Speciation of structural fragments in crude oil by means of isotope exchange in near-critical water and Fourier transform mass spectrometry

The structures of individual molecules in crude oil remain largely unknown despite the considerable amount of research dedicated to this topic. The extreme complexity of crude oil (recently Marshall reported the observation of more than 400,000 unique compounds in one sample) makes it impossible to separate crude oil into individual compounds and determine their structure by NMR or X-ray spectroscopy. Recently, isotope exchange, performed both in solution and in the gas phase, combined with high-resolution mass spectrometry was used for speciation of certain structural fragments of individual molecules in crude oil and humic substances. O-16/O-18 exchange allows enumeration of =O groups and speciation of furans, whereas H/D exchange allows enumeration of -OH groups, -NH groups, aromatic hydrogens, alpha hydrogens, etc. Unfortunately, crude oil is insoluble in water (the most available and cleanest source of isotopes), so performance of the exchange in solution requires harsh conditions, such as concentrated acids or bases, which could considerably modify the sample. Here we describe the use of a cheap and simple analytical approach for performing both H/D and O-16/O-18 exchange in crude oil using only water as the source of the isotopes. Crude oil was incubated in near-critical water and the reaction was monitored by high-resolution Fourier transform mass spectrometry. Although isotope exchange results in complication of the spectrum, the resolving power of modern mass spectrometers is sufficient to determine the number of exchanges for each molecule simultaneously. We determined the number of O-16/O-18 exchanges in 276 species and the number of H/D exchanges in 150 species. Our results allow deeper investigation of crude oil and other nonpolar samples on the molecular level.