Labelled microbial culture as a calibration medium for C-13-isotope measurement of derivatized compounds: application to tert-butyldimethylsilyl amino acids

International audienceRationaleCompound-specific stable carbon isotope analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) is widely used in studies of environmental or biological functioning. In the case of derivatized molecules, a calibration might be required due to added non-analyte carbon and in some cases non-stoichiometric recovery by the mass spectrometer. MethodsTwo biological materials of known isotopic composition were produced by microbial cell cultures on either C-13-labelled glucose or non-labelled glucose as sole source of carbon. Subsequent hydrolyzed amino acids were derivatized as tert-butyldimethylsilyl (tBDMSi) derivatives and analyzed by GC/C/IRMS. The C-13-enrichment measurements were used as a direct calibration to calculate the original C-13/C-12 ratios of individual amino acids. We tested this calibration on both known and unknown samples. ResultsFor the main proteinogenic amino acids we could determine the number of non-analyte added carbon atoms and assess the non-stoichiometrical recovery of tBDMSi carbon atoms, due to their incomplete oxidation in the combustion step of GC/C/IRMS. The calibration enabled the determination of the natural abundances (C-13 values) of amino acids with an average accuracy of 1.1 parts per thousand. We illustrate the application of the calibration to determine the C-13/C-12 ratios of amino acids, and the associated uncertainty, in biological and plant materials. ConclusionsThe analysis of a labelled microbial cell culture offers a straightforward, rapid and reliable estimate of non-analyte carbon contribution to stable isotope composition. We recommend this method as a calibration or a control in artificial or natural C-13-tracing experiments. Copyright (c) 2016 John Wiley & Sons, Ltd