Rapid and accurate determination of chlorine isotopic ratios with ICP-MS/MS using O2 reaction gas†

Abstract

Chlorine isotopic ratio measurements are useful for stable isotope tracing, isotopic abundance measurements in nuclear chemistry, and accurate determination of concentrations using isotope dilution methods. Accurate and precise determination of Cl isotopic ratios using inductively coupled plasma mass spectrometry (ICP-MS) methods is challenging due to major polyatomic interferences of ¹⁶O¹⁸O¹H⁺ and ³⁶Ar¹H⁺ on ³⁵Cl⁺ and ³⁷Cl⁺, respectively. Previous work has demonstrated that using tandem mass spectrometry (ICP-MS/MS) with either H₂ or O₂ gas in the collision/reaction cell can significantly improve the precision, but not necessarily the accuracy, of chlorine isotopic measurements over single-quadrupole techniques. In this work, we further investigate ICP-MS/MS, using O₂ as a reaction gas, as a technique for accurate determination of Cl isotopic ratios. Using the methodology developed herein we measure both natural and enriched chlorine isotopic ratios in diverse samples matrices, targeting ³⁷Cl isotope enrichment efforts, without the need for complex front-end chemistry (i.e., ion exchange chromatography), while maintaining a typical accuracy and precision better than ∼1%. The reduced need for time-consuming sample processing afforded by this method results in higher sample throughput (>80 measurements/day) relative to other analytical techniques (e.g., thermal ionization mass spectrometry, accelerator mass spectrometry, etc.). This work demonstrates that ICP-MS/MS with O₂ as a reaction gas can be a useful tool for making rapid and accurate chlorine isotopic ratio measurements.