Standardless determination of Nd and Sr isotope ratios in geological samples using LA-MC-ICP-MS with a low-oxide molecular yield interface setup

We investigated an appropriate instrumental setup for a laser-ablation multiple-collector inductively coupled plasma mass spectrometer (LA-MC-ICP-MS) and found that a reduced oxide setting allowed accurate and precise analyses of Sr and Nd isotope ratios in geological samples with concomitant interfering elements (Kr and Rb on Sr and Sm on Nd). We used an Aridus II solution-excimer laser dual-intake system. The ICP interface used normal sample and skimmer cones with torch shield switched-OFF and an additional large interface rotary pump. The setting accomplished reduced oxide levels NdO+/Nd+ <0.01%, without significant sacrifice of the instrumental sensitivity (c.a. 70%). Oxide molecular ions for the lighter elements were negligible and accurate internal mass bias corrections were achieved for Sr, Sm, and Nd using isotopic ratios derived from thermal ionization mass spectrometry measurements. However, elemental fractionation between Rb and Sr and Nd and Sm still exists due perhaps to elemental fractionation in the ICP preventing standardless determination of parent-daughter ratios. For Sr isotope measurement, a new analytical protocol was developed for correcting Kr baseline-induced biases. Residual analytical biases of 84Sr/86Sr and 87Sr/86Sr were observed after applying on-peak background subtractions and mass-fractionation corrections using internal normalization. The residual biases occurred only for samples analysed with LA and not for solution analyses using Aridus II with the same instrumental setup. We concluded that this was due to suppression and enhancement of the Kr baseline by loading of the LA sample aerosols and by the introduction of Kr from the samples, respectively. We found that both the 84Sr/86Sr and 87Sr/86Sr isotope ratios were affected proportionally by the baseline biases of the LA analyses of an isotopically homogeneous anorthite plagioclase, and similar result were seen in theoretical calculations. A theoretical bias correction for the 87Sr/86Sr ratios was, therefore, available using the shift in the simultaneously measured 84Sr/86Sr from the natural ratio. The novel interface setup improved reproducibility of Nd isotope ratios by a factor of four and the new baseline correction technique along with the interface setup improved reproducibility of Sr isotope ratios by a factor of four providing a basis for accurate Sr and Nd isotope analyses using LA-MC-ICP-MS. REFERENCES Sr isotope: Kimura J.-I. et al. (2013a) JAAS. 28, 945-957, DOI:10.1039/C3JA30329B. Nd isotope: Kimura J.-I. et al. (2013b) JAAS. DOI:10.1039/C3JA50109D. Element fractionation in ICP: Kimura J.-I. et al. (2012) JAAS 27, 1549-1559, DOI: 10.1039/c2ja10344c.