Mass-Spectrometric Assay of Elementary Nitrogen

Abstract

THE method generally used for the estimation of nitrogen-15 in an isotopically enriched organic compound involves the conversion of the organic nitrogen, first to ammonia, and afterwards (by oxidation of the ammonia with alkaline sodium hypobromite) to molecular nitrogen, which is assayed in the mass-spectrometer. Such an assay depends upon the measurement of the intensities of the peaks at mass numbers 28 and 29 due to the ionic species ¹⁴N¹⁴N⁺ and ¹⁴N¹⁵N⁺, respectively. Other substances, which can give rise to ions having similar mass/charge ratios, such as acetone (Me₂CO⁺⁺) and carbon dioxide or carbon monoxide (¹²C¹⁶O⁺ and ¹³C¹⁶O⁺), will also contribute to the observed intensities of these peaks ; and corrections for these have to be applied before the intensities of the peaks can be used for calculating the nitrogen-15 content of the nitrogen sample. A correction is also required to allow for the normal nitrogen introduced by the inclusion of air in the gas sample. The accuracy of the determination of nitrogen-15 therefore depends upon the elimination of contaminants so far as possible, and upon the application of adequate corrections for those contaminants which cannot be removed.