Two very similar expressions for  are in wide use in hydrology.  Both have the form  ‰ (‰ = permil or parts per thousand) where s and a are the relative abundances of N in respectively the sample and the atmosphere.  The difference is whether the relative abundance is with respect to all the nitrogen, i.e. N plus N, or just to N.  Since the atmosphere is 99.6337% N and 0.3663% N, a is 0.003663 in the former case and 0.003663/0.996337 = 0.003676 in the latter.  However s varies similarly; for example if in the sample N is 0.385% and N is 99.615%, s is 0.003850 in the former case and 0.00385/0.99615 = 0.003865 in the latter.  The value of  is then 51.05‰ in the former case and 51.38‰ in the latter, an insignificant difference in practice given the typical range of -20 to 80 for .