Comparison of mass spectrometric and atomic emission spectrometric measurements on the plasma extracted from an analytical inductively coupled plasma

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2000
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Zhai, Yan
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Houk, R. S.
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Abstract
There are several processes taking place before analytes can be detected by inductively coupled plasma-mass spectrometer: ionization, sampling, skimming, focusing, separating and detection. There always is a question of whether the ion extraction process changes the atomic ion composition in the plasma. Studies on the gas kinetics in this process indicates that atomic ion ratios before ion extraction equal that after ion extraction. This work is to answer this question experimentally. To relate mass spectrometric signals to the ion density in the plasma, electron number density (n[Subscript e]), ionization temperature (T[Subscript ion]) and gas temperature (T[Subscript g]) were measured in a 1.3kW inductively coupled plasma (ICP). T[Subscript ion], was measured with both mass spectrometry (MS) and emission spectrometry (AES). The emission measurements were taken while the plasma was sampled by the mass spectrometer. To avoid the variation of the plasma from day to day, these three measurements were made in the same run of the plasma. The situation when 0-2000ppm NaNO3 was present was also examined. When no Na matrix was present, both MS and AES measurements yielded the same T[Subscript ion] but gave quite different T[Subscript ion] values with matrix present. The results indicated that mass spectrometer draws a representative sample from the plasma for a low ionic strength but not for a high ionic strength, at least for this particular instrument.
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