Low-femtogram sensitivity analysis of polyaromatic hydrocarbons using GC-APLI-TOF mass spectrometry: Extending the target window for aromatic steroids in early Proterozoic rocks

Abstract Reading the molecular record of early life on Earth can be challenging as a consequence of the thermal decomposition of biomarker hydrocarbons during extended and deep sedimentary burial. Biomarkers that appear absent may still exist in concentrations far below the detection limit of regular analytical techniques. Hence thermal maturation and analytical limitations form a major hurdle for the unambiguous reconstruction of Earth’s earliest ecosystems. Specific degradation products of hopanes and steranes might still carry diagnostic value for the reconstruction of past biota when found in mature oils and sediment extracts, but their concentrations are often exceedingly low. We here report on the use of gas chromatography–time-of-flight-mass spectrometry with laser ionization at atmospheric pressure (GC-APLI-TOF-MS) for the identification and quantification of aromatic steroids. GC-APLI-TOF-MS analyses of a reference oil revealed that triaromatic steroids (TAs) can be detected with significantly higher sensitivity and selectivity compared to commonly used electron impact ionization GC-MS in selected ion monitoring mode, while the limit of detection for retene was as low as 25 fg on column. Paleoproterozoic (1.64 Ga) sedimentary rocks of the Barney Creek Formation yield the so far oldest unambiguous aromatic steroids. GC-APLI-TOF-MS applied to these rocks revealed the presence of TAs in even deeper parts of this succession, where they are represented by short chain homologues only. While the TA-I/(TA-I + TA-II) parameter is principally controlled by thermal degradation of longer chain homologues, the complete disappearance of TA-II already at 435–442 °C T max , may point towards an uncommon thermal gradient in the Barney Creek Formation or towards other influencing factors. GC-APLI-TOF-MS proves to be a highly promising tool that might extend molecular geobiological approaches to older sedimentary basins that were hitherto precluded from organic geochemical approaches by their elevated thermal maturity.