d e r a t i o n i n the f i e l d o f P A H a n a l y s i s o f e n v i r o n m e n t a l samples is the issue o f s a m p l e preparation. T y p i c a l l y , most f o r m s o f P A H a n a l y s i s require that the samples are cleaned and the P A H s r e m o v e d from the s o l i d m a t r i x . T h i s process is neither i n e x p e n s i v e nor fast, and certainly not t r i v i a l because one s a m p l e often contains P A H s i n a variety o f p h y s i c o - c h e m i c a l states [94]. O n c e the P A H s have b e e n extracted f r o m their native m a t r i x , the analyst must also be c o n c e r n e d w i t h issues o f s o l u b i l i t y , storage, b i o t r a n s f o r m a t i o n , and photo-degradation. A s a s o l u t i o n to these persistent p r o b l e m s several groups have attempted direct a n a l y s i s . S o m e o f the m o r e successful a p p l i c a t i o n s i n c l u d e secondary i o n mass spectrometry ( S I M S ) [40], fast a t o m b o m b a r d m e n t ( F A B ) [42], and laser d e s o r p t i o n ( L D ) [41]. These techniques h o w e v e r , are often non-selective and p r o d u c e c o m p l e x m a s s spectra w i t h a v a r i e t y o f peaks r e s u l t i n g from m o l e c u l a r fragmentation o f b o t h the analyte a n d the m a t r i x . T w o - l a s e r m a s s spectrometry has been a p p l i e d to P A H analysis s u c c e s s f u l l y b y several groups [72, 9 5 , 9 6 ] . These groups have p e r f o r m e d P A H a n a l y s i s i n b o t h a r t i f i c i a l [96] a n d natural matrices [95]. Perhaps the b e s t - k n o w n e x a m p l e o f this t e c h n i q u e w a s the e x a m i n a t i o n o f M a r t i a n meteorites b y Z a r e ' s group [97, 98]. It s h o u l d be noted that i n a l l o f these e x a m p l e s , the m o d e o f M S used was a l w a y s a T O F . W h i l e a T O F is v e r y useful w h e n used i n a t w o - l a s e r experiment, it is not capable o f M S " - a feature that m a y p r o v e useful for o b t a i n i n g structural and p o s s i b l y i s o m e r i c i n f o r m a t i o n . F i n a l l y , f r o m a m e t h o d d e v e l o p m e n t p o i n t o f v i e w , the fact the P A H s had been p r e v i o u s l y e x a m i n e d w i t h the t w o - l a s e r technique w i t h a T O F p r o v i d e s an e x c e l l e n t base for the d e v e l o p m e n t and c o m p a r i s o n to this instrument.  65  T h i s chapter w i l l describe the early stages o f the d e v e l o p m e n t o f the instrument for e n v i r o n m e n t a l analysis. T h i s i n c l u d e s a s m a l l section devoted to d e m o n s t r a t i n g the effectiveness Of the current i o n trap software b y operating w i t h the e l e c t r o n g u n m o d e o f i o n i z a t i o n - this process was important for c a l i b r a t i o n because the i o n trap software u s e d here w a s n e w l y created. T h e remainder o f the chapter w i l l be dedicated to d e s c r i b i n g the effectiveness o f the t w o - l a s e r i o n trap s y s t e m f o r the a n a l y s i s o f P A H s d i r e c t l y o n s o l i d materials. T h e b u l k o f the m a t e r i a l reported i n this chapter w a s p r e v i o u s l y p u b l i s h e d b y Specht a n d B l a d e s i n the  Journal of the American Society for Mass Spectrometry  [99].  3.2 Experimental The overall system design was described i n  C H A P T E R 2. Therefore this s e c t i o n  w i l l be l i m i t e d to a s p e c i f i c d e s c r i p t i o n o f the e x p e r i m e n t a l techniques a n d materials u s e d d u r i n g the w o r k d e s c r i b e d i n this.chapter. I n this chapter, t w o distinct set-ups were e m p l o y e d . T h e i o n trap w a s i n i t i a l l y operated w i t h the " t r a d i t i o n a l " e l e c t r o n g u n m o d e o f i o n i z a t i o n , w h e r e b y analyte m o l e c u l e s w e r e l e a k e d into the v a c u u m c h a m b e r and i o n i z e d b y e l e c t r o n i m p a c t i o n i z a t i o n b y a n e l e c t r o n g u n located b e h i n d one o f the end caps. T h i s m e t h o d is useful f o r p r o v i d i n g mass c a l i b r a t i o n and demonstration o f the i n i t i a l effectiveness o f the n e w i o n trap software.. T h e i o n trap w a s also operated i n the n e w l y d e v e l o p e d two-laser set-up f o r the d i r e c t a n a l y s i s o f s o l i d samples. I n the e l e c t r o n g u n m o d e o f operation, the p r i m a r y s a m p l e u s e d w a s c a r b o n tetrachloride - C C 1 ( O m n i s o l v e grade, B D H C h e m i c a l s , T o r o n t o , Ont.). F o r the s o l i d 4  s a m p l i n g L 2 M S m e t h o d the s o l i d P A H samples were created b y first p r e p a r i n g standard s o l u t i o n s o f the P A H s i n H P L C grade hexane. T h e hexane and a l l o f the P A H s w e r e 66  acquired  from  S i g m a - A l d r i c h ( M i l w a u k e e , W I , U S A ) a n d w e r e u s e d as r e c e i v e d . T h e s i x  P A H s u s e d i n this chapter w e r e acenaphthene (mass 154 amu), phenanthrene (178 a m u ) , pyrene (202 a m u ) , chrysene (228 amu), benzo(a)pyrene (252 a m u ) , a n d coronene (300 a m u ) . T h e p r o d u c t i o n a n d creation o f the s o l i d samples w a s part o f the w o r k d e s c r i b e d i n this chapter a n d therefore i n f o r m a t i o n c o n c e r n i n g this is located i n the ' R e s u l t s a n d D i s c u s s i o n ' Section 3.3.2.  3.3 Results and Discussion 3.3.1  Test Of Instrument Effectiveness  T h e first stage i n the d e v e l o p m e n t o f this s y s t e m w a s to test the effectiveness o f the n e w l y b u i l t i o n trap system. T h i s w a s a c h i e v e d t h r o u g h the use o f a standard e l e c t r o n g u n arrangement. T h e electron g u n w a s used to i o n i z e a test gas ( C C L ) that h a d b e e n 4  l e a k e d into the v a c u u m c h a m b e r at a pressure o f 0.1 m T o r r . Figure 3.2 s h o w s a mass s p e c t r u m (average o f one h u n d r e d spectra) that results f r o m the electron i m p a c t ionization o f C C 1  4  u s i n g a 70 e V e l e c t r o n gun. T h i s spectrum w a s c o l l e c t e d under the  t i m i n g r e g i m e s h o w n at the b o t t o m o f the figure (this convention the thesis).  will be used  throughout  T h e s p e c t r u m contains 4 p r i m a r y peaks r e s u l t i n g f r o m the 4 p o s s i b l e  c o m b i n a t i o n s o f the c h l o r i n e isomers o f mass 35 (75.77 % abundance) a n d mass 37 (24.23 % abundance). T h e average r e s o l u t i o n o f these peaks is (m/AmFWHivi) is 2 4 6 . T h e reader s h o u l d also note that under this s i m p l e m o d e o f o p e r a t i o n the mass range w a s l i m i t e d to 2 8 0 T h as the h i g h mass l i m i t . O n c e the n o r m a l m o d e o f operation w a s established, the m o r e s o p h i s t i c a t e d resonance e j e c t i o n m o d e o f operation w a s e x a m i n e d . T h i s m e t h o d o f operation, w h i c h is  67  Figure 3.2 Mass spectrum of CCI ionized by electron impact ionization, resulting in the formation of CCI ions. 4  +  3  119 .  117  RF Voltage  68  n o w standard o n m o s t c o m m e r c i a l d e v i c e s , utilizes, a s i n g l e frequency w a v e f o r m , i n c o n j u n c t i o n w i t h the R F v o l t a g e r a m p i n order to eject species at a q v a l u e o f less t h a n 0.908. T h i s s h o u l d , i n theory, increase the mass range o f the d e v i c e because i o n s are ejected to the detector at a l o w e r  q-value,  a n d thus at the m a x i m u m v a l u e o f the R F  potential a p p l i e d to the trap, a h i g h e r mass i o n m a y be ejected. A l s o , this m o d e o f o p e r a t i o n s h o u l d p r o d u c e higher r e s o l u t i o n peaks because the ions are ejected d u r i n g a n a r r o w range o f R F voltages (i.e. they are m o r e coherently ejected).  Figure 3.3 is the mass spectrum (average, o f one h u n d r e d spectra) that results f r o m the e l e c t r o n i m p a c t i o n i z a t i o n o f C C I 4 under i d e n t i c a l c o n d i t i o n s to those u s e d for  Figure 3.2 except that a single frequency w a v e f o r m is a p p l i e d between the end caps d u r i n g the R F r a m p . T h i s w a v e f o r m has a frequency o f 247 k H z a n d e x c i t a t i o n v o l t a g e o f 6.25 Vpeak-to-peak- T h e a d d i t i o n o f this w a v e f o r m causes the i o n s to be ejected at a r e d u c e d q v a l u e o f 0.62, a n d consequently an increase i n the mass range to a v a l u e o f ~ 405 T h . A l s o , note that the average r e s o l u t i o n o f the C C l 3 peaks has i n c r e a s e d f r o m 246 +  to 301  m/AniFWHM-  A n o t h e r important feature f o u n d i n most i o n trap mass spectrometers is the a b i l i t y to p e r f o r m i o n i s o l a t i o n w i t h i n the trap b y the a d d i t i o n o f s u p p l e m e n t a l w a v e f o r m s . T h i s a b i l i t y w a s demonstrated o n the CCI4 sample b y a p p l y i n g a n o t c h e d b r o a d b a n d w a v e f o r m ( N B B W ) d u r i n g the i o n - c o o l i n g p e r i o d before the R F r a m p .  Figure 3.4 s h o w s  a mass s p e c t r u m (average o f one h u n d r e d spectra) that resulted f r o m the a p p l i c a t i o n o f a N B B W w a v e f o r m after a C C L  4  sample w a s i o n i z e d b y electron i m p a c t i o n i z a t i o n . T h e  N B B W w i n d o w w a s c h o s e n to have a n o t c h w i t h n o frequency c o m p o n e n t s b e t w e e n 84.5 k H z a n d 86.5 k H z a n d the o v e r a l l w a v e f o r m h a d a voltage  69  Figure 3.3 Mass spectrum of CCI ionized by electron impact ionization and ejected from the ion trap by the resonance ejection mode of operation. 4  • 130  150  Mass/Charge  E  -° 1 ' U N  fl  '  :  Resonance Ejection  4  RF Voltage  70  J  Figure 3.4 Mass spectrum of CCI ionized by electron impact ionization with a single NBBW cycle after ionization. The NBBW cycle caused the removal of all mass components below 119 Th and above 120 Th. 4  50  70  90  110  130  Mass/Charge  E-GUN I |  1  RF Voltage I  71  150  170  190  range n o larger than 1.4 V . . T h i s n o t c h w a v e f o r m effectively r e m o v e d a l l other i o n s i n p  p  the i o n trap except that at 119 T h . F i n a l l y , one a d d i t i o n a l a b i l i t y was r e q u i r e d to be demonstrated for this i o n trap system. A f t e r the N B B W i o n i s o l a t i o n w a v e f o r m is a p p l i e d , t y p i c a l l y , one w o u l d a p p l y a secondary s i n g l e frequency c o m p o n e n t at the secular frequency o f the i o n o f interest i n order to i n d u c e c o l l i s i o n a l d i s s o c i a t i o n and thus g a i n daughter i o n i n f o r m a t i o n . T h e l o g i c a l extreme o f this a p p l i c a t i o n occurs w h e n the single frequency w a v e f o r m v o l t a g e a p p l i e d is large e n o u g h so that n o daughter i o n s r e m a i n , but rather the parent i o n is c o m p l e t e l y r e m o v e d . F o r demonstration purposes, a single frequency w a v e f o r m o f this nature w a s a p p l i e d to an electron i m p a c t i o n i z e d CCI4 sample, but w i t h n o N B B W w a v e f o r m a p p l i e d . T h i s a l l o w s one to e x a m i n e the selectiveness o f the a p p l i e d w a v e f o r m . F i g u r e 3.5 s h o w s a m a s s spectrum (average o f one h u n d r e d spectra) that results f r o m the e l e c t r o n i m p a c t i o n i z a t i o n o f CCI4 f o l l o w e d b y the a p p l i c a t i o n a single frequency w a v e f o r m at 85.20 k H z , w i t h a voltage o f 1 . 5 V . . N o t e that this w a v e f o r m e f f e c t i v e l y P  P  r e m o v e s the peak at 119 T h w h i l e l e a v i n g its neighbors untouched. T h i s single frequency w a s useful not o n l y i n that it s h o w e d that a single frequency/voltage c o m b i n a t i o n c o u l d be c h o s e n to c o m p l e t e l y r e m o v e a single i o n w i t h / - 2 T h u n i t r e s o l u t i o n , but it also +  a l l o w e d the user to determine the precise q v a l u e under w h i c h the i o n s w e r e trapped. D u e to the nature o f the R F c i r c u i t r y it is v i r t u a l l y i m p o s s i b l e to d i r e c t l y measure the exact R F v o l t a g e a p p l i e d to an i o n trap. A s a result, d e t e r m i n i n g the t r a p p i n g R F l e v e l , a n d thus the trapping q v a l u e is v e r y difficult. A s a s o l u t i o n to this p r o b l e m m o s t w o r k e r s instead deduce a q v a l u e and thus the trap R F l e v e l b y d e t e r m i n i n g the frequency o f a k n o w n i o n b y selective i o n ejection. F o r e x a m p l e , u s i n g the data  72  Figure 3.5 Mass spectrum of CCI ionized by electron impact ionization and with removal of 119 Th by a single frequency ejection after ionization. 4  50  70  90  110  130  Mass/Charge  RF Voltage.  73  150  170  190  d e s c r i b e d i n F i g u r e 3.5 one c a n calculate the t r a p p i n g q v a l u e for the 119 T h i o n w a s 0.23 a n d b y k n o w i n g the i o n trap dimensions,that the R F t r a p p i n g v o l t a g e w a s a p p r o x i m a t e l y 283 v o l t s .  3.3.2 Sample Preparation O n c e the i n i t i a l p r o b l e m s o f software d e s i g n and i o n trap f u n c t i o n a l i t y w e r e w o r k e d out, the next major c h a l l e n g e c o n c e r n e d that o f sample preparation. A s m e n t i o n e d p r e v i o u s l y , the two-laser m e t h o d has been k n o w n for s o m e t i m e , and n a t u r a l l y , a n u m b e r o f s a m p l e preparation schemes have d e v e l o p e d o v e r the years [78, 100-102]. T h e r e are several requirements that must be met w i t h respect to s a m p l e preparation i n this system. I d e a l l y , a system s h o u l d be d e s i g n e d so that a s a m p l e m a y be inserted d i r e c t l y into the mass spectrometer w i t h o u t m u c h pre-treatment or preparation. M e c h a n i c a l l y , this presents a challenge as the samples to be investigated ( s o i l or sediments for e x a m p l e ) are often i n a loose p o w d e r f o r m . T h i s p r o b l e m is c o m p o u n d e d b y the fact that, i n the case o f this instrument (and m a n y others), that the s a m p l e m u s t be inserted p e r p e n d i c u l a r to the h o r i z o n t a l . T h i s i m p l i e s that some means must be d e v e l o p e d i n order to c o n t a i n a p o w d e r sample o n a v e r t i c a l surface. In a d d i t i o n to the issue o f preparing " r e a l " p o w d e r samples, s u c h as s o i l s a n d sediments, the question o f what an appropriate "test" sample s h o u l d consist o f w a s also addressed. I d e a l l y , the test sample w o u l d p r o v i d e a means o f p r o d u c i n g a s i m p l e a n d r e p r o d u c i b l e s i g n a l so that the m u l t i t u d e o f instrumental parameters c o u l d be o p t i m i z e d w i t h ease. M a n y groups have confronted the "test" sample p r o b l e m b y s i m p l y d i s s o l v i n g the analyte o f interest i n a n appropriate solvent, then d e p o s i t i n g a s m a l l d r o p o f the 74  s o l u t i o n onto a s a m p l e probe and a l l o w i n g the solvent to evaporate [103]. A l t e r n a t i v e l y , other groups have gone the route o f p r o d u c i n g t h i n p o l y m e r f i l m s w h i c h w e r e i m p r e g n a t e d w i t h the sample o f interest [100, 101]. W h i l e b o t h o f these techniques p r o d u c e v e r y stable and r e p r o d u c i b l e sources o f analytes, neither addresses the p r o b l e m o f p r e p a r i n g a real e n v i r o n m e n t a l sample, i.e. a s o l i d p o w d e r s u c h as a s o i l or sediment. It w a s therefore d e c i d e d early i n this w o r k that the m e t h o d o f s a m p l e i n t r o d u c t i o n a n d test s a m p l e selection s h o u l d be engineered s i m u l t a n e o u s l y so that the f i n a l p r o c e d u r e w o u l d be a m e n a b l e to " r e a l " p o w d e r samples a n d that the test samples w o u l d be a n accurate representation o f the same. N u m e r o u s p r o t o c o l s were e x a m i n e d before a suitable m e t h o d w a s d e c i d e d u p o n . I n terms o f addressing the p r o b l e m o f s a m p l i n g a p o w d e r m a t e r i a l o n a v e r t i c a l surface, a n u m b e r o f procedures were attempted. These i n c l u d e d the use o f g l y c e r o l as a b i n d i n g agent w h i c h w a s u s e d to adhere the p o w d e r sample to a v e r t i c a l surface. It has also been suggested b y L u b m a n that the use o f g l y c e r o l enhances the d e s o r p t i o n process because o f a favorable a b s o r p t i o n b y the a l c o h o l groups o f the g l y c e r o l at the d e s o r p t i o n laser w a v e l e n g t h [102]. W h i l e this m a y be true.in the case o f d e s o r p t i o n w i t h a CO2 laser (10.6 u m ) it w a s f o u n d to be unsuitable w h e n used w i t h the N d : Y A G d e s o r p t i o n laser ( 1 0 6 4 n m ) . M o r e i m p o r t a n t l y , the g l y c e r o l sample preparation m e t h o d w a s f o u n d to be i m p r a c t i c a l a n d i n t r o d u c e d numerous t e c h n i c a l p r o b l e m s s u c h as c o n t a m i n a t i o n , u n p r e d i c t a b l e s a m p l e m o d i f i c a t i o n , and m o s t i m p o r t a n t l y samples s l i p p e d o f f the p r o b e . T h e s e c o n d m e c h a n i s m for sample preparation w h i c h w a s attempted c o n c e r n e d the use o f d o u b l e s i d e d tape. It has been suggested that a p o w e r e d s a m p l e c o u l d be s i m p l y adhered to one side o f the tape and then attached to the probe surface. W h i l e w i t h  75  this procedure it w a s p o s s i b l e to o b t a i n a 2-laser mass spectrum, the q u e s t i o n o f the p o s s i b l e interferences b y the adhesive m a t r i x l i m i t e d the effectiveness o f this a p p r o a c h . T h e m e t h o d that w a s the most successful i n v o l v e d m e c h a n i c a l l y p r e s s i n g the p o w d e r into a s m a l l s a m p l e cup m a c h i n e d into the end o f the s a m p l i n g probe. T h i s m e t h o d , w h i c h is routine i n F T - I R measurements (i.e. w i t h the creation o f K B r pellets), w a s able to r e l i a b l y p r o d u c e stable samples that m a i n t a i n e d their p h y s i c a l structure w h e n h e l d v e r t i c a l l y . T h e p h y s i c a l d e s i g n and m a n u f a c t u r i n g o f the s a m p l e probe c u p and the press were done i n c o o p e r a t i o n w i t h the m e c h a n i c a l services g r o u p at U B C . T h e  final  d e s i g n is s h o w n i n F i g u r e 3.6. T h e p o w d e r samples were first p l a c e d into the s a m p l e c u p w h i c h w a s then inserted into the press. T h e press was d e s i g n e d w i t h m e c h a n i c a l stops i n p l a c e to insure that every sample was c o m p r e s s e d to the same extent. O n c e the sample preparation question h a d been r e s o l v e d the next c o n c e r n i n v o l v e d sample d e v e l o p m e n t . I d e a l l y , w h e n testing and d e s i g n i n g a n e w instrument, there are several requirements w h i c h must be met for selecting a "test" sample. F o r e x a m p l e , the i d e a l s a m p l e w o u l d p r o d u c e a l o n g lasting stable s i g n a l , so that instrument parameters m a y be v a r i e d and the r e s u l t i n g effect o n the s i g n a l c a n be investigated. S e c o n d l y , a test sample s h o u l d also m i m i c , as c l o s e l y as p o s s i b l e , potential analytes a n d matrices. S i n c e the i n i t i a l g o a l o f this instrument was to test for P A H s o n e n v i r o n m e n t a l matrices, this o b v i o u s l y i m p l i e d the creation o f p o w d e r sample that h a d been s p i k e d w i t h PAHs. S e v e r a l s o l i d matrices were c o n s i d e r e d and tested, but the one that w a s f i n a l l y u s e d w a s activated c h a r c o a l . A c t i v a t e d c h a r c o a l is a n i n e x p e n s i v e , c a r b o n based m a t e r i a l  76  Figure 3.6 Mechanical press and sample cup used in the creation of solid samples for the two-laser system at UBC.  s i m i l a r i n structure to graphite, w h i c h is easily attainable and k n o w n to h a v e e x c e l l e n t c h e m i c a l a d s o r p t i o n properties [104]. Test s a m p l e s were p r o d u c e d b y first c r e a t i n g a hexane s o l u t i o n o f the P A H s o f interest and then s p i k i n g a k n o w n v o l u m e o f the s o l u t i o n onto a w e i g h e d a m o u n t o f activated c h a r c o a l . T h e samples were then sonicated i n a c l o s e d container for 3 0 m i n u t e s to insure g o o d m i x i n g and the s o l v e n t then a l l o w e d to s l o w l y evaporate o f f at r o o m temperature l e a v i n g the P A H s deposited o n the c h a r c o a l m a t r i x . T h i s s i m p l e m e t h o d p r o d u c e d v e r y r e p r o d u c i b l e samples, w h i c h were l o n g l i v e d (for thousands o f laser shots), e a s i l y managed, and h a d r o u g h l y the same m e c h a n i c a l properties as s o i l or sediment.  3.3.3  Two Laser Ion Trap Mass Spectra  S e v e r a l h u n d r e d different c h a r c o a l / P A H standards were e x a m i n e d u s i n g the s a m p l e preparation procedure d e s c r i b e d above. A t y p i c a l e x a m p l e o f the mass s p e c t r u m o b t a i n e d f r o m one o f these samples c o n t a i n i n g five P A H s (acenaphthene,  phenanthrene,  pyrene, chrysene, a n d benzo(a)pyrene) a l l at a concentration o f a p p r o x i m a t e l y 25 p i n o l e P A H / g r a m o f c h a r c o a l is s h o w n i n F i g u r e 3.7. T h e operating c o n d i t i o n s u n d e r w h i c h this data w a s c o l l e c t e d is s h o w n at the b o t t o m o f the figure. N o t e that the s p e c t r u m is r e l a t i v e l y " c l e a n " i n that is s h o w s v e r y f e w peaks other than for the 5 P A H s . It s h o u l d also be noted that v e r y little fragmentation was o b s e r v e d i n any o f the trials p e r f o r m e d . A n e x p a n d e d v i e w o f this spectrum w h i c h s h o w s the detail a r o u n d the P A H central m a s s peaks i s also s h o w n i n F i g u r e 3 . 