MOLECULAR-STRUCTURES OF SEDIMENTARY 8,14-SECOHOPANES INFERRED FROM THEIR GAS-CHROMATOGRAPHIC RETENTION BEHAVIOR

The branched and cyclic alkane fractions from a biodegraded crude oil and the solvent extract of a mature sediment were analysed for extende d 17 alpha-hopanes/moretanes and extended 8,14-secohopanes using gas c hromatography-mass spectrometry techniques. The biodegraded crude oil contained abundant 8,14-secohopanes in the form of pseudohomologous se ries with carbon numbers ranging from 31 to 35. Comparison of the chro matograms of the extended 8,14-secohopanes with those of the extended 17 alpha-hopanes and moretanes from the sediment extract revealed some interesting similarities. Firstly, the C-31 moretanes (22S, 22R) were not resolved but the corresponding C-31 17 alpha-hopanes were well re solved. In the case of the 8,14-secohopanes three of the six groups of C-31 compounds were either poorly resolved into doublets or appeared as single peaks while the other three compound groups were well-resolv ed into doublets. This suggests that one group of 8,14-secohopanes beh ave in a similar manner to the moretanes and another group have a beha viour similar to that of the 17 alpha-hopanes. Another feature of the distribution of extended moretanes, namely the greater relative abunda nce of the second eluting isomer of each pair (22S and 22R diasteromer s), was also observed in the ''moretane-like'' 8,14-secohopanes. These similarities in GC behaviour and in the relative abundances of isomer s has been interpreted as evidence for ''hopane-type'' and ''moretane- type'' extended 8,14-secohopanes.