Comparison of N(H₂) and the CO isotopologues column density. Each group of sources is shown with a different symbol and colour, as indicated. The stars refer to sources of subsample S3, while their colour still identifies the group (red = IRB, green = RMS, black = D8, and blue = D24). The dashed lines indicate the canonical abundance assuming local isotopic ratios. A typical uncertainty (for subsamples S1 and S2) is shown in the top left corner of each panel.