Mass spectrometric and theoretical investigations into the formation of argon molecular ions in plasma mass spectrometry

Abstract

The abundance and distribution of argon molecular ions (e.g., ArH⁺, ArO⁺, ArN⁺, Ar₂⁺ and MAr⁺; M = metal) in plasma MS (ICP-MS, LA-ICP-MS and rf-GDMS) were investigated and compared. In ICP-MS the non-metal argon molecular ions were formed with higher intensity compared with the metal argide ions. This could be explained by theoretical calculations of the binding energies. The ArH⁺ ion can be viewed as an isoelectronic system, comparable to HCl. The intensities of diatomic metal argide ions relative to metal ions in ICP-MS are less than 10^–4. A correlation between the intensities of metal argide ions with the bond dissociation energies of diatomic ions was found. The highest intensity of metal argide ions, of the order of per cent. values, were observed in rf-GDMS. The intensity of the argon molecular ions in an rf-GD varied by up to three orders of magnitude as a function of the plasma parameters (e.g., argon pressure in the GD ion source). The characteristic distribution of diatomic argide ions of REEs in ICP-MS was found to be comparable to the distribution of rare earth oxide ions.