Organic geochemistry of Walvis Bay diatomaceous ooze—III. Structural analysis of the monoenoic and polycyclic fatty acids

Abstract

Fatty acids isolated from four sections of a core sampled in diatomaceous ooze off the shore of Walvis Bay (S.W. Africa) were re-analysed with the object of determining the position and geometry of double bonds in the monoenoic fatty acids. The complete fatty acid mixtures were derivatised in such a way that unsaturated acids could be analysed as trimethylsilyloxy methyl esters and the saturated acids as methyl esters. These mixtures were analysed by capillary gas chromatography-mass spectrometry connected to a computerized data acquisition system. Assignment of the double bond position is based on mass spectral data, while relative retention time is used for the determination of the cis-trans geometry. Monoenoic fatty acids in the sediment, range in chain length from C 12 to C 26. Double bonds in even and odd straight chain and branched chain (iso and anteiso) monoenoic fatty acids with a chain length shorter than C 20 are found to be located at the Δ 7, Δ 9 and Δ 11 positions. These acids are considered to be of algal and microbial origin. Some straight chain acids have double bonds at the ° 6 position. Monoenoic acids with a chain length longer than C 20 have double bonds at the even straight chain Δ 13 and Δ 15 positions and are thought to be derived from yeast. The polycyclic fatty acids in the sediment consist of steroid and triterpenoid acids. The 5α(H) cholanic acid was identified in several core sections. The triterpenoid acids belong to the hopane family. The 17β(H),21β(Hrbishomo-hopanic acid is present as the major triterpenoid acid in a mixture which consists of various homologues with 17β(H),21β(H)-, but also l7α(H),21β(H)- and 17β(H),21α(H)stereochemistry.