8 . T h e r e are s m a l l peaks at M - l and M - 2 a r o u n d e a c h parent peak that represents the loss o f one or t w o h y d r o g e n atoms f r o m e a c h o f the P A H s . T h i s type o f fragmentation is t y p i c a l i n t w o laser mass spectrometry o f P A H s and is dependant o n the laser p o w e r a n d w a v e l e n g t h [ 5 1 , 7 2 , 9 5 , 105]. 78  Figure 3.7 Typical mass spectrum collected with the two-laser mode of ionization of a sample of charcoal spiked with five PAHs.  50'  100  150  200 250 Mass/Charge  IR UV  RF Voltage  79  300  350  400  Figure 3.8 Expanded view of Figure 3.7 between mass 130-265 Th.  130  140  150  IR  160  170 . 180  190 200 210 Mass/Charge  UV  1 RF Voltage  80  220  230  240  250  260  In order to test the l o n g e v i t y , stability, and r e p r o d u c i b i l i t y o f the standard "test" s a m p l e several trials were p e r f o r m e d where the one standard was e x p o s e d to several t h o u s a n d laser shots. A n e x a m p l e o f this type o f experiment is s h o w n i n  Figure 3.9, w h i c h is a  p l o t o f the m a g n i t u d e (integrated pyrene peak areas) o f the pyrene s i g n a l as a f u n c t i o n o f the n u m b e r o f laser shots for a sample c o n t a i n i n g 10 u m o l p y r e n e / g r a m o f c h a r c o a l . T h i s p l o t , t y p i c a l o f these experiments, s h o w s a r e l a t i v e l y dramatic i n i t i a l d r o p i n p y r e n e s i g n a l w h i c h then l e v e l s o f f to f o r m a stable constant s i g n a l . T h i s data suggests, that there are l i k e l y t w o types o f pyrene e n v i r o n m e n t s i n the c h a r c o a l sample. F o r one a d s o r p t i o n site, the pyrene is w e a k l y b o u n d and it is desorption f r o m this site that is p r i m a r i l y r e s p o n s i b l e for the pyrene s i g n a l d u r i n g the first 500 laser shots. F o r the other type o f a d s o r p t i o n site pyrene is b o u n d m o r e strongly and produces the r e l a t i v e l y constant s i g n a l o b s e r v e d i n the later tens o f thousands o f mass spectra. W h i l e there i s n o further e v i d e n c e p r o v i d e d here to c o n f i r m this hypothesis, the result is nonetheless v e r y useful. T h e flat r e g i o n o f the curve i n  Figure 3.9 i m p l i e s that this s a m p l e preparation c a n  p r o v i d e a stable source o f s i g n a l so that instrumental parameters and operating c o n d i t i o n s m a y be o p t i m i z e d w i t h o u t fear o f other s a m p l e effects.  3.3.4  Effect of IR Power on Observed Signal  It is w e l l k n o w n that the magnitude o f the I R p o w e r p l a y s a n important r o l e i n the d e s o r p t i o n process, therefore the effect o f I R intensity o n the P A H signals w a s investigated. A n e x a m p l e o f this experiment is s h o w n i n  Figure 3.10 for the case o f  phenanthrene at a c o n c e n t r a t i o n o f 10 u m o l phenanthrene/gram o f c h a r c o a l .  Figure 3.10  s h o w s a n e x p e r i m e n t where, l i k e above, a sample was e x p o s e d to several t h o u s a n d laser shots. F o r the first 2 5 0 0 shots ( I R laser p o w e r a p p r o x i m a t e l y 1.2* 1 0 W / c m ) the s i g n a l 5  81  2  Figure 3.9 Magnitude of the pyrene peak area as a function of the number of laser cycles for a charcoal sample spiked with pyrene.  0  10  20 100's of laser shots  82  30  40  Figure 3.10 Effect of IR power on observed signal for phenanthrene as a function of the number of laser shots for three IR laser powers. • Measured IR Power (Left Hand Units). • Integrated Peak Areas (Right Hand Units).  20  40  60  100's of laser shots  83  80  100  w a s o b s e r v e d to decrease and l e v e l o f f as before. A t this p o i n t , h o w e v e r , the I R laser intensity w a s increased b y 3 3 % ( I R laser p o w e r a p p r o x i m a t e l y 1.6*10 W / c m ). A s a results o f this increase i n I R p o w e r "the phenanthrene s i g n a l increased s h a r p l y , a n d then b e g a n the d o w n w a r d trend as before. T h e same p h e n o m e n o n w a s o b s e r v e d a t h i r d t i m e w h e n the laser p o w e r w a s increased yet a g a i n b y 8 0 % w i t h respect to the o r i g i n a l ( I R laser p o w e r a p p r o x i m a t e l y 2.2* 1 0 W / c m ) . 5  2  T h e s e observations are consistent w i t h b u l k s a m p l i n g theory [106-108]. B r i e f l y , i n order for a P A H to be desorbed f r o m the surface o f a b u l k s o l i d , the temperature m u s t increase e n o u g h to m a k e the process statistically probable. D u r i n g the course o f a 10 ns laser pulse, the laser induces a v e r y r a p i d heating i n the s o l i d . T h e rate o f heating as w e l l as the t e m p o r a l a n d spatial properties o f the temperature i n the s o l i d are affected b y a n u m b e r o f factors i n c l u d i n g the absorption coefficient o f the m a t e r i a l at the laser w a v e l e n g t h , the t h e r m a l c o n d u c t i v i t y o f the sample, and the heat c a p a c i t y o f the m a t e r i a l [109,  110]. S e v e r a l e x c e l l e n t treatments o f this heating p h e n o m e n o n s h o w that, t y p i c a l l y ,  the b u l k s a m p l e w i l l rise to a temperature that m a k e s the d e s o r p t i o n p r o b a b l e for o n l y a few tens o f nanoseconds d u r i n g and after the laser p u l s e [108, 111]. A l s o , w h e n a greater I R p o w e r is a p p l i e d to the sample, a h i g h e r temperature w i l l be a c h i e v e d at a greater b u l k depth. Therefore, w e m a y c o n c l u d e that i n the case o f a stepped I R p r o f i l e , w e are o b s e r v i n g s a m p l i n g f r o m greater and greater depths w i t h each increase i n laser p o w e r . T h e signals a l l q u i c k l y decay, as seen p r e v i o u s l y , because as the n u m b e r o f laser shots increases, the s a m p l i n g area b e c o m e s depleted i n w e a k l y b o u n d P A H s . F i n a l l y , it is n o t e d that i n the t h i r d I R p o w e r increase step, the increase i n phenanthrene s i g n a l is not  84  as p r o n o u n c e d as the s e c o n d step. T h i s is most l i k e l y due to the fact that the P A H m u s t desorb f r o m a greater depth i n order to be s a m p l e d .  3.3.5 Effect of UV power on Observed Signal T h e U V p o w e r i s also w e l l k n o w n to affect the type a n d m a g n i t u d e o f o b s e r v e d signals i n t w o laser mass spectrometry. A s a result, a systematic study w a s undertaken to e x a m i n e the effect o f the U V p o w e r o n the s i g n a l f r o m several P A H s . V i r t u a l l y i d e n t i c a l results were o b s e r v e d for a l l o f the five P A H s e x a m i n e d . F i g u r e 3.11 is a p l o t o f the U V p o w e r v s . integrated peak areas for the five P A H s . T h e graphs e x h i b i t a l i n e a r r e l a t i o n s h i p b e t w e e n s i g n a l a m p l i t u d e and laser energy at energies b e l o w about 80 p J , h o w e v e r , at h i g h e r energies, a l l o f the signals b e g i n to l e v e l o f f (note: the line drawn  through the data has no physical meaning and is only meant to guide the readers eye). A t p u l s e energies h i g h e r than about 100 u J , fragmentation products f r o m the P A H s b e g i n to appear i n the mass spectra. T h i s w o u l d indicate that at h i g h p h o t o n f l u x e s , the neutrals are either a b s o r b i n g three photons c a u s i n g fragmentation, or i o n s f o r m e d p r e v i o u s l y are a b s o r b i n g a p h o t o n to i n d u c e fragmentation.  F i n a l l y , it s h o u l d also be noted, that at these  h i g h fluxes, the space charge l i m i t w a s easily reached. S o any a d d i t i o n a l i o n s w h i c h w e r e f o r m e d w e r e faced w i t h a large C b u l o m b i c r e p u l s i o n . I n general, the r e l a t i o n s h i p between the s i g n a l intensity and U V energy is s i m i l a r to that o b s e r v e d b y Z e n o b i ' s group [96] and is consistent w i t h the theoretical treatment b y J o h n s o n a n d O t i s [112]. B r i e f l y , the results c a n be e x p l a i n e d i f o n e considers the process i n v o l v e d i n resonance enhanced t w o - p h o t o n i o n i z a t i o n . I f the a b s o r p t i o n cross section at the e x c i t a t i o n w a v e l e n g t h used is i d e n t i c a l for b o t h the g r o u n d state a n d the  85  Figure 3.11 UV power (uJ) vs. integrated peak areas for five PAHs. The xaxis is in the units of uJ of UV energy in the trap volume and the y-axis is in terms of integrated peak areas.  86  Figure 3.11 continued.  Chrysene  900  Benzo(a)Pyrene 5 -. 4 _  0  100  200  300  400  500  87  600  700  800  900  e x c i t e d state o f the m o l e c u l e , then one w o u l d expect to see a quadratic dependence o f the s i g n a l o n the U V energy. If, h o w e v e r , the a b s o r p t i o n cross section at the e x c i t a t i o n w a v e l e n g t h w a s m u c h greater for one state than the other, a "rate d e t e r m i n i n g step" w o u l d be o b s e r v e d , a n d the signal w o u l d b e c o m e l i n e a r w i t h U V intensity. T h e latter scenario is w h a t w a s o b s e r v e d i n the case o f a l l five P A H s e x a m i n e d . It is i m p o s s i b l e to k n o w , w i t h this e x p e r i m e n t a l set u p , w h i c h absorption process w a s rate d e t e r m i n i n g . T h e a n a l y t i c a l l y important result f r o m this experiment is that it a l l o w s us to select the r e g i o n a r o u n d ~ 4 0 pJ/pulse as the o p t i m u m U V laser energy for a n a l y t i c a l determinations o f P A H s because the signal/energy ratio i n this r e g i o n is linear a n d thus e a s i l y a c c o u n t e d for.  3.3.6 Semi-Quantitative A n a l y s i s It has l o n g b e e n k n o w n that laser s a m p l i n g is a less than ideal means o f p e r f o r m i n g quantitative a n a l y s i s . T h e shot-to-shot fluctuations i n the lasers a n d n o i s e i n d u c e d b y u n e v e n s a m p l e preparation t y p i c a l l y renders f u l l quantitative a n a l y s i s an i m p o s s i b i l i t y . R e g a r d l e s s , it w a s still a n important parameter to investigate w i t h this n e w l y created s y s t e m since semi-quantitative a n a l y s i s is p o s s i b l e . T o evaluate this aspect, a series o f standards o f k n o w n concentration were prepared c o v e r i n g the c o n c e n t r a t i o n range o f 1 to 25 u m o l p y r e n e / g r a m o f c h a r c o a l . T h e m e a s u r e d s i g n a l v s . k n o w n c o n c e n t r a t i o n o f the sample is p l o t t e d i n F i g u r e 3.12. T h i s p l o t e x h i b i t s a g o o d linear r e l a t i o n s h i p between c o n c e n t r a t i o n a n d o b s e r v e d s i g n a l w i t h a n d R v a l u e o f 0.99. 2  T h e error bars are based o n m e a s u r i n g five replicates o f the same sample a n d are ± 5 % . T h e m e a s u r e d signals w e r e c o l l e c t e d b y averaging the s i g n a l (integrated p e a k area) for  88  Figure 3.12 Concentration vs. measured signal for a series of pyrene/charcoal standards.  1000 mass spectra after the sample h a d been e x p o s e d to 500 laser shots (i.e. once the s i g n a l w a s i n the flat part o f the l i f e t i m e curve).  It s h o u l d be noted, that this c a l i b r a t i o n  c u r v e demonstrates o n l y the linear response o f the system and not the absolute d e t e c t i o n l i m i t s . T h i s c a n be appreciated w h e n one c o n s i d e r s that the m a g n i t u d e o f the p e a k area i s a f u n c t i o n o f b o t h I R a n d U V laser energy as w e l l as the c o n c e n t r a t i o n o f the analyte a n d the nature o f the m a t r i x . A s a result, the m a g n i t u d e o f the analyte s i g n a l c a n be s i g n i f i c a n t l y c h a n g e d for a g i v e n concentration b y s i m p l y i n c r e a s i n g the I R a n d U V power. C a l i b r a t i o n curves have been o b s e r v e d p r e v i o u s l y for the two-laser t e c h n i q u e . H o w e v e r , m o s t have u s e d a n internal standard to compensate for shot to shot fluctuations i n the laser energy [53]. R e g a r d l e s s , a useable c a l i b r a t i o n c u r v e w a s o b t a i n e d i n this e x p e r i m e n t , w h i c h demonstrates that the technique is useful for, at least, s e m i quantitative analysis. O f course, i n cases where a m a t r i x m a t c h w a s h a r d to find, the m e t h o d o f standard additions w o u l d be preferred.  3.3.7 Selective Ion A c c u m u l a t i o n T o demonstrate the u n i q u e c a p a b i l i t i e s o f the i o n trap, a series o f e x p e r i m e n t s w e r e p e r f o r m e d to evaluate the p o s s i b i l i t y o f s e l e c t i v e l y a c c u m u l a t i n g the results o f several laser shots. E x t e r n a l w a v e f o r m s were used to isolate s p e c i f i c l o w c o n c e n t r a t i o n species i n the i o n trap. These species were pre-concentrated i n the gas phase b y p e r f o r m i n g m u l t i p l e laser s h o t / N B B W sequences w h i l e m a i n t a i n i n g the R F v o l t a g e at a steady t r a p p i n g l e v e l . T h i s procedure begins w i t h d e s o r p t i o n a n d i o n i z a t i o n laser shots f o l l o w e d b y a short N B B W pulse that is used to clear the trap o f a l l species but the l o w c o n c e n t r a t i o n analyte. S u c c e s s i v e laser s h o t / N B B W pulse c o m b i n a t i o n s are then u s e d to 90  b u i l d u p a large analyte p o p u l a t i o n i n the trap. W i t h o u t the N B B W pulses b e t w e e n the laser shot c o m b i n a t i o n s , a l l species i n the trap w o u l d b u i l d u p e q u a l l y , a n d as a result the l o w intensity s i g n a l w o u l d be s w a m p e d and space charge w o u l d render the r e s u l t i n g spectrum meaningless. A s a d e m o n s t r a t i o n o f this m e t h o d , a c h a r c o a l sample w a s prepared w h i c h c o n t a i n e d five P A H s : acenaphthene, phenanthrene, pyrene, benzo(a)pyrene, a n d c o r o n e n e a l l at a c o n c e n t r a t i o n o f a p p r o x i m a t e l y 25 p m o l P A H / g r a m c h a r c o a l . A s i x t h P A H , chrysene, w a s also s p i k e d o n this sample, at a concentration o f a p p r o x i m a t e l y 1 u m o l P A H / g r a m c h a r c o a l . A n e x a m p l e o f a mass spectrum o f this sample c o l l e c t e d i n the n o r m a l m a n n e r is s h o w n as the s o l i d l i n e i n  Figure 3.13. T h e chrysene s i g n a l is seen to  be o n l y s l i g h t l y a b o v e the base l i n e i n this spectrum. T h e gray l i n e i n  Figure 3.13 s h o w s  a s p e c t r u m w h e r e seven laser s h o t / N B B W c y c l e s were u s e d to a c c u m u l a t e a large chrysene i o n p o p u l a t i o n . I n this spectrum, the chrysene signal i s n o w far a b o v e base l i n e and e a s i l y quantifiable. T h e c o n d i t i o n s under w h i c h these t w o spectra w e r e a c q u i r e d are s h o w n at the b o t t o m o f the figure.  Figure 3.14 s h o w s the average chrysene peak height ( n o r m a l i z e d relative to a s i n g l e laser c y c l e ) as a f u n c t i o n o f the n u m b e r o f c y c l e s . T h i s l i n e has a slope o f 0.9991 and a R o f 0.996 i n d i c a t i n g that the b u i l d - u p is v e r y linear a n d that s u c c e s s i v e laser shots do not substantially interfere w i t h the trapped i o n s . T h i s procedure c o u l d b e c o m e a n a l y t i c a l l y useful i n situations where l o w concentration species are to be e x a m i n e d b y MS/MS.  O b v i o u s l y , a signal b u i l d - u p l i k e this w o u l d be i m p o s s i b l e i n a T O F M S .  91  Figure 3.13 Mass spectrum from a sample containing 6 PAHs (chrysene depleated) [black line]. After chrysene gas phase pre concentration [gray line].  150  100  200  250  Mass/Charge N o pre concentration  IR  RF  U V  Voltage  W i t h pre concentration  IR UV .  ,|R UV  92  300  350  Figure 3.14 Average chrysene peak height normalized relative to a single laser cycle as a function of the number of laser cycles.  0  1  2  3  4  5  6  7  8  Number of Cycles  93  9  10  11  12  3.3.8 SRM 1994 Analysis T h e f i n a l sets o f experiments were a i m e d at d e t e r m i n i n g whether or not real s o l i d samples c o u l d be a n a l y z e d w i t h the p r o p o s e d p r o t o c o l . T o this end, a standard reference m a t e r i a l , S R M 1944 N e w Y o r k - N e w Jersey R i v e r W a t e r S e d i m e n t , w a s e x a m i n e d i n a m a n n e r i d e n t i c a l to the determinations o f the c h a r c o a l standard. T h e r e s u l t i n g s p e c t r u m p r o d u c e d w i t h n o s a m p l e p r e t r e a t m e n t is s h o w n i n F i g u r e 3.15. T h i s figure s h o w s the u n c o r r e c t e d mass spectral data that results w h e n a real sediment s a m p l e c o n t a i n i n g a series o f P A H s a l o n g w i t h several other classes o f contaminants is a n a l y z e d . T a b l e 3.1 s h o w s a list o f some o f the P A H c o m p o u n d s k n o w n to exist i n the s a m p l e w i t h their certified concentrations. It was b e y o n d the scope o f this i n i t i a l study to quantify a n d c o n f i r m the concentrations o f a l l the P A H s i n this sample. Instead this s h o u l d s i m p l y be accepted as a p r o o f o f concept to stimulate further i n v e s t i g a t i o n . I n the case o f this experiment, n o real secondary layer o f s e l e c t i v i t y , b e y o n d o p t i c a l s e l e c t i v i t y , is r e q u i r e d i n the analysis o f the spectra; h o w e v e r , it w o u l d be a r e l a t i v e l y t r i v i a l matter to p e r f o r m N B B W i s o l a t i o n o n selected m a s s p e a k s f o r further daughter a n a l y s i s and structural c o n f i r m a t i o n . T h i s c o n f i r m a t i o n o f mass selected p e a k s is i m p o s s i b l e w i t h m o s t c o n v e n t i o n a l two-laser systems because they a l l a l m o s t e x c l u s i v e l y use T O F mass spectrometers for analysis.  94  Figure 3.15 Two laser mass spectrum observed from a sample of standard reference material 1944a.  ii  I ii  100.  50  150  200  250  300  Mass/Charge  IR  UV  R F Voltage  95  350  iLlLilU j 400  450  500  Table 3.1 PAH compounds certified to be contained in SRM 1944a.  P A H s certified i n S R M 1944a - N e w Y o r k / N e w Jersy R i v e r water sediment Mass  Mass Fraction Component  (mg/kg)  (amu) 128  Naphthalene  1.65  154  Acenaphthene  0.57  178  Phenanthrene  5.27  178  Anthracene  1.77  192  Methylphenantherene  7.88  202  Fluorantherene  8.92  202  Pyrene  9.7  206  Dimethylphenantherene  11.94  228  • Chrysene  4.86  228  Benz(a)anthracene  4.72  252  Benz(x)fluoranthene where x = bj,k,a  9.04  252  . Benzo(x)pyrene where x=a,e  7.58  252  Perylene  1.17  276  Benzo(ghi)perylene  2.84  276  Indeno(l ,2,3-cd)pyrene  2.78  96  3.4 Conclusions R E M P I - I T M S c a n be effectively used to d i r e c t l y o b t a i n mass spectra for P A H s i n soils a n d s i m i l a r materials. T h e R E M P I m e t h o d p r o v i d e s e x c e l l e n t s e l e c t i v i t y for P A H s c o u p l e d w i t h v e r y h i g h i o n i z a t i o n efficiency. T h e i o n trap was demonstrated to h a v e m a n y advantages o v e r traditional T O F instruments w i t h respect to t w o - l a s e r a n a l y s i s ; h o w e v e r , a f e w p r o b l e m s w i t h i o n traps r e m a i n . I o n traps t y p i c a l l y have a l o w e r r e s o l u t i o n , mass a c c u r a c y , and mass range than reflectron T O F ' s . W h i l e the samples e x a m i n e d here were r e l a t i v e l y h i g h i n concentration there is n o reason to b e l i e v e that b y s i m p l y i n c r e a s i n g the laser i r r a d i a t i o n area or i o n i z a t i o n t i m e that s e n s i t i v i t y o n par w i t h other e n v i r o n m e n t a l mass spectrometric m e t h o d s c o u l d n ' t be achieved. F i n a l l y , the reason w h y n o M S / M S data was c o l l e c t e d o n any o f the P A H s s h o w n i n this chapter s h o u l d be noted. P A H s are one o f a v e r y s m a l l class o f c o m p o u n d s for w h i c h M S / M S is p o o r at d i s t i n g u i s h i n g between i s o m e r s because m o s t i s o m e r s have r e l a t i v e l y s i m i l a r fragmentation patterns. Therefore, future chapters w i l l address these t w o issues. A d e m o n s t r a t i o n o f the M S / M S capabilities o f this t w o - l a s e r i o n trap s y s t e m w i l l be s h o w n w i t h a n a n a l y t i c a l l y relevant sample i n C H A P T E R 5. A l s o , the issue o f d i s t i n g u i s h i n g between P A H isomers w i t h this s y s t e m w i l l be addressed i n C H A P T E R S 6 a n d 7.  