Measurement of strontium isotope ratios by MC-ICP-MS after on-line Rb–Sr ion chromatography separation

Abstract

Large volume injection ion chromatography has been evaluated for the precise and accurate measurement of strontium isotope ratios by multicollector ICP-MS. On-line coupling of the ion chromatograph to the MC-ICP-MS instrument allowed the complete separation of isobaric rubidium and other matrix elements (Ca, Mg, Na and K) and the measurement of strontium isotope ratios on flat-topped chromatographic peaks of ca. 3 minutes duration. No drift in the strontium isotope ratios was observed during strontium elution and mass bias could be corrected using either the ⁸⁶Sr/⁸⁸Sr isotope ratio (internal correction) or the observed ⁸⁷Rb/⁸⁵Rb isotope ratio (external correction) in the real samples. Typical internal precisions obtained with the selected measurement and correction conditions were about 2 times lower than the precisions obtained with continuous sample introduction for a solution 50 ng g^–1 of Sr NIST SRM 987 in HNO₃ 2% (typical standard deviations of 0.00022–0.00036 for the corrected ⁸⁷Sr/⁸⁶Sr ratio in comparison to 0.00014–0.00020 for continuous sample introduction). The method was applied to the measurement of strontium isotope ratios in cider samples and the results agreed with previous data obtained after an off-line Rb–Sr separation procedure using the Sr-Spec chelating material. The precision obtained for the real samples was poorer than previous data due to the low concentration of strontium in the measured samples. The method was also applied to other sample types (soil leachates, apple leaves and apples) to study the transference of strontium isotope ratios in the soil–tree–cider system.