Elution of Adsorbed n-alkanes by 5Å Molecular Sieve Using Solvent Mixtures for Compound-Specific Hydrogen Isotopic Analysis

The extraction procedure using solvent mixtures has been used to elute n-alkanes adsorped by 5Å molecular sieve and subsequent stable carbon isotope ratio analysis of individual n-alkanes. However, a 5Å molecular sieve may catalyze the hydrogen exchange of organic molecules during long-term heating. Therefore, the usage of this method for hydrogen isotopic analysis needs to be evaluated. Two different types of crude oil saturated hydrocarbon and n-alkanes mixed standard samples were used as the object for this study. The extraction method using cyclohexane and n--heptane solvent mixtures and another method evolving the digestion with hydrofluoric-acid and solvent extraction were both used to elute adsorbed n-alkanes by the 5Å molecular sieve. The separation efficiency of the two methods was compared, and both carbon and hydrogen isotopes of obtained n-alkanes have been analyzed. Results show that the two methods enable excellent separations of n-alkanes, and yield variations of the hydrogen isotope ratios smaller than 4‰, within uncertainty. The average recovery of n-alkanes obtained by three-time solvent extraction was 58%, whereas that obtained by hydrofluoric acid dissolution and solvent extraction was 68%. Although cyclohexane and n-heptane solvent mixture has lower extraction efficiency and involves multiple heating processes, these two factors do not result in significant fractionation or exchange of hydrogen isotopes, indicating that this method can be applied to the separation and purifying of n-alkanes and subsequent compound-specific hydrogen isotopic analysis.