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 μg 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 μg N) was <0.0001 atom % ¹⁵N. Very little memory was observed when natural abundance samples (0.366 atom % ¹⁵N) were analysed following samples containing 40 atom % ¹⁵N. Analyses in the range, 0.2 to 1 atom % ¹⁵N (50–150 μg 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 % ¹⁵N, automated analyses were slightly lower than manual analyses, which was attributed to outgassing of N₂ from the plastic microplate used to contain samples.