DETERMINATION OF STABLE ISOTOPIC ENRICHMENT IN INDIVIDUAL PLASMA AMINO-ACIDS BY CHEMICAL IONIZATION MASS-SPECTROMETRY

Using chemical ionization-selected ion monitoring-gas chromatography-mass spectrometry (CI-SIM-GCMS) of the N-acetyl, n-propyl ester amino acid derivatives, isotopic enrichment can be measured in individual plasma amino acids regardless of choice of label (15N, 13C, 18O, 2H) or position in the molecule. Results using 15N-labeled amino acids show that CI-SIM-GCMS can detect 0.08 atom percent excess isotope in plasma amino acids from less than 100 .mu.l of [dog] plasma.