Nitrogen isotope ratio analysis of small samples: sample preparation and calibration

A method for the preparation of samples in the range of 14-100 ..mu..g of organic and precipitated N for mass spectrometric isotope ratio analysis is described. In vacuo combustion and molecular sieve trapping of the sample gas into a 0.5 mL mass spectrometer inlet volume are employed. Samples can be processed at a rate of two to three per hour. The standard deviation of 12 analyses of a peptone preparation was 0.10%. The results of an interlaboratory calibration effort are presented. The materials tested were peptone, thiourea, histidine, kerogen, and two (NH/sub 4/)/sub 2/SO/sub 4/ standards. The results of these comparisons were with the range +/-0.12 delta/sup 15/N or less. Peptone was found to provide a more rigorous test of the combustion and preparation system than nitrogen salts and organic compounds of lower molecular weight. 20 references, 1 figure, 1 table.