NITROGEN AND N-15 ANALYSIS USING AUTOMATED MASS AND EMISSION SPECTROMETERS

Analysis of large numbers of samples has been a major limitation in the use of N-15. Two new instruments-an automatic N analyzer coupled to a mass spectrometer (ANA/MS), and a microprocessor-controlled emission spectrometer (NOI-6e)-were therefore compared with a longer established procedure of the International Atomic Energy Agency Laboratories involving N-15 analysis with a Micromass (MM) 602E mass spectrometer. For 10-mg plant samples ranging from natural abundance (NA) to 1.341 atom % N-15, the ANA/MS agreed with the MM 602E to within 0.002 atom %, and the NOI-6e to within 0.014 atom %. In the range 1.895 to 2.398 atom %, data from the ANA/MS and the NOI-6e agreed with the MM 602E within 0.022 and 0.093 atom %, respectively. Relative standard deviations (RSD) for six replications of a NA plant sample were: MM 602E, 0.22%; ANA/MS, 0.47%; NOI-6e, 2.3%. For a plant sample with 2.398 atom % N-15 the RSDs were: MM 602E, 0.03%; ANA/MS, 0.08%, and NOI-6e, 0.3%. With finely ground soil samples (40 mg, 0.449 atom %) the RSD for N-15 analysis with the ANA/MS was 0.30%. The ANA/MS was comparable to the Kjeldahl method (500-mg samples) for plant N analyses and was found suitable for routine N-15 analysis of plant and soil samples, but additional care in sample grinding was needed. The NOI-6e analyzed N-15 samples in one-tenth the time required with previous emission spectrometers and its precision would be adequate for many agricultural experiments. The lower precision is a limitation, however, for measuring the N-15 content in soil samples.