Simultaneous determination of compound-specific isotopic compositions of acyclic isoprenoids and n-alkanes in light crude oil.

Abstract

Stable carbon and hydrogen isotope ratios of individual n-alkanes and acyclic isoprenoids are important tools in petroleum geochemistry. However, the analysis requires baseline separation and peak profiles using gas chromatography-isotope ratio mass spectrometry to obtain accurate compound-specific isotope data. Time-consuming isolation or purification is typically conducted to separate the compounds to avoid co-elution with other compounds or matrices in crude oils. We developed a simple gas chromatography separation method to simultaneously measure the compound-specific carbon or hydrogen isotope compositions of n-alkanes and acyclic isoprenoids. It was achieved by direct injection of the whole crude condensate and light oil or the saturated fractions of different types of crude oils using a 60m DB-17ms column. This method simplifies the pre-treatment of compound-specific isotope analysis, saves manpower and time, and reduces the use of organic solvents to be more environmentally friendly.