Molecular Sieve Isolation Technique for Use in Stable Carbon Isotope Analysis of Individual Long-Chain n-Alkanes in Crude Oil.

An isolation procedure of microgram amounts of long-chain n-alkanes from crude oil using molecular sieve was examined for its applicability to stable carbon isotope analysis by gas chromatography/isotope ratio mass spectrometry (GC/IRMS).The procedure examined is as follows: molecular sieve (type 5A, 200 mg) in 1 ml of isooctane solvent are mixed with a saturated hydrocarbon fraction extracted from an appropriate amount (approx. 20 mg) of crude oil and stayed at room temperatures for more than 3 hours. Long-chain n-alkanes are isolated by extraction with n-hexane after dissolution of the resulting molecular sieve with 47% hydrofluoric acid solution. The recoveries were 90±6% for C15~C34 n-alkanes when their total amounts applied do not exceed 1.4 mg. No effect of the isolation procedure on carbon isotope ratios of n-alkanes was observed.