CN/CO(1–0) vs. HNC/CO(1–0) (top), HCO⁺/CO(1–0) (middle), and HCN/CO(1–0) (bottom) line luminosity ratios for the different line components of Mrk 231 (narrow core, blue wing, red wing, and total emission), including literature samples of nearby galaxies (grey circles) and GMCs (grey stars). Measurements corresponding to extragalactic molecular outflows (NGC 253 and NGC 3256, see Table B.1) are highlighted with purple circles. The CN(1–0) luminosity corresponds to the sum of the two spingroup components. When only the brightest J = 3/2−1/2 spingroup line flux is available in the literature, we extrapolated the total CN(1–0) flux by multiplying this value by 1.5. The dashed line indicates the 1:1 relation. All error bars include an 10% calibration uncertainty, which was added in quadrature to the statistical error (1σ). Upper limits are shown at the 3σ level. Sources with are labelled in the plot.