97  Chapter 4 Desorption Profiles of PAHs from Activated Charcoal 4.1 Introduction D u r i n g the course o f w o r k described i n C H A P T E R 2 and C H A P T E R 3 the q u e s t i o n o f the o p t i m u m t i m e d e l a y b e t w e e n the f i r i n g o f the d e s o r p t i o n laser a n d the i o n i z a t i o n laser arose. K n o w l e d g e o f the effect o f this delay is important for t w o reasons. P r i m a r i l y , the d e t e r m i n a t i o n o f the i d e a l d e l a y is important for instrument d e v e l o p m e n t i n that it helps o p t i m i z e the r e s u l t i n g s i g n a l . In a d d i t i o n , this data c a n also be u s e d to p r o v i d e i n s i g h t into the d e s o r p t i o n process. T h e study o f desorption o f m o l e c u l e s and atoms f r o m surfaces is a r e l a t i v e l y mature field [106, 113]. H o w e v e r , w h e n this d e s o r p t i o n is i n d u c e d b y a laser p u l s e there are further factors w h i c h must be considered [54]. O w i n g to the great v a r i e t y o f p o s s i b l e c o m b i n a t i o n s o f laser w a v e l e n g t h , t e m p o r a l w i d t h s , and pulse energies, it is d i f f i c u l t to describe a l l d e s o r p t i o n events w i t h a single c o m p r e h e n s i v e m o d e l . A d d i t i o n a l l y , the c o m p o s i t i o n o f the desorbate m a t r i x further c o m p l i c a t e s analysis. F o r e x a m p l e , the e l e c t r i c a l c o n d u c t i v i t y , t h e r m a l properties, and o p t i c a l cross s e c t i o n a l l p l a y a r o l e i n d e t e r m i n i n g the m o d e o f desorption. In terms o f laser w a v e l e n g t h s , t w o d e s c r i p t i v e r e g i m e s are c o m m o n ; either resonant laser d e s o r p t i o n b y direct v i b r a t i o n a l or e l e c t r o n i c e x c i t a t i o n o f the adsorbates [114, 115], or e x c i t a t i o n b y laser-induced thermal desorption ( L I T D ) (i.e. i n d i r e c t h e a t i n g o f the analytes). In general, w h e n the d e s o r p t i o n is p e r f o r m e d w i t h a p u l s e d I R laser (as  98  i n the case o f this w o r k ) a l l adsorbed species, regardless o f structure, are affected due to the i n t e r m o l e c u l a r c o u p l i n g and surface heating effects b y adsorbate-surface c o u p l i n g s [116]. A s a result o f this n o n - s e l e c t i v i t y , L I T D is preferred for routine a n a l y s i s o f u n k n o w n analytes a n d matrices [64]. I n the literature, n u m e r o u s m e c h a n i s m s have been presented to e x p l a i n the o b s e r v a t i o n that t h e r m a l l y l a b i l e , polar, and n o n - v o l a t i l e c o m p o u n d s c a n be desorbed w i t h o u t fragmentation b y L I T D . Z a r e and L e v i n e suggested a bottleneck i n the f l o w o f energy f r o m a r a p i d l y heated surface t h r o u g h the surface-adsorbate b o n d to the i n t e r n a l b o n d s o f the d e s o r b i n g m o l e c u l e s [117, 118]. T h i s effect is p r o p o s e d based o n a m i s m a t c h i n frequency between internal v i b r a t i o n a l m o d e s o f the m o l e c u l e a n d the l o w frequency surface-adsorbate m o d e . T h i s effect w o u l d most l i k e l y o n l y b e c o m e i m p o r t a n t for situations w i t h h i g h heating rates ( 1 0  1 1  K / s ) and v e r y w e a k l y b o u n d m o l e c u l e s .  T h e b u l k o f the current literature, h o w e v e r , suggests that m o s t L I T D e x p e r i m e n t s can be e x p l a i n e d u s i n g c l a s s i c a l transition state theory [57, 7 3 , 119-121]. It is p r o p o s e d , that, under these c i r c u m s t a n c e s , t h e r m a l e q u i l i b r i u m is m a i n t a i n e d , and t h e r m a l d e s o r p t i o n is i n c o m p e t i t i o n w i t h thermal d e c o m p o s i t i o n where the relative rates are d e t e r m i n e d b y frequency factors ( A ) and a c t i v a t i o n energies ( E A ) . F o r m o s t systems, at moderate temperatures, d e c o m p o s i t i o n dominates o v e r desorption. H o w e v e r , at h i g h temperatures, d e s o r p t i o n is expected to be the p r e d o m i n a n t m o d e . Therefore, i n situations s u c h as L I T D , w h e r e the temperature rises v e r y q u i c k l y , it is postulated that it is p o s s i b l e to desorb t h e r m a l l y l a b i l e m o l e c u l e s w i t h little d e c o m p o s i t i o n . T h e w o r k i n this chapter w a s o r i g i n a l l y p e r f o r m e d for p r a c t i c a l reasons; to determine the i d e a l t i m e b e t w e e n the I R desorption laser and the U V i o n i z a t i o n laser.  99  T h i s w a s a c h i e v e d b y i n c r e m e n t i n g the delay t i m e between laser shots o v e r m a n y thousands o f c o l l e c t e d m a s s spectra. T h i s delay data, or t i m e - o f - f l i g h t data ( T O F ) as it is often c a l l e d , p r o v i d e s s o m e insight into the desorption process. It also suggests p o s s i b l e fundamental l i m i t s to future w o r k .  4.2 Experimental T h e experiments d e s c r i b e d i n this chapter were p e r f o r m e d o n the t w o - l a s e r system d i s c u s s e d i n  C H A P T E R 2 and C H A P T E R 3. I n this experiment, a s i n g l e  s a m p l e was a n a l y z e d o v e r several thousand-laser c y c l e s . T h i s sample w a s prepared as described i n  C H A P T E R 3, and c o n t a i n e d f i v e P A H s ; A c e n a p h t h e n e (154 a m u ) ,  Phenanthrene (178 a m u ) , P y r e n e (202 a m u ) , C h r y s e n e (228 a m u ) , a n d B e n z o ( a ) p y r e n e (252). T h e d e l a y b e t w e e n the d e s o r p t i o n laser ( N d : Y A G @ 1064 n m ) and the i o n i z a t i o n laser ( N d : Y A G @ 2 6 6 n m ) w a s p r e c i s e l y r e g i m e n t e d b y c o n t r o l o f e a c h laser's Q SWITCH.  T h i s y i e l d e d excellent t i m i n g r e s o l u t i o n w i t h jitters o f less than l p s . T o  ensure accurate t i m i n g , each laser shot was recorded u s i n g the d e t e c t o r / o s c i l l o s c o p e system d e s c r i b e d i n  C H A P T E R 2. A s a result the actual d e l a y b e t w e e n every laser shot  w a s r e c o r d e d ; it w a s this v a l u e w h i c h was used i n further c a l c u l a t i o n s .  4.3 Results and Discussion T h e p r i m a r y g o a l o f this chapter w a s to determine the i d e a l t i m e d e l a y b e t w e e n the f i r i n g o f the d e s o r p t i o n laser a n d i o n i z a t i o n laser. T h i s e x p e r i m e n t w a s p e r f o r m e d b y first p r e p a r i n g a c h a r c o a l standard c o n t a i n i n g five P A H s .  T h e sample w a s then e x p o s e d  to t w o t h o u s a n d laser shots i n order to reach the "steady s i g n a l " seen p r e v i o u s l y  100  ( C H A P T E R 3). T h e U V laser d e l a y w a s then v a r i e d b e t w e e n - 5 to 4 0 0 ps w i t h respect to the I R f i r i n g t i m e . T h e delay settings for a c q u i r i n g the data w e r e set n o n - s e q u e n t i a l l y i n order to a v o i d systematic effects. A "reference" delay t i m e o f 30 ps w a s also e x a m i n e d several times throughout the course o f the experiments i n order to track any s i g n a l decrease caused b y sample degradation. M a s s spectral data were a c q u i r e d for at least 100 laser shots at e a c h d e l a y setting. T h e integrated peak areas for e a c h o f the f i v e P A H s as a f u n c t i o n o f d e l a y t i m e between d e s o r p t i o n and i o n i z a t i o n is p l o t t e d as data points i n F i g u r e 4.1. S e v e r a l interesting features m a y be n o t i c e d i n the r a w data. T h e p r i m a r y use o f this data is that it a l l o w s us to determine the o p t i m u m d e l a y times b e t w e e n laser shots for each P A H ( T a b l e 4.1). T h e average o p t i m u m d e l a y t i m e w a s a p p r o x i m a t e l y 27 ps b e t w e e n laser shots; h o w e v e r , a s m a l l mass dependant v a r i a t i o n w a s o b s e r v e d . F o r e x a m p l e the lightest P A H used, acenaphthylene, had ah o p t i m u m d e l a y t i m e o f 2 5 . 9 ps w h i l e the heaviest P A H , benzo(a)pyrene h a d an o p t i m u m d e l a y t i m e o f 28.4 ps. A n o t h e r interesting feature, w h i c h w a s o b s e r v e d i n a l l five figures, o c c u r r e d at a p p r o x i m a t e l y 6 0 ps. A t this t i m e delay, a secondary peak w a s o b s e r v e d as a shoulder. T h i s structure w a s m o s t l i k e l y due to the r e f l e c t i o n o f neutral species f r o m the e n d cap electrodes a n d b a c k into the center o f the trap. N e u t r a l reflections o f this type h a v e b e e n p r e v i o u s l y reported i n s i m i l a r experiments [122]. I n a d d i t i o n to the p r a c t i c a l aspect o f this experiment, this data m a y be u s e d to g a i n s o m e i n s i g h t into the desorption process. E x p e r i m e n t s o f the type d e s c r i b e d i n this chapter h a v e r e c e n t l y been u s e d to p r o v i d e a means o f o b s e r v i n g the energy d i s t r i b u t i o n o f m o l e c u l e s d e s o r b i n g f r o m surfaces [56, 58, 7 1 , 7 3 , 107, 122-126]. H o w e v e r , these  101  Figure 4.1 Integrated peak areas vs. IR-UV delay time for Acenaphthene, Phenanthrene, Pyrene, Chrysene, and Benzo[a]pyrene.  100  300  200  400  Time Delay (usee) Acenaphthene  50  100  150  200  250  300  350  Time Delay (usee) Phenanthrene  100  200 Time Delay (usee) Pyrene  102  300  400  F i g u r e 4.1 C o n t i n u e d .  400  100  200  300  400  Time Delay (usee) Benzo(a)pyrene  Table 4.1 Experimentally determined ideal delay times between laser events. PAH  Mass  Acenaphthylene  154  Phenanthrene.'  178  I d e a l T i m e D e l a y (us) 25.5 : ' \  25.9  Pyrene  202  27.5  Chrysene  228  27.5  Benzo(a)pyrene  254  28.4  103  w o r k e r s c o l l e c t e d this data o n instruments designed w i t h this e x p e r i m e n t i n m i n d (i.e. d e s o r p t i o n events w e r e detected b y laser i o n i z a t i o n i n the e x t r a c t i o n r e g i o n o f a t i m e o f flight M S ) .  A s i m i l a r analysis c a n be p e r f o r m e d here, h o w e v e r , there are several caveats  because the data w a s c o l l e c t e d i n a less than i d e a l e n v i r o n m e n t . In this experiment, the analyte species are desorbed f r o m a c h a r c o a l m a t r i x o n the r i n g electrode o f the i o n trap. O n c e desorption occurs, the neutral m o l e c u l e s are free to traverse the trap v o l u m e . T y p i c a l angular spreads for d e s o r p t i o n o f this type are a p p r o x i m a t e l y 4 0 ° ( s o l i d angle representing 90 % o f material) [107]. D e p e n d i n g o n the sample (concentration o f analyte, m a t r i x type, etc.) the d e s o r b i n g species m a y c o l l i d e w i t h each other. I f this occurs, the c o l l i s i o n s w i l l t y p i c a l l y have a net f o r w a r d m o m e n t u m a n d thus increase the v e l o c i t y p e r p e n d i c u l a r to the surface (this effect is t e r m e d "stream v e l o c i t y " ) [ 1 2 7 ] . I n a d d i t i o n to c o l l i s i o n s w i t h ejected neutrals, the analytes m a y also experience c o l l i s i o n s w i t h the H e buffer gas w i t h i n the trap. H o w e v e r , at the pressures used i n this w o r k , the m e a n free p a t h is o n the order o f 3 0 c m , so c o l l i s i o n s d u r i n g the o r i g i n a l e x p a n s i o n are u n l i k e l y . A f t e r a set d e l a y t i m e the U V i o n i z i n g laser is passed t h r o u g h the i o n trap a l o n g the asymptote b e t w e e n the end caps a n d the r i n g electrode. O b v i o u s l y , o n l y neutrals that are i n the laser path d u r i n g the 10 ns w i n d o w i n w h i c h the laser fires c a n be i o n i z e d . O n c e the analytes are i o n i z e d , the q u e s t i o n o f i o n t r a p p i n g b e c o m e s important. T h e t r a p p i n g e f f i c i e n c y depends o n several factors i n c l u d i n g the i o n ' s k i n e t i c energy,.angular v e l o c i t y , a n d the R F phase angle w h e n i o n i z a t i o n o c c u r s [ 1 2 8 - 1 3 0 ] . S i n c e 100 averages w e r e a c q u i r e d at each t i m e delay, w e c a n assume that the gains a n d losses associated w i t h phase angle w i l l balance each other out at a l l l o c a t i o n s except near  104  the center o f the trap. Therefore, w e w i l l m a k e the s i m p l i f y i n g a s s u m p t i o n that m o s t o f the o b s e r v e d s i g n a l results from i o n s created at the center o f the i o n trap; 10 m m f r o m the probe surface. T h e s e c o n d issue related to i o n t r a p p i n g concerns p o s s i b l e d i s c r i m i n a t i o n based o n k i n e t i c energy. T r a p p i n g efficiency is greater at the center o f the trap for i o n s w i t h l o w e r k i n e t i c energy [74]. Therefore, one m i g h t expect a n o b s e r v e d s i g n a l d i s c r i m i n a t i o n as a f u n c t i o n o f k i n e t i c energies (i.e. a r e d u c e d s i g n a l at short d e l a y t i m e s ) . O v e r the t y p i c a l range o f k i n e t i c energies o b s e r v e d here, between 0.01 to 5 e V , this is m o s t l i k e l y not important. H o w e v e r , i n the w o r s t p o s s i b l e case (i.e. fast, h e a v y i o n s ) , s u c h as w h e n benzo(a)pyrene arrives i n the center o f the trap i n 1 ps or less, the i o n w o u l d have a k i n e t i c energy o f a p p r o x i m a t e l y 130 e V and w o u l d m o s t l i k e l y not be trapped. W i t h the above caveats relating to the l i m i t a t i o n s o f the c o l l e c t e d data i n p l a c e w e m a y b e g i n to a n a l y z e the d e s o r p t i o n profiles. W h i l e a variety o f e x p e r i m e n t a l data a n d theories r e l a t i n g to laser i n d u c e d desorption have been p r e v i o u s l y o b s e r v e d , w i t h i n the last five years m a n y groups have settled o n a c o m m o n m e c h a n i s m for L I T D . T h e s e researchers have p r o p o s e d that laser desorption o f analytes f r o m a surface is s i m p l y a t h e r m a l process [57, 122]. I n fact, Z a r e et a l . have recently s h o w n that i n the case o f a n i l i n e d e s o r b i n g f r o m a sapphire surface, the analytes were i n a l m o s t c o m p l e t e translational, v i b r a t i o n a l , and e l e c t r o n i c t h e r m a l e q u i l i b r i u m w i t h the surface [122]. I n order to p r o c e e d w i t h a t h e r m a l a n a l y s i s o f the d e s o r p t i o n event a f e w a d d i t i o n a l s i m p l i f y i n g assumptions must be made: (1) the laser induces a r a p i d heating o f the s a m p l e o n the order o f 1 0 - 1 0 K / s [111]. T h i s heating is fast e n o u g h that the 7  9  d e s o r p t i o n p a t h w a y c a n b e c o m e favorable i n preference to the d e c o m p o s i t i o n process  105  [131]. T h e s a m p l e then c o o l s almost as r a p i d l y as it is heated. S i n c e d e s o r p t i o n is o n l y statistically favorable w h i l e the temperature is elevated, i n essence, this i m p l i e s that a l l d e s o r p t i o n events are almost concurrent w i t h the laser shot [132, 133]. (2) T h e r m a l e q u i l i b r i u m is m a i n t a i n e d b y the m o l e c u l e s o n the s o l i d d u r i n g the process o f laser heating [125]. (3) F i n a l l y , analytes desorb w i t h m i c r o s c o p i c r e v e r s i b i l i t y , m e a n i n g that the translational energy w i t h w h i c h the m o l e c u l e s desorb is a n accurate representation o f the s a m p l e surface [122]. W i t h these assumptions i n place, w e m a y b e g i n to e x a m i n e the T O F p r o f i l e s for the P A H analytes. I f the analytes desorb i n e q u i l i b r i u m w i t h the surface, a n d i f w e assume that i o n i z a t i o n a n d efficient capture o f i o n s o n l y occurs at the center o f the i o n trap, then w e w o u l d expect to see a M a x w e l l - B o l t z m a n n ( M - B ) l i k e d i s t r i b u t i o n o f the d e s o r b i n g analytes w i t h a translational temperature representative o f the surface temperature w h e n d e s o r p t i o n occurred. S i n c e w e k n o w the flight t i m e and the distance each analyte travels, w e c o u l d , i n p r i n c i p l e , t u r n each T O F d i s t r i b u t i o n into a k i n e t i c energy p r o f i l e (as is seen i n most M - B distributions). Instead, w e w i l l re-write the M - B e q u a t i o n to v a r y signal.intensity (gas phase density) as a f u n c t i o n o f t i m e instead o f energy. T h i s m a n i p u l a t i o n has been p e r f o r m e d p r e v i o u s l y b y A s s c h e r et a l . [134], a n d u s e d b y Z a r e et a l . [122]. T h e m a n i p u l a t e d equation has the f o r m b e l o w :  m(d - tv ) 2kTt J  Equ. 4.1  s  2  V  W h e r e C is a constant, t is the t i m e delay between laser pulses, m is the m a s s o f the desorbate m o l e c u l e , k is the B o l t z m a n n constant, T is the temperature, v is the stream s  v e l o c i t y t e r m , d is the distance between d e s o r p t i o n and i o n i z a t i o n events, a n d g(t) the 106  o b s e r v e d s i g n a l . T h e c u r v e - f i t t i n g p r o g r a m M i c r o c a l O r i g i n ( M i c r o c a l Software Inc., N o r t h h a m p t o n , M A , U S A ) w a s used to best fit the M - B distributions. T h e s e fits are s h o w n as the s o l i d lines o n F i g u r e 4.1.  T h i s data was w e l l fit w i t h o u t the n e e d for the  "stream v e l o c i t y " t e r m i n d i c a t i n g that f e w c o l l i s i o n s b e t w e e n analytes o c c u r r e d after d e s o r p t i o n a n d before i o n i z a t i o n . T h e M - B temperature for each o f these P A H s as f o u n d b y the c u r v e - f i t t i n g p r o g r a m is s h o w n i n T a b l e 4.2. T h e M - B fits w o r k v e r y w e l l for a l l analytes but one. T h e lightest P A H u s e d , acenaphthene, tended to have an u n u s u a l l y h i g h s i g n a l for the t a i l i n g edge o f the t i m e d i s t r i b u t i o n . T h i s is most l i k e l y due to the fact that acenaphthene, the smallest P A H used, is the P A H w h i c h is easiest to desorb. O n e m a y assume, therefore, that d u r i n g the c o o l i n g p e r i o d after the laser pulse, acenaphthene c o n t i n u e d to be desorbed at s o m e s m a l l rate. I n fact, samples w h i c h c o n t a i n e d the s l i g h t l y s m a l l e r P A H , naphthalene, c o u l d not be u s e d i n p r a c t i c a l a p p l i c a t i o n s because the naphthalene desorbed f r o m the activated c h a r c o a l under the r e d u c e d pressure o f the i o n trap, and large signals were o b s e r v e d b y laser i o n i z a t i o n w i t h o u t any need for desorption. F o l l o w i n g this l o g i c a l argument, that s m a l l e r P A H s are easier to desorb, then one w o u l d expect that larger P A H s , h a v i n g greater mass and m o r e i n t e r m o l e c u l a r forces, w o u l d be harder to desorb. I n this context, w e w o u l d not expect the h e a v i e r P A H s to desorb a p p r e c i a b l y u n t i l the surface temperature w a s h i g h e n o u g h to m a k e this process statistically favorable. B u t since the analytes desorb i n t h e r m a l e q u i l i b r i u m , one m a y expect this h i g h e r temperature w e i g h t i n g to be reflected i n the translational energy p r o f i l e s (i.e. the T O F d i s t r i b u t i o n s ) . I n fact, as s h o w n i n F i g u r e 4.2, a p l o t o f the M - B translational temperature vs. P A H mass is linear. T h i s graph c a n p r o v i d e i n s i g h t into  107  Table 4.2 Table of PAH mass and experimentally determined MaxwellBoltzmann temperature for laser desorption from activated charcoal. PAH  Mass  Temperature (K)  Acenaphthylene  154  686 7 - 2 4  Phenanthrene  178  765 7 - 23  Pyrene  202  817 7 - 25  Chrysene  228  923 7 - 3 0  Benzo(a)pyrene  254  963 7 - 39  Figure 4.2 Chart of experimentally determined Maxwell-Boltzmann temperature vs. PAH mass.  100  150  200 Molar Mass  108  250  300  i n t e r m o l e c u l a r interactions and m a y p r o v i d e a clue as to the ultimate mass l i m i t for d e s o r p t i o n . It s h o u l d be noted that a p l o t o f mass v s . temperature is l i n e a r i n this case because the analytes are a l l part o f a h o m o l o g o u s series. I f the analytes w e r e w i l d l y different, then the graph w o u l d most l i k e l y not be linear. I n this case, h o w e v e r , the g r a p h points the w a y towards w h a t c o u l d be a mass l i m i t for d e s o r p t i o n w i t h respect to the temperature e l e v a t i o n i n d u c e d b y the I R laser. P r e s u m a b l y , there are s o m e P A H s for w h i c h the i n t e r m o l e c u l a r b o n d i n g is greater than the energy p r o v i d e d b y the laser heating. In these situations, two-laser mass spectrometry m a y not be a p p l i c a b l e .  4.4 Conclusions T h e g o a l o f the w o r k i n this chapter w a s to determine the o p t i m u m t i m e d e l a y b e t w e e n d e s o r p t i o n and i o n i z a t i o n events i n the t w o laser system. T h e e x p e r i m e n t a l data presented here suggests that the o p t i m u m t i m e delay is a p p r o x i m a t e l y 27 ps. I n a d d i t i o n to the p r a c t i c a l aspects o f these experiments, this data c a n p r o v i d e s o m e i n s i g h t i n to the desorption process o f P A H s f r o m a n activated c h a r c o a l surface. T h i s data, w h i l e severely l i m i t e d b y e x p e r i m e n t a l constraints suggests that the d e s o r p t i o n process i n d u c e d b y a 10 ns pulse N d : Y A G laser is p u r e l y t h e r m a l i n nature. T h i s is consistent w i t h results observed b y Z a r e [122] and m o r e recently b y Z e n o b i [57, 58, 7 3 , 111, 131]. F u r t h e r m o r e , there appears to be a linear r e l a t i o n s h i p b e t w e e n the m o l a r m a s s o f the P A H a n d the M - B based translational temperature.  T h i s type o f l i n e a r r e l a t i o n s h i p  has b e e n p r e v i o u s l y o b s e r v e d for a series o f p o l y e t h y l e n e g l y c o l units, a n d is m o s t l i k e l y due to the v a r i a t i o n i n d e s o r p t i o n rates as a f u n c t i o n o f temperature for the h o m o l o g o u s series [73]. 109  F i n a l l y , this type o f experiment m a y p r o v e useful i n the f i e l d o f a t m o s p h e r i c science, w h e r e P A H s are o f great interest. S p e c i f i c a l l y , the k n o w l e d g e o f P A H a d s o r p t i o n and d e s o r p t i o n rates f r o m graphite and soot partials are a k e y to m a n y p o l l u t i o n m o d e l s [135-137]. T h e laser desorption data p r o v i d e d b y e x p e r i m e n t s s i m i l a r to that d e s c r i b e d here m a y p r o v i d e a means o f a c c e s s i n g these important constants.  