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 13C- and 15N(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 (13C-urea: 0-1% APE; 15N(2)-urea: 0-7% MPE) enrichments were established in single point measurements; for 15N(2)-urea as values+/-0.15% MPE (95% confidence interval); for 13C-urea as values+/-0.02% APE (95% confidence interval). 15N(1)-urea enrichments were determined by measurement of the same sample with GC-c-IRMS and GC-MS. Subtraction of the 13C specific GC-c-IRMS data from the nondiscriminating GC-MS data for the sum of 13C- and 15N(1)-urea resulted in 15N(1)-urea enrichments+/-0.15% MPE (95% confidence interval). Application of the method in a combined 13C-urea bolus and 15N(2)-urea primed constant infusion experiment in piglet was demonstrated.