The abundance of the nitrogen isotopes in several sources of nitrogen have been determined. In atmospheric nitrogen the absolute ratio, N14/N15, is 272.0 ± 0.3. Small variations were observed for various sources of compressed gas but the isotopic composition of the nitrogen isotopes of the atmosphere was constant, to 1 part in 7000, in samples collected at different geographical sites and altitudes above these sites.
Junk, Gregor Albert and Svec, H. J., "Nitrogen isotope abundance measurements" (1958). Ames Laboratory ISC Technical Reports. 208.