110  Chapter 5 Detection of the Drug Spiperone on Biological Matrices 5.1 Introduction D u r i n g the process o f p h a r m a c e u t i c a l research a n d d e v e l o p m e n t there are several k e y c h e m i c a l attributes that must be c o n s i d e r e d w h e n selecting l e a d c o m p o u n d s for further study. These i n c l u d e receptor/drug s p e c i f i c i t y , p o t e n c y , potential t o x i c i t y at therapeutic concentrations, and f i n a l l y , the issue o f transport and a v a i l a b i l i t y o f the agent at the site o f a c t i o n . T y p i c a l l y , i n v i t r o c e l l - b a s e d experiments are used to e x a m i n e the b i n d i n g efficiencies o f a d r u g w i t h a receptor site. W h i l e these studies p r o v i d e a g o o d i n d i c a t i o n o f d r u g a c t i v i t y , little" k n o w l e d g e is gained c o n c e r n i n g transport and a c c u m u l a t i o n . S i m i l a r l y , m e t a b o l i c p r o f i l i n g o f b o d y fluids p e r f o r m e d u s i n g standard a n a l y t i c a l techniques ( N M R , M S , etc.) produces i n f o r m a t i o n o n b i o a v a i l a b i l i t y o f specific drugs, but y i e l d s little i n f o r m a t i o n regarding the transport o f the d r u g into tissues i n the i n t r a c e l l u l a r / e x t r a c e l l u l a r spaces. T o address the important issue o f whether a d r u g reaches a site o f a c t i o n a variety o f sensitive m i c r o p r o b e techniques have b e e n d e v e l o p e d [ 1 3 8 - 1 4 1 ] . Indeed, the m a p p i n g o f targeted c o m p o u n d s i n tissues has m a n y a p p l i c a t i o n s b e y o n d j u s t those i n d r u g d i s c o v e r y . F o r e x a m p l e , altered c h e m i c a l distributions are d i a g n o s t i c for diseases s u c h as stroke [142], cancer, and A l z h e i m e r ' s disease [143]. F u r t h e r m o r e , m i c r o p r o b e techniques have also f o u n d use i n the detection o f trace metals i n i n f l a m e d tissues s u r r o u n d i n g replacement j o i n t s [144].  Ill  T h e t e r m m i c r o p r o b e s i m p l y defines a type o f instrument that p r o v i d e s spatial a n a l y t i c a l i n f o r m a t i o n c o n c e r n i n g s o l i d samples o n the m i c r o s c o p i c scale. T y p i c a l l y these d e v i c e s c o m b i n e a n x - y stage for rastering a t w o - d i m e n s i o n a l s a m p l e a n d a detection element used to r e c o r d the a n a l y t i c a l i n f o r m a t i o n . T h e s e m e t h o d s have b e e n p e r f o r m e d w i t h a v a r i e t y o f a n a l y t i c a l schemes i n c l u d i n g o p t i c a l methods a n d e l e c t r o n scattering techniques [145]. B y far, the most c o m m o n technique u s e d to date i n v o l v e s secondary i o n mass spectrometry [146, 147]. O n e o f the m o s t p o w e r f u l c o m b i n a t i o n s o f these a n a l y s i s techniques has b e e n the advent o f the laser/mass spectrometer based m i c r o p r o b e [138, 140, 1 4 8 - 1 5 0 ] . T h e laser p e r m i t s e x c e l l e n t c o n t r o l o f the sample spot size a n d input energy w h i l e the mass spectrometer p r o v i d e s v e r y sensitive detection w i t h i n f o r m a t i o n o n m o l e c u l a r c o m p o s i t i o n . V i r t u a l l y , a l l types o f mass spectrometer have been d e s c r i b e d as detectors for laser m i c r o p r o b e s . B y f a r , t h e y most c o m m o n l y u t i l i z e t i m e - o f - f l i g h t systems [ 1 5 1 , 152] h o w e v e r , successful devices have also been reported w i t h F T - I C R [149, 153, 154], and t r i p l e q u a d r u p o l e systems [155]. T h e u s u a l pros a n d cons a p p l y to the a p p l i c a t i o n s o f the v a r i o u s mass spectrometric methods to this p r o b l e m . F o r e x a m p l e t y p i c a l T O F d e v i c e s p r o v i d e e x c e l l e n t mass range a n d sensitivity, but have no a b i l i t y to p e r f o r m M S / M S - s o m e t h i n g that c a n be v e r y useful i n a n a l y s i s o f c o m p l e x samples l i k e tissues. S i m i l a r l y , F T - I C R s are superb at M S / M S and have excellent r e s o l u t i o n , but are e x p e n s i v e . T o date, the most u n d e v e l o p e d m e t h o d i n this f i e l d i n v o l v e s the c o m b i n a t i o n o f the i o n trap mass spectrometer w i t h laser i o n i z a t i o n . In terms o f the l a s e r - s a m p l i n g step, these methods a l m o s t a l l e x c l u s i v e l y use M A L D I as the means o f i o n i z a t i o n [62, 6 3 ] . T h e M A L D I technique is k n o w n to be a  112  v e r y soft i o n i z a t i o n source and excellent at v o l a t i l i z i n g / i o n i z i n g large m o l e c u l a r w e i g h t c o m p o u n d s . H o w e v e r , the M A L D I m e t h o d is l i m i t e d b y the fact that the tissue surface m u s t first be c h e m i c a l l y m o d i f i e d b y the a d d i t i o n o f the m a t r i x m a t e r i a l . A d d i t i o n a l l y , o n c e the m a t r i x is a p p l i e d , the sample surface often b e c o m e s opaque so c o r r e l a t i o n w i t h v i s i b l e surface features b e c o m e s difficult. A s a n alternative to the M A L D I m e t h o d , straight laser d e s o r p t i o n o f analytes f r o m b i o l o g i c a l surfaces has been attempted. W h i l e this technique is useful i n the analysis o f transition m e t a l i o n s [148], the i o n i z a t i o n e f f i c i e n c y for m o s t o r g a n i c s is v e r y l o w [139]. T o address this issue, the Y o s t group at the U n i v e r s i t y o f F l o r i d a , has constructed a m i c r o p r o b e s y s t e m based o n laser d e s o r p t i o n f o l l o w e d b y c h e m i c a l i o n i z a t i o n for use i n a n i o n trap [156]. T h i s m e t h o d has p r o v e d useful i n the detection o f p h a r m a c e u t i c a l agents f r o m b i o l o g i c a l materials. A s an alternative to c h e m i c a l i o n i z a t i o n , this chapter w i l l focus o n the use o f the two-laser m o d e o f i o n i z a t i o n for the a n a l y s i s o f the d r u g S p i p e r o n e o n b i o l o g i c a l samples d i r e c t l y i n the v o l u m e o f the i o n trap. T h e p h a r m a c e u t i c a l c o m p o u n d Spiperone was i n i t i a l l y d e v e l o p e d as a n a n t i p s y c h o t i c d r u g . It is part o f a class o f c o m p o u n d s k n o w n as a z i p i r o n e s , w h i c h are s i m i l a r i n structure to serotonin, and so b i n d to 5 - H T receptors i n the central nervous s y s t e m [157]. It has been s h o w n t h r o u g h i n v i v o studies, that S p i p e r o n e not o n l y b i n d s to the 5 - H T I A receptors, but also 5-HT2, and dopamine2 receptors [158]. S i n c e these receptors are at v a r i o u s densities throughout the b r a i n , there is considerable interest i n d e t e r m i n i n g the concentration o f this d r u g i n different cerebral regions. T h i s chapter describes the use o f the two-laser i o n trap s y s t e m for the detection o f the d r u g S p i p e r o n e d i r e c t l y o n a b i o l o g i c a l m a t r i x . T h i s study had t w o goals. P r i m a r i l y ,  113  the d e t e c t i o n o f p h a r m a c e u t i c a l c o m p o u n d s o n c o m p l e x matrices is an important p r o b l e m i n a n a l y t i c a l c h e m i s t r y , a n d one that is w e l l addressed u s i n g the two-laser m e t h o d . T h e gentle d e s o r p t i o n process c o m b i n e d w i t h specific i o n i z a t i o n and the M S / M S c a p a b i l i t i e s o f the i o n trap are a perfect s o l u t i o n to the p r o b l e m o f detection o f analytes f r o m a complicated solid matrix. T h e s e c o n d r o l e o f this chapter w a s to demonstrate the potential scope o f the instrument a n d m e t h o d o l o g y d e s c r i b e d i n this thesis.  C H A P T E R 3 d e s c r i b e d the use o f  the t w o - l a s e r s y s t e m for e n v i r o n m e n t a l samples. H o w e v e r , because the analytes u s e d w e r e not c o m p a t i b l e w i t h M S / M S , a full demonstration o f the potential was not p o s s i b l e . In the case o f the S p i p e r o n e detection o n a b i o l o g i c a l m a t r i x , h o w e v e r , the use o f M S / M S is essential for c o m p l e t e characterization.  5.2 Experimental T h e experiments d e s c r i b e d i n this chapter were p e r f o r m e d o n the h o m e b u i l t t w o laser i o n trap mass spectrometer system d e s c r i b e d i n  C H A P T E R 2 a n d C H A P T E R 3.  T h e b a s i c e l e c t r o n i c and o p t i c a l systems were c o n s e r v e d . H o w e v e r the v a r i o u s w a v e f o r m s u s e d for i o n m a n i p u l a t i o n were changed as required. A s has already been m e n t i o n e d the p h a r m a c e u t i c a l c o m p o u n d u s e d for the study d e s c r i b e d i n this chapter w a s S p i p e r o n e ( S i g m a C h e m i c a l , S t . L o u i s , M o , U S A ) .  Slightly  a c i d i c ( 1 % acetic a c i d b y v o l u m e ) aqueous solutions o f S p i p e r o n e were prepared as s p i k e s for the s o l i d samples. Standard c h a r c o a l standards o f S p i p e r o n e were prepared as described i n  C H A P T E R 3.  114  T w o types o f b i o l o g i c a l tissues were e x a m i n e d i n this study. T h e samples u s e d i n this w o r k w e r e f r o m the b r a i n a n d l i v e r o f m a l e Sprague D a w l e y rats ( 2 2 5 - 2 5 0 grams). T h e a n i m a l s w e r e p r o v i d e d b y G r e g B o w d e n o f the U B C department o f N e u r o s c i e n c e t h r o u g h the U B C A n i m a l C a r e Center. T h e a n i m a l s w e r e s a c r i f i c e d b y a s l o w increase i n CO2 l e v e l s i n their enclosures. T h e l i v e r a n d b r a i n o f four rats w e r e r e m o v e d a n d i m m e d i a t e l y flash f r o z e n i n - 3 0 ° C 2-methyl-butane. T h e frozen tissue w a s then stored i n a - 7 0 ° C freezer. F i g u r e 5.1 a n d F i g u r e 5.2 s h o w representative samples o f the b r a i n a n d l i v e r tissues as c o l l e c t e d b y the author. S a m p l e s were prepared b y first s l i c i n g a 2 m m t h i c k p i e c e o f f r o z e n tissue. T h e tissue w a s t h e n sonicated i n the S p i p e r o n e s o l u t i o n (~0.7g S p i p e r o n e / k g f r o z e n tissue) for 15 m i n u t e s . T h e solutions were then p o u r e d off, a n d a tissue p l u g w a s inserted into the probe t i p . T h e sample w a s then a l l o w e d to d r y i n a fume h o o d for 1 h o u r p r i o r to i n s e r t i o n into the i o n trap.  5.3 Results and Discussion T h e p r i m a r y g o a l o f this chapter w a s to investigate whether the t w o - l a s e r m e t h o d c o m b i n e d w i t h a n i o n trap mass spectrometer possess e n o u g h s e l e c t i v i t y a n d s p e c i f i c i t y to observe a p h a r m a c e u t i c a l c o m p o u n d i n the presence o f a real b i o l o g i c a l m a t r i x . H o w e v e r , before this c o u l d be a c h i e v e d , the first task w a s s i m p l y to observe a t w o - l a s e r mass s p e c t r u m o f S p i p e r o n e o n the standard c h a r c o a l m a t r i x .  115  Figure 5.1 Photo of brain tissue extracted from a male Sprague Dawley rat used in this work.  116  A t w o laser mass spectrum o f a c h a r c o a l based Spiperone sample w a s c o l l e c t e d and is s h o w n i n Figure 5.3. T h e spectrum s h o w s the m o l e c u l a r peak o f S p i p e r o n e at 3 9 5 T h , i n a d d i t i o n to a s m a l l fragment peak at 3 4 0 T h . T h e peaks o b s e r v e d b e l o w 3 3 0 T h w e r e caused b y l i n g e r i n g c o n t a m i n a t i o n i n the i o n trap f r o m the m u l t i t u d e o f p r e v i o u s experiments u s i n g P A H s . U n f o r t u n a t e l y , the P A H s a n d other o r g a n i c contaminants released f r o m the p u m p o i l were a persistent p r o b l e m throughout this project. T h i s c o n t a m i n a t i o n speaks to t w o w e l l - k n o w n facts about this type o f w o r k ; (1) P A H s are k n o w n to be " s t i c k y " a n d are excellent at c o n t a m i n a t i n g mass spectrometer  .  systems, a n d (2) the two-laser m e t h o d is e x t r a o r d i n a r i l y efficient at i o n i z i n g P A H s . O f course routine c l e a n i n g w a s performed to help m i n i m i z e this p r o b l e m . U n f o r t u n a t e l y , the i d e a l s o l u t i o n to this c o n t a m i n a t i o n p r o b l e m , w h i c h i n v o l v e s heating the v a c u u m s y s t e m , w a s u n a v a i l a b l e due to p r o b l e m s i n the v a c u u m c h a m b e r design. R e g a r d l e s s o f the c o n t a m i n a t i o n problem,-Figure 5.3 does i n d e e d demonstrate that S p i p e r o n e c a n be o b s e r v e d b y the two-laser m e t h o d . H o w e v e r , i n cases w h e r e the s a m p l e c o n c e n t r a t i o n is s m a l l , it w o u l d be preferable to increase the o b s e r v e d s i g n a l b y p r e concentrating the i o n p o p u l a t i o n i n the gas phase. Figure 5.4 demonstrates the effect o f 1, 5, a n d 10 laser c y c l e s f o l l o w e d b y a N B B W pulse o n the o b s e r v e d S p i p e r o n e s i g n a l . In this case the N B B W pulse w a s a p p l i e d at the e n d o f the laser c y c l e s rather than b e t w e e n c y c l e s . T h e experiments were p e r f o r m e d i n this w a y because it w a s e m p i r i c a l l y d e t e r m i n e d that the net i m p r o v e m e n t w a s greater w i t h a single N B B W c y c l e rather t h a n m u l t i p l e N B B W c y c l e s . T h i s is most l i k e l y because S p i p e r o n e is a r e l a t i v e l y fragile m o l e c u l e , a n d e v e n a w i d e N B B W w i n d o w caused a degree o f s i g n a l loss.  117  Figure 5.3 Two laser mass spectrum of Spiperone on a charcoal matrix.  RF Voltage  118  Figure 5.4 Two laser mass spectrum of Spiperone on a charcoal matrix with 1, 5, and 10 laser cycles followed by a single NBBW pulse.  10 laser cycles 5 laser cycles 1 laser cycle  375  385  395  405  415  Mass/Charge  IR UV IR UV IR UV IR UV IR UV  RF Voltage  mm 119  #  O n c e a large S p i p e r o n e i o n p o p u l a t i o n w a s a c q u i r e d a n d isolated, the next stage w a s to c o l l e c t a M S / M S spectrum.  Figure 5.5 demonstrates the result o f a c o l l i s i o n  i n d u c e d d i s s o c i a t i o n ( C I D ) c y c l e after N B B W i s o l a t i o n . T h e r e are f i v e p r i m a r y daughter i o n s that result f r o m C I D o f the S p i p e r o n e sample. T h e s e o c c u r at 2 8 0 T h , 261 T h , 165 T h , 146 T h , a n d 109 T h . O n e p o s s i b l e fragmentation is s h o w n i n  Figure 5.5. It s h o u l d  also be n o t e d that t w o pairs o f peaks are o b s e r v e d (280 T h , 261 T h a n d 165 T h , 1 4 6 T h ) w h i c h are b o t h separated b y 19 mass units. These m o s t l i k e l y result f r o m the loss o f f l u o r i n e d u r i n g fragmentation. T h i s suggests that the charge center is m o s t l i k e l y o n the a r o m a t i c r i n g c o n n e c t e d to the fluorine. T h e d i f f i c u l t y i n f u l l y a s s i g n i n g the daughter s p e c t r u m is t y p i c a l o f c o m p l e x m o l e c u l e s o f this type. D u r i n g the C I D process a n u m b e r o f u n u s u a l fragmentation p a t h w a y s are p o s s i b l e [159]. A d d i t i o n a l l y , rearrangement reactions f o l l o w e d b y e l i m i n a t i o n s are often o b s e r v e d ; this further c o m p l i c a t e s fragment assignments [159]. R e g a r d l e s s o f any m i s s e d assignments, this M S / M S spectrum c a n be u s e d as a useful d i a g n o s t i c t o o l for c o n f i r m i n g the presence o f S p i p e r o n e i n a c o m p l i c a t e d m a t r i x . T h e next step i n the m e t h o d d e v e l o p m e n t w a s the a n a l y s i s o f b l a n k b i o l o g i c a l tissues.  Figure 5.6 a n d Figure 5.7 s h o w t y p i c a l 2-laser mass spectra o f rat b r a i n a n d  l i v e r tissues. T h e s e samples represent v e r y c o m p l e x matrices w i t h a n abundance o f m o l e c u l a r d i v e r s i t y . It is important to note that w h e n a l i v e r sample w a s e x a m i n e d b y the m e t h o d o f laser d e s o r p t i o n / c h e m i c a l i o n i z a t i o n a peak w a s o b s e r v e d at v i r t u a l l y e v e r y mass [156]. I n contrast, the s e l e c t i v i t y p r o v i d e d b y the U V i o n i z a t i o n scheme u s e d here p r o d u c e s at least some r e l i e f f r o m the o v e r abundance o f m a t r i x i o n s .  ...120  Figure 5.5 Two laser mass spectrum of Spiperone on a charcoal matrix with five laser cycles followed by a single NBBW pulse and a collision induced dissociation (CID) waveform.  50  100  150  200  250  .  Mass/Charge  RF Voltage  121  300  350  400  450  Figure 5.6 Two laser mass spectrum of a slice of brain tissue from a male Sprague-Dawley rat.  RF Voltage  122  Figure 5.7 Two laser mass spectrum of a slice of liver tissue from a male Sprague-Dawley rat.  F i n a l l y , samples o f b r a i n a n d l i v e r w h i c h h a d b e e n sonicated i n a S p i p e r o n e s o l u t i o n o f ~ 0 . 7 m g / k g tissue w e r e e x a m i n e d .  Figure 5.8 represents the o b s e r v e d spectra  o f a rat b r a i n s a m p l e s p i k e d w i t h S p i p e r o n e c o l l e c t e d b y t w o - l a s e r mass spectrometry.  Figure 5.9 s h o w s the same sample after i o n a c c u m u l a t i o n a n d i s o l a t i o n w i t h a N B B W pulse. F i n a l l y ,  Figure 5.10 demonstrates the r e s u l t i n g M S / M S spectra f r o m the p e a k at  395 T h . S i m i l a r l y , data c o l l e c t e d for a s p i k e d l i v e r sample are s h o w n i n  Figure 5.11,  Figure 5.12, and Figure 5.13. Figure 5.8 a n d Figure 5.11 b o t h s h o w a s m a l l p e a k i n the mass s p e c t r u m r e s u l t i n g f r o m S p i p e r o n e at 3 9 5 T h , h o w e v e r , i s w o u l d be d i f f i c u l t to c o n f i r m the analytes presence u s i n g this data alone. B y the a p p l i c a t i o n o f m u l t i p l e laser c y c l e s f o l l o w e d b y a s i n g l e N B B W pulse a large i o n p o p u l a t i o n at 395 T h c o u l d be a c q u i r e d . F i n a l l y , C I D o f the 395 T h peak i n  Figure 5.10 a n d Figure 5.13 c o n f i r m s the presence o f  S p i p e r o n e i n the tissue.  5.4 Conclusions The g o a l o f this chapter w a s to demonstrate the u n i q u e c a p a b i l i t i e s o f the t w o laser s y s t e m w i t h a n ion-trap mass spectrometer. T h i s w a s a c h i e v e d b y c o l l e c t i n g spectra and i d e n t i f y i n g the m o l e c u l a r l y fragile p h a r m a c e u t i c a l c o m p o u n d S p i p e r o n e o n one o f the m o s t c o m p l e x matrices p o s s i b l e , b i o l o g i c a l tissue. B y w a y o f c o m p a r i s o n , this a n a l y s i s w o u l d have b e e n i m p o s s i b l e w i t h the t r a d i t i o n a l t w o - l a s e r T O F set-up, as a T O F d e v i c e c o u l d neither precoricentrate ions nor c o l l e c t M S / M S data.  124  Figure 5.8 Two laser mass spectrum of a slice of brain tissue from a male Sprague-Dawley rat which had been spiked with a Spiperone solution.  395  Mass/Charge  R  UV  RF Voltage  125  Figure 5.9 Two laser mass spectrum with 5 laser cycles followed by a single NBBW pulse of a slice of brain tissue from a male Sprague-Dawley rat that had been spiked with a Spiperone solution.  50  100  150  200  250  300  Mass/Charge  IR UV IR UV IR UV IR UV IR UV  RF Voltage*  126  350  400  450  Figure 5.10 Two laser mass spectrum with 5 laser cycles followed by a single NBBW pulse and a single CID waveform of a slice of brain tissue from a male Sprague-Dawley rat which had been spiked with a Spiperone solution.  50  100  150  200  250  300  Mass/Charge  127  350  400  450  Figure 5.11 Two laser mass spectrum of a slice of liver tissue from a male Sprague-Dawley rat which had been spiked with a Spiperone solution.  Figure 5.12 Two laser mass spectrum with 5 laser cycles followed by a single NBBW pulse of a slice of liver tissue from a male Sprague-Dawley rat that had been spiked with a Spiperone solution.  200  250  300  Mass/Charge  IR UV IR UV IR UV IR UV IR UV  RF Voltage  129  450  Figure 5.13 Two laser mass spectrum with 5 laser cycles followed by a single NBBW pulse and a single CID waveform of a slice of liver tissue from a male Sprague-Dawley rat which had been spiked with a Spiperone solution.  50  100  150  200  250  300  Mass/Charge  130;  350  400  450  F u r t h e r m o r e , the data c o l l e c t e d here, w h i l e useful as an instrument c h a r a c t e r i z a t i o n t o o l , m a y also have some potential use i n real p h a r m a c e u t i c a l a p p l i c a t i o n s . T h e d r u g e x a m i n e d here, S p i p e r o n e , for e x a m p l e is o f current interest i n the f i e l d o f n e u r o b i o l o g y [158]. Interestingly, the S p i p e r o n e sample concentrations u s e d i n this p r e l i m i n a r y w o r k were not totally out o f l i n e w i t h those o f p h a r m a c o l o g i c a l studies. F o r e x a m p l e i n this chapter samples were created w i t h 0 . 7 m g S p i p e r o n e / k g o f tissue, w h i l e a recent report o b s e r v e d effects o n discharge rates o f seven m e d u l l a r y 5 - H T c e l l s i n a cat w i t h i n t r a v e n o u s doses o f S p i p e r o n e at l e v e l s o f l m g d r u g / k g o f a n i m a l [157]. O f course, to be a true m i c r o p r o b e technique the m e t h o d s h o u l d be able to s c a n o v e r a range o f tissue l o c a t i o n s . W i t h the current set-up this c o u l d o n l y be a c h i e v e d b y c o l l e c t i n g a series o f s a m p l e p l u g s for sequential analysis. H o w e v e r , a future d e v i c e c o u l d p e r f o r m true m i c r o s c a n n i n g b y p l a c i n g the sample outside the i o n trap a n d r e l y i n g o n i o n o p t i c s to transport the analyte ions. T h i s w o u l d , o f course, c o m e at the expense o f r e d u c e d s e n s i t i v i t y , so a trade o f f w o u l d have to be made. I n a d d i t i o n to l o o k i n g at tissue samples, these techniques m a y also p r o v e i n v a l u a b l e for the direct analysis o f bacteria. T h e r e has been great interest i n recent years i n the f i e l d o f r a p i d bacterial detection, s p e c i f i c a l l y as it relates to potential cases o f t o x i c i t y (i.e. b i o t e r r o r i s m ) . T h e two-laser m e t h o d c o m b i n e d w i t h an i o n trap m a y be useful as a r a p i d probe because o f its a b i l i t y to p r o v i d e r e l a t i v e l y selective i o n i z a t i o n c o m b i n e d w i t h the a b i l i t y to c o l l e c t M S / M S data.  131  Chapter 6 Optical Spectroscopy in an Ion Trap 6.1 Introduction T o this p o i n t i n the thesis, the g o a l o f the w o r k w a s to investigate v a r i o u s facets o f t w o - l a s e r i o n trap mass spectrometry. T h i s chapter describes the a d d i t i o n o f a t h i r d laser to the s y s t e m i n a n attempt to broaden the already w i d e c a p a b i l i t i e s o f the d e v i c e . P u r e mass spectrometry alone w i l l a l w a y s be l i m i t e d i n its a b i l i t i e s b y the fact that it p r o v i d e s little structural i n f o r m a t i o n d i r e c t l y . M u l t i p l e stages o f M S ( M S ) m a y be 11  u s e d to address this p r o b l e m . H o w e v e r , direct gas phase m o l e c u l a r structures are s t i l l d i f f i c u l t to determine. R E M P I i o n i z a t i o n d e s c r i b e d i n this thesis a n d as a p p l i e d b y m a n y others helps address this p r o b l e m ; b y u s i n g a tunable laser source and m o n i t o r i n g the appearance o f the r e s u l t i n g i o n s i g n a l a large amount o f s p e c t r o s c o p i c (and structural) i n f o r m a t i o n about the parent m o l e c u l e c a n be gained [69, 7 0 ] . It turns out, h o w e v e r , that w h e n i o n i z a t i o n occurs d i r e c t l y after the laser d e s o r p t i o n process, v i r t u a l l y a l l o f the s p e c t r o s c o p i c i n f o r m a t i o n is lost due to the large internal temperature o f the analytes [96].  