Plot of ¹²C/¹³C isotope ratios as a function of galactocentric distance, R_GC (kpc). The filled red circles represent the ¹²C/¹³C ratio obtained using CH (this paper), while the empty black and purple triangles, grey pentagons, yellow circles, dark blue squares, and green, light blue diamonds and tan stars are those obtained using isotopologues of C¹⁸O (J = 1− 0 and J = 2−1 transitions) (Wouterloot & Brand 1996; Giannetti et al. 2014), CN (Savage et al. 2002; Milam et al. 2005), CO (Langer & Penzias 1990), CH⁺ (Ritchey et al. 2011), H₂CO (Henkel et al. 1985; Yan et al. 2019), and complex organic molecules (Halfen et al. 2017), respectively. The black solid line represents the weighted fit to the data, and the grey shaded region demarcates the 1σ interval of this fit. For comparison, the fit obtained by Halfen et al. (2017) is displayed by the dashed blue line.