Plan view of the Milky Way. The Galactic centre (red asterisk) is at (0,0), and the Sun (red Sun symbol) is at (0,8.34). The background grey discs correspond to the Galactic bar region (~4 kpc), the solar circle (~8 kpc), co-rotation of spiral pattern (~12 kpc), and the edge of major star-formation regions (~16 kpc). The solid black lines indicate the centre of spiral arms traced by masers, and the dotted lines the 1σ widths. For more details, see Reid et al. (2014). The filled circles represent the sources studied in this work and in Colzi et al. (2018b), and the three colours are the ¹⁴N/¹⁵N ratios measured for HCN: in red values < 250, in green values in between 250 and 450, and in blue ratios > 450.