Effects of cone combinations on the signal enhancement by nitrogen in LA-ICP-MS

Abstract

The effects of assorted cone combinations on the signal enhancement of nitrogen in LA-ICP-MS are presented. In Ar plasma, the standard sample cone + X skimmer cone (S + X cone) increases the sensitivity for most of the investigated elements by a factor of 1.5–9.7 compared to the standard sample cone + H skimmer cone (S + H cone). The effect of the addition of nitrogen on the sensitivities of elements obtained by LA-ICP-MS was minimal for the S + H cone but significant for the S + X cone. The degree of the signal enhancement by the addition of nitrogen is positively correlated with the oxide bond dissociation energy of the element. Compared to Ar plasma, the oxide ratio in Ar–N₂ mixed plasma (N₂ = 4 ml min⁻¹) is reduced from 31.5% to 1.8% in the S + H cone and from 88.7% to 12.6% in the S + X cone. The hydride yield is also reduced by a factor of 10 and 46 in the S + H cone and S + X cone by the addition of nitrogen (4 ml min⁻¹), respectively. Our results indicate that the geometry of the skimmer cone is an important factor for signal enhancement and the reduction of oxide and hydride yields by the addition of nitrogen. In addition, the increased Zr/Rb ratio by the addition of nitrogen indicated the reduction of ICP-related elemental fractionation. We successfully apply the S + X cone with Ar–N₂ mixed plasma to the analysis of low content REEs in four geological reference materials at a 16 μm single-hole ablation mode by LA-ICP-MS.