Determination of C-13- and N-15-enrichment of glutamine by gas chromatography/mass spectrometry and gas chromatography/combustion/isotope ratio mass spectrometry after N(O,S)-ethoxycarbonyl ethyl eater derivatisation

To measure C-13- and N-15-Gln enrichments in plasma samples by gas chromato graphy/mass spectrometry (GC/MS) and gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) we investigated the use of the N(O,S)-e thoxycarbonyl ethyl ester derivative. We found that this derivative is very stable at room temperature, even after 5 days storage. It allows reproduci ble and accurate C-13- and N-15-isotopic enrichment determinations with val ues of RSD below 0.8 and 3.2% for GC/MS and GC/C/IRMS, respectively. This d erivative enables high sample throughput analyses when automated GC/MS and GC/C/IRMS are used. The methodology was applied to measure isotopic enrichm ents in rat plasma after oral force-feeding with [2,5-N-15(2)]-Gln and in h uman plasma samples obtained after intravenous infusion of [1-C-13]-Gln. In rats after oral force-feeding, we found that the maximal appearance of [2, 5-N-15(2)]-Gln occurred between 40 and 80 min in the two compartments studi ed (portal and peripheral blood). In humans infused intravenously with [1-C -13]-Gln for 4 h, the plateau in C-13-Gln enrichments in plasma was reached after 3 h of infusion and coefficients of variation between samples were 1 .7 and 12.4% for the two subjects. Using the N(O,S)ethoxycarbonyl ethyl est er derivative, C-13- and N-15-isotopic enrichment can be determined in the range 2-100 +/- 0.3 MPE by GC/MS and 0.3-2 +/- 0.02 MPE by GC/C/IRMS. Copyr ight (C) 2001 John Wiley & Sons, Ltd.