Determination of13C Enrichment by Conventional GC-MS and GC-(MS)-C-IRMS

Isotopic enrichment of branched-chain L-amino acids (BCAA) and branched-chain 2-oxo acids (BCOA) in standard preparations and in human plasma samples withdrawn after oral loads with 1-13C labelled BCAA were measured on a conventional GC-MS system and an on-line GC-C-IRMS employing O-TMS quinoxalinol derivatives. It was concluded that the recently introduced GC-C-IRMS, owing to its high sensitivity, is the adequate analytical tool when tracer doses of stable isotope labelled compounds of low enrichment are to be used in biomedical in vivo studies.