Investigation into the behavior of metal-argon polyatomic ions (MAr+) in the extraction region of inductively coupled plasma-mass spectrometry

The abundances of metal-argon polyatomic ions (MAr+) are determined in inductively coupled plasma-mass spectrometry (ICP-MS). The ratios of MAr+ abundance to that for M+ ions are measured experimentally. These ratios are compared to expected values, calculated for typical plasma conditions using spectroscopic data. For all metals studied (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), the measured ratios are significantly lower than the calculated ratios. Increasing the plasma potential (and thereby increasing the ion kinetic energy) by means of a homemade guard electrode with a wide gap further reduces the MAr+/M+ ratio. Implementing a skimmer cone designed for high transmission of light ions increases the MAr+ abundance. Considering this evidence, the scarcity of MAr+ ions is attributed to collision induced dissociation (CID), likely due to a shock wave at the tip of or in the throat of the skimmer cone.

► MAr+ ions are less abundant in the mass spectrum than expected from the ICP.
► Increasing the plasma potential reduces their abundance further.
► The extraction lens voltage does not greatly affect the MAr+ abundances.
► The weakly-bound MAr+ ions are probably dissociated by collisions during extraction.