REFINEMENT AND EVALUATION OF AN AUTOMATED MASS-SPECTROMETER FOR NITROGEN ISOTOPE ANALYSIS BY THE RITTENBERG TECHNIQUE

An apparatus designed to automatically perform hypobromite oxidations of ammonium salt samples for nitrogen isotope analyses with a mass spectrometer was modified to improve performance and reduce analysis time. As modified, reference N2 is admitted to the mass spectrometer between samples from a dedicated inlet manifold, for calibration at the same pressure as that of the preceding sample. Analyses can be performed on samples containing 10-mu-g to 1 mg of N (or more), at a rate of up to 350 samples/day. When operated with a double-collector mass spectrometer, the standard deviation at the natural abundance level (10 analyses, 50-150-mu-g N) was < 0.0001 atom % N-15. Very little memory was observed when natural abundance samples (0.366 atom % N-15) were analysed following samples containing 40 atom % N-15. Analyses in the range, 0.2 to 1 atom % N-15 (50-150-mu-g N), were in good agreement with manual Rittenberg analyses (1 mg N) using a dual-inlet system, and precision was comparable. For enrichments of 2 to 20 atom % N-15, automated analyses were slightly lower than manual analyses, which was attributed to outgassing of N2 from the plastic microplate used to contain samples.