Spatial profiling of ion distributions in a nitrogen–argon plasma in inductively coupled plasma mass spectrometry

Abstract

Spatial profiling was used to investigate the effect of nitrogen on non-spectroscopic interferences (matrix effects) in inductively coupled plasma mass spectrometry. Nitrogen was introduced as 5.9% of the outer gas flow. Sixteen elements and several analyte oxides and doubly charged ions were monitored in the absence and presence of 0.01 M Na and 0.01 M K matrices. While matrix-induced enhancement was seen for most analytes in the Ar plasma, the effect was greatly reduced in the mixed N₂–Ar plasma, albeit with a sacrifice in sensitivity (which is a small price to pay for the freedom from non-spectroscopic interferences). Analyte oxides were also reduced by over an order of magnitude. A comparison of the analyte profiles with those of background ions suggests that electron-impact ionization is the predominant ionization mechanism in the Ar plasma whereas, in the mixed-gas plasma, charge-transfer with Ar⁺ was suggested by the close match between the Ar₂⁺ profiles and those of the analytes. The identical radial profiles of background polyatomic ions also suggest that Ar-containing ions, including the Ar dimer, all have Ar⁺ as the precursor ion in the Ar plasma. In contrast, the different profiles observed for these ions compared with Ar₂⁺ in the mixed-gas plasma suggest that these ions (but obviously not Ar₂⁺) more likely originate from the combination of neutral Ar with an ion.