HCO⁺/HCN and HNC/CO luminosity ratios in our sample (solid circles). We also include data from Yang et al. (2023) for comparison (empty squares). The shaded regions indicate HCO⁺/HCN and HNC/HNC ratios for the EMPIRE sample (Jiménez-Donaire et al. 2019). HCN is the brightest line in most of our targets. SDP.81 shows strong HCO⁺ enhancement, potentially due to low metallicity (Rybak et al. 2023). The HCN(3–2) emission line in SDP.11 is unusually bright, potentially due to AGN-driven chemistry.