The effects of cell-gas impurities and kinetic energy discrimination in an octopole collision cell ICP-MS under non-thermalized conditions

Abstract

The fundamental mechanisms for reducing isobaric interference in octopole collision cell ICP-MS were investigated in the light of the effects of impurities and kinetic energy discrimination, along with the consideration of the ion–molecule reactions involved. The cell was operated under non-thermalized conditions, where kinetic energies of the analyte ions were maintained above thermal energies after collisions. Although water cluster ions H(H₂O)_n⁺ (n = 2, 3) were formed in the cell when low quality hydrogen was used, the effect of cell gas impurities such as water vapor was negligible in the reduction of Ar⁺, ArO⁺ and Ar₂⁺ realized by the hydrogen reaction cell. The in-cell product ions were attenuated efficiently by energy discrimination, which was also found to reduce interfering polyatomic ions that do not react with the cell gas or with any other species in the cell. The mechanism for this reduction was explained by the experimantal results that showed lower kinetic energies of polyatomic ions than monatomic ions, after a number of collisions with the cell gas. In the helium collision cell, dissociation of ArO⁺ was observed but not ArH⁺ and Ar₂⁺. This observation is consistent with the magnitude of bond-dissociation energy versus collision energy for these argide ions.