protoplanetary disc. ]{} [The photodissociation rate of in a Draine field assuming 30K excitation is $1.73\times 10^{-10}\,\mathrm{s}^{-1}$, within 4% of the rate for , and the rate due to cosmic ray induced photons [assuming an ionisation rate of $\zeta=10^{-16}$s$^{-1}$]{} is about $10^{-15}\,\mathrm{s}^{-1}$, with up to a factor of 10 difference between isotopologues. Shielding functions for by , , and H are presented. [Incorporating these into an interstellar cloud model, an enhancement]{} of the atomic $\mathrm{{}^{15}N/{}^{14}N}$ ratio over the elemental value is obtained due to the self-shielding of external radiation at an extinction of about 1.5mag. This effect is larger where assumed grain growth has reduced the opacity of dust to ultraviolet radiation. The transfer of photolytic isotopic fractionation of N and N$_2$ to other molecules is demonstrated to be significant [in a protoplanetary disc model with grain growth]{}, and is species dependent with enhancement approaching a factor of 10 for HCN. [The cosmic ray induced dissociation of CO is revisited employing a more recent photodissociation cross section, leading to a rate that is $\sim$40% lower than previously calculated.]{} ]{}

Introduction {#sec:intro}
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Nitrogen is the fifth most abundance element in the universe and comprises two isotopes with relative populations ${}^{14}\mathrm{N}/{}^{15}\mathrm{N}\simeq 