Non-linear effects in the determination of paleotemperature U37k′ alkenone ratios by chemical ionization mass spectrometry

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 U-37(k') re-evaluated. In some conditions, the discrepancies observed in the measurement of the 37 index with this technique as compared with GC equipped with flame ionization detection (GC-FID) cannot be explained by differences in sensitivity Uk(37)(k') between the tri- and diunsaturated alkenones. Thus, at low (currently <0.3) or high (currently >0.4) 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. (C) 2003 Elsevier B.V. All rights reserved.