Example spectra for each of the profile classes (in Wm² µm⁻¹), normalised to the integrated flux of the 7.7 µm feature. The solid vertical lines drawn at 7.7 and 8.2 µm illustrate how the peak wavelength of the 7.7 µm complex changes with profile class. Spectra belonging to Class A peak shortward of 7.7 µm and spectra belonging to Class C peak around 8.2 µm whereas spectra belonging to Classes B or D peak in between. These examples are representative for Classes A and C as those classes only exhibit small variations, but not for Classes B and D.