GC-MS DETERMINATION OF RATIOS OF STABLE-ISOTOPE LABELED TO NATURAL UREA USING [(CN2)-C-13-N-15]UREA FOR STUDYING UREA KINETICS IN SERUM ANDAS A MEANS TO VALIDATE ROUTINE METHODS FOR THE QUANTITATIVE ASSAY OF UREA IN DIALYSATE

A GC-MS determination of urea in serum or spent dialysate is described , using (CN2)-C-13-N-15-labelled urea and assaying the area ratio of l abelled to natural urea by mass fragmentographic monitoring of fragmen ts m/e 153 and 156, after its eventual conversion into the trimethylsi lylether-derivative of 2-hydroxypyrimidine. The procedure can be succe ssfully applied in the follow-up of the disappearance of labelled urea in serum after intravenous injection in man, enabling kinetic paramet ers of urea to be established, e.g. for purposes of studying the effec tiveness of dialysis procedures. Furthermore the method can be used fo r validation of routine methods for measuring urea in other fluids, in particular dialysate. Examples are given of both applications of the GC-MS method described.