HYDROGEN ISOTOPE FRACTIONATION OF LOW MOLECULAR WEIGHT N-ALKANES DURING PROGRESSIVE VAPORIZATION

Abstract

We determined the stable hydrogen isotope fractionation during progressive vaporization of three n-alkanes (heptane or C7, octane or C8, nonane or C9) at 24+/-1<space>°C, in an effort to understand the environmental isotopic fractionation of low molecular weight petroleum hydrocarbons. The measurements were carried out using continuous flow gas chromatography- high temperature conversion-isotope ratio mass spectrometry (GC/TC/IRMS). During the course of vaporization, the remaining liquid compounds show progressive deuterium (D)-depletion, indicating a preferential vaporization of D-enriched species. Up to a 14%% change in δD values was observed during the course of vaporization, with vapor-liquid fractionation factors (α) of 1.0044+/-0.0002 for heptane, 1.0043+/-0.0001 for octane, and 1.0040+/-0.0002 for nonane. The observed Δ is significantly greater than the Δ13C reported previously for different compounds. Our data are fundamental for evaluating the isotopic fractionation of organic contaminants during natural evaporation processes. The results have potential applications for the study of petroleum hydrocarbons at contaminated sites. The large isotopic change during the experimental vaporization also highlights the importance of avoiding evaporation losses during the collection and preparation of low molecular weight compounds for hydrogen isotope analyses.