Comparisons of column densities in 0.8 km s^–1 velocity channel bins for seven molecular species; (a) HCN versus HCO⁺, (b) HNC versus HCO⁺, (c) C₂H versus HCO⁺, (d) c-C₃H₂ versus HCO⁺, (e) CS versus HCO⁺, and (f) H₂S versus HCO⁺. The color codes correspond to specific spiral arms (local arm in cyan, Perseus arm in black, Aquila-Rift in blue, Sagittarius-Carina arm in magenta, Scutum-Centaurus arm in green, inter-arm regions in orange). All symbols without black edges indicate upper limits for nondetections, while the data points above the detection threshold are marked with black edges. In addition, the blue-green colored points indicate clouds that belong to the 135 km s^–1 arm located beyond the Galactic center, toward G10.47 + 0.03. The black dotted lines represent the median value of the column density ratio in each panel. Uncertainties in the derived channel-wise column densities are presented as error bars. The red lines present a best-fit taking into account uncertainties of column densities. The gray areas are 2σ confidence interval for the best fit (red lines) with errors.