Origins of polyatomic ions in laser ablation-inductively coupled plasma-mass spectrometry: An examination of metal oxide ions and effects of nitrogen and helium in the aerosol gas flow

Differences in the origins of polyatomic ions in wet plasma conditions, as exist in solution inductively coupled plasma-mass spectrometry (ICP-MS), versus dry plasma conditions, found in laser ablation (LA)-ICP-MS, are investigated. Silicate and metal samples are ablated and gas kinetic temperature (T-gas) is measured to establish the origins of metal oxide (MO+) ions. MO+ ion abundances observed from both the ablation of silicate samples and metallic samples are found to correspond to formation of the polyatomic ion in the ICP. The same strategy is applied to oxide ratios measured when additional gases (N-2 and He) are introduced into the aerosol gas flow. N-2 is found to decrease the MO+/M+ signal ratio by maximizing atomic sensitivity at lower total gas loads. The addition of He reduces polyatomic ions throughout the plasma. Use of both N-2 and He produces the lowest MO+/M+ signal ratio and highest T-gas values at the position sampled in the ICP. By gaining a better understanding of the origin of polyatomic ions in LA-ICP-MS and their behavior in mixed gas plasmas, perhaps strategies can be developed to minimize polyatomic interferences in the mass spectrum.