Method for optical 15N analysis of small amounts of nitrogen gas released from an automatic nitrogen analyzer

A method of optical 15N analysis is proposed for application to small amounts of nitrogen gas released from an automatic nitrogen analyzer (model ANA-1300, Carlo Erba, Milano) subjected to certain set modifications. The ANA-1300 was combined with a vacuum line attached by a molecular sieve 13X column. The nitrogen gas released from the ANA-1300 was introduced with a carrier gas of helium into the molecular sieve column which was pre-evacuated at 10-4 Torr and cooled with outer liquid nitrogen. After removal of the helium by evacuation, the nitrogen gas fixed on the molecular sieve was released by warming the column, and then, it was sealed into pre-evacuated pyrex glass tubes at 4.5 - 5.0 Torr. In the preparation of discharge tubes, contamination of unlabelled nitrogen occurred from the carrier gas of standard grade helium, and the relative lowering of the 15N value by it was estimated to be less than 1% when over 700 μg nitrogen was charged on the ANA-1300; when 200 μg nitrogen was charged, it was about 3.5%. However, the effect of the contamination could be corrected for by knowing the amount of contaminant nitrogen. In the analysis of plant materials by the proposed method, the coefficient of variation was less than 2%, and no significant difference was observed between results given by the present method and by the ordinary method in which samples were directly pyrolyzed in the discharge tubes by the Dumas method. The present method revealed about 1.5 μg of cross-contaminated nitrogen and was applicable to more than 200 μg of sample nitrogen. (author)