Production of barium and light rare earth element oxides during LA-ICP-MS microanalysis

Abstract

Production rates of molecular oxides (M¹⁶O⁺/M⁺) of Ba and light rare earth elements (LREE: La, Ce, Pr and Nd), have been measured by Laser Ablation ICP-MS (LA-ICP-MS) analysis via ablation of a Ba and LREE-doped synthetic silicate glass. Our work confirms that oxide production is related to the strength of the M–O bond in the MO⁺ ion, and in agreement with solution-based measurements and theoretical considerations, M–O bond strength correlates linearly with log(MO⁺/M⁺). Oxide production is also strongly dependent on plasma conditions and increases markedly at higher nebulizer gas flow rates (and corresponding lower plasma temperatures), although relative differences between oxide production rates for Ba and LREE do not remain constant. Oxide production is also influenced by availability of oxygen within the plasma. Immediately after opening the ablation chamber to the atmosphere, oxide production rates can be 50–100% higher than those measured after the ablation chamber has been purged of atmospheric oxygen. Differences in plasma load related to differences in ablation rates appear to have little influence on oxide production. Calculations based on our data confirm that the widespread approach of tuning plasma conditions so that measured ThO⁺/Th⁺ ratios are less than a few percent is largely suitable for reducing isobaric interferences from Ba and LREE oxides on middle light rare earth element isotopes to levels suitable for trace element analysis for most geological materials. However contributions to Gd from LREE oxides may exceed analytical uncertainties in moderately to highly LREE-enriched materials ([La/Gd]_N > ∼3). Contributions from BaO to Eu are only significant for materials with Ba/Eu ratios ≫1000.