Matrix dependency for oxide production rates by LA-ICP-MS

Abstract

The production rates of polyatomic oxygen interferents (MO⁺/M⁺) during LA-ICP-MS analysis were investigated in a range of silicate materials and metals. The total amount of oxygen in the ICP is significantly lower for laser ablation analysis compared to solution nebulisation analysis resulting in lower oxide production rates. However, these interferents can still be significant for some elements. The contribution of oxygen from the material being ablated was found to influence the oxide production rate (OPR). When using a well degassed system to minimise the entrainment of atmospheric oxygen, the OPR for Al, Si and W was up to 4 times lower when ablating the elements as a metal compared to when ablating oxygen-bearing minerals. There is a relationship between the MO⁺/M⁺ production rate and the cation–oxygen dissociation energy for elements measured by solution and by laser ablation ICP-MS. However, for Hf and Th the OPR varied significantly depending on the mineral being ablated under the same analytical conditions (0.007–0.02% for Hf and 0.09–0.2% for Th), whereas UO⁺/U⁺ was more consistent (0.058–0.063%). The effects of carrier gas flow rate and resulting differences in aerosol breakdown and ionisation in the ICP were investigated for U and Th oxides in NIST610, NIST612, zircon (ZrSiO₄), monazite ([REE,Th]PO₄) and uraninite (UO₂). Increasing the Ar flow rate had a larger effect on the Th OPR (0.05 to 0.5%) compared to U (0.04 to 0.09%) when ablating the NIST610 glass. The relative differences in the OPR between minerals compared to NIST610 were small for U, with all minerals having the same OPR except for uraninite at high carrier gas flow rates (35% higher). In contrast the OPR for Th was highly variable between all minerals and showed differing responses to changes in the Ar flow. This study highlights the complexities in oxide production for LA-ICP-MS compared to solution analyses, and that the OPR for some elements is strongly dependent on the material being ablated. Also, that there can be a significant contribution to the MO⁺ production from ionisation of an incompletely atomised sample aerosol in the plasma.