A s a s o l u t i o n to this persistent p r o b l e m , m o s t w o r k e r s n o w use a s u p e r s o n i c b e a m  to c o o l analyte m o l e c u l e s after desorption. U s i n g this c o o l i n g technique, c o m p r e h e n s i v e spectral l i b r a r i e s have b e e n a c q u i r e d o v e r the last thirty years [65, 160]. T h e r e r e m a i n , h o w e v e r , s o m e analytes that have b r o a d spectral features e v e n under jet c o n d i t i o n s either because their spectra are diffuse and u n - r e s o l v a b l e , or because m o l e c u l a r species b e c o m e i n c r e a s i n g l y d i f f i c u l t to c o o l as they increase i n s i z e [68, 1 6 1 , 162].  A s a n alternative to r e l y i n g o n the spectroscopic properties o f the R E M P I process  132  to p r o v i d e structural i n f o r m a t i o n and s e l e c t i v i t y , the p o s s i b i l i t y o f p r o b i n g the m o l e c u l a r i o n s w i t h a t h i r d p u l s e d laser has been investigated. T h e analytes that were selected for this study, the P A H s , were c h o s e n l a r g e l y because they are o f great interest i n t w o fields o f science. A s d i s c u s s e d i n  C H A P T E R 1,  P A H s are o f interest to e n v i r o n m e n t a l scientists because they are k n o w n c a r c i n o g e n s , a n d are u b i q u i t o u s l y f o u n d throughout the e n v i r o n m e n t [79, 80]. P A H s , a n d s p e c i f i c a l l y P A H cations, have also e x p e r i e n c e d m u c h interest b y the a s t r o p h y s i c a l c o m m u n i t y . T h i s is due to the current s p e c u l a t i o n that P A H s are expected to represent a large p o r t i o n o f the c a r b o n present i n interstellar space [163, 164]. S p e c t r o s c o p i c a l l y , w h i l e neutral P A H s absorb l i g h t i n the U V , their cations e x h i b i t a b s o r p t i o n bands i n the v i s i b l e and near I R range. A s t r o p h y s i c i s t s are interested i n these p a r t i c u l a r features as they m a k e P A H s g o o d candidates as a p o s s i b l e source o f the diffuse interstellar bands ( D I B s ) [165]. T h e D I B s consist o f a large n u m b e r o f a b s o r p t i o n lines s u p e r i m p o s e d o n the interstellar e x t i n c t i o n c u r v e [166]. S i n c e their d i s c o v e r y i n the 1 9 2 0 ' s [167], the identity o f these D I B carriers has r e m a i n e d a n i m p o r t a n t and d i f f i c u l t p r o b l e m i n astronomy [168]. These ~ 300 a b s o r p t i o n features are g e n e r a l l y correlated w i t h dust extinction, and are p r e s u m e d to relate to interstellar dust c l o u d s . T h e detection o f substructure i n s o m e D I B s has l e a d to the c o n c l u s i o n that m o l e c u l e s or i o n s m a y be the source o f s o m e o f these bands [169]. T h e D I B p r o b l e m is made m o r e difficult b y the fact that a s t r o n o m i c a l s u r v e y s o v e r large w a v e l e n g t h ranges a n d d i r e c t i o n s i n space suggest that a l l m e a s u r e d D I B s originate f r o m different carriers (i.e. there are no t w o locations i n space w h e r e the ratios o f the peak heights are i d e n t i c a l ) . H o w e v e r , some D I B s s h o w s i m i l a r b e h a v i o r i n  133  different a s t r o n o m i c a l e n v i r o n m e n t s and m a y arise f r o m structurally related species [170]. D u e to the h i g h U V f l u x and l o n g m e a n free path i n interstellar space, it has b e e n suggested that P A H cations m a y be the source o f m a n y o f these D I B s . A s a result, m a n y groups have sought to g a i n an understanding o f the p h y s i c a l c h e m i s t r y o f gas phase P A H cations. T h e r e a c t i v i t y a n d photo stability o f these m o l e c u l e s and cations has been studied e x t e n s i v e l y b y photo fragmentation studies e x p e r i m e n t a l l y f r o m the V U V [171], to the U V w i t h l a m p i r r a d i a t i o n [172], s y n c h r o t r o n r a d i a t i o n [173], a l l the w a y into the I R w i t h m u l t i p h o t o n d i s s o c i a t i o n [174]. T h e photo stability o f these cations has also b e e n i n v e s t i g a t e d theoretically [173, 175, 176]. V i r t u a l l y a l l c a t i o n spectroscopic techniques have been a p p l i e d to the P A H s .  The  m o s t c o m m o n o f a l l o f these (by far) i n v o l v e s c o l l e c t i o n o f the c a t i o n spectra i n a raregas s o l i d m a t r i x [177]. T h i s m e t h o d i n v o l v e s c o - d e p o s i t i n g analyte i o n s w i t h a n o b l e gas onto a n u l t r a - c o l d w i n d o w . A n o p t i c a l absorption s p e c t r u m is then a c q u i r e d i n the n o r m a l w a y u s i n g a l a m p source and a m o n o c h r o m a t o r . T h i s technique, h o w e v e r , has m a n y p r o b l e m s ; it is d i f f i c u l t ( i f not i m p o s s i b l e ) to identify e x a c t l y the nature o f the i o n s d e p o s i t e d o n the surface. F o r e x a m p l e fragments f r o m the i o n i z a t i o n process c a n also be deposited. F u r t h e r m o r e , the n o b l e gas m a t r i x tends to i n d u c e peak l o c a t i o n shifts d e p e n d i n g o n the m e t h o d o f m a t r i x f o r m a t i o n and the p h y s i c a l l o c a t i o n o f the analyte i n the m a t r i x . A t t e m p t s are made to account for this b y c y c l i n g t h r o u g h a series o f different matrices ( A r , N e , H e ) a n d t r y i n g to predict the l o c a t i o n o f a m a t r i x free peak. W h i l e this m e t h o d is not totally satisfactory (the peak l o c a t i o n s i n free space result o n l y f r o m  134  c a l c u l a t i o n s ) , due to its ease o f use, it has b e c o m e v e r y routine and a c o m m o n source o f a s t r o p h y s i c a l data. A s a n alternative to this m e t h o d , a n u m b e r o f gas phase methods have b e e n attempted.  R e c e n t advances i n c l u d e u s i n g c a v i t y r i n g - d o w n spectroscopy o n m o l e c u l a r  beams [178] a n d a rare-gas C o m p l e x p h o t o d i s s o c i a t i o n c o m p l e x technique [179, 180]. R e c e n t l y it has e v e n been suggested that is m a y be p o s s i b l e to observe florescence d i r e c t l y f r o m large gas phase cations [181].. O n e p o s s i b l e m e t h o d o f o b t a i n i n g gas phase spectra, w h i c h has l a r g e l y been i g n o r e d for P A H c a t i o n a n a l y s i s , is the m e t h o d o f p h o t o d i s s o c i a t i o n ( P D ) spectroscopy. V i s i b l e p h o t o d i s s o c i a t i o n studies have been p e r f o r m e d i n the gas phase s i n c e the late 1 9 7 0 ' s u s i n g a v a r i e t y o f techniques [182]. I n fact, o p t i c a l p r o b i n g o f trapped gas phase i o n s w a s one o f the o r i g i n a l uses o f the i o n trap [11]. M o s t o f the recent w o r k i n this f i e l d has f o c u s e d o n o b s e r v i n g w e a k l y - b o u n d c o m p l e x e s w h e r e the a b s o r p t i o n o f a s i n g l e p h o t o n induces d i s s o c i a t i o n . T h i s w o r k has p r i m a r i l y f o c u s e d o n clusters o f s m a l l gaseous m o l e c u l e s or m o l e c u l a r - r a r e gas and m e t a l - l i g a n d c o m p l e x e s w h e r e n o n - v o l a t i l e species c a n be v a p o r i z e d b y laser a b l a t i o n and the gaseous l i g a n d i n t r o d u c e d t h r o u g h a supersonic b e a m [ 1 8 3 ] . study o f F e  + 3  R e c e n t l y , e v e n electrospray has been u s e d as a source for the  c o m p l e x e s b y P D spectroscopy [184]. T h e technique, as u s e d b y the m o s t  p r o l i f i c w o r k e r i n this f i e l d , R . D u n b a r , i n v o l v e s p e r f o r m i n g P D s p e c t r o s c o p y o f v a r i o u s m o l e c u l a r gaseous ions i n t r o d u c e d into a n I C R trap w i t h i o n i z a t i o n b y e l e c t r o n i m p a c t , a n d then u s i n g a tunable dye laser for d i s s o c i a t i o n [182, 185]. T h e advantage o f these traps is the low-pressure c o n d i t i o n s a l l o w for v i r t u a l l y c o l l i s i o n l e s s studies - for e x a m p l e t i m e dependant energy r e l a x a t i o n experiments. I C R was also used b y the g r o u p o f  135'  B o i s s e l to trap laser desorbed and i o n i z e d P A H cations, u s i n g the b r o a d b a n d e m i s s i o n o f a X e l a m p for d i s s o c i a t i o n [105, 186-189]. B y c o m p a r i n g a statistical m o d e l w i t h the results f r o m m u l t i p h o t o n a b s o r p t i o n i n d u c e d fragmentation o f the isolated i o n s they w e r e able to o b t a i n o s c i l l a t o r strengths o f the v i s i b l e a b s o r p t i o n processes. A d d i t i o n a l l y , P D spectroscopic studies have also been p e r f o r m e d o n m o l e c u l a r i o n s i n a n i o n trap b o t h w i t h l a m p sources and tunable lasers [12]. T h i s chapter demonstrates the use o f an R P i o n trap for the c o l l e c t i o n o f v i s i b l e a b s o r p t i o n spectra o f large gas phase cations o f n o n - v o l a t i l e organics t h r o u g h R e s o n a n c e E n h a n c e d M u l t i p h o t o n D i s s o c i a t i o n ( R E M P D ) . T h e a d d i t i o n o f a t h i r d tunable laser to the i o n trap s y s t e m a l l o w s for the p o s s i b i l i t y o f p e r f o r m i n g spectroscopy o n the trapped cations. T h e spectra w e r e observed b y s c a n n i n g the laser w a v e l e n g t h and o b s e r v i n g the fragmentation products that result f r o m the trapped ions. A s w i t h R E M P I , the R E M P D process is selective based o n resonant a b s o r p t i o n o f a single p h o t o n b y the trapped i o n , f o l l o w e d b y several successive non-resonant p h o t o n absorptions that l e a d to fragmentation. T h e b u l k o f the material presented i n this chapter has been p r e v i o u s l y p u b l i s h e d b y R o l l a n d , Specht, B l a d e s , and H e p b u r n , i n Chemical Physics Letters [190].  6.2 Experimental T h e experiments described i n this chapter used m o d i f i e d e q u i p m e n t based o n the t w o laser s y s t e m d e s c r i b e d i n C H A P T E R 2. T h e p r i m a r y a d d i t i o n to this s y s t e m w a s a t h i r d laser; a Spectra P h y s i c s P D L - 3 D y e L a s e r (Spectra P h y s i c s , M o u n t a i n V i e w , C A , U S A ) p u m p e d b y a Quantel Y G 6 6 0 (Quantel Lasers, L e s U l i s , France) N d : Y A G .  In  a d d i t i o n to this d y e laser, the electronics r e q u i r e d to define the t i m i n g r e g i m e s w e r e also added. 136  T h e s e lasers w e r e added to the o p t i c a l table set up as s h o w n i n  Figure 6.1 a n d  Figure 6.2. T h e output f r o m the d o u b l e d N d : Y A G ( Y G 6 6 0 ) b e a m w a s first split u s i n g a d i e l e c t r i c c o a t e d m i r r o r that reflected o n l y the 5 3 2 n m l i n e w h i l e a l l o w i n g the f u n d a m e n t a l ( @ 1064 n m ) to pass t h r o u g h into a b e a m d u m p . T h i s 5 3 2 n m b e a m w a s then d i r e c t e d into the d y e laser b y a 9 0 ° quartz p r i s m . T h e output f r o m the d y e laser w a s passed into the v a c u u m m a n i f o l d b y three quartz p r i s m s ( M e l l e s G r i o t ) after b e i n g shaped b y a telescope external to the dye laser (two lenses s u p p l i e d b y M e l l e s G r i o t ) . T h e d y e laser b e a m entered the v a c u u m c h a m b e r t h r o u g h a quartz w i n d o w at the top o f the m a n i f o l d . It then interacted w i t h the i o n c l o u d v o l u m e t h r o u g h a 2.0 m m diameter h o l e d r i l l e d i n the r i n g electrodes 9 0 ° f r o m the s a m p l e probe. T h i s i s s c h e m a t i c a l l y represented i n  Figure 6.3.  T y p i c a l energies used w e r e o n the order o f 1 m J / p u l s e i n the v i s i b l e a n d near I R ranges. A s i n  C H A P T E R 2, the laser p o w e r w a s measured for e v e r y c y c l e b y p l a c i n g a  p y r o e l e c t r i c p o w e r meter i n - l i n e w i t h a r e f l e c t i o n o f the d y e laser. T h e s i g n a l f r o m this meter w a s r e c o r d e d a n d stored o n d i s k w i t h e a c h mass s p e c t r u m u s i n g the o s c i l l o s c o p e system detailed i n  C H A P T E R 2. A p o w e r meter w a s u s e d i n this w o r k rather than the  p h o t o d i o d e s u s e d earlier because the p h o t o d i o d e response is related to w a v e l e n g t h ; w h i c h o f course b e c o m e s a p r o b l e m w h e n u s i n g a tunable d y e laser. S i n c e the i o n trap operates o n a 1 H z c y c l e , a n d the N d : Y A G / D y e L a s e r c a n operate at 2 0 H z , it w a s p o s s i b l e to a c c u m u l a t e the products o f a n u m b e r o f p h o t o fragmentation laser shots, thus i n c r e a s i n g the fragmentation y i e l d . E x p e r i m e n t s , therefore, w e r e p e r f o r m e d w i t h five or m o r e photo fragmentation laser shots, o b t a i n e d b y  137  Figure 6.1 Diagram of the three-laser set-up at UBC.  138  Figure 6.2 Enhanced photo of the three-laser set-up at UBC.  139  Figure 6.3 Diagram of the IR desorption, UV photoionization, and visible photo fragmentation lasers interacting with the interior of the ion trap.  140  a c t i v e l y t r i g g e r i n g the Q - S w i t c h o f the N d : Y A G p u m p e d laser w i t h a B N C m o d e l 555 d i g i t a l d e l a y generator ( B e r k l e y N u c l e o n i c s C o r p . , S a n R a f a e l , C A . U S A ) . F o r these experiments, the d y e laser was scanned i n 2 n m steps o v e r a 4 5 0 n m range. A t e a c h w a v e l e n g t h , 50 mass spectra were recorded. T h e w a v e l e n g t h scan range o f 575 n m to 9 8 0 n m was a c h i e v e d b y the use o f 8 laser dyes ( a l l laser dyes were s u p p l i e d b y E x c i t o n Inc., D a y t o n , O H , U S A ) . T h e y were: K i t o n R e d ( 5 7 8 - 6 0 6 n m ) , D C M ( 6 0 5 - 6 7 0 n m ) , L D S 678 ( 6 6 0 - 7 2 0 n m ) , L D S 750 ( 7 0 5 - 7 5 0 n m ) , L D S 751 ( 7 1 5 - 7 9 2 n m ) , L D S 821 (785-85 l n m ) , L D S 867 ( 8 3 0 - 9 1 6 n m ) , L D S 925 ( 8 9 0 - 9 8 0 n m ) .  6.3 Results and Discussion 6.3.1 Photodissociation of Trapped Cations T h e g o a l o f this chapter w a s to demonstrate the p o s s i b i l i t y o f g a i n i n g s p e c t r o s c o p i c i n f o r m a t i o n o f P A H cations i n a n i o n trap. T o demonstrate the usefulness o f this m e t h o d , t w o P A H isomers were chosen for analysis. T h e y were anthracene ( D 2 h s y m m e t r y ; 7 . 4 5 e V I P ; 178 amu) and its i s o m e r i c partner phenanthrene ( C 2 s y m m e t r y ; V  7.86 e V I P ; 178 a m u ) s h o w n i n Figure 6.4 F i g u r e 6.5 i s a mass s p e c t r u m o f a sample o f pure phenanthrene f o l l o w i n g n o r m a l 2-laser mass spectrometry under the c o n d i t i o n s s h o w n at the b o t t o m o f the figure. T h i s s p e c t r u m s h o w s the parent i o n at 178 T h i n a d d i t i o n to some fragmentation products that result f r o m the 2-laser process o n top o f the regular b a c k g r o u n d s i g n a l . C l e a r l y , w h e n d e v e l o p i n g this technique (where the o b s e r v a t i o n o f any s m a l l s i g n a l change is useful) it w a s essential to first isolate the parent i o n and r e m o v e any and a l l other i o n s f r o m the i o n trap i n order to observe any change w h i c h results f r o m the a p p l i c a t i o n o f the t h i r d laser.  141  Figure 6.4 The PAH isomers at 178 Th phenanthrene and anthracene.  Phenanthrene  Anthracene  i78 amu  178 a m u "  142  Figure 6.5 Two laser mass spectrum of a sample of phenanthrene on activated charcoal.  R F Voltage  143  Figure 6.6 s h o w s the s p e c t r u m that is o b s e r v e d after the a p p l i c a t i o n o f a N B B W p u l s e before the i o n s w e r e r a m p e d out o f the i o n trap. W i t h the a d d i t i o n o f this w a v e f o r m , the i o n trap w a s ready to b e g i n to c o l l e c t fragments f r o m the i n t r o d u c t i o n o f the t h i r d laser. It turns out, that w i t h the current d y e laser p o w e r s a v a i l a b l e , that a s i n g l e t h i r d laser shot p r o d u c e d o n l y a m i n o r change i n the total i o n p o p u l a t i o n s . Therefore, i t w a s d e c i d e d early o n i n this w o r k , that m u l t i p l e t h i r d laser shots c o u l d be u s e d i n order to b u i l d u p a n appreciable daughter i o n s i g n a l .  Figure 6.7 is the s p e c t r u m that is o b s e r v e d  after 5 laser shots at 8 9 2 n m interact w i t h the i s o l a t e d phenanthrene i o n s . F o u r m a i n fragmentation channels were o b s e r v e d for phenanthrene (and anthracene); they w e r e due to loss o f - 1 H , - 2 H ,  -C2H2, a n d p o s s i b l y - 1 0 H . T h e fragmentation w i l l be d i s c u s s e d i n  the Section 6.3.2. S i m i l a r l y , a sample o f anthracene w a s e x p o s e d to 5 laser shots at 648 n m after a n i d e n t i c a l i s o l a t i o n routine a n d the result is s h o w n i n  Figure 6.8.  In order to j u s t i f y the use o f m u l t i p l e laser shots and to insure that there w e r e n o other effects o f the t h i r d laser o n the daughter i o n p o p u l a t i o n , a study w a s p e r f o r m e d w h e r e the n u m b e r o f laser shots w a s v a r i e d between 0-4. T h e effect o f the n u m b e r o f shots at 648 n m o n a s a m p l e o f isolated anthracene cations is s h o w n i n  Figure 6.9 a n d  Figure 6.10. Figure 6.9 focuses o n the daughter i o n p o p u l a t i o n a r o u n d 152 T h whereas Figure 6.10 focuses o n the daughters w h i c h result f r o m the loss o f one or t w o h y d r o g e n s . C l e a r l y , it c a n be seen that a d d i t i o n o f m u l t i p l e laser shots o n l y i m p r o v e s the s i g n a l a n d has v i r t u a l l y n o effect o n any n e w l y created daughter i o n s . T o demonstrate this p o i n t , the area o f the daughter s i g n a l at 152 T h is plotted v s . the n u m b e r o f laser shots i n 6.11.  Figure  T h i s figure suggests that there is no c u m u l a t i v e effect o f m u l t i p l e laser shots (other  than the o b v i o u s one) a n d that it is not l i k e l y that the parent i o n s have any m e m o r y o f  144  Figure 6.6 Two laser mass spectrum of a sample of phenanthrene on activated charcoal with the addition of a NBBW isolation pulse.  100  110  120  130  140  150  160  Mass/Charge  RF Voltage  145  170  180  190  200  Figure 6.7 Two laser mass spectrum of phenanthrene on activated charcoal with the addition of a NBBW isolation pulse followed by 5 photodissociation laser shots (892 nm).  146  Figure 6.8 Two laser mass spectrum of a sample of anthracene on activated charcoal with the addition of a NBBW isolation pulse followed by 5 photodissociation laser shots (682 nm).  120  140  160  Mass/Charge  RF Voltage  147  180  200  Figure 6.9 Daughter ion population observed over 0-4 photodissociation laser shots (focusing around 152 Th) on a sample of anthracene.  i ^  AJ 145  /  4 Pulses  I  3 Pulses  I 1/  2 Pulses  ' .11  1 Pulse  VA  0 Pulses  150  155  160  Mass/Charge  Figure 6.10 Daughter ion population observed over 0-4 photodissociation laser shots (focusing around 178 Th) on a sample of anthracene.  4 Pulses  I  3 Pulses  2 Pulses 1 Pulse  ]  0 Pulses  ni XJ  i  160  165  170  175 Mass/Charge  148  180  185  Figure 6.11 Normalized areas for the ratio of 152/178 vs. the number of photodissociation laser shots on a sample of anthracene.  7 -,  0  1  2  3  4  Number of Photodisociation Laser Shots  149  5  6  p r e v i o u s laser shots; i.e. the parent i o n s l i k e l y do not retain any energy b e t w e e n laser shots, a n d that the 50 m s between laser shots is e n o u g h t i m e for the i o n s to r a d i a t i v e l y or c o l l i s i o n a l l y relax.  6.3.2 Fragmentation Pathways in Anthracene and Phenanthrene E x p e r i m e n t s c o n c e r n i n g the photofragmentation o f gas phase cations have b e e n p e r f o r m e d for s o m e t i m e . S p e c i f i c a l l y , the photo stability o f P A H m o n o c a t i o n s h a v e r e c e i v e d a great d e a l o f interest due to their potential a s t r o p h y s i c a l r o l e [165]. It has e v e n b e e n p r o p o s e d that certain fragments o f P A H c a t i o n m a y i n fact be m o r e stable than the parent i o n (and thus m o r e abundant i n deep s p a c e ) [ l 7 3 ] . In this w o r k , the fragmentation o f an isolated c a t i o n begins w i t h the a b s o r p t i o n o f a p h o t o n . A f t e r the a b s o r p t i o n o f this first p h o t o n , a c o m p e t i t i o n takes p l a c e , b e t w e e n the r e e m i s s i o n o f the energy (fluorescence) and the r e d i s t r i b u t i o n o f this energy a m o n g the v i b r a t i o n a l degrees o f freedom o f the electronic g r o u n d state (internal c o n v e r s i o n ) . W h e n internal c o n v e r s i o n o c c u r s , the p h o t o n energy is stored i n the i o n , and c a n be r e m o v e d one o f t w o w a y s i n an i o n trap, either r a d i a t i v e l y v i a infrared v i b r a t i o n a l fluorescence or c o l l i s i o n a l l y w i t h b a c k g r o u n d buffer gas. B o t h o f these loss processes are c o n s i d e r e d s l o w , e s p e c i a l l y w i t h respect to the w i d t h o f a N d : Y A G laser p u l s e (10 ns). T h e a b s o r p t i o n o f a s e c o n d p h o t o n is then p o s s i b l e before the energy o f the first has dissipated. T h i s process c a n continue u n t i l the net energy w i t h i n the i o n is large e n o u g h for the d i s s o c i a t i o n rate to b e c o m e c o m p a r a b l e w i t h the c o o l i n g rate. W e m a y state that the m a g n i t u d e o f p h o t o d i s s o c i a t i o n for a P A H c a t i o n depends o n : (a) the e x c i t e d state energies b e i n g attainable b y the i n c i d e n t r a d i a t i o n , (b) the e x c i t e d 150  '• .'  