785 nm femtosecond laser ablation for improved precision and reduction of interferences in Sr isotope analyses using MC-ICP-MS

Abstract

The femtosecond (fs) laser ablation (LA) MC-ICP-MS Sr isotope analysis of calcium-rich materials is reported. Independent Sr isotope ratio determination using solution nebulization (SN) MC-ICP-MS after chromatographic separation of Sr was also conducted. Material studied includes a marine carbonate saggital otolith certified (for trace metals) reference material FEBS-1, and igneous fluorite. Analyses of the FEBS-1 using LA analyses at repetition rates of 5 to 200 Hz with energies from 0.1 to 0.6 mJ pulse⁻¹ and SN analyses demonstrate that fs LA Sr isotope analysis produced similar within-run precision to the SN Sr isotope analysis when the same total number of ions was collected in an analysis. The results demonstrate that the precision for fs LA Sr isotope analyses is more controlled by Poisson counting statistics than for nanosecond (ns) LA analyses, and improvements (≥ 2 fold) in external precision, within-run precision, and accuracy can be obtained in comparison with reported ns LA-based results. Good external precision (1SD, n = 17) ± 0.000007, good within-run precision (1SE) ± 0.000010, as well as good accuracy (0.000006) determinations of Sr isotope ratios in carbonate materials can be obtained in situ with a spatial resolution of 60 to 90 μm when using a Faraday detector signal intensity at mass 88 of greater than 10 V and a total integration time of 151 s. In this contribution, it is further shown that an accurate ⁸⁴Sr/⁸⁶Sr ratio of 0.056560 ± 0.000015 to 0.056525 ± 0.000007 can be measured in fs LA Sr isotope analysis of FEBS-1. Previously reported Ca argide and Ca dimer polyatomic ion interferences were not observed to be significant in this study. In addition, this study reports the criteria necessary for effective interference corrections from doubly-charged ions of Er and Yb in LA Sr isotope analysis of fluorite samples, which have Sr concentrations > 650 ppm, and a rare-earth element (REE) Er/Sr or Yb/Sr concentration ratio of less than 0.01.