An online method to determine chlorine stable isotope composition by continuous flow isotope ratio mass spectrometry (CF-IRMS) coupled with a Gasbench II

Abstract

An online method using continuous flow isotope ratio mass spectrometry (CF-IRMS) interfaced with a Gasbench II was presented to determine chlorine stable isotope composition. Silver chloride (AgCl) was quantitatively derived from chloride by using silver nitrate (AgNO₃), and then was reacted with iodomethane (CH₃I) to produce methyl chloride (CH₃Cl). A GasBench II equipped with a PoraPlot Q column was used to separate CH₃Cl from any other gas species. Finally, chlorine stable isotope analysis was carried out on CH₃Cl introduced to the IRMS in a helium stream via an active open split. The minimum amount of Cl used in this method is of the order of 1.4 μmol. Inter-laboratory and inter-technique comparisons show that the total uncertainty incorporating both the precision and accuracy of this method is better than 0.007%. Furthermore, ten seawaters sampled from different locations have a narrow δ³⁷Cl value range from −0.008% to 0.010%, with a mean value of (0.000±0.006)%. This supports the assumption that any seawater can be representative of standard mean ocean chloride (SMOC) and used as an international reference material.

Keywords

chlorine stable isotope / continuous flow isotope ratio mass spectrometry / CF-IRMS / GasBench II

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