Non-linear effects in the determination of paleotemperature Uk′37 alkenone ratios by chemical ionization mass spectrometry☆

Abstract The performance of gas chromatography coupled to mass spectrometry in the positive chemical ionization mode using ammonia as reagent gas (GC–PCI-MS) in the analysis of C 37 alkenones for paleotemperature estimation has been re-evaluated. In some conditions, the discrepancies observed in the measurement of the U k′ 37 index with this technique as compared with GC equipped with flame ionization detection (GC–FID) cannot be explained by differences in sensitivity between the tri- and diunsaturated alkenones. Thus, at low (currently 0.4) U k′ 37 values the GC–PCI-MS determinations may be observed to be lower or higher, respectively, than those measured with GC–FID. As shown by analysis of a series of synthetic C 37 alkenone standards these discrepant results can be explained by non linear effects in the GC–PCI-MS response factors. Second-order polynomial functions provide equations that describe better the signal to amount of analyte ratios. Users of GC–PCI-MS should calibrate their instruments with standards of known C 37 alkenone composition in order to minimize non-linear effects.