state l i f e t i m e s are s u f f i c i e n t l y l o n g to ensure m u l t i p h o t o n absorption, and (c) a sufficient density o f e x c i t e d states l e v e l s to enable m u l t i p h o t o n a b s o r p t i o n to o c c u r [172]. T h e c o n d i t i o n d e s c r i b e d i n (a) i s u s e d to g a i n spectroscopic i n f o r m a t i o n , because the fragmentation e f f i c i e n c y i s related to the a b i l i t y o f the i o n to g a i n energy (i.e. the laser w a v e l e n g t h i s i n resonance w i t h a n electronic transition i n the cation). D u r i n g the w o r k p e r f o r m e d i n this chapter, v i r t u a l l y i d e n t i c a l fragmentation patterns w e r e o b s e r v e d for b o t h the anthracene a n d phenanthrene cations. T h e t w o p r i m a r y patterns m a y be described as b e l l o w :  c  c  C  u 'o  -> i4#8  u 'o  -> u 9  H  u  H  H  w  c  c  H  +  +  + H  +  H  i-  Equ.  6.1  \  Equ.  6.2  Eq _ 6.3  -+C Hl +C H +  X2  2  U  2  T h e s e r e a c t i o n patterns are w e l l established and have p r e v i o u s l y b e e n e x p e r i m e n t a l l y o b s e r v e d [171, 191, 192] and theoretically d e s c r i b e d [173]. M u c h w o r k has b e e n done e x a m i n i n g the h y d r o g e n loss process, but to date, it i s s t i l l unclear as to whether b o t h h y d r o g e n s depart s i m u l t a n e o u s l y (Equ. 6.1) o r sequentially (Equ. 6.2)[171]. U n f o r t u n a t e l y , w i t h the experiments d i s c u s s e d here, i t w a s i m p o s s i b l e to shed a n y l i g h t o n this debate. H o w e v e r , future w o r k m a y be p e r f o r m e d to isolate the s i n g l e H loss c a t i o n i n order to e x a m i n e its photo stability independently. P r e v i o u s w o r k has s h o w n that the A p p e a r a n c e E n e r g i e s ( A E ) for the processes i n Equ.6.1 a n d Equ.6.3 are v i r t u a l l y i d e n t i c a l to each other and s i m i l a r for b o t h i s o m e r s  151  [171]. T h i s A E v a l u e o f - 1 5 . 0 e V w a s d e r i v e d f r o m a series o f experiments w h e r e the a r r i v a l o f fragments w a s m o n i t o r e d as a f u n c t i o n o f p h o t o n energy (for a s i n g l e p h o t o n  experimental 71]. W e m a y c o n c l u d e , therefore, that i n this w o r k , fragmentation requires a m i n i m u m o f b e t w e e n 6 to 11 (visible/near I R ) photons, d e p e n d i n g o n the laser w a v e l e n g t h used. A s e c o n d issue o f m u c h debate c o n c e r n i n g these fragmentation processes i s that o f i s o m e r i z a t i o n . It is p o s s i b l e that, f o l l o w i n g the loss o f acetylene (C2H2), the r e s u l t i n g fragment f r o m b o t h phenanthrene a n d anthracene i s o m e r i z e to a n i d e n t i c a l f o r m .  Again,  to date, there has been no agreement o n the identity o f this species, o r e v e n i f i s o m e r i z a t i o n o c c u r s . H o w e v e r density f u n c t i o n a l theory suggests that the m o s t l i k e l y c o m m o n p r o d u c t is either b i p h e n y l e n e  #+  o r acenaphthylene* [193]. T h i s q u e s t i o n is o f +  p a r t i c u l a r a s t r o p h y s i c a l i m p o r t a n c e , because it is this type o f stable fragment w h i c h s o m e suggest m a y represent a large p o r t i o n o f a s t r o n o m i c a l P A H s [194]. A m o d i f i e d v e r s i o n o f this e x p e r i m e n t w o u l d be able to u n a m b i g u o u s l y s o l v e this p r o b l e m a n d m a n y l i k e it because the daughter i o n s that result f r o m photo fragmentation c a n be further mass selected a n d subjected to a d d i t i o n a l laser spectroscopy. T h e mass spectra i n  Figures 6.6 a n d 6.7 s h o w one a d d i t i o n a l interesting feature.  In b o t h cases, the photo-fragmentation results i n the appearance o f a m i n o r peak at 168 T h . T h i s peak m a y c o r r e s p o n d to the loss o f a l l 10 h y d r o g e n s f r o m the parent c a t i o n . W h i l e this u n u s u a l result has never been reported for phenanthrene o r anthracene it has b e e n o b s e r v e d i n the case o f coronene a n d naphtha[2,3-a]pyrene [195]. It is d i f f i c u l t to e x p l a i n this o b s e r v a t i o n , but it m a y be attributed to the c o l l i s i o n a l c o o l i n g process i n the i o n trap; m o s t c o m p a r a b l e experiments (where H - loss w a s not observed) w e r e  152  p e r f o r m e d i n I C R mass spectrometers w h e r e the pressure w a s three orders o f m a g n i t u d e l o w e r than i n the i o n trap. There m a y be a m e c h a n i s m w h e r e b y c o l l i s i o n s w i t h the r e l a t i v e l y s m a l l H e buffer gas atoms favors energy transfer into C - H b o n d stretching m o d e s , h o w e v e r , at this point, this is o n l y speculation. F u r t h e r experiments w i t h a v a r i e t y o f different buffer gases ( N e o n , A r g o n , etc.) at a variety o f pressures m a y shed l i g h t o n this o b s e r v a t i o n .  6.3.3  Visible Spectra of Phenanthrene and Anthracene cations  T h e use o f the R E M P D process as a spectroscopic t o o l has a 3 0 year h i s t o r y [182]. I n o r d e r f o r p h o t o d i s s o c i a t i o n to take p l a c e , the c a t i o n m u s t absorb e n o u g h energy to m a k e the rate o f d i s s o c i a t i o n c o m p a r a b l e to the r e l a x a t i o n process. I n theory, this rate o f energy a b s o r p t i o n is a f u n c t i o n o f the e x c i t a t i o n w a v e l e n g t h . W h e n the e x c i t i n g laser w a v e l e n g t h b e c o m e s s i m i l a r i n energy to an electronic t r a n s i t i o n o f a c a t i o n o f interest a dramatic i m p r o v e m e n t i n the energy transfer w i l l occur. Therefore, a p h o t o d i s s o c i a t i o n s p e c t r u m (i.e. a p l o t o f the ratio o f daughter ions/parent ions v s . w a v e l e n g t h ) s h o u l d y i e l d an accurate representation o f the c a t i o n i c absorption spectra; a s s u m i n g a f e w caveats are met. T h e m o s t important c o n d i t i o n a l requirement is that the p h o t o n f l u x m u s t b y large e n o u g h so that sufficient energy is deposited into the c a t i o n to i n d u c e fragmentation.  The  s e c o n d c o n d i t i o n is that w e must also assume that fragmentation is the p r i m a r y result o f energy d e p o s i t i o n (i.e. instead o f fluorescence). A s a first d e m o n s t r a t i o n o f the w a v e l e n g t h s e l e c t i v i t y o f the p h o t o d i s s o c i a t i o n process t w o w a v e l e n g t h s were c h o s e n to e x a m i n e the fragmentation e f f i c i e n c y o f the anthracene c a t i o n . A t each o f these wavelengths (720 n m and 7 4 0 n m ) the laser p o w e r  153  w a s v a r i e d and the fragmentation efficiency recorded. T h e r e s u l t i n g data, F i g u r e 6 . 1 2 , demonstrates t w o k e y points. F i r s t , that the fragmentation process is linear w i t h respect to laser p o w e r . T h i s result c o n f i r m s that the fragmentation rate depends p r i m a r i l y o n the a b s o r p t i o n o f a s i n g l e p h o t o n . T h i s is important for t w o reasons; it validates the w o r k i n g theory that s i n g l e p h o t o n a b s o r p t i o n is the r a t e - l i m i t i n g step, but it is also useful e x p e r i m e n t a l l y because it m e a n s that s m a l l changes i n laser p o w e r c a n be e a s i l y a c c o u n t e d for. T h e s e c o n d p o i n t that F i g u r e 6.12 demonstrates is that the fragmentation e f f i c i e n c y i s a f u n c t i o n o f w a v e l e n g t h . P r e s u m a b l y , m o r e fragmentation o c c u r s at 7 2 0 n m rather than at 7 4 0 n m because it is closer to b e i n g i n resonance w i t h a r e a l e l e c t r o n i c t r a n s i t i o n o f the c a t i o n . Therefore, energy is m o r e efficiently transferred into the c a t i o n ; the result b e i n g m o r e fragmentation. T h e l o g i c a l e x t e n s i o n o f this experiment w a s to scan the d y e laser o v e r a large range o f w a v e l e n g t h s at a constant p o w e r and r e c o r d the fragmentation e f f i c i e n c y o f the anthracene c a t i o n . T h e result o f these experiments is s h o w n i n F i g u r e 6.13. E a c h p o i n t a l o n g the c u r v e represents the m a g n i t u d e o f the peak area for a fragment at 152 T h d i v i d e d b y the peak area at 178 T h averaged o v e r 50 mass spectra. I n this s p e c t r u m , data p o i n t s were taken every 2 n m due the large t i m e r e q u i r e d to p e r f o r m these e x p e r i m e n t s m a n u a l l y . T h e lines c o n n e c t i n g the points i n this figure are five p o i n t m o v i n g averages f r o m e a c h d y e laser scan. A n interesting aspect o f this spectrum is its s i m i l a r i t y to that recorded i n a rare gas m a t r i x at l o w temperature. F o r c o m p a r i s o n , the v i s i b l e spectra o f anthracene r e c o r d e d i n  154  Figure 6.12 Fragmentation efficiency (ratio of 152/178 Th) vs. energy at two different photodissociation laser wavelengths (720 nm and 740 nm) for a sample of anthracene.  1.2 -,  -i  0  1  2  :  i  i  i  i  4 6 8 10 12 Energy Measured External to Trap (mj)  155,  i  14  i  16  Figure 6.13 Top spectrum- Photodissociation spectra of the anthracene cation (ratio of 152/178 Th vs. wavelength). Bottom spectrum - Anthracene cation spectrum acquired in a frozen argon matrix at 12 K [196]  712 nm  |  500  I I , I I I M  | i i I II  I I i | i I I I M  600  I M  I I i I I I i I I I  |l ' I  I M  I I ! I I I i M  700 wavelength (nm)  156  I M  I | I I I I . I I I I I I M  800  I i M  I I | I M  900  I i M  I I I M  I I I  I  an A r m a t r i x at 12 K has been i n c l u d e d o n F i g u r e 6.13 [196]. T h e s i m i l a r i t i e s b e t w e e n the t w o sets o f experiments are e n c o u r a g i n g . B e s i d e s a s m a l l shift i n peak l o c a t i o n s a n d relative intensities, the t w o sets o f data are i n g o o d agreement c o n s i d e r i n g the v a s t l y different m e t h o d s o f determination. Shifts o f this magnitude are t y p i c a l i n f r o z e n matrices. M a n y other w o r k e r s u s i n g a variety o f methods and solvent m e d i a have r e c o r d e d s i m i l a r data. A peak at 7 2 2 n m was first observed b y A n d r e w s and c o w o r k e r s [197] a n d a s c r i b e d b y t h e m to the a l l o w e d D 2 ( A )<— D o ( B 2 ) transition i n the anthracene c a t i o n . u  g  P h o t o e l e c t r o n spectral bands recorded i n the gas phase suggest a t r a n s i t i o n at 704.5 n m [196]. H a r t r e e - F o c k c a l c u l a t i o n s p e r f o r m e d b y V a l a and c o - w o r k e r s agree w i t h this assignment a n d predict a peak between 717 n m - 7 2 4 n m [196]. O w i n g to the s i m i l a r i t y i n spectral structure, the peak at 712 n m w i l l be assigned i n this w o r k to the t r a n s i t i o n A 2  <— B  2 g  U  . T e n a d d i t i o n a l c o m p o n e n t bands o f decreasing intensity, b u i l t o n the 7 2 2 n m  b a n d ( 7 1 2 n m i n this case) have also been p r e v i o u s l y observed [196]. C o n s e q u e n t l y , the p e a k s at 6 4 8 n m a n d 6 0 0 n m w i l l , for n o w , be attributed to v i b r o n i c bands. In a s i m i l a r manner, the gas phase photo fragmentation spectrum for the c a t i o n o f phenanthrene was also recorded. T h e resulting spectrum is s h o w n i n F i g u r e 6.14. A g a i n , the b o t t o m trace represents the v i s i b l e spectra o f the phenanthrene c a t i o n , this t i m e r e c o r d e d i n a n e o n m a t r i x at 4.2 K b y the A l l a m a n d o l a group [165]. H e r e , as above, the t w o sets o f data bare r e m a r k a b l e resemblances.  In the frozen m a t r i x , the longest  w a v e l e n g t h peak is f o u n d at 898.3 n m where as this w o r k has f o u n d a p e a k m a x i m u m at 892 n m . O w i n g to the s i m i l a r i t i e s between the t w o sets o f data, the peak at 892 n m is assigned to the same t r a n s i t i o n as that at.898 n m b y the A l l a m a n d o l a g r o u p :  157  Figure 6.14 Top spectrum- Photodissociation spectrum of the phenanthrene cation (ratio of 152/178 Th vs. wavelength). Bottom spectraPhenanthrene cation spectrum acquired in a frozen neon matrix at 4.2 K [165].  892 nm  700  750  800  850  wavelength (nm)  158  900  950  1000  D2( A2)<—Do( Bi). T h e r e m a i n i n g peaks at higher energies are assigned to a v i b r a t i o n a l 2  2  p r o g r e s s i o n b u i l t o n the 8 9 2 n m peak as above. O n e d e v i a t i o n between the t w o spectra is the relative increase i n m a g n i t u d e o f the peaks at 874 n m (880 n m i n the m a t r i x ) . Interestingly, the A r - p h e n a n t h r e n e  +  p h o t o d i s s o c i a t i o n spectra recorded b y B r e c h i g n a c and P i n o s h o w v e r y s i m i l a r results to those reported here (their p r i m a r y transition occurs at 891 n m ) h o w e v e r their r e c o r d e d data s h o w s n o peak at 8 7 6 n m [ l 8 0 ] . A t this p o i n t it i s difficult to account for these differences.  6.4 Conclusions T h e g o a l o f (his chapter was to demonstrate this s y s t e m ' s a b i l i t y to acquire direct gas phase spectroscopic i n f o r m a t i o n relating to large P A H cations. F i g u r e 6.13 and F i g u r e 6.14 demonstrate that the P A H cations phenanthrene a n d anthracene c a n be s p e c t r o s c o p i c a l l y evaluated i n this manner. T h e o b s e r v e d o p t i c a l spectra e x h i b i t spectral features v e r y s i m i l a r to those o b t a i n e d b y m a t r i x i s o l a t i o n methods. H o w e v e r , w i t h m a t r i x techniques, the exact l o c a t i o n o f the peaks i n the gas phase must be h y p o t h e s i z e d because o f unpredictable m a t r i x shifts. F u r t h e r m o r e , w i t h the m a t r i x m e t h o d the exact identity o f the a b s o r b i n g species is never certain. I n contrast, w i t h this technique the user has e x c e l l e n t c o n t r o l o v e r the identity o f the species a n a l y z e d . T h e p o s s i b i l i t y e v e n exists o f mass selecting daughter i o n s or other species that exist o n l y as r a d i c a l cations as candidates for further analysis. F i n a l l y , because the source o f i o n s is the two-laser s a m p l i n g m e t h o d a v e r y w i d e v a r i e t y o f potential analytes is a v a i l a b l e for analyses.  159  T h e R E M P D m e t h o d does, unfortunately have a f e w l i m i t a t i o n s . P r a c t i c a l l y , the technique, as it is c u r r e n t l y p e r f o r m e d ( w i t h a d y e laser), is c u m b e r s o m e a n d t i m e c o n s u m i n g . H o w e v e r , the next generation o f this d e v i c e c o u l d be greatly i m p r o v e d w i t h the a d d i t i o n o f a solid-state tunable laser - an o p t i c a l parametric o s c i l l a t o r ( O P O ) - , w h i c h has the a b i l i t y to scan o v e r a large w a v e l e n g t h range under c o m p u t e r c o n t r o l . It w o u l d t h e n be p o s s i b l e to acquire data for b o t h spectroscopic purposes as w e l l as a n a l y t i c a l a p p l i c a t i o n s . A scenario c o u l d e v e n be i m a g i n e d w h e r e real e n v i r o n m e n t a l samples w e r e e x a m i n e d a n d certain isomers were mass selected a n d t h e n a n a l y z e d b y o p t i c a l means d i r e c t l y i n the trap; the i o n trap, i n this case, a c t i n g as a cuvette. W h i l e the o p t i c a l bands o b s e r v e d i n this chapter w e r e o f s i m i l a r l i n e w i d t h s to those i n the s o l i d matrices, they were certainly m u c h w i d e r than those o b s e r v e d i n a free jet e x p a n s i o n . I n the rare gas m a t r i x , the analyte w a s r e l a t i v e l y c o o l ( 1 0 K ) . H o w e v e r ; the spectral features w e r e broadened b y c o l l i s i o n s w i t h the m a t r i x . O n the other h a n d , i n this w o r k , the b r o a d e n i n g w a s due to the large internal temperatures o f the analytes; a s s u m i n g the analyte i o n s are f u l l y e q u i l i b r a t e d w i t h the b a c k g r o u n d buffer gas, the i n t e r n a l temperature o f the i o n s is most l i k e l y ~ 3 0 0 K [198-202]. I n contrast, w i t h free jet e x p a n s i o n techniques (i.e. c a v i t y r i n g d o w n spectroscopy), the ions are not o n l y super c o o l e d but are also i n c o l l i s i o n - f r e e e n v i r o n m e n t s . Therefore the spectra are often v e r y sharp a n d the peaks o c c u r at the true gas phase l o c a t i o n s . T h e one disadvantage o f b e a m m e t h o d s , h o w e v e r , is that a r e l a t i v e l y s m a l l f l u x o f ions is a v a i l a b l e for analyses, so the s e n s i t i v i t y is often l i m i t e d . A s a result, experiments o f this type are far f r o m routine. T h e R E M P D technique for P A H p h o t o d i s s o c i a t i o n experiments, w h i l e mature i n the f i e l d o f I C R spectrometry, is r e l a t i v e l y n e w w i t h the i o n trap. W i t h the current  160  d e v e l o p m e n t o f s m a l l i n e x p e n s i v e solid-state lasers, a w i d e variety o f future p o s s i b i l i t i e s exist. F o r e x a m p l e , a series o f laser diodes c o u l d be i n c l u d e d i n the v a c u u m m a n i f o l d at c a r e f u l l y selected w a v e l e n g t h s w h i c h a l l o w the user to r a p i d l y a n d r o u t i n e l y g a i n o p t i c a l l y i n f o r m a t i o n o n the trapped cations s e a m l e s s l y w i t h e v e r y mass spectra; thus a c h i e v i n g true m u l t i d i m e n s i o n a l analyses.  161  Chapter 7 Semi-Quantitative Determination of PAH Isomers Directly from Solid Matrices 7.1 Introduction M a s s spectrometry p r o v i d e s a fast and effective means o f a n a l y z i n g a w i d e v a r i e t y o f samples o v e r a large range o f c o n d i t i o n s and concentrations. E v e n t h o u g h m a s s a n a l y z e r s , p a r t i c u l a r l y h i g h r e s o l u t i o n m a c h i n e s s u c h as F T - I C R , are capable o f e x q u i s i t e s e l e c t i v i t y they are not able to resolve m i x t u r e s o f structural i s o m e r s because they o c c u r at p r e c i s e l y the s a m e m a s s to charge. A v a r i e t y o f supplemental techniques have b e e n d e v e l o p e d h a n d i n h a n d w i t h the mass spectrometer to a l l o w structural c h a r a c t e r i z a t i o n ( M S / M S ) [12], h o w e v e r , there r e m a i n a f e w types o f samples a n d classes o f m o l e c u l e s w h e r e a n acceptable strategy has yet to be d e v e l o p e d . O n e important class o f these m o l e c u l e s , w h e r e structural d e t e r m i n a t i o n is c r i t i c a l to the c o m p l e t e a n a l y s i s , is the f a m i l y o f p o l y c y c l i c aromatic h y d r o c a r b o n s ( P A H s ) . P A H s are a n e n v i r o n m e n t a l l y important class o f c o m p o u n d s w h e r e i s o m e r i c determinations are important [79, 80]. A s discussed i n C H A P T E R 3 these m o l e c u l e s are p r o d u c e d m a i n l y b y the c o m b u s t i o n o f h y d r o c a r b o n s and are f o u n d u b i q u i t o u s l y throughout the e n v i r o n m e n t . T h e l e v e l o f t o x i c i t y o f the v a r i o u s P A H s c a n v a r y w i d e l y b e t w e e n i s o m e r s , and as a result, i s o m e r d i s c r i m i n a t i o n is essential to w h a t e v e r s o l i d a n a l y s i s t e c h n i q u e is used. F o r e x a m p l e , benzo[a]pyrene is k n o w n to be a strong c a r c i n o g e n , w h i l e its i s o m e r benzo[e]pyrene is n o n c a r c i n o g e n i c [203]. A d d i t i o n a l l y , the a b i l i t y to e a s i l y d i s c r i m i n a t e between isomers m a y p r o v e useful w h e n t r y i n g to d e t e r m i n e  162  the source o f contaminants, as the i s o m e r i c ratio is often i n d i c a t i v e o f the p r o d u c t i o n process [79]. C u r r e n t l y there are a n u m b e r o f methods a v a i l a b l e for the a n a l y s i s o f P A H s i n s o l i d samples. T h e technique that is most r o u t i n e l y used t y p i c a l l y i n v o l v e s a l i q u i d e x t r a c t i o n o f the P A H s f r o m the s o l i d , f o l l o w e d b y separation v i a gas c h r o m a t o g r a p h y ( G C ) w i t h detection b y mass spectrometry [89].  