Analysing Sr isotopes in low-Sr samples such as single insects with inductively coupled plasma tandem mass spectrometry using N₂O as a reaction gas for in-line Rb separation

Abstract

Rationale: Strontium isotopes are valuable markers of provenance in a range of disciplines. Limited amounts of Sr in low-mass samples such as insects mean that conventional Sr isotope analysis precludes their use for geographic origins in many ecological studies or in applications such as biosecurity. Here we test the viability of using inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) with N₂O as a reaction gas for accurately determining Sr isotopes in insects with Sr < 100 ng. Methods: Strontium isotopes were determined in solution mode using ICP-MS/MS with 0.14 L/min N₂O as a reaction gas to convert Sr⁺ into SrO⁺ for in-line separation of ⁸⁷Sr from ⁸⁷Rb. The Sr isotope reference standards NIST SRM 987, NIST SRM 1570a and NIST SRM 1547 were used to assess accuracy and reproducibility. Ten insect species collected from the wild as a proof-of-principle application were analysed for Sr concentration and Sr isotopes. Results: Using ICP-MS/MS we show for the first time that internal mass bias correction of ⁸⁷Sr¹⁶O/⁸⁶Sr¹⁶O based on ⁸⁸Sr¹⁶O/⁸⁶Sr¹⁶O works to give for NIST SRM 987 a ⁸⁷Sr/⁸⁶Sr ratio of 0.7101 ± 0.012 (RSD = 0.17%) and for NIST SRM 1570a a ⁸⁷Sr/⁸⁶Sr ratio of 0.7100 ± 0.009 (RSD = 0.12%), which are within error of the accepted values. The first ⁸⁷Sr/⁸⁶Sr ratio of NIST SRM 1547 is 0.7596 ± 0.0014. Strontium analyses were run on 0.8 mL of 0.25–0.5 ppb Sr, which equates to 2–4 ng of Sr. Strontium isotope analysis with a precision of >99.8% can be achieved with in-line separation of ⁸⁷Sr from ⁸⁷Rb at least up to solutions with 25 ppb Rb. Conclusions: A minimum of 5 mg of insect tissue is required for Sr isotope analysis. This new ICP-MS/MS method enables Sr isotope analysis in single insects, allowing population-scale studies to be feasible and making possible applications with time-critical uses such as biosecurity.