Measurement of δ13C values of soil amino acids by GC–C–IRMS using trimethylsilylation: a critical assessment

This study has been supported by the French Dynamos project [ANR-07-BLAN-0222-01] and the Italian POLIGRID project [CUP B65B0900002007]; International audience; In this study, we evaluated trimethylsilyl (TMS) derivatives as derivatization reagents for the compound-specific stable carbon isotope analysis of soil amino acids by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). We used non-proteinogenic amino acids to show that the extraction-derivatization-analysis procedure provides a reliable method to measure delta C-13 values of amino acids extracted from soil. However, we found a number of drawbacks that significantly increase the final total uncertainty. These include the following: production of multiple peaks for each amino acid, identified as di-, tri- and tetra-TMS derivatives; a number of TMS-carbon (TMS-C) atoms added lower than the stoichiometric one, possibly due to incomplete combustion; different TMS-C delta C-13 for di-, tri- and tetra-TMS derivatives. For soil samples, only four amino acids (leucine, valine, threonine and serine) provide reliable delta C-13 values with a total average uncertainty of 1.3 parts per thousand. We conclude that trimethylsilyl derivatives are only suitable for determining the C-13 incorporation in amino acids within experiments using C-13-labelled tracers but cannot be applied for amino acids with natural carbon isotope abundance until the drawbacks described here are overcome and the measured total uncertainty significantly decreased.