Dissociation of polyatomic ions in the inductively coupled plasma

A general method for identifying the origin of a particular polyatomic ion is described. Based on a postulated dissociation reaction, measured ion signal ratios (e.g. Ar-2(+)/Ar+) are combined with mass bias corrections and estimates of the density of the neutral product (usually Ar, O or H atoms) to determine a gas kinetic temperature T-gas. The temperature can also be measured by the reduction in pressure when the ICP is sampled (compared to room temperature argon), or by other means. Dissociation energies and spectroscopic constants for the ions are necessary. For the particular instrument used, some of the findings of this study are: (a) ArO+ and ArN+ can be either dissociated (if the plasma potential is high) or created (if the plasma potential is low) by collisions between the sampler and skimmer; (b) the strongly-bound oxide ions O-2(+) and MO+ for the rare earths are observed at levels consistent with T-gas similar to 5300 K in a 'hot' plasma, but ClO+ is formed in excess; and (c) the abundances of most other polyatomic ions like H2O+ and ArH+ correspond to higher densities than would be expected in the ICP itself. (C) 2001 Elsevier Science B.V. All rights reserved.