Compound‐specific hydrogen isotope analysis of fluorine‐, chlorine‐, bromine‐ and iodine‐bearing organics using gas chromatography–chromium‐based high‐temperature conversion (Cr/HTC) isotope ratio mass spectrometry

Abstract

The conventional high-temperature conversion (HTC) approach towards hydrogen compound-specific isotope analysis (CSIA) of halogen-bearing (F, Cl, Br, I) organics suffers from incomplete H2 yields and associated hydrogen isotope fractionation due to generation of HF, HCl, HBr, and HI byproducts. Moreover, the traditional off-line combustion of highly halogenated compounds results in incomplete recovery of water as an intermediary compound for hydrogen isotope ratio mass spectrometry (IRMS), and hence also leads to isotope fractionation. This study presents an optimized chromium-based high-temperature conversion (Cr/HTC) approach for hydrogen CSIA of various fluorinated, chlorinated, brominated and iodinated organic compounds. The Cr/HTC approach is fast, economical, and not affected by low H2 yields and associated isotope fractionation. The performance of the modified gas chromatography/chromium-based high-temperature conversion (GC-Cr/HTC) system was monitored and optimized using an ion trap mass spectrometer. Quantitative conversion of organic hydrogen into H2 analyte gas was achieved for all halogen-bearing compounds. The corresponding accuracy of CSIA was validated using (i) manual dual-inlet (DI)-IRMS after off-line conversion into H2 , and (ii) elemental analyzer (EA)-Cr/HTC-IRMS (on-line conversion). The overall hydrogen isotope analysis of F-, Cl-, Br- and I-bearing organics via GC-Cr/HTC-IRMS achieved a precision σ≤3 mUr and an accuracy within ±5 mUr along the VSMOW-SLAP scale compared with the measured isotope compositions resulting from both validation methods, off-line and on-line. The same analytical performance as for single-compound GC-Cr/HTC-IRMS was achieved compound-specifically for mixtures of halogenated organics following GC separation to baseline resolution. GC-Cr/HTC technology can be implemented in existing analytical equipment using commercially available materials to provide a versatile tool for hydrogen CSIA of halogenated and non-halogenated organics. Copyright © 2017 John Wiley & Sons, Ltd.