Dissociation of analyte oxide ions in inductively coupled plasma mass spectrometry

Analyte oxide ions have been studied in inductively coupled plasma mass spectrometry as a function of the sampling position, the carrier gas flow rate and the efficiency of the energy transfer from the plasma to the sample, in order to support the hypothesis of analyte oxide formation in the plasma. Lanthanum was selected as the test species owing to its high oxide bond strength, and its behaviour was compared to that of Pb as this element has a low oxide bond strength. A prototype ICP mass spectrometer was used. The LaO+: La+ ratio was found to be in the 0.2–13 000% range under the operating conditions used. Energy transfer was either degraded by adding a sheathing gas or improved by adding hydrogen. The LaO+: La+ ratio was minimized for low carrier gas flow rates and an efficient energy transfer. This was corroborated by temperature optical measurements. The results are in good agreement with those found in other recent work. In any instance, the role of the sampling position was found to be crucial in studying analyte oxide ion behaviour and optimizing analytical performance.