The traditional high-temperature conversion (HTC) approach toward compound-specific stable isotope analysis (CSIA) of hydrogen for heteroatom-bearing (i.e., N, Cl, S) compounds has been afflicted by fractionation bias due to formation of byproducts HCN, HCl, and H₂S. This study presents a chromium-based high-temperature conversion (Cr/HTC) approach for organic compounds containing nitrogen, chlorine, and sulfur. Following peak separation along a gas chromatographic (GC) column, the use of thermally stable ceramic Cr/HTC reactors at 1100–1500 °C and chemical sequestration of N, Cl, and S by chromium result in quantitative conversion of compound-specific organic hydrogen to H₂ analyte gas. The overall hydrogen isotope analysis via GC–Cr/HTC–isotope ratio mass spectrometry (IRMS) achieved a precision of better than ± 5 mUr along the VSMOW-SLAP scale. The accuracy of GC–Cr/HTC–IRMS was validated with organic reference materials (RM) in comparison with online EA–Cr/HTC–IRMS and offline dual-inlet IRMS. The utility and reliability of the GC–Cr/HTC–IRMS system were documented during the routine measurement of more than 500 heteroatom-bearing organic samples spanning a δ²H range of −181 mUr to 629 mUr.

Hydrogen stable isotope analysis and the interpretation of resulting δ²H values provide a powerful tool in many disciplines, e.g., in earth sciences, ecology, forensics, and biochemistry.(1) Hydrogen isotope analysis in organic compounds originally required two offline conversion steps, namely, first oxidation to water, and then reduction of water to molecular hydrogen (H₂) analyte gas using reducing metals such as zinc, uranium, chromium, magnesium, or tungsten.(2-4) Subsequently, δ²H values were determined in manual dual-inlet mode using isotope-ratio mass spectrometry (DI-IRMS). Direct pyrolytic conversion of organically bound hydrogen to H₂ analyte gas via high-temperature conversion (HTC) at temperatures of > 1050 °C resulted in much-enhanced utility of continuous flow (CF) online CF-IRMS.(3, 5) Modern stable isotope analysis of organic hydrogen uses isotope-ratio mass spectrometry (IRMS) where the H₂ analyte gas is generated online via (1) direct HTC at 1050 °C in an elemental analyzer (EA) or (2) via compound-specific stable isotope analysis (CSIA) in combination with GC separation of mixtures and subsequent HTC of the target compounds at 1400–1450 °C.(1, 6) However, these methods yield the best results for hydrocarbons and become more challenging for nitrogen-, chlorine-, and sulfur-containing organics, where HTC-derived H₂ yields are incomplete due to the formation of hydrogen-containing byproducts (HCN, HCl, and H₂S).(7-10)

A chromium-based reactor system can overcome interferences by quantitatively scavenging heteroatoms. Chromium was first employed in hydrogen online EA–IRMS by Morrison et al.(11) and Kelly et al.(12) The analysis of water with chromium entails quantitative conversion to H₂ and accurate δ²H values. EA conversion of polyhalogenated compounds with chromium at 1000 °C, however, resulted in incomplete H₂ yields and limited the accuracy and suitability of chromium-based reactor systems for some substrates at relatively low temperatures.(13) Efforts to trap or eliminate the byproducts with a cold trap, stainless steel, or additional reduction with hot chromium in tubular reactors at 800–1000 °C could not establish a reliable and technically simple GC–IRMS method for compound-specific hydrogen isotope analysis of polyhalogenated compounds.(7, 14-16)

Gehre et al. introduced EA–Cr/HTC–IRMS (previously named Cr-EA) as an accurate tool for hydrogen stable isotope-ratio analysis of organic compounds bearing heteroelements.(17) Quantitative H₂ yields and accurate δ²H values were derived from several nitrogen-, chlorine-, and sulfur-containing compounds, as documented by the comparison of several stable isotope laboratories using chromium-based reactor systems of different designs and conversion conditions as well as offline conversion and analysis by dual-inlet mode using isotope-ratio mass spectrometry (DI-IRMS).(17)

This study builds on the proven chromium-based EA reactor design and introduces an interface for compound-specific hydrogen isotope analysis using a chromium-based reactor for GC–IRMS. In contrast to earlier approaches by Shouakar-Stash and Drimmie(16) and Kuder and Philp,(15) a ceramic reactor tube was packed with chromium and used as an HTC reactor at > 1100 °C. Our system allows temperatures of up to 1500 °C and is therefore termed ‘chromium-based high-temperature conversion’ (Cr/HTC). Our reactor design can be implemented in existing equipment by replacing the HTC reactor with a Cr/HTC reactor, using commercially available components for GC–HTC–IRMS hydrogen isotope analysis.