Rapid gas chromatographic-mass spectrometric analysis of [15N]urea: application to human metabolic studies

A rapid gas chromatographic-mass spectrometric method for [15N]urea analysis is described which is based on the formation of a TFA-urea derivative. The method is extremely sensitive, being capable of detecting less than 0.2 atom % excess [15N]urea in as little as 50 microliter of plasma and 10 microliter of urine with a coefficient of variation of 11%. The method was used to measure in vivo the disappearance of [15N]urea from plasma and the incorporation of [15N]ammonium acetate into [15N]urea. The values obtained for urea pool size and flux are 4.5 +/- 0.30 g nitrogen/total body weight and 16.2 +/- 1.34 g nitrogen per 24 h per 1.73 m2. Following the ingestion of [15N]ammonium acetate nearly 90% of the 15N label was incorporated into [15N]urea by 30 min.