Elemental abundance and their ratios along the Galactocentric radius provide key information about the chemical evolution of the Milky Way. The 12C/13C ratio, in particular, is widely adopted as a proxy to infer other isotopic ratios, such as 16O/18O and 14N/15N. Traditionally, emission lines of 12CN and 13CN – with their optical depths corrected based on their hyper-fine structure lines – have traditionally been exploited to measure 12C/13C isotopic ratios. However, such a method has major pitfalls, with oversimplified (and overlooked) assumptions in both theory and observational interpretations. The optically-thin satellite-lines of 12CN and 13CN transitions present the advantages over the 'classical method', but still suffers bias from non-trivial effects from non-Local Thermal Equilibrium conditions. Our new measurements of 13C18O and C18O emission lines provide a promising new avenue for isotopic ratio determinations. With it, we further obtained the virial states of low-metallicity molecular clouds in the outer Galactic disk. Our analysis shows that the turbulent kinetic energy cannot support self-gravitational energy in these clouds, suggesting that the magnetic field likely play a dominant role in supporting clouds in such conditions.
