An improved method for the analysis of archaeal and bacterial ether core lipids

In recent decades, microbial membrane lipids have become a focus of geoscientific research because of their proxy potential. The aim of this study was to develop new methods for ultra high performance liquid chromatography (UHPLC) separation of isomers of archaeal and bacterial membrane ether lipids, in particular glycerol dialkyl glycerol tetraethers (GDGTs), because of their tendency to co-elute with related but incompletely characterized derivatives. Our newly developed protocol, involving analysis using two Acquity BEH HILIC amide columns in tandem, enables chromatographic separation of several of these co-eluting compounds, such as the isoprenoid GDGT with four cyclopentyl moieties and other chromatographic shoulders often observed in GDGT analysis. Additionally, resolved peaks were observed for isoprenoid GDGTs, branched GDGTs and isoprenoid glycerol dialkanol diethers (GDDs); these have typically the same molecular mass as the corresponding major compound. Multiple stage mass spectrometry (MS2) indicated that the shoulder peaks represent either regioisomers or other structural isomers with different ring or methyl positions. In some samples, these isomers can be even more abundant than their “regular” counterparts, suggesting that previously hidden clues regarding source organisms and/or community response to environmental forcing factors may be encoded in the distributions.