We describe a method for obtaining the specific activity of 14C in urea, essential in the measurement of the synthesis rate of a plasma protein in vivo, which is simpler than the original procedure. The principle is the measurement of 14CO2 and NH4+ separately, after incubation with urease. A simple alteration gives samples of 13CO2 for mass spectrometry. The ‘recoveries’ of 14C and 13C in urea were invariably between 90 and 96% and the CV was 3%.
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