Determination of the isotopic composition of sub-ng amounts of Sr in Antarctic snow by multi-collector ICP-mass spectrometry

Abstract

Determination of the ⁸⁷Sr/⁸⁶Sr isotope ratio in Antarctic snow can provide useful information on the sources of the atmospheric particulates in present and ancient times. However, precise and accurate determination of the ⁸⁷Sr/⁸⁶Sr ratio is challenging because of the low analyte concentration and the limited amount of sample typically available. In this work, Sr was first pre-concentrated 30-fold via evaporation, after which Sr was chromatographically isolated from the matrix (and thus separated from Rb, avoiding isobaric overlap of ⁸⁷Sr and ⁸⁷Rb). Subsequently, the sensitivity of the multi-collector ICP-mass spectrometer was maximized by using an Aridus II membrane desolvating system, a high-transmission interface and 10¹² Ω amplifiers instead of the standard 10¹¹ Ω ones. With this instrumental setup, the repeatability for ⁸⁷Sr/⁸⁶Sr ratio measurements was 0.3 and 0.5‰ RSD at 0.8 ng mL⁻¹ (or for 320 pg in the 0.4 mL of sample solution consumed) and at 0.4 ng mL⁻¹ (or for 160 pg) of Sr, respectively. The procedural blank was 2.5 ± 0.5 pg and did not significantly affect the ⁸⁷Sr/⁸⁶Sr ratio measured using 160 pg of analyte (at 0.4 ng mL⁻¹). The accuracy of the procedure was evaluated via replicate analysis of NIST SRM 987 SrCO₃ isotopic reference material. The experimental data agreed with the accepted value within the experimental uncertainty both at 0.8 ng mL⁻¹ (for 320 pg) and at 0.4 ng mL⁻¹ (for 160 pg) of Sr. The method was applied to the analysis of Antarctic snow samples (30 g) containing 0.5–2.1 ng of Sr. The precision was fit for the purpose of discriminating between potential source areas (PSAs). The entire procedure required ∼30 g of snow per sample, potentially enabling the contributions of PSAs to be unravelled with high temporal resolution.