Determination of vinegar acidity by attenuated total reflectance infrared measurements through the use of second-order absorbance-pH matrices and parallel factor analysis.

Univariate (zero-order), multivariate (first-order) and multiway (second-order) calibrations were assayed for the determination of vinegar acidity using a mechanized procedure based upon vibrational spectroscopy and the emerging multicommutation methodology. The second-order methodology relies on the use of a flow system based on multicommutation and binary sampling. The flow network comprises a set of three-way solenoid valves, computer-controlled to provide facilities to handle the sample and to generate a time-dependent pH gradient using two carrier solutions. The procedure is based on the volumetric fraction variation approach that maintains the same volume of sample solution and dynamically varies its pH. The analysis of second-order absorbance-pH matrices was performed using parallel factor analysis (PARAFAC). Results were compared with first-order absorbance data analyzed with linear calibration and partial least squares regression (PLS) and they were employed for a discussion of the relative advantages of the applied chemometric tools. As excellent accuracy is obtained without the need of any sample pre-treatment, the procedure can be fully mechanized (i.e., by means of an auto-sampler device). Accuracy of the different strategies assayed was assessed by comparing the results achieved with those obtained by titration reference procedure.