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

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 (N(2)O) instead of Freon (CCl(2)F(2) or CHClF(2)) for the purging of air prior to hypobromite oxidation of ammonium-N to N(2) 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 % (15)N, isotope-ratio analyses of 50 to 200 mug 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 20mug of N were accurate to within 2%, and automated analyses of 10 mug of N were accurate to within 7%. The relative standard deviation for measurements at the natural abundance level (10 analyses, 20-200 mug of N) was < 0.04 %.