the relation between $N$ and $\tau$ depends only mildly on the temperature. Hence, the retrieved column density values are not much affected by the inaccuracies in the corresponding $T_{\ensuremath{\mathrm{ex}}}$ determination.

The derivation of sound values of the isotopic ratios involving is less straightforward, as various approaches can be adopted to estimate the overall column density from the available observational results. By assuming that the two spin species are in thermal equilibrium, total $N$ values of $0.70\times 10^{13}$cm$^{-2}$ and $0.98\times 10^{13}$cm$^{-2}$ can be obtained from the $o$- and $p$- results, respectively. A direct sum of the *ortho* and *para* column density with no assumptions gives instead $0.90\times 10^{13}$cm$^{-2}$. Based on the resulting 30% discrepancies, we conservatively quote $N = (0.9\pm 0.3)\times 10^{13}$cm$^{-2}$ as the total column density of . This yields $[\ce{ND2}]/[\ce{NHD}] = 0.19\pm 0.07$ and $[\ce{ND2}]/[\ce{NH2}] = 0.17\pm 0.06$, that is, deuteration levels in the 10–30% range. The amidogen D/H ratio measured in the low-mass protostar IRAS16293 is very high and is comparable to the one derived for the related species imidogen (\[\]/\[\]$=30-70$%, @bacmann2010first), while ammonia shows a lower level (\[\]/\[\]$\sim 10$%, @van1995molecular). The species is very abundant, yielding a remarkably high deuteration ratio, even higher than the one found 