A new high-precision method for determining stable chlorine isotopes in halite and igneous rock samples using UV-femtosecond laser ablation multiple Faraday collector inductively coupled plasma mass spectrometry

We report a new, rapid method for the high-precision determination of chlorine isotope ratios in halite and AgCl pellets formed from seawater and igneous rock samples. Use of 266 nm ultra violet-femtosecond laser ablation (UV-FsLA) allowed quantitative sampling of halite and AgCl and enabled precise determination of Cl-37/Cl-35 isotope ratios (delta Cl-37) when coupled with a multiple Faraday collector-inductively coupled plasma mass spectrometer (MFC-ICPMS). We used Ar-36(+)/Ar-38(+) as an external standard for the mass bias corrections between K-39+-K-41+,(ArH+)-Ar-36-H-1-(ArH+)-Ar-38-H-1-(ArH+)-Ar-40-H-1, and Cl-35(+)-Cl-37(+) with isobaric overlap corrections between K+, ArH+, and Cl+ ions. Overlap of the sulfur (S-36(+)) isobar with Ar-36(+) was indirectly monitored and corrected by baseline modelling using the Ar-36(+)/Ar-38(+) measurement. These procedures collectively helped accomplish an accurate and high-precision measurement of Cl-37(+)/Cl-35(+) ratios. Using the proposed analytical method, delta Cl-37 ratios in natural halite samples were analysed by direct laser ablation. delta Cl-37 in JB-1a and JB-3 igneous rocks were analysed as AgCl powder pellets produced by pyrohydrolytic separation and co-precipitation of the separated Cl with silver. The external reproducibility of the delta Cl-37 measurements was +/-0.2 parts per thousand 2SD (2 standard deviations) for halite and +/-0.4 parts per thousand 2SD for AgCl precipitate rivalling that of gas source isotope ratio mass spectrometry. The new analytical protocol enabled a precise and rapid delta Cl-37 analysis of igneous rock samples as AgCl with as little as 4 mg chlorine from the similar to 500 mg chlorine separated. This is the first time that high-precision in situ determination of delta Cl-37 from halite using 4 mu g of chlorine has been reported.