Branched hydrocarbons, biomarker

The separated hydrocarbon fraction is then analyzed by capillary column gas chromatography, which simultaneously separates the mixture of hydrocarbons into its components and measures how much of each compound is present. Individual n-alkanes can usually be identified by their relative retention times in gas chromatograms, although sometimes branched hydrocarbon molecules will share the same retention time as an n-alkane. For this reason, positive identification of biomarker hydrocarbons require the combination of gas chromatography and mass spectrometry using verified mass spectra. 

The characterization of a novel series of biomarkers is illustrated with the g 7 2-dialkylalkanes in bitumen from a hydrothermal system on the Mid-Atlantic Ridge. The total bitumen consists of hydrocarbons, a major UCM (unresolved complex mixture of branched and cyclic compounds) and mature biomarkers (e.g. hopanes) (Fig. 13a). The bitumen contains a series of cyclopentylalkanes Cfi 2n) that range from n = 14 to 34, with only even-chained pseudohomologs and a concentration maximum (Cmax) at n = 18. Their source is biogenic, based on the presence of only even-carbon number homologs, but the precursors are unknown. 

The author s group in Jerusalem is involved in both GC and GC/MS/C of fossil fuels. The availability of MS (VG ZABIIF) linked to a GC allowed various operation modes. Some of the data presented in this review have not been published previously, except in industrial reports. The main advantage in the use of GC/MS/C for saturated hydrocarbons lies in the ability to identify the branched and cyclic bio -alkanes, also called biomarkers . The basic analytical approach to the study of these compounds was discussed in Section III.A.3. However, the GC/MS/C method provides the possibility to work on the whole saturated hydrocarbon fraction, without employing extensive... 

Usually, fluid inclusion oils are measured by GC-MS as whole oils (i.e., without prior fractionation), because extraction yields are usually very low (too low for gravimetric analysis). However, fluid inclusion oils can be fractionated into total hydrocarbon fractions and nitrogen-sulfur-oxygen (NSO) fractions to avoid unwanted interference of NSO compound peaks with aliphatic and aromatic hydrocarbon peaks. Furthermore, samples that have significant n-alkane interference, in particular in their biomarker distributions, can be further separated into n-alkanes and branched and cyclic hydrocarbon fractions by using molecular sieves (e.g., ZSM-5 sieve such as silicalite, a zeolitlic form of silica). 

---