Spectra toward selected positions in the GC in HCO⁺, H¹³CO⁺, HCN, H¹³CN, HNC, and HN¹³C. The positions are indicated in the HCO⁺ integrated intensity map from Riquelme et al. (2010). The main isotope is plotted in black solid line. The ¹³C isotope is plotted in red dashed line. The ¹³C substitution is scaled by a factor of 2 or 5 to allow an easy visualization of the spectra. The factor is indicated in each spectrum.