Comparison of inferred column densities over the size of the emitting region and upper limits with shock model results from Neufeld & Dalgarno (1989) for a pre-shock density of 10⁶ cm^-3 and shock velocity of 80 km s^-1. Observations are marked with black dots and arrows are for upper limits. For the cases where both a black dot and arrow are present (OH⁺ and C⁺) the dot marks the detection and the arrow the upper limit towards the other sources; when two arrows are present (CH and HCO⁺) they illustrate the range of upper limits. Model results are shown as red circles and are normalised to the inferred CO column density.