Determination of Alkenone Unsaturation Ratios by Direct-High Resolution Mass Spectrometry

Abstract

The degree of unsaturation in long chain (C37) alken2-ones (alkenones) (* Unsaturation is expressed a s : U3K7 = [C37:2]/[C37:2+C37:3]) biosynthesized by algae of the haptophyte order Isochrysidales has previously been shown to be correlated with growth temperature and exp lo i t ed in p a l a e o c e a n o g r a p h y as a 'palaeothermometer' to study past changes in sea surface temperature (SST) (Brassell et al., 1986). The conventional measurement typically involves extraction and isolation of an alkenone-containing fraction from the sediment prior to analysis by Gas Chromatography (GC) with flame ionization detection (FID). The GC-based method is both sensitive and precise. However, insufficient purification, or degradation of chromatographic performance can both result in the presence of co-eluting GC peaks which can significantly degrade the accuracy of the determined ratios (and hence SST estimates). Moreover, the speed of sample analysis is limited by the GC cycle time which is typically in excess of l h r . In an attempt to overcome these analytical limitations we have investigated the potential of direct high resolution mass spectrometry (HRMS) to determine alkenone unsaturation ratios. Previous experiments using direct temperature-resolved mass spectrometry at nominal mass resolution indicated that thermal desorption of lipid extracts yielded similar information to that derived from GC-based analyses (Eglinton et al., 1996).