Identification of some single branched alkanes in crude oils

A series of crude oils of varying ages, source types and depositional environments were analysed for T-branched alkanes by comparison of mass spectra and gas chromatographic retention behaviour with authentic reference compounds. Single branched C18 alkanes with ethyl-, propyl-, butyl- and pentyl- substituents were synthesised in order to establish their mass spectral fragmentation patterns. Using magnet selected ion recording mass spectrometry, all open chain structural isomers with a single n-alkyl branch larger than methyl in the range C10–C20 were identified at each carbon number in the crude oils. Also, C21–C25 structural isomers containing an ethyl branch were identified in all of these samples. This represents a total of 163 compounds. T-branched alkanes comprise approx. 3% of the crude oil alkanes with GC retention times between n-C19 and n-C20.