Automatic emission spectrometer for the determination of nitrogen-15

An automatic nitrogen-15 analyser, employing a novel use of a rhodium-platinum catalyst for the generation of nitrogen and capable of analysing 60 samples per hour, is described. Nitrogen compounds of biological origin are first converted into ammonium chloride by conventional Kjeldahl digestion and distillation methods. The ammonium chloride sample (about 5 µl containing about 10 µg of nitrogen) is injected into a soda-lime reactor at 590 °C through which flows a stream of purified helium. Ammonia that is released passes directly into the catalyst tube and the generated nitrogen and hydrogen are separated by passage through a gas-chromatographic column, which also retains the water.After passing through a pressure restrictor the nitrogen flows in the helium stream through a Spectrosil discharge tube located in a microwave cavity. The emitted radiation is analysed by means of a specially constructed dual-wavelength monochromator and the intensities of the 14N14N (297.7 nm) and 14N15N (298.3 nm) bands are measured simultaneously by two photomultipliers. Amplified signals, proportional to the peak intensities, are fed through phase-sensitive detectors into a ratiometer, the output from which is fed to a digital voltmeter and printed out in terms of nitrogen-15 abundance. A peak detector indicates the total nitrogen content of each sample and actuates the nitrogen-15 print-out.The response of the instrument is slightly curvilinear but may be regarded as linear over limited ranges. Calibration can therefore be achieved by running suitably chosen standards to fix upper and lower set points. Carryover between samples is very small and is eliminated by running duplicates. Standard deviations of replicate measurements of natural abundance and enriched standards are less than 0.01 atom-%, while determinations of nitrogen-15 in biological samples were shown to be accurate to ± 0.01 atom-% by comparison with a Statron NOI-4 nitrogen-15 analyser.