NITROGEN ISOTOPIC ANALYSES BY ISOTOPE-RATIO-MONITORING GAS-CHROMATOGRAPHY MASS-SPECTROMETRY

Amino acids containing natural-abundance levels of N-15 were derivatized and analyzed isotopically using a technique in which individual compounds are separated by gas chromatography, combusted on-line, and the product stream sent directly to an isotope-ratio mass spectrometer. For samples of N2 gas, standard deviations of ratio measurement were better than 0.1 parts per thousand (Units for delta are parts per thousand or per million (parts per thousand).) for samples.larger than 400 pmol and better than 0.5 parts per thousand for samples larger than 25 pmol (0.1 parts per thousand N-15 is equivalent to 0.00004 atom % N-15). Results duplicated those of conventional, batchwise analyses to within 0.05 parts per thousand. For combustion of organic compounds yielding CO2/N2 ratios between 14 and 28, in particular for N-acetyl n-propyl derivatives of amino acids, delta values were within 0.25 parts per thousand of results obtained using conventional techniques and standard deviations were better than 0.35 parts per thousand. Pooled data for measurements of all amino acids produced an accuracy and precision of 0.04 and 0.23 parts per thousand, respectively, when 2 nmol of each amino acid was injected on column and 20% of the stream of combustion products was delivered to the mass spectrometer.