Results of molecular line analysis of the GCMF. Left: ¹²C/¹³C ratio of various species vs corresponding cloud velocities. The yellow dotted line corresponds to a carbon isotope ratio of 24, the value expected at the GC. The dash-dotted line corresponds to ¹²C/¹³C = 40 and the dashed line corresponds to ¹²C/¹³C = 65, the carbon isotope ratio of the local ISM in the solar neighbourhood. Right: ¹²C/¹³C derived from I[C¹⁸O]/I[¹³C¹⁸O] (for GCMF; red triangles) as a function of galactocentric distance. Distances to cloud components are determined using the ¹²C/¹³C gradient across the Galaxy (Yan et al. 2023). The shaded area corresponds to the 1σ interval of the fit. The grey and black dots correspond to isotope ratios from C¹⁸O and its isotopologue from Giannetti et al. (2014) and Langer & Penzias (1990), respectively.