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  • Title: Assay of stable isotope labelled urea in biological fluids by selected ion monitoring.
    Author: Nelson JE, Ruo TI.
    Journal: Clin Chim Acta; 1988 Jun 30; 175(1):59-65. PubMed ID: 3168284.
    Abstract:
    A selected ion-monitoring method, incorporating an internal standard, was developed to allow the direct and simultaneous measurement of stable isotope-labelled urea and unlabelled urea concentrations in biological specimens. After cyclization of urea with malonaldehyde bis(dimethylacetal) to form 2-hydroxypyrimidine, the volatile heptafluorobutyryl derivative was made in a two-step process utilizing a trimethylsilyl intermediate. The internal standard, 4-methyl-2-hydroxypyrimidine, was found to be stable under the cyclization conditions. The structures of the internal standard and urea derivatives were confirmed by electron impact and chemical ionization mass spectrometry. This method was applied to the measurement of [15N2]urea concentrations in dog plasma.
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