The most c o m m o n ionization scheme i n  these analyses is e l e c t r o n i m p a c t ( E I ) . H o w e v e r , the E I mass spectra o f m o s t P A H i s o m e r s are v i r t u a l l y i d e n t i c a l . A s a result, i s o m e r differentiation relies o n d i f f e r i n g G C retention t i m e s a n d matches w i t h pure standards. E v e n i f the mass spectrometry w e r e done i n a d e v i c e capable o f M S / M S s u c h as an i o n trap o r triple q u a d u r p o l e , P A H i s o m e r s often have i d e n t i c a l fragmentation patterns [204]. A l t e r n a t i v e l y , s o m e success has b e e n r e a l i z e d i n P A H i s o m e r differentiatiori b y o b s e r v i n g the results o f gas phase c h e m i s t r y [205-207]. T h e p h y s i c a l process o f e x t r a c t i o n a n d G C analysis is often t i m e c o n s u m i n g a n d e x p e n s i v e . A s a result a n u m b e r o f direct s o l i d analysis methods have b e e n attempted. S o m e o f the m o r e successful direct s o l i d s a m p l i n g techniques a p p l i e d to this p r o b l e m i n c l u d e secondary i o n mass spectrometry ( S I M S ) [40], fast a t o m b o m b a r d m e n t ( F A B ) [ 4 2 ] , a n d laser d e s o r p t i o n ( L D ) [ 4 1 ] . T h e s e techniques, h o w e v e r , are n o n - s e l e c t i v e i o n sources a n d t y p i c a l l y p r o d u c e c o m p l e x mass spectra w i t h a v a r i e t y o f m o l e c u l a r fragments a n d little i s o m e r i c i n f o r m a t i o n . O n e direct s o l i d s a m p l i n g technique that has r i s e n to p r o m i n e n c e o v e r the last t w e n t y years is that o f laser desorption, resonant t w o - p h o t o n i o n i z a t i o n mass spectrometry ( L 2 M S ) [ 7 2 , 9 6 ] . T h i s technique, relies o n the n o n - s p e c i f i c d e s o r p t i o n  163  ( t y p i c a l l y w i t h a p u l s e d I R laser) o f analyte f r o m the s o l i d f o l l o w e d b y selective p h o t o i o n i z a t i o n o f the analyte b y a s e c o n d p u l s e d laser ( t y p i c a l l y a p u l s e d U V laser). T h e r e s u l t i n g i o n s are then, i n v i r t u a l l y every reported case, e x a m i n e d b y a t i m e o f flight mass spectrometer ( T O F ) [208].  T h e i o n i z a t i o n stage p r o v i d e s a h i g h degree o f  s e l e c t i v i t y because i o n i z a t i o n w i l l o n l y o c c u r i f the m o l e c u l e has a strong a b s o r p t i o n at the i o n i z a t i o n laser w a v e l e n g t h . T h u s , the p o s s i b i l i t y exists o f p r o v i d i n g a s e c o n d d i m e n s i o n to the M S because this i o n i z a t i o n step depends i n t r i n s i c a l l y o n the m o l e c u l a r s p e c t r o s c o p y o f the m o l e c u l e s . W h i l e this step m a y be used to d i s c r i m i n a t e b e t w e e n i s o m e r s i n theory, most groups have not pursued this and instead have been content to o n l y use one w a v e l e n g t h ( 2 6 6 n m ) for the i o n i z a t i o n step[64]. T h i s is understandable f r o m a p r a c t i c a l standpoint (tunable laser sources i n the U V are c u m b e r s o m e ) and f r o m a n a n a l y t i c a l p o i n t o f v i e w because the goals o f the experiments are often to get a p i c t u r e o f the o v e r a l l P A H content (and m o s t P A H s have strong absorptions at 2 6 6 n m ) . T o the extent o f u s i n g the s e l e c t i v i t y o f the i o n i z a t i o n step, Z e n o b i and c o w o r k e r s have e x a m i n e d the a b s o r p t i o n spectra o f P A H s d i r e c t l y after laser d e s o r p t i o n , a n d f o u n d these w a r m m o l e c u l e s to p r o d u c e spectra w h i c h were e x t r e m e l y b r o a d due to the large i n t e r n a l temperatures o f the m o l e c u l e s f o l l o w i n g laser desorption. Isomer d i s c r i m i n a t i o n d i r e c t l y thus m a y not be p o s s i b l e [96]. O n the other e n d o f the temperature r e g i m e , L u b m a n and c o w o r k e r s h a v e a c h i e v e d h i g h r e s o l u t i o n , h i g h l y s p e c i f i c i o n i z a t i o n o f peptides and neurotransmitters b y i n s e r t i n g a supersonic jet between i o n i z a t i o n and d e s o r p t i o n [209]. S i m i l a r l y , S h l a g ' s g r o u p o b s e r v e d i s o m e r selective i o n i z a t i o n o f p o l y c h l o r i n a t e d b i p h e n y l s ( P C B s ) b y s i m i l a r means [160]. T h e supersonic jet p r o v i d e s e x t r e m e l y efficient c o o l i n g o f the  164  analytes. H o w e v e r , this apparatus c o m e s at the expense o f b e c o m i n g s o m e w h a t m o r e analytically unpractical. A s a n alternative to creating the s e c o n d d i m e n s i o n o f a n a l y s i s i n the i o n i z a t i o n step, the p o s s i b i l i t y o f p r o b i n g the m o l e c u l a r ions w i t h a t h i r d p u l s e d laser has b e e n investigated. T o a c c o m m o d a t e this, a system was b u i l t , w h i c h a l l o w s L 2 M S to o c c u r d i r e c t l y i n the v o l u m e o f a n i o n trap. T h e analyte i o n s o f interest c a n be mass selected i n the i o n trap and p r o b e d w i t h a t h i r d laser. C H A P T E R 6 demonstrated that the P A H i s o m e r cations phenanthrene and anthracene (178 T h ) have strong distinct a b s o r p t i o n i n the v i s i b l e a n d near I R i n the gas phase ( F i g u r e 7.1). These spectra w e r e r e c o r d e d b y the m e t h o d o f R e s o n a n c e E n h a n c e d M u l t i p h o t o n D i s s o c i a t i o n ( R E M P D ) , w h e r e the degree o f fragmentation w a s o b s e r v e d as a f u n c t i o n o f w a v e l e n g t h . T h e a b s o r p t i o n spectra alone c a n a l l o w d i s c r i m i n a t i o n b e t w e e n i s o m e r s ; the i o n trap a c t i n g as a cuvette f o r the samples. H o w e v e r , e v e n w i t h a state o f the art solid-state laser, it w o u l d take a p p r o x i m a t e l y 2 0 m i n to acquire a useful s p e c t r u m o f each P A H .  A s a result, this  chapter describes a secondary m e t h o d o f u s i n g the gas phase c a t i o n a b s o r p t i o n spectral i n f o r m a t i o n to identify isomers. T h e m e t h o d relies o n j u d i c i o u s l y c h o o s i n g one w a v e l e n g t h a n d m o n i t o r i n g the fragmentation e f f i c i e n c y o f the mass selected isomers. I f a s a m p l e c o n t a i n e d t w o i s o m e r s then the i s o m e r that is m o r e i n resonance w i t h the laser w a v e l e n g t h w o u l d have a greater degree o f fragmentation. T h u s , b y e x a m i n i n g the relative degree o f fragmentation, the c o n t r i b u t i o n o f each i s o m e r to a specific mass peak c a n be determined. I n this chapter, a n internal standard is also used, to p r o v i d e s e m i quantitative c o n c e n t r a t i o n i n f o r m a t i o n o n t w o P A H i s o m e r s d i r e c t l y f r o m a s o l i d sample i n less than five minutes.  165  Figure 7.1 Anthracene and phenanthrene photofragmentation spectra acquired in an ion trap by the REMPD method.  166  7.2 Experimental T h e experiments d e s c r i b e d i n this chapter were c a r r i e d out o n a n i n house b u i l t i o n trap d e s i g n e d for the direct analysis o f s o l i d samples u s i n g the t w o - l a s e r m e t h o d o f s o l i d s a m p l i n g . T h e use o f this instrument for the direct a n a l y s i s o f P A H s o n s o l i d matrices w a s d e s c r i b e d i n  CHAPTER 2 and CHAPTER 3.  A t h i r d laser w a s added to  this s y s t e m to a l l o w w a v e l e n g t h selective photofragmentation and w a s d e s c r i b e d i n  C H A P T E R 6. T h e methods for c o l l e c t i n g and m a n i p u l a t i n g the i o n s i n this s y s t e m w e r e a l l p r e v i o u s l y k n o w n , h o w e v e r , the c o m b i n a t i o n presented here is s o m e w h a t u n i q u e . S a m p l e m o l e c u l e s are first desorbed f r o m a s o l i d m a t r i x into the gas phase b y laser d e s o r p t i o n u s i n g the u n f o c u s s e d 1064 n m b e a m o f a N d : Y A G laser ( D C R - 2 A , S p e c t r a P h y s i c s , M o u n t a i n V i e w , C A ) . T h e I R p o w e r r e a c h i n g the probe surface w a s t y p i c a l l y 1 0 W / c m . T h e I R laser induces a r a p i d heating o f the s a m p l e surface a n d p r o d u c e s a s  2  p l u m e o f desorbed neutrals. A p p r o x i m a t e l y 3 0 ps after the desorption event; the m a j o r i t y o f the neutrals have reached the center o f the i o n trap.  These neutrals are then  s e l e c t i v e l y p h o t o i o n i z e d i n the center o f the i o n trap b y 1+1 R e s o n a n c e E n h a n c e d M u l t i p h o t o n i o n i z a t i o n ( R E M P I ) . T h i s i o n i z a t i o n is a c h i e v e d b y p a s s i n g the 4  t h  harmonic  o f a s e c o n d N d : Y A G laser ( L u m o n i c s , H Y 4 0 0 , ~ 4 0 p j / p u l s e ) t h r o u g h the neutral p l u m e . T h e i o n s were trapped u s i n g an R F quadrupole p o w e r s u p p l y operating at 0 . 9 6 7 M H z at a h e l i u m buffer gas pressure o f - 10" T o r r . O n c e trapped, a 4 0 m s N B B W , p u l s e w a s a p p l i e d to the end caps to isolate a s p e c i f i c mass range. I n the case o f these e x p e r i m e n t s the p u l s e w a s d e s i g n e d to e l i m i n a t e a l l species w i t h mass l o w e r that 178 a m u a n d h i g h e r  167  t h a n 2 0 2 a m u . T h i s w a v e f o r m p r o v e d useful for p r a c t i c a l reasons d u r i n g the e x p e r i m e n t s (it a l l o w e d easy v i e w i n g o f any fragment ions p r o d u c e d ) but w a s not essential. F i n a l l y , the t h i r d laser b e a m , a N d : Y A G ( Q u a n t e l Y G 6 6 0 - 5 3 2 n m ) p u m p e d d y e laser ( S p e c t r a P h y s i c s P D L - 3 ) w a s sent v e r t i c a l l y t h r o u g h the r i n g electrode to interrogate the i o n c l o u d . T y p i c a l energies w e r e 1 m J / p u l s e for the v i s i b l e to near I R ranges. T h e e n e r g y f r o m e a c h o f these laser shots w a s r e c o r d e d a n d b u n d l e d w i t h e a c h mass s p e c t r u m , thus a l l o w i n g for p o w e r fluctuations to be a c c o u n t e d for d u r i n g data a n a l y s i s . E x p e r i m e n t s were p e r f o r m e d b y a l l o w i n g the i o n c l o u d to interact w i t h 2 0 laser shots each c y c l e . T h e r e s u l t i n g ions w e r e then resonantly ejected f r o m the i o n trap. T h e samples used i n this w o r k w e r e a l l manufactured i n the l a b a n d are o f the t y p e p r e v i o u s l y characterized i n C H A P T E R 3. B r i e f l y , samples w e r e prepared b y d i s s o l v i n g a P A H ( A n t h r a c e n e , Phenanthrene, P y r e n e ) a l l f r o m S i g m a A l d r i c h ( M i l w a u k e e , W I ) i n hexane ( H P L C grade, S i g m a A l d r i c h ) . These solutions were then s p i k e d o n to a p r e v i o u s l y w e i g h e d a m o u n t o f activated c h a r c o a l . T h e samples w e r e then sonicated for 30 m i n , a n d then the hexane a l l o w e d to evaporate off. T h e c h a r c o a l samples w e r e then m e c h a n i c a l l y pressed into a s m a l l sample cup a n d inserted into the mass spectrometer. T h e samples u s e d i n this study were a l l i n the c o n c e n t r a t i o n range o f b e t w e e n 5 - 2 5 0 pmol/gram charcoal.  7.3 Results and Discussion E x p e r i m e n t s w e r e p e r f o r m e d to determine the potential u t i l i t y o f u s i n g a s i n g l e w a v e l e n g t h laser to d i s c r i m i n a t e between i s o m e r i c P A H cations stored i n a n i o n trap. T h e f o l l o w i n g equations describe h o w a single laser w a v e l e n g t h c o u l d be u s e d to d i s c r i m i n a t e b e t w e e n t w o i s o m e r s ; h o w e v e r , i n p r i n c i p l e (n) w a v e l e n g t h s c o u l d be u s e d 168  to d i s c r i m i n a t e b e t w e e n (n+1) isomers. T h e f o l l o w i n g analysis describes the s p e c i f i c case o f i d e n t i f y i n g the concentrations o f phenanthrene a n d anthracene (178 amu) d i r e c t l y i n a s o l i d that has been s p i k e d w i t h a k n o w n concentration o f an internal standard, pyrene. I f w e assume that the observed peak intensity r e s u l t i n g f r o m t w o laser m a s s spectrometry o f pyrene (Ip ) is, o n average, the product o f the pyrene c o n c e n t r a t i o n , [ P y ] , y  t i m e s the d e s o r p t i o n / i o n i z a t i o n / t r a p p i n g efficiencies ( g i v e n the s y m b o l : ap ). T h e n w e y  m a y m a t h e m a t i c a l l y w r i t e that:  i„ =VPy}(*„ ) y  y  E  q  u  E  q  u  7  1  S i m i l a r l y , for anthracene ( A ) and phenanthrene (P) w e m a y w r i t e :  I =[A](a ) A  A  /,=m(«;0  7  -  2  Equ. 7.3  A f t e r the i o n c l o u d has been irradiated b y the t h i r d laser, a certain percentage o f e a c h o f the P A H s w i l l have been r e m o v e d based o n the fragmentation e f f i c i e n c y o f that P A H at the s p e c i f i e d w a v e l e n g t h . F o r e x a m p l e , the s i g n a l observed for p y r e n e after the f i r i n g o f the t h i r d laser w i l l be:  Ipy = [Py](apy)(JragPy)  '  E  q  u  . 7.4  W e m a y therefore w r i t e that the ratio o f signals o b s e r v e d at m a s s 178 (for a m i x t u r e o f anthracene p l u s phenanthrene) vs. m a s s ' 2 0 2 (pyrene) w i t h t w o - l a s e r m a s s spectrometry w i t h o u t the t h i r d laser to be:  Ratio of observed  signals at mass 1 7 8 / 2 0 2 =  Anthracene (I,) + Phenantherene (I,,) —  K  Pyrene (I,, ) y  169  E q u . 7.5  Or in equation format: Ratio of observed signals at mass 178/202 =  Equ. 7.6  +  [Py](a J P  Similarly, after the third laser interacts with the ion cloud and a certain amount of fragmentation has occurred the ratio of signals would be as follows: Equ. ~i  Ratio of observed signals at ](a )(fragA) + [P](cc„)(fragP) mass 178/202 with third laser on = A/^I & / L J I I/U <s / [PyYa )(fragPy) [A  A  iy  In principle, the only two unknowns that are being observed in Equ.7.6 and Equ.7.7  are the concentrations of the isomers phenanthrene and anthracene. Therefore,  since we have two equations and two unknowns this problem should in theory be tractable. From a practical point of view, many of the constants listed above must be determined for the specific experimental system being used. For example, the term (ap ) y  the desorption/ionization/trapping efficiency of the PAH is system dependant and must be determined empirically. This can be achieved in a straightforward manner, by simply performing a few routine calibration curves. For example, if a series of standards were made which contained only phenanthrene and pyrene and where the ratio of the concentrations of phenanthrene/pyrene were systematically varied, then a plot of the ratio of observed signals vs. the ratio of concentrations would produce a straight line, and Equ.7.6 would be reduced to: ( „ V r mA Ratio of signals 178/202 with the laser off = P JZ1 \ Py a  170  J  Equ. 7.8  T h e slope o f this graph then w o u l d s i m p l y be (a )/(a ). p  py  S i m i l a r l y , i f a series o f  anthracene a n d p y r e n e standards were p r o d u c e d , then the slope o f the ratio o f o b s e r v e d signals v s . ratio o f concentrations w o u l d y i e l d  (a^/fapy).  I f b o t h sets o f the c a l i b r a t i o n curves were also a c q u i r e d w i t h the t h i r d fragmentation laser o n , then, t w o a d d i t i o n a l constants c o u l d be acquired. F o r e x a m p l e , i f y o u p l o t t e d the ratio o f observed signals i n the phenanthrene/pyrene m i x t u r e v s . the ratio o f concentrations w i t h the t h i r d laser o n then:  Equ. 7.9  (a )(fragP) P  Ratio of signals 1 7 8 / 2 0 2 with laser on  y(a )(fragPy) Py  Therefore, the slope o f the phenanthrene/pyrene ratio o f o b s e r v e d signals v s . the phenanthrene/pyrene ratio o f concentrations w i t h the t h i r d laser o n w o u l d y i e l d a straight line n u m e r i c a l l y equal to [(o.p)(fragP)]/[(ap )(fragPy)]. y  S i m i l a r l y for samples w h i c h  c o n t a i n o n l y anthracene a n d pyrene, w i t h the t h i r d laser o n , the ratio o f o b s e r v e d signals at m a s s l 7 8 / m a s s 2 0 2 v s . ratio o f concentrations o f anthracene/pyrene w o u l d p r o d u c e a straight l i n e graph w i t h the slope equal to  [(aA)(fragA)]/[(ap )(jragPy)]. y  To  s u m m a r i z e , the slope o f the f o l l o w i n g c a l i b r a t i o n curves y i e l d s the constants d e s c r i b e d i n T a b l e 7.1. F i n a l l y , i f w e define t w o a d d i t i o n a l terms, n a m e l y , the ratio o f o b s e r v e d peaks at mass 178/202 for a n u n k n o w n m i x t u r e w i t h the laser o f f ( R N L ) a n d w i t h the laser o n ( R L O ) , then w e m a y b e g i n to rewrite equations (Equ. 7.6) a n d (Equ. 7.7). A f t e r several pages o f algebra, w e c a n rearrange the above equations to y i e l d the concentration o f phenanthrene a n d anthracene d i r e c t l y f r o m a s i n g l e series o f measurements. T h e phenanthrene concentration becomes:  171  Table 7.1 Summary of symbols and reference slopes used in this chapter.  O b s e r v e d Slopes  Physical Meaning  S y m b o l used  S l o p e Phenanthrene/Pyrene N O L A S E R  (a,)/ /(«/>)  SPPyN  S l o p e Phenanthrene/Pyrene L A S E R O N  (a,,)(fragP)  SPPyO  (a,> )(fragPy) y  Slope Anthracene/Pyrene N O L A S E R  (<*A)/  Slope Anthracene/Pyrene L A S E R O N  (a )(fragA) A  (a )(fragPy) Py  172  SAPyN SAPyO  Unkown Phenanthrene Concentration [P]  (SAPyN)(RLO)[Py] - (SAPyO)(RNLJ[Py]  Equ. 7.10  (SPPyO)(SAPyO)-(SPPyN)(SAPyO)  T h e anthracene c o n c e n t r a t i o n also b e c o m e s :  Unkown Anthracene Concentration [A] =  (RNL)[Py]-[P](SPPyN)  Equ. 7.11  SAPyN  I d e a l l y , the c a l i b r a t i o n curves w o u l d be at least five o r s i x points i n scope. H o w e v e r , w i t h the s y s t e m currently e m p l o y e d , the p o w e r drift o f the t h i r d laser o v e r the t i m e r e q u i r e d to take several h u n d r e d mass spectra l i m i t s the n u m b e r o f c a l i b r a t i o n p o i n t s that c a n be u s e f u l l y e m p l o y e d . Therefore a l l c a l i b r a t i o n c u r v e s s h o w n are based o n o n l y three s a m p l e p o i n t s . E a c h p o i n t is the average o f 50 mass spectra. Figure 7.2 s h o w s a (50 c y c l e averaged) mass s p e c t r u m demonstrating the effect o f the t h i r d fragmentation laser o n the phenanthrene s i g n a l . T h e laser w a v e l e n g t h (892 n m ) w a s c h o s e n to be i n resonance w i t h a phenanthrene c a t i o n electronic absorption hence the relative s i g n a l o f phenanthrene i s r e d u c e d b y 5 0 % w h e n the t h i r d laser interacts w i t h the i o n c l o u d . B y w a y o f c o m p a r i s o n , Figure 7.3 s h o w s a sample o f anthracene a n d p y r e n e w i t h a n d w i t h o u t the t h i r d laser w h e r e the third laser reduces the s i g n a l o f anthracene b y o n l y 2 0 % . Figures 7.4 s h o w s the c a l i b r a t i o n curves for anthracene a n d phenanthrene w i t h respect to the p y r e n e concentrations w i t h the t h i r d laser o n a n d off. T h e s l o p e s o f these c u r v e s p r o v i d e the values for the constants f r o m Table 7.1. W i t h these constants d e t e r m i n e d , w e m a y n o w a n a l y z e a sample that contains a m i x t u r e o f the t w o i s o m e r s (anthracene a n d phenanthrene).  Figure 7.5 s h o w s the mass spectra o f the m i x t u r e s a m p l e  173  Figure 7.2 Two laser mass spectrum of a sample of phenanthrene and pyrene on activated charcoal with the addition of a NBBW laser pulse. The gray line is with the addition of 20 photofragmentation laser shots at 892 nm.  170  180  190  200  Mass/Charge  IR UV  VISIBLE  RF Voltage  174  210  Figure 7.3 Two laser spectrum of a sample of anthracene and pyrene on activated charcoal with the addition of a swift laser pulse. The gray line is with the addition of 20 photofragmentation laser shots at 892 nm.  175  Figure 7.4 Calibration curve for anthracene and phenanthrene measured relative to pyrene with and without the addition of the photofragmentation laser.  176  Figure 7.5 Two laser mass spectrum of an "unknown" sample of anthracene, phenanthrene, and pyrene on activated charcoal with the addition of a NBBW laser pulse. The gray line is with the addition of 20 photofragmentation laser shots at 892 nm.  170  180  200  190 Mass/Charge  RF Voltage  177  210  w i t h and w i t h o u t the t h i r d laser o n . T h e t h i r d laser effectively reduces the ratio o f peaks b y about 3 9 % - consistent w i t h a sample c o n t a i n i n g b o t h anthracene a n d phenanthrene. F i n a l l y , b y inserting the relevant values into E q u . 7 . 1 0 and E q u . 7 . 1 1 w e m a y calculate the c o n c e n t r a t i o n o f the P A H s . S i n c e this " u n k n o w n " m i x t u r e w a s p r o d u c e d i n the l a b , w e c a n c o m p a r e the c a l c u l a t e d values to the actual concentrations. T h e results are presented i n T a b l e 7.2. B y e x a m i n i n g this chart, w e c a n determine the relative effectiveness o f this procedure.  T h e v a l u e s reported here are r e l a t i v e l y close to those o f the actual s o l u t i o n  concentrations. T h e absolute errors i n this measurement were o n the order o f 1-2 u m o l e / g r a m o f c h a r c o a l . T h i s translates into percent errors o f b e t w e e n 2 - 2 3 % .  While  these errors are r e l a t i v e l y large i n c o m p a r i s o n to those obtained i n G C / M S experiments, the speed at w h i c h this data was a c q u i r e d places the v a l u e i n a different area. W h e r e G C / M S is useful for quantitative analysis, this technique w o u l d instead f i n d use w h e n large n u m b e r s o f s o l i d samples w e r e i n need o f analysis, a n d a s i m p l e " Y E S / N O " a n s w e r relevant. F o r e x a m p l e s , i n cases where a hazardous s p i l l o c c u r r e d , a n d a large n u m b e r o f samples needed to be pre-screened for further analysis and p o s s i b l e site r e m e d i a t i o n .  7.4 Conclusion In this chapter, a t h i r d laser was added to the two-laser i o n trap s y s t e m , w i t h the hopes that it c o u l d be used for semi-quantitative analysis o f P A H isomers. C H A P T E R 6 demonstrated that it w a s p o s s i b l e to obtain spectroscopic i n f o r m a t i o n o n t w o P A H i s o m e r s d i r e c t l y i n the v o l u m e o f a n i o n trap. W h i l e this m a y be s p e c t r o s c o p i c a l l y useful, the m e t h o d , as it w a s currently performed, w a s a n a l y t i c a l l y u n p r a c t i c a l because o f the t i m e i n v o l v e d to scan the w a v e l e n g t h range o f interest. Instead, this chapter f o c u s e d o n 178  Table 7.2 Summary of observed data for the "unknown" sample.  