DETERMINATION OF (C-13)UREA ENRICHMENT BY GAS-CHROMATOGRAPHY MASS-SPECTROMETRY AND GAS-CHROMATOGRAPHY ISOTOPE RATIO MASS-SPECTROMETRY

We present gas chromatographic/mass spectrometric and gas chromatograp hic/isotope ratio mass spectrometric assays of the C-13 enrichment of plasma urea converted to its dimethylaminomethylene derivative. The li mits of sensitivity of the two techniques are 0.2% and 0.02%, respecti vely. The techniques were tested in rats and humans infused with (C-13 )urea or (3-C-13)lactate. (C-13)Urea enrichment during the infusion of (3-C-13)lactate in humans was not detectable by gas chromatography/ma ss spectrometry but was easily measured by gas chromatography/isotope ratio mass spectrometry. These assays should be useful for clinical in vestigations, in which the incorporation of a (C-13)gluconeogenic subs trate into glucose must be corrected for the incorporation of (CO2)-C- 13 derived from the oxidation of the substrate. This correction involv es measuring the low-level C-13 enrichment of urea.