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Effect of Oxide Formation on the Accuracy of Nd Isotopic Ratios Measurements in MC-ICP-MS

by E. FRÈRES, D. WEIS, K. NEWMAN, M. AMINI AND K. GORDON

Isotopic ratios can be measured with ppm levels of precision using MC-ICP-MS. The measurement of accurate isotope ratios depends on the adequate correction of the instrumental mass bias. The origin of this bias is element dependent and not fully understood. Previous studies [1,2] document that an increased oxide formation rate for Nd can yield a non-linear mass dependent component to mass bias that cannot be corrected using standard mass bias models and can compromise the accuracy of the corrected ratios.

We investigated the effects of changing the instrument operating conditions, sample introduction system, and sampler/ skimmer cones’ geometry on the formation of NdO+ species. The most relevant parameters to oxide formation are torch position, nebulizer pressure, sample gas flow, and skimmer geometry. Varying these parameters allowed us to map how Nd isotopic ratios are affected by oxide formation. The general trend for 142Nd, 143Nd and 145Nd /144Nd ratios is that increasing oxides cause the ratios to decrease, while 148Nd/144Nd ratios increase (e.g. Table 1). The rate of oxide formation for Nd was found to be isotope dependent.

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