Determination of 13C and 15N enrichments of urea in plasma by gas chromatography-combustion isotope ratio mass spectrometry and gas chromatography-mass spectrometry using the 2-methoxypyrimidine derivative

We describe a GC-MS and GC-c-IRMS method for the determination of labeled urea tracer enrichments in plasma as a result of combined C-13- and N-15(2)-urea infusion experiments in piglets. Urea was converted into 2-methoxypyrimidine, a stable derivative, suited for analyses by both GC-MS and GC-c-IRMS. Using calibration curves for the respective working ranges (C-13-urea: 0-1% APE; N-15(2)-urea: 0-7% MPE) enrichments were established in single point measurements; for N-15(2)-urea as values+/-0.15% MPE (95% confidence interval); for C-13-urea as values 0.02% APE (95% confidence interval). N-15(1)-urea enrichments were determined by measurement of the same sample with GC-c-IRMS and GC-MS. Subtraction of the C-13 specific GC-c-IRMS data from the nondiscriminating GC-MS data for the sum of C-13- and N-15(1)-urea resulted in N-15(1)-urea enrichments +/-0.15% MPE (95% confidence interval). Application of the method in a combined C-13-urea bolus and N-15(2)-urea primed constant infusion experiment in piglet was demonstrated. (C) 2003 Elsevier Science B.V. All rights reserved.