Percent E r r o r  Calculated  Actual  Absolute  concentration  concentration  Error  (|j.mole/gram  (u.mole/gram  ((rmole/gram  charcoal)  charcoal)  charcoal)  Phenanthrene  7.433  9.773  2.34  23.94%  Anthracene  51.08  49.98  1.10  2.212%  PAH  179  j u d i c i o u s l y c h o o s i n g a single w a v e l e n g t h , a n d then carefully m e a s u r i n g the relative degree o f fragmentation for the isomers o f interest. S i n c e the fragmentation e f f i c i e n c y at a selected w a v e l e n g t h depends o n the spectral properties o f the analyte it a l l o w e d i s o m e r i c differentiation. T h e results f r o m this chapter s h o w e d that it i s p o s s i b l e to o b t a i n at least s e m i quantitative i n f o r m a t i o n c o n c e r n i n g i s o m e r i c c o m p o u n d c o n c e n t r a t i o n d i r e c t l y f r o m a s o l i d s a m p l e i n less than five minutes. T h i s a b i l i t y to r a p i d l y s a m p l e s o l i d materials m a y p r o v e useful i n a n u m b e r o f e n v i r o n m e n t a l a p p l i c a t i o n s . T h e technique, as it w a s presented here, p r o d u c e d measurement errors m u c h larger t h a n those t y p i c a l i n traditional G C / M S m e t h o d o l o g i e s . T h e s e are m o s t l i k e l y acceptable, because the g o a l o f this technique w a s to be a r a p i d c o m p l e m e n t to G C / M S for s o l i d samples rather t h a n a replacement. T h i s b e i n g s a i d , the technique c o u l d s t i l l be v a s t l y i m p r o v e d . These i m p r o v e m e n t s w o u l d m o s t l y stem f r o m e n g i n e e r i n g issues rather than scientific ones. F o r e x a m p l e the stability o f the a l l three lasers p l a y s a huge r o l l i n the s i g n a l measurement. T h e use o f a n internal standard helps m i n i m i z e this p r o b l e m , but s t i l l , m o r e stable lasers are desirable. S e c o n d l y , m u c h w o r k c o u l d be done i n the s e l e c t i o n o f the i d e a l w a v e l e n g t h a n d p o w e r o f the fragmentation laser. T h e w a v e l e n g t h u s e d i n this w o r k w a s c h o s e n because it represented the longest w a v e l e n g t h ( l o w e s t energy) that m a t c h e d a resonance t r a n s i t i o n , w i t h the i d e a that this w o u l d m i n i m i z e non-resonant fragmentation. H o w e v e r , n o attempt w a s made to determine the i d e a l n u m b e r o f laser shots o r laser p o w e r that p r o d u c e d the best result. F o r e x a m p l e , a perfectly engineered s y s t e m w o u l d c o m p l e t e l y r e m o v e one i s o m e r w h i l e l e a v i n g the other t o t a l l y u n t o u c h e d . F i n a l l y , i n a  180  dedicated c o m m e r c i a l system, it c o u l d be p o s s i b l e to insert a single d i o d e laser for a m i n i m a l cost that c o u l d be used to selectively photo- d i s t i n g u i s h a select i s o m e r pair.  181  Chapter 8 Conclusions 8.1 Generalities T h i s thesis presented the d e v e l o p m e n t o f a n a n a l y t i c a l d e v i c e capable o f p r o v i d i n g mass spectrometric data d i r e c t l y f r o m s o l i d samples. T h i s n e w instrument is based o n the c o u p l i n g o f the i o n trap mass spectrometer w i t h the m e t h o d o f t w o - l a s e r s o l i d s a m p l i n g . T h e n o v e l c o m b i n a t i o n o f these methods increases the already w i d e range o f c a p a b i l i t i e s o f the i o n trap. T h e i n i t i a l g o a l o f this w o r k was to s i m p l y b u i l d and characterize a n o v e l instrument. H o w e v e r , as it often happens i n d o c t o r a l research, d u r i n g the process o f i n s t r u m e n t a l characterization, m a n y m o r e questions arose f r o m the results than c o u l d p o s s i b l y be a n s w e r e d d u r i n g the n o r m a l tenure o f a graduate student. Therefore, c h o i c e s h a d to be m a d e c o n c e r n i n g w h i c h paths to f o l l o w and w h i c h to leave for the next group. T h e r e t w o s c h o o l s o f thought r e g a r d i n g these c h o i c e s . S o m e w o u l d argue, that one s h o u l d f o l l o w a s i n g u l a r l i n e o f q u e s t i o n i n g o n l y , and pursue this l i n e to the end. O n the other h a n d , one c o u l d instead, f o l l o w m a n y paths, and therefore g a i n a m u c h broader u n d e r s t a n d i n g o f a d i v e r s e range o f p h e n o m e n a . O v e r the last five years, I have g e n e r a l l y c h o s e n the later o p t i o n . B e c a u s e o f this c h o i c e , three separate facets o f research h a v e e v o l v e d f r o m this w o r k . These i n c l u d e : e n v i r o n m e n t a l a n a l y s i s , b i o l o g i c a l a n a l y s i s , a n d a n o v e l s p e c t r o s c o p i c t o o l . In the f o l l o w i n g sections each o f these areas w i l l be d i s c u s s e d i n terms o f results a c h i e v e d , c o n c l u s i o n s d r a w n , and potential for future w o r k .  182  8.2 Environmental Analysis T h e m o s t natural a p p l i c a t i o n o f the t w o - l a s e r i o n trap instrument is f o r d i r e c t a n a l y s i s o f s o l i d e n v i r o n m e n t a l samples. T h e m e t h o d o f t w o laser s o l i d s a m p l i n g is a useful t o o l for the e x a m i n a t i o n o f e n v i r o n m e n t a l l y relevant materials because it features a gentle a n d n o n - s e l e c t i v e d e s o r p t i o n process f o l l o w e d b y a h i g h l y selective a n d sensitive i o n i z a t i o n scheme i n laser based R E M P I . T h i s p u l s e d i o n source is a perfect c o m p l e m e n t to the i o n trap mass spectrometer because its s e l e c t i v i t y helps m i n i m i z e space charge concerns a n d i o n i z a t i o n c a n o c c u r i n the center o f the i o n trap w h e r e t r a p p i n g e f f i c i e n c y is greatest. A d d i t i o n a l l y , the i o n trap is an ideal d e v i c e to c o u p l e to this v e r y gentle i o n i z a t i o n m e t h o d because it e a s i l y a l l o w s for M S / M S to be p e r f o r m e d , thus p r o v i d i n g e x c e l l e n t structural i n f o r m a t i o n o f the i o n i z e d species. A s demonstrated i n C H A P T E R 3, the instrument d e v e l o p e d here, w a s s h o w n to be capable o f detecting a w i d e v a r i e t y o f P A H m o l e c u l e s d i r e c t l y f r o m a s o l i d s a m p l e . T h e s e test samples w e r e useful i n c h a r a c t e r i z i n g the instrument, but also h a d s o m e e n v i r o n m e n t a l a p p l i c a t i o n s . T o demonstrate the a p p l i c a b i l i t y o f this d e v i c e to real w o r l d a n a l y s i s , a test sample o f N e w Y o r k R i v e r water sediment w a s a n a l y z e d a n d the presence o f a range o f contaminants w a s c o n f i r m e d . In terms o f e n v i r o n m e n t a l m o n i t o r i n g , there are several advantages to this t w o laser i o n trap s y s t e m . F r o m an analysis p o i n t o f v i e w , perhaps the greatest advantage is speed a n d ease o f s a m p l e preparation. T h e t w o laser m e t h o d , because o f it s e l e c t i v i t y , is capable o f d i r e c t l y s a m p l i n g materials i n their r a w f o r m w i t h m i n i m u m sample preparation. A d d i t i o n a l l y , the i o n trap i s able to a c q u i r e m e a n i n g f u l data d i r e c t l y f r o m this i o n i z a t i o n technique i n seconds. F o r e x a m p l e , the r i v e r water s a m p l e a n a l y s i s w a s  183  a c c o m p l i s h e d i n less than five m i n u t e s (from s a m p l e preparation to the t i m e data w a s c o l l e c t e d ) . B y c o m p a r i s o n , t r a d i t i o n a l methods for the a n a l y s i s o f P A H s i n s o l i d materials t y p i c a l l y i n v o l v e several l a b o r i o u s steps, i n c l u d i n g sample c l e a n up, s o l v e n t e x t r a c t i o n , a n d G C / M S a n a l y s i s w h i c h take o v e r 24 hours to c o m p l e t e . O f course a f u l l G C / M S a n a l y s i s w o u l d p r o v i d e a greater w e a l t h o f sample i n f o r m a t i o n , but at the cost o f i n c r e a s e d t i m e a n d expense. Therefore, these t w o techniques s h o u l d be v i e w e d as b e i n g c o m p l e m e n t a r y . W i t h the two-laser s y s t e m used to r a p i d l y pre-screen samples for further analysis b y G C / M S . T h e t w o - l a s e r i o n trap s y s t e m is capable o f p r o v i d i n g a v e r y r a p i d i d e n t i f i c a t i o n o f a s o l i d s a m p l e , a n d as s h o w n i n C H A P T E R 3, m a y be u s e d to p r o v i d e at least s e m i quantitative i n f o r m a t i o n . A d d i t i o n a l l y , w i t h the M S / M S c a p a b i l i t i e s o f the trap, structural c o n f i r m a t i o n m a y be a c c o m p l i s h e d d i r e c t l y . E v e n i n the w o r s e cases, w h e r e M S / M S is not a p p l i c a b l e , and the semi-quantitation not v a l i d , this instrument w o u l d be v a l u a b l e as a r a p i d pre-screening d e v i c e to determine i f a s a m p l e s h o u l d be subjected to further a n a l y s i s . T h e d e v e l o p e d s y s t e m is, o f course, not w i t h o u t s o m e disadvantages. I n t e r m s o f cost a n d size, the current v e r s i o n o f the instrument is neither cheap n o r s m a l l . T h e d e v i c e , as it stands n o w , contains c o m p o n e n t s w i t h a n o r i g i n a l cost o f a p p r o x i m a t e l y t w o h u n d r e d t h o u s a n d d o l l a r s . A d d i t i o n a l l y , this apparatus alone fills a n entire r o o m a n d requires several 2 0 - a m p p o w e r l i n e s a n d t w o sources o f w a t e r - c o o l i n g . H o w e v e r , m u c h o f this cost a n d size m a y be r e d u c e d d r a m a t i c a l l y b y u t i l i z i n g current d e v e l o p m e n t s i n the field  o f e l e c t r o n i c s a n d lasers. F o r e x a m p l e , d e s o r p t i o n a n d i o n i z a t i o n c o u l d be a c h i e v e d  w i t h a s i n g l e w e l l d e s i g n e d N d : Y A G laser. I n fact current laser t e c h n o l o g y has reached a  184  p o i n t where the o p t i c a l specifications r e q u i r e d for this w o r k are a v a i l a b l e i n a n air c o o l e d laser the size o f a text b o o k w h i c h runs o f f l i n e voltage. F u r t h e r m o r e , w e l l - e n g i n e e r e d p o w e r supplies n o w a l l o w an entire i o n trap system to be e n c l o s e d i n a structure r o u g h l y the size o f t w o c o m p u t e r cases. Therefore, it is not d i f f i c u l t to i m a g i n e the p o s s i b i l i t y o f c o n s t r u c t i n g a v e h i c l e - p o r t a b l e d e v i c e capable o f r a p i d analysis i n the f i e l d . In terms o f future e n v i r o n m e n t a l applications, one important q u e s t i o n concerns the issue o f s e n s i t i v i t y . M e c h a n i s m s for i m p r o v i n g s e n s i t i v i t y c a n best be r e a l i z e d b y c o n s i d e r i n g current areas o f s i g n a l loss. T h e o b s e r v e d s i g n a l c a n be d e s c r i b e d as:  Observed signal = [Concentration of Sample'.on Surface] [Desorption Efficiency] [Ionization Efficiency] [Trapping Efficiency] [Detection Efficiency] M a n y o f these factors are due to the i n t r i n s i c properties o f the materials (desorption e f f i c i e n c y is l a r g e l y related to the identify o f the analyte and m a t r i x for e x a m p l e ) , h o w e v e r , there is one area w h i c h m a y be greatly i m p r o v e d ; the i o n i z a t i o n e f f i c i e n c y . A s s h o w n i n CHAPTER 4, after desorption, analytes continue to pass t h r o u g h the center o f the i o n trap for a p p r o x i m a t e l y 300 ps. H o w e v e r , the i o n i z i n g laser p u l s e lasts for o n l y 10 ns. A d d i t i o n a l l y , the laser b e a m intersects o n l y a v e r y s m a l l p o r t i o n o f the total d e s o r p t i o n p l u m e . Therefore, a n o b v i o u s a n d i m m e d i a t e w a y to increase the s e n s i t i v i t y o f the d e v i c e is to i m p r o v e the spatial and t e m p o r a l o v e r l a p b e t w e e n the analyte ions and the laser b e a m . There are m a n y methods that m a y be e m p l o y e d to i m p r o v e this overlap, for e x a m p l e , a set o f t w o m i r r o r s c o u l d be arranged so that the laser b e a m i s reflected m u l t i p l e t i m e s t h r o u g h the center o f the trap i n a w a y analogous to that o f c a v i t y r i n g d o w n spectroscopy.  185  Future w o r k o n this d e v i c e s h o u l d first focus o n i m p r o v i n g the s e n s i t i v i t y a n d ease o f use for e n v i r o n m e n t a l applications. O n c e this is a c c o m p l i s h e d there are a n u m b e r o f p o s s i b l e c o l l a b o r a t i o n and a p p l i c a t i o n projects that w o u l d be o f interest to the scientific c o m m u n i t y . F o r e x a m p l e , i n results not presented here, the t w o - l a s e r i o n trap has b e e n s h o w n to be effective i n detecting p o l y c h l o r i n a t e d b i p h e n y l s ( P C B s ) d i r e c t l y o n s o l i d matrices. T h i s a b i l i t y c o u l d be o f use to w o r k e r s s t u d y i n g b i o r e a c t o r - i n d u c e d degradation o f P C B s o n arctic s o i l s . C u r r e n t l y , reactor e f f i c i e n c y cannot be m e a s u r e d i n real t i m e ; instead samples must be a n a l y z e d after a lengthy sample pretreatment a n d e x t r a c t i o n procedure. H e r e , the two-laser i o n trap, w i t h its a b i l i t y to g a i n a n a l y t i c a l i n f o r m a t i o n d i r e c t l y f r o m solids o n a t i m e scale o f minutes c o u l d p r o v e useful.  8.3 Biological Analysis T h e second a p p l i c a t i o n , w h i c h naturally g r e w out o f the d e v e l o p m e n t o f the t w o laser i o n trap, is that o f detection o f b i o l o g i c a l l y relevant c o m p o u n d s . H e r e , the s e l e c t i v i t y afforded b y the two-laser process p r o v e d v a l u a b l e i n s i m p l i f y i n g the o b s e r v e d spectra that resulted f r o m direct analysis o f c o m p l e x matrices. I n this w a y c o m p l e x b i o l o g i c a l matrices s u c h as tissues c o u l d . b e assayed d i r e c t l y . A d d i t i o n a l l y , the i m p l e m e n t a t i o n o f the i o n trap as the mass spectfometric detector a l l o w e d for c o n f i r m a t i o n o f analyte c o m p o s i t i o n b y M S / M S . T h i s s y s t e m has m a n y advantages o v e r traditional means o f s p a t i a l l y r e s o l v e d p h a r m a c e u t i c a l a n a l y s i s . C o m p a r e d to other direct analysis techniques ( S I M S , M A L D I ) the t w o laser i o n source is v e r y r a p i d , selective, a n d sensitive to o r g a n i c c o m p o u n d s . A d d i t i o n a l l y , because o f the p u l s e d nature o f the d e v i c e , it is p o s s i b l e to preconcentrate  186  i o n s d i r e c t l y i n the gas phase to i m p r o v e s e n s i t i v i t y . F i n a l l y , because a n a l y s i s is p e r f o r m e d i n the i o n trap, the a b i l i t y to acquire M S data is a l w a y s a v a i l a b l e . N  T h e s y s t e m as it is currently i m p l e m e n t e d does have some l i m i t a t i o n s . T h e s a m p l e d e l i v e r y m e c h a n i s m l i m i t s sample sizes to those less than 2 m m square. A l s o , i n order for efficient i o n i z a t i o n to o c c u r the analyte o f interest must have a strong a b s o r p t i o n at the laser w a v e l e n g t h . W i t h the fixed w a v e l e n g t h s y s t e m c u r r e n t l y a v a i l a b l e , this l i m i t s a n a l y s i s to those species w h i c h are either h i g h l y conjugated or c o n t a i n a r o m a t i c r i n g s (and absorb at 2 6 6 n m ) . F u t u r e w o r k i n this area s h o u l d first focus o n i m p r o v i n g the d i v e r s i t y o f a p p l i c a b l e samples and analytes. T h i s m a y be a c h i e v e d b y p e r f o r m i n g the laser desorption/laser i o n i z a t i o n steps outside o f the trap v o l u m e . B y l o c a t i n g the s a m p l e p r o b e outside the v o l u m e o f the i o n trap the sample size m a y be increased d r a m a t i c a l l y . I m p l e m e n t i n g this change w o u l d require the a d d i t i o n o f a stage o f i o n o p t i c s to transport the i o n s into the trap. I n a d d i t i o n to m o v i n g the s a m p l i n g l o c a t i o n , the a d d i t i o n o f a tunable solid-state laser w o u l d greatly increase the v e r s a t i l i t y o f the d e v i c e . I n this w a y , the i o n i z i n g laser c o u l d be tuned to an absorption o f the analyte o f interest a n d thus increase the s e n s i t i v i t y a n d s e l e c t i v i t y . W i t h the above changes i n place, a n u m b e r o f s a m p l e types a n d e x p e r i m e n t a l p r o t o c o l s b e c o m e p o s s i b l e . F o r e x a m p l e , a d e v i c e o f this type m a y p r o v e useful i n t o x i c o l o g i c a l or p h a r m a c e u t i c a l studies. T h e . s p a t i a l r e s o l u t i o n p r o v i d e d b y the laser s a m p l i n g c o u l d also be useful i n p r o b i n g b i n d i n g affinities for a variety o f b i o l o g i c a l l y active agents d i r e c t l y f r o m relevant, matrices. F i n a l l y , this type o f set-up m a y e v e n h a v e  187  use i n the a n a l y s i s o f w h o l e c e l l s i n cases o f f o o d p o i s o n i n g or e v e n b i o - t e r r o r i s m w h e r e a r a p i d a n a l y s i s is c r i t i c a l .  8.4 Spectroscopy of Trapped Ions T h e f i n a l course o f research that was pursued d u r i n g this d o c t o r a l w o r k i n v o l v e d the c r e a t i o n o f a u n i q u e t o o l for o p t i c a l spectroscopy. T h i s w a s a c h i e v e d b y the a d d i t i o n o f a t h i r d , tunable, laser to the p r e v i o u s l y established t w o - l a s e r i o n trap s y s t e m . T h e a d d i t i o n o f this t h i r d laser a l l o w e d the c o l l e c t i o n o f spectroscopic data o f m a s s selected gas phase i o n s .  C H A P T E R 6 demonstrated, for the first t i m e , v i s i b l e spectral features for the P A H i s o m e r cations phenanthrene and anthracene d i r e c t l y i n the gas phase b y the m e t h o d o f resonance e n h a n c e d m u l t i p h o t o n d i s s o c i a t i o n ( R E M P D ) . T h e c o l l e c t e d spectra w e r e o b s e r v e d to be v e r y s i m i l a r to those recorded i n frozen n o b l e gas matrices. T h i s n o v e l c o m b i n a t i o n o f techniques m a y have s o m e advantages as a s p e c t r o s c o p i c t o o l for large gas phase ions. F o r e x a m p l e , because the analyte ions are created b y t w o - l a s e r s o l i d s a m p l i n g , a w i d e variety o f sample m o l e c u l a r w e i g h t s are p o s s i b l e . A d d i t i o n a l l y , because the i o n s are created and stored i n a n i o n trap m a s s spectrometer, it is p o s s i b l e to mass select ions o f interest for spectroscopic e v a l u a t i o n . F u r t h e r m o r e , the fragment i o n that result f r o m photofragmentation m a y also be trapped for further a n a l y s i s , thus i n c r e a s i n g the magnitude o f potential uses i n the f i e l d o f physical chemistry. T h e d e v i c e as it currently stands is not w i t h o u t some l i m i t a t i o n s . F o r e x a m p l e , the c o l l e c t i o n o f data i n this w o r k w a s v e r y l a b o r i o u s . T h e experiments presented i n  C H A P T E R 6 were p e r f o r m e d w i t h a c h e m i c a l d y e laser, where a different d y e h a d to be 188  u s e d r o u g h l y every 20 n m . Further m o r e , the large n u m b e r o f data files h a d to be c o l l e c t e d a n d processed i n d i v i d u a l l y b y hand. Therefore, future w o r k o n this d e v i c e s h o u l d focus a r o u n d the m o d e r n i z a t i o n and a u t o m a t i o n o f the instrument. F o r e x a m p l e , the a d d i t i o n o f a tunable solid-state laser, such as an o p t i c a l parametric o s c i l l a t o r , w o u l d greatly i m p r o v e the ease o f use and speed o f the d e v i c e . W i t h this type o f laser it w o u l d be p o s s i b l e to f u l l y automate data c o l l e c t i o n because w a v e l e n g t h s e l e c t i o n c a n be computer controlled. F i n a l l y , future w o r k s h o u l d focus o n the analysis o f a s t r o p h y s i c a l l y m e a n i n g f u l samples that are d i f f i c u l t to analyze i n standard w a y s . F o r e x a m p l e it has been suggested that one o f the p o s s i b l e carriers o f the D I B s are r a d i c a l fragments o f larger P A H s . T h i s instrument is i d e a l l y suited for s u c h analysis because analyte species c a n be e x a m i n e d first o n a m a s s basis a n d then o p t i c a l l y . I n a d d i t i o n to u n i q u e fragments, this t o o l w o u l d also be useful for the e x a m i n a t i o n o f larger i o n i c species that are not e a s i l y v a p o r i z e d into the gas phase. F o r e x a m p l e , the large aromatic c a r b o n structure B u c k m i n s t e r f u l l e r e n e , Cgo, w a s e x a m i n e d b y two-laser mass spectrometry w i t h a n eye for future e x a m i n a t i o n b y the o p t i c a l R E M P D m e t h o d . T h e resulting two-laser spectrum for C 6 o is s h o w n b e l o w i n +  F i g u r e 8.1. C l e a r l y , the parent c a t i o n or any o f its daughter i o n s are a v a i l a b l e for future a n a l y s i s b y o p t i c a l means. T h i s large m o l e c u l e is t y p i c a l o f those that are not e a s i l y a n a l y z e d b y other gas phase methods, because o f its l o w v a p o r pressure. In a d d i t i o n to the p h y s i c a l c h e m i s t r y a p p l i c a t i o n s o f this d e v i c e , another l i n e o f research c o u l d also focus o n the a n a l y t i c a l capabilities o f the instrument. R e s u l t s f r o m  CHAPTER 7 suggest  that it m a y be p o s s i b l e to use the gas phase o p t i c a l properties o f  different i s o m e r s to d i s t i n g u i s h samples where the isomers have s i m i l a r M S / M S C I D  189  Figure 8.1 Two laser mass spectrum of Buckminsterfullerene.  'c ro u  ^0 -t—'  c  (D 1_  3 O  400  500  600 mass/charge  700  800  spectra. 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