Estimation of nitrogen-15 levels in derivatized amino acids using gas chromatography quadrupole mass spectrometry with chemical ionization and selected ion monitoring

The data indicate that a commercial, low resolution mass spectrometer can be used with chemical ionization and selected ion monitoring, without a data system, to estimate nitrogen-15 isotopic enrichment with a sensitivity and precision adequate for many biological studies. This was demonstrated using derivatized amino acid mixtures with separation and direct introduction into the mass spectrometer via a gas chromatograph. The method can discriminate between mean levels separated by 0.1 atom % 15N with 68% confidence at the 0.1 nmol level. Labeled dansylamide samples, inserted by probe, were also measured for 15N content.