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 analysis by the Rittenberg tech nique 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 N-2 in a plastic microplate. Analytical per formance 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 % N-15, isotope-ratio analyses of 50 to 2 00 mu g of N using the automated Rittenberg apparatus (ARA) with a dou ble-collector mass spectrometer were accurate to within 0.7%, as compa red to manual Rittenberg analyses of 1 mg of N using the same mass spe ctrometer with a dual-inlet system. Automated analyses of 20 mu g of N were accurate to within 2%, and automated analyses of 10 mu g of N we re accurate to within 7%. The relative standard deviation for measurem ents at the natural abundance level (10 analyses, 20-200 mu g of N) wa s <0.04%.