PARTIAL LEAST-SQUARES FOURIER-TRANSFORM INFRARED SPECTROMETRIC DETERMINATION OF METHANOL AND ETHANOL BY VAPOR-PHASE GENERATION

A partial least-squares (PLS)-FTIR procedure is proposed for the direc t determination of methanol and ethanol in liquid samples, based on va pour generation from small injected volumes of untreated samples, vola tilized inside a Pyrex glass reactor, transported into a multi-pathlen gth cell with nitrogen carrier gas and then continuously measured by F TIR spectrometry, The spectra were processed using PLS regression, Sev eral sets of standards and different wavenumber ranges were tested for calibration. A calibration set integrated by aqueous solutions of eth anol from 2.5 to 22.5% v/v and aqueous solutions of methanol from 0.5 to 3.5% v/v provided the lowest prediction errors. Average relative pr ediction errors for the PLS model of about 3% for ethanol and 2% for m ethanol were obtained for aqueous mixtures of the two compounds studie d, with RSDs of 2.7 and 2,3%, respectively, calculated from four injec tions of 1 mu l of sample, providing the method with a sampling freque ncy of 69 h(-1). Results predicted by PLS-FTIR were compared with thos e found using a proportional equations approach. The method was applie d to the determination of methanol and ethanol in high-level alcoholic beverages and cosmetic products, obtaining average recoveries of 107 +/- 6 and 98 +/- 2%, respectively.