also a tracer of the $^{12}$C/$^{13}$C ratio. Although CN has a distinct hyperfine structure, which can be used to directly evaluate opacities (@2002ApJ...578..211; @2005ApJ...634...1126), it is still difficult to obtain a true $^{12}$C/$^{13}$C value from $^{12}$CN/$^{13}$CN because of sometimes large opacities of $^{12}$CN. Milam et al. ([-@2005ApJ...634...1126]) proposed a gradient of $^{12}$C/$^{13}$C with Galactocentric distance, from a combination of all previously published measurements of CN, C$^{18}$O and H$_2$CO. The goal of our study is to critically review their results using new distances and H$_2$CO collision rates as well as a larger sample of sources and to examine in how far previous results have to be modified.

Thus we have performed a more systematic study of H$_2$CO and H$_2$$^{13}$CO toward a big sample of Galactic molecular clouds to determine $^{12}$C/$^{13}$C isotope ratios more accurately. Based on a TMRT formaldehyde survey at C-band ($\sim 5$ GHz; Li et al. 2019, in preparation), we chose sources with strong H$_2$CO signals as our targets for deep integration. Some previously studied strong H$_2$CO sources with absolute flux densities larger than 0.5 Jy (Wilson et al. ; Henkel et al. ,,; Araya et al. [-@2007ApJS...170..152]) are also included in our sample. H$_2$CO collision rates were taken 