Fractionation ratios DCO⁺/HCO⁺ and N₂D⁺/N₂H⁺ as functions of ${\zeta }_{{\text{H}}_2}$ (left) and time (right) according to our chemistry model. The upper row (a and b) corresponds to a model with the grain radius a = 0.1 µm, whereas the simulation used for the bottom row (c and d) assumes a = 0.3 µm. The cyan curve shows the prediction from a steady-state formula derived for a chemical network that includes multiply deuterated forms (see Sect. 7). The dashed red curve shows the ratio $1/3\times {\text{H}}_2{\text{D}}^{+}/{\text{H}}_3^{+}$.