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Development and characterization of furnace atomization plasma excitation spectrometry

University of British Columbia

Furnace Atomization Plasma Excitation Spectrometry (FAPES) is a new emission spectrochemical method which employs a graphite furnace for analyte atomization and an atmospheric pressure plasma sustained inside the furnace for analyte excitation. The primary objective of this work was to characterize the fundamental processes that are occurring within the plasma during the analyte atomization, vaporization, and excitation. Background spectra are dominated by emission features from CO⁺, N₂⁺, OH, NH, and N₂. The plasma background emission is most intense near the radio frequency (RF) electrode and less intense near the graphite furnace wall. The Fe and Pb-excitation temperatures are in the range of 3000 - 5000 K at RF powers between 10 and 100 W. The Fe-excitation temperature also exhibits a spatial dependence. The emission features of CO⁺ and N₂ indicate that this plasma source is capable of exciting energy levels as high as 20 eV. Both atomic absorption and emission experiments show a non-uniform temperature distribution along the length of the RF electrode. The temperature lag of the RF electrode relative to the furnace wall causes condensation of analytes on the RF electrode and subsequent second-surface vaporization resulting in two peaks in the temporal response of the emission signal. Analyte condensation on the RF electrode is severe at low RF powers and can be observed when high amounts of analyte are deposited. Similar temporal responses are observed for simultaneously measured atomic absorption and emission signals for Ag and Pb. The time-resolved Pb-excitation temperature also suggests that the temporal emission profiles of these analytes in FAPES are determined by atomization and vaporization characteristics of the analyte rather than by excitation characteristics. Results obtained for Pb also show an early shift in the appearance and peak temperatures in FAPES compared with those in Graphite Furnace Atomic Absorption Spectrometry (GFAAS), probably because of a shift in the dissociation equilibria of Pb species in the gas-phase. Experimental results show the presence of high levels of CO in the FAPES source due to the enhanced oxidation of graphite on the RF electrode and on the graphite furnace wall in the presence of the plasma. At RF powers higher than 50 W, the Pb emission intensity decreases. The highest signal-to-noise and signal-to-background ratios are observed at relatively low RF powers (about 20 W). For both Pb and Ag, the major cause of interference effects from NaCl is the formation of volatile molecular species which are lost prior to atomization, and consequently, leads to a decrease in the atomic emission intensity in FAPES. The interference effect for both Pb and Ag in a NaNO₃ matrix is complex and exhibits both condensed and gas-phase effects. The work presented in this thesis demonstrates that the FAPES source has the potential to be a potent excitation source for atomic emission spectrometry. This work also identifies the limitations of FAPES and suggests further improvements to the instrumentation.

Thesis/Dissertation

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2008-07-31

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http://hdl.handle.net/2429/1230

Chemistry

University of British Columbia

UBCV

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