N(¹²C)/N(¹³C) column density ratio as a function of the galactocentric distance R_G. The red squares are from this work, and therefore evaluated with measurements of the N(¹²CH⁺)/N(¹³CH⁺) column density ratio. The blue circles are from previous measurements of the N(¹²CN)/N(¹³CN), N(¹²CO)/N(¹³CO), and column density ratios (Milam et al. 2005 and references therein) while the green curve corresponds to a linear least-squares fit applied to these data.