Application of a Supercritical Fluid Chromatography quadrupole Time of Flight system to the detection of Glycerol Dialkyl Glycerol Tetraethers

Due to their response to environmental parameters and global distribution, the bacterial and archaeal lipids Glycerol Dialkyl Glycerol Tetraethers (GDGTs) have been used as proxies for environmental studies in marine and terrestrial settings for the past decades. Since these molecules were first discovered, improvements in the analytical method have been made, from the initial method that allowed for the identification of these compounds using cyanocolumns run in an HPLC-MS system, to the separation of their different isomers using two silica columns in series. While currently the most widely used method, using the silica columns, has been shown to produce very reproducible results across different laboratories, the runtime of the program of approximately 90 minutes makes it burdensome to analyze large arrays of samples, and the long runtime also requires large volumes of solvent. These are significant limitations for the application of this analysis in paleoclimate studies where large numbers of samples are required. Furthermore, since this method is run in Single Ion Monitoring mode, other related compounds are not currently being analyzed along the GDGTs.

Here we present the initial results of implementing a Supercritical Fluid Chromatography coupled to quadrupole Time of Flight mass spectrometry (SFC-qTOF) system to measure GDGTs. We report that this method produces results with comparable distribution of the GDGTs to the classical HPLC-MS method, where the GDGTs are correctly detected although no isomer separation is observed. Nevertheless, our total runtime is 25 minutes and also uses less than a third of the solvent from the standard method, as it primarily uses supercritical CO₂ as mobile phase, supplemented with no more than 40% of methanol:ethanol (1:1). Furthermore, while other alternative techniques, such as reverse phase chromatography have also reduced the runtime of the GDGT analysis, since our system is running in normal phase, our results are easily comparable with those of the commonly made analyses. The implementation of a faster analysis would allow for an easier application of GDGT analyses to high resolution paleoreconstruction analyses.

While improvements in this method are still required, the versatility of the SFC-qTOF system, as demonstrated in its use for lipidomic studies, could provide a useful tool to not only measure the commonly studied GDGTs, but also by expanding the analysis to less studied structures such as intact polar lipids and unidentified GDGTs. Previous work has shown that the measurement of additional non-traditional GDGTs can provide extra paleoecological information, thus allowing for more robust and reliable paleoreconstructions.