The samples were measured in duplicate. The spectra were collected continuously over a wavelength range of 600–4000 cm1 with a data resolution of 4 cm1. Each spectrum consists in the average of 256 individual readings. Every 10 min a new air background spectrum was collected. The procedure to clean the ATR crystal consists of 3 steps: (a) to remove the sample from the surface of the ATR crystal using a tissue; (b) to clean the ATR crystal with an tissue wet with ethanol; (c) to dry the ATR crystal with a tissue.