Use of nitrous oxide as a purge gas for automated nitrogen isotope analysis by the Rittenberg technique

Abstract

An apparatus that operates with an isotope-ratio mass spectrometer to automatically perform nitrogen isotope analyses by the Rittenberg technique was modified to permit the use of nitrous oxide (N₂O) instead of Freon (CCl₂F₂ or CHClF₂) for the purging of air prior to hypobromite oxidation of ammonium-N to N₂ in a plastic microplate. Analytical performance was unaffected by the modifications. Up to 768 samples can be processed in a single loading, at a rate of 6 to 12 samples/h. Within the range of 0.2 to 20 atom % ¹⁵N, isotope-ratio analyses of 50 to 200 μg of N using the automated Rittenberg apparatus (ARA) with a double-collector mass spectrometer were accurate to within 0.7%, as compared to manual Rittenberg analyses of 1 mg of N using the same mass spectrometer with a dual-inlet system. Automated analyses of 20μg of N were accurate to within 2%, and automated analyses of 10 μg of N were accurate to within 7%. The relative standard deviation for measurements at the natural abundance level (10 analyses, 20-200 μg of N) was < 0.04 %.