Top panels: column density of HCN compared with that of HC¹⁵N (left) and of HNC compared with that of H¹⁵NC (right). Bottom panels: comparison between the ¹⁴N/¹⁵N isotopic ratios derived from the column density ratios N(HCN)/N(HC¹⁵N) and N(HNC)/N(H¹⁵NC) (left) and comparison between the H/D and ¹⁴N/¹⁵N isotopic ratios in HNC (right). In all panels, the filled circles represent the detected sources (black = HMSCs; red = HMPOs; blue = UC HIIs). The open triangles in the top panels are the upper limits on N(H¹⁵NC) (right) and N(HC¹⁵N) (left), while in the bottom panels the open triangles indicate lower limits on either N(HCN)/N(HC¹⁵N) or N(HNC)/N(H¹⁵NC). The filled squares represent tentative detections. The solid lines in the top panels indicate the mean atomic composition as measured in the terrestrial atmosphere (TA) and in the protosolar nebula (PSN), while in the bottom left panel, the solid line indicates the locus of points where N(HCN)/N(HC¹⁵N) is equal to N(HNC)/N(H¹⁵NC).