Protocols to assist deconvolution of co-eluting 5β– and 5α–steranes by gas chromatography–mass spectrometry

Abstract

Molecular parameters based on the relative abundance of various sterane isomers contribute valuable information about sources and thermal history of organic-matter-rich sediments. However, this application is limited for 5β– and 5α–steranes due to their co-elution on common gas chromatography (GC) columns: e.g., relative retention index of 5β20R and 5α20S cholestanes on Agilent DB1 columns = 28.081 and 28.065, respectively. Selected ion monitoring (SIM) and tandem (MS/MS) mass spectral protocols were developed to improve the analytical resolution of these compounds, exploiting slight differences in mass fragmentation patterns. Protocols were optimised on a 1:1 mixture of authentic 5α20R and 5β20R cholestane standards (5α20R in place of 5α20S). Parameters offering 5α:5β selectivity (e.g., SIM: 149/151 Da; MRM: [149 → 79]/[151 → 79]) were tested on a suite of 5α20R:5β20R standards and show an excellent correlation (R² > 0.99) with substrate composition. The capacity to quantify the relative contribution of co-eluting 5α20S and 5β20R in more complex samples was tested by analyses of a crude oil (Blina-4, Canning Basin, Western Australia) known to have high abundances of C_27-29 5α20S steranes (20S/[20S + 20R] ≈ 0.5) and spiked with 5β20R cholestane. The relative proportions of 5α20S and 5β20R were determined using 5α20R:5β20R calibration curves with good consistency and accuracy. Values deviated from the target when either analyte was particularly dominant (≥80 %), probably due to differences in ion density compared to the resolved standards. This issue may be improved with further method refinement and the true sterane deconvolution value offered by this approach will become apparent with further application to a diverse range of samples.