Measurement of 14N : 15N ratios by Fourier transform infrared spectrometry

Abstract

An approach to the measurement of isotopic composition is described, illustrated by the measurement of ¹⁴N : ¹⁵N ratios in enriched biological materials. The nitrogen in the samples is converted to ammonia gas which is analysed by Fourier transform infrared spectrometry (FTIR). When spectra are recorded at a resolution of 1 cm^–1 the isotopic shift is sufficient for the areas of corresponding lines in the v₂ bands of ¹⁴NH₃ and ¹⁵NH₃ to be measured precisely, thus enabling the composition of samples to be determined by reference to standard calibration spectra. The standard deviation at the natural abundance level of 0.366%¹⁵N is 0.002 at.-%. This paper also describes the decomposition of the sample and the conversion of the nitrogen to ammonia gas.