Measurement of Nitrogen and Nitrogen Isotopic Ratios Using Reduction Pyrolysis Coupled with Mass Spectrometry

Abstract

An analytical system for measuring total nitrogen and its isotopic abundance in a variety of environmental samples has been developed. A reductive pyrolysis system and a directional focusing 6-inch gas mass spectrometer were combined into the analytical system. In the reductive part of the system, nitrogen species are converted to ammonia with an atmosphere of hydrogen in the presence of a heated nickel catalyst. Five percent of the gas stream is split away for measuring total nitrogen by a conductivity detector. The ammonia is removed from the gas stream employing a cold finger reaction vessel.

The hydrogen-free ammonia is decomposed thermally to nitrogen and hydrogen at 1000°C, employing a hot rhenium filament. The N₂ produced from the decomposition is used for measuring the abundance of masses 28 and 29 by mass spectrometry. From this ratio, the ¹⁵N atom fraction is calculated.

Standard samples of N₂, ammonia, orchard leaves and urea have been successfully analyzed to determine isotopic compositions. Samples containing as little as 20 μg of total nitrogen can be analyzed by this system. By the addition of multi-reaction vessels, three samples may be completed per hour.