Measurement system for total nitrogen and 15N/14N in plant tissue, soil and water

The ability to follow the behaviour of nitrogen in environmental systems has led to the development of an analytical system which can measure the quantity and isotopic abundance of nitrogen in a variety of samples. A reductive pyrolysis unit and a directional focussing 6-inch radius gas mass spectrometer were combined into the system. In the reductive pyrolysis part of the system, nitrogen species are converted to ammonia in an atmosphere of hydrogen in the presence of a heated nickel catalyst. A small amount (approximately 5%) of the resulting ammonia is directed into a cold finger reaction vessel where it is converted to nitrogen for isotopic analysis by mass spectrometry. The ammonia is decomposed thermally to nitrogen and hydrogen employing a rhenium filament heated to 1050degC. The nitrogen from the decomposition is used for measuring the abundance of masses 28 and 29; the 15N abundance is calculated from the measured ratios. The system has been used to analyse vegetation and water samples with precisions of 0.5% of the 15N/14N value at the normal isotopic range and 6% for the quantitative analysis. The minimum sample size required for a complete analysis is 20 μg of nitrogen. Three samples can be completed in an hour using multiple reaction vessels. (author)