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

Determination of stable hydrogen isotopic compositions (delta H-2) is currently challenged to achieve a high detection limit for reaching the linear range where delta H-2 values are independent of concentration. Therefore, it is difficult to assess precise delta H-2 values for calculating the hydrogen isotope enrichment factor (epsilon(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 delta H-2 values below the linear range. The core concept was to use a logarithmic function to fit the delta H-2 values below the linear range and then adjust the delta H-2 values below the linear range into the linear range by using the fitted logarithmic equation. Moreover, the adjusted delta H-2 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 delta H-2 values from a TCEP transformation experiment with OH radicals. Comparable delta H-2 values and epsilon(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.