Determination of Stable Hydrogen Isotopic Composition and Isotope Enrichment Factor at Low Hydrogen Concentration

Determination of stable hydrogen isotopic compositions (δ²H) is currently challenged to achieve a high detection limit for reaching the linear range where δ²H values are independent of concentration. Therefore, it is difficult to assess precise δ²H values for calculating the hydrogen isotope enrichment factor (ε_H) and for field application where the concentrations of contaminants are relatively low. In this study, a data treatment approach was developed to obtain accurate δ²H values below the linear range. The core concept was to use a logarithmic function to fit the δ²H values below the linear range and then adjust the δ²H values below the linear range into the linear range by using the fitted logarithmic equation. Moreover, the adjusted δ²H values were calibrated by using laboratory reference materials, e.g., n-alkanes. Tris(2-chloroethyl) phosphate (TCEP) and hexachlorocyclohexane (HCH) isomers were selected as examples of complex heteroatom-bearing compounds to develop the data treatment approach. This data treatment approach was then tested using δ²H values from a TCEP transformation experiment with OH radicals. Comparable δ²H values and ε_H between the low-concentration experiment and the reference experiment were obtained using the developed approach. Therefore, the developed data treatment approach enables a possibility of determining the hydrogen isotopic compositions of organic components in low concentrations. It is especially valuable for determining organic contaminants in environmental samples, which are usually present in low concentrations.