Nitrogen isotopic analyses by isotope-ratio-monitoring gas chromatography / mass spectrometry

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