Equivalent-width velocity (V_EW) versus upper-level energy (E_u) for HCO⁺, H¹³CO⁺, SO⁺, N₂H⁺, and H₃O⁺ transitions (filled symbols) and ¹³CO, for comparison (empty circles; ¹³CO profiles and line parameters are given in the Appendix). The dashed line is a fit to the V_EW of the ¹³CO lines. For all ions, except for H₃O⁺, the values of V_EW are systematically larger than for ¹³CO, indicating a larger wing-to-core emission contribution to the observed profile; the H₃O⁺ emission, in contrast, is dominated by the narrow, core emission component as indicated by the small V_EW.