¹²C/¹³C isotope ratios from C³⁴S/¹³C³⁴S, CN/¹³CN, C¹⁸O/¹³C¹⁸O, , CH⁺/ ¹³CH⁺, and CH/¹³CH are plotted as functions of the distance from the Galactic center. The red symbol ⊙ indicates the ¹²C/¹³C isotope ratio of the Sun. The filled black circles are the results obtained from C³⁴S with corrections of opacity in the current work, and the resulting first-order polynomial fit is plotted as a solid line, with the gray-shaded area showing the 1σ interval of the fit. The open black circles are the 3σ lower limits obtained from nondetections of ¹³C³⁴S in the current work. The blue triangles, orange pentagons, yellow stars, green squares, and green diamonds are values determined from CN (Savage et al. 2002; Milam et al. 2005), C¹⁸O (Langer & Penzias 1990; Wouterloot & Brand 1996; Keene et al. 1998), H₂CO (Henkel et al. 1980, 1982, 1983, 1985; Yan et al. 2019), CH⁺ (Ritchey et al. 2011), and CH (Jacob et al. 2020), respectively, using the most up-to-date distances. The red crosses visualize the results from the GCE model of Kobayashi et al. (2011, 2020, see also Sect. 4.9).