Toward Improved Accuracy in Chlorine Isotope Analysis: Synthesis Routes for In-House Standards and Characterization via Complementary Mass Spectrometry Methods

Increasing applications of compound-specific chlorine isotope analysis (CSIA) emphasize the need for chlorine isotope standards that bracket a wider range of isotope values in order to ensure accurate results. With one exception (USGS38), however, all international chlorine isotope reference materials (chloride and perchlorate salts) fall within the narrow range of one per mille. Furthermore, compound-specific working standards are required for chlorine CSIA but are not available for most organic substances. We took advantage of isotope effects in chemical dehalogenation reactions to generate (i) silver chloride (CT16) depleted in ³⁷Cl/³⁵Cl and (ii) compound-specific standards of the herbicides acetochlor and S-metolachlor (Aceto2, Metola2) enriched in ³⁷Cl/³⁵Cl. Calibration against the international reference standards USGS38 (-87.90 ‰) and ISL-354 (+0.05 ‰) by complementary methods (gas chromatography-isotope ratio mass spectrometry, GC-IRMS, versus gas chromatography-multicollector inductively coupled plasma mass spectrometry, GC-MC-ICPMS) gave a consensus value of δ³⁷Cl_CT16 = -26.82 ± 0.18 ‰. Preliminary GC-MC-ICPMS characterization of commercial Aceto1 and Metola1 versus Aceto2 and Metola2 resulted in tentative values of δ³⁷Cl_Aceto1 = 0.29 ± 0.29 ‰, δ³⁷Cl_Aceto2 = 18.54 ± 0.20 ‰, δ³⁷Cl_Metola1 = -4.28 ± 0.17 ‰ and δ³⁷Cl_Metola2 = 5.12 ± 0.27 ‰. The possibility to generate chlorine isotope in-house standards with pronounced shifts in isotope values offers a much-needed basis for accurate chlorine CSIA.