A novel method for isolation of diamondoids from crude oils for compound-specific isotope analysis

A novel, effective and simple method based on adsorption chromatography on silica gel, adsorption on molecular sieves, and inclusion in β-cyclodextrin (β-CD) was developed to separate and enrich diamondoids directly from crude oil. In this method, diamondoids ranging from adamantanes to triamantanes were purified and isolated from the branched/cyclic hydrocarbon fraction of crude oil for further accurate analysis of stable carbon isotope ratios (δ13C) by gas chromatography-isotope ratio mass spectrometry (GC-irMS). A 5 mg/ml solution of β-CD stirred for 2 h was found to be sufficient for the inclusion of diamondoids and the inclusion complex completely breaks down by acidolysis in 1 mol/l hydrochloric acid at 80 °C for 16 h. The recovery of diamantanes was higher than that of other diamondoids. Our major finding is that the specific inclusion effect between β-CD and diamondoids can be utilized to isolate and concentrate diamondoid compounds from crude oil in a fast and effective way. It is a potential method to yield the diamondoid fraction amenable for compound-specific isotope analysis (CSIA), which markedly improves accuracy of molecular isotopic proxies of oil-oil and oil-source correlation.