Exploration of Case 1 fitting of the whole IRDC clump sample, including trends with density, using models with gas temperature of T = 15 K, A_v = 20 mag, CO ice binding energy of 1100 K and exploring effects of cosmic ray induced desorption parameter T_crid. Left column panels show f_D versus n_H; middle column panels show [HCO+]/[CO] versus n_H; right column panels show [N2H+]/[CO] versus n_H. In all panels, black points are the actual observed data (with HCO⁺ obtained from HC¹⁸O⁺), while the grey points assume CO abundance is reduced by a factor of 2 (due to CO envelope contamination). (a) First row: high CRIR case with ζ = 1.0 × 10⁻¹⁶ s⁻¹. Solid lines are models with T_crid = 80 K and dotted lines are those with T_crid = 85 K. As shown in the legend, the various colours correspond to models from 0.1 to 2 Myr. (b) Second row: case with ζ = 1.0 × 10⁻¹⁷ s⁻¹. Solid lines are models with Tcrid = 100 K and dotted lines are those with T_crid = 90 K. (c) Third row: case with ζ = 4.6 × 10⁻¹⁸ s⁻¹. Solid lines are models with T_crid = 100 K and dotted lines are those with T_crid = 95 K. (d) Fourth row: case with ζ = 2.2 × 10⁻¹⁸ s⁻¹. Solid lines are models with T_crid = 100 K and dotted lines are those with T_crid = 105 K.