Factors Affecting the Stability of Hydrogen Isotopes Measurements by Gas Chromatography-Thermal Conversion-Isotope Ratio Mass Spectrometry

The compound-specific hydrogen isotopes were detected by gas chromatography-thermal conversion-isotope ratio mass spectrometry (GC-TC-IRMS). High accuracy and precision of hydrogen isotope analysis was the basis and guarantee for the in-depth study and development of compound-specific hydrogen isotopes. Therefore, better long-term stability of GC-TC-IRMS system was needed for compound-specific hydrogen isotopes analysis to same time and experimental materials, meanwhile, to acquire better accuracy. C₂₁, C₂₅, C₂₇, C₂₉, C₃₁ and C₃₃ n-alkanes were used to prepare a mixed solution in toluene of a concentration approximately 300 mg/L as a working standard. Conditioning was carried out at a new ceramic tube by passing methane gas through the tube in backflushing mode for 3 s prior to each sample determination. In experiment, 1 working standard (C₂₁, C₂₅, C₂₇, C₂₉, C₃₁, C₃₃ n-alkanes) was measured in 5 natural samples to monitor stability of the system. Through tracking analysis for almost 1 100 samples in more than 4 months, it was found that: 1) Under strict experimental conditions, the measurements of working standard could be kept stable and accurate for a long time. The average δ²H of C₂₁, C₂₅, C₂₇, C₂₉, C₃₁ and C₃₃ n-alkanes was (-216±1.6)‰, (-129±1.6)‰, (-180±1.5)‰, (-244±1.8)‰, (-146±1.7)‰ and (-195±1.9)‰, respectively. 2) Different n-alkanes in working standard needed different time to reach their long-term stable state with a new ceramic tube by GC-TC-IRMS. It took the shortest time for C₂₁ n-alkane to reach a long-term stable state. 3) It was found that the long-term stability of GC-TC-IRMS for analyzing compound-specific hydrogen isotopes could be acquired by effective method. Controlling the experimental conditions strictly, selecting high-quality ceramic tubes for hydrogen isotopes determination, and acquiring clean, good separated gas chromatograms of samples were important factors to guarantee accurate and stable hydrogen isotope results. This study has guiding significance for improving long-term stability and accuracy of measurements by use of GC-TC-IRMS system for hydrogen isotope analysis of organic compounds.