Spectra of HN¹³C(1–0) obtained for the sample of sources. For each spectrum the x axis represents a velocity interval of ±20 km s⁻¹ around the velocity, v₀, obtained from H₂CO observations by Blair et al. (2008). The y axis shows the intensity in main beam temperature units. The red curves are the best LTE fits obtained with MADCUBA for detections and tentative detections (indicated with an asterisk in the source name). The blue curves correspond to the upper limits obtained for non-detections.