Intensities of the 6.9 (top) and 7.25 µm (bottom) aliphatic bands compared to the 7.7 µm feature intensities normalised to the intensity of the 6.2 µm feature. Filled markers represent sources where both aliphatic features have been detected, while unfilled markers represent sources where only the 6.9 µm feature has been detected. The correlation coefficient is 0.84 and 0.38 for the 6.9 and 7.25 µm bands, respectively.