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Studies of analyte ionization in a furnace atomization plasma excitation spectrometry source Lu, Shengyong

Abstract

Furnace Atomization Plasma Excitation Spectrometry (FAPES) is a relatively new atomic emission spectrochemical method which employs a conventional graphite furnace for analyte atomization and an atmospheric-pressure helium plasma sustained inside the furnace for analyte excitation. The generation of the plasma is achieved by applying radio frequency (rf) power to an electrode located inside, and coaxial with, the graphite furnace. The primary objective of this thesis was to characterize the fundamental properties of the plasma, study analyte excitation and ionization processes, and seek ways to improve analyte ionization efficiency. Background emission characteristics have been observed in a new FAPES source, and temporally resolved emission profiles of background species (He, N₂⁺, OH and CO⁺) have been measured. The ionization mechanisms of major background species are also discussed. Plasma temperatures have been measured in order to characterize the helium plasma. During an atomization cycle, rotational temperatures for N₂⁺ and OH at 40 W have been found to be 1300 K and 1400 K, respectively, and the excitation temperature for He at 40 W is about 3600 K. Plasma temperatures can be substantially affected by plasma operating conditions. Thus, the effects of conditions such as rf power, gas flow rate, atomization temperature and the dimensions of the center electrode on plasma temperatures and analyte ionization were studied. The temporal atomic and ionic emission behaviors of Cr, Mg, Cd, Fe and Zn have been measured, and analyte atomization mechanisms have been proposed based on the measurements. The effects of operating conditions on analyte ionization have been studied, and an appropriate atomization temperature was found for optimum analyte ionization. An optimum gas flow rate can also maximize analyte ionization. Compared with a "continuousflow" mode, a "stop-flow" mode can improve analytical sensitivity. Increasing rf power was found to be the best way to achieve a high degree of ionization. A variety of center electrodes with different physical dimensions were used to modify the FAPES source in an effort to improve the ionization capability. However, the larger electrode size acted to reduce the voltage drop across the plasma sheath compromising the analytical performance. The effects of varying the counter ion (MgCl₂, MgO, MgNO₃ and MgSO₄ ) on Mg atomization, excitation, and ionization were studied, and it was found that these compounds exhibit different atomization mechanisms. It was also observed that Mg ionization in the FAPES source could be improved by the addition of a minute amount of Pd modifier. Figures of merit for magnesium demonstrate improved analytical performance for this new FAPES source. An ionization temperature of about 7000 K at 80 W rf power level was measured. The calculation of electron number densities in the FAPES source shows that in addition to helium ionization, secondary electron emission and thermal or thermionic electron emission from the graphite walls and center electrode inside the source also contribute to the total electron number density.

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