Direct determination of ethanol and methanol in liquid samples by means of vapor phase-Fourier transform infrared spectroscopy

A FTIR procedure is proposed for the direct determination of methanol and ethanol in liquid samples, such as alcoholic beverages and eau-de-cologne, based on vapor generation from small injected volumes of untreated samples into a heated Pyrex glass reactor in which, at a temperature of 80°C, ethanol and methanol are volatilized and introduced into a long-path IR gas cell by means of a N2 carrier flow. The IR spectra obtained present two characteristic ethanol bands (1050 and 880 cm−1) and a single characteristic methanol band (1030 cm−1). The measurement of the area of the transient recording obtained for the wavenumber range between 1025–950 and 950–820 cm−1 allows us the determination of ethanol and methanol in the same sample by using a simple proportional equations approach. The limit of detection values found for an injection volume of 1 μl were 0.21% v/v for ethanol and 0.04% v/v for methanol and the relative standard deviation values were between 0.5 and 1.4% for five analyses of samples containing between 30–80% v/v of ethanol and 0.5–3.5% v/v of methanol. Acetone, ethyl acetate, 1-propanol and 2-propanol do not interfere with this determination, at concentration levels about those of methanol.