Top panel: spectrum of a given pixel (in black) along with the XCLASS fit for CH₃CN (12_K−11_K), K = 0−6 and lines (in solid red). The corresponding fit parameters are provided in the panel. Bottom panel: spectrum of the same pixel as above (in black) with the XCLASS fit for CH₃CN (12_K−11_K), K = 4−6 and (in solid red). The dashed red line corresponds to the predicted spectrum for the CH₃CN lines that were not used in the fitting process. This shows that the exclusion of the low-K lines in the fitting process allows XCLASS to provide a better fit for the optically thinner high-K lines. The bright line detected between the K = 4 and 5 components is C₂H₅CN.