Spectra of lines from CH₃OH, N₂H⁺, H₂CO, and CH₂NH, observed simultaneously on 2019 July 10 (B4a, in red) and August 17 (B4b, in blue). The difference spectra (B4b–B4a) are shown in green, multiplied by a factor two and shifted to unity. The best Gaussian fit to each of the difference spectra is overlaid as a dotted line, the cross indicating the velocity centroid, amplitude, and full-width at half maximum of the Gaussian.