Sheath-flow Fourier transform infrared spectrometry for the simultaneous determination of citric, melic and tartaric acids in soft drinks

A new approach for the determination of organic acids in soft drinks based on pH modulation and Fourier transform infrared spectroscopic detection is presented. The analytical readout was taken from the spectral changes induc ed by the pH change of the sample. The pH modulation was carried out in a n ovel sheath-flow cell which was connected to a sequential injection analysi s system. The sheath-flow cell comprised three stream lines flowing adjacen t to each other in a strongly laminar fashion, The sample (pH similar to 9) was introduced in the central channel whereas reagent (hydrochloric acid) was introduced in the outer channels. As a consequence of the laminar flow profile, hardly any mixing between the stream lines was observed while main taining the flow, hence allowing the measurement of the sample spectrum at alkaline pH (similar to 9). Upon stopping the flow, diffusion of protons fr om the outer stream lines into the central line occurred resulting in a com plete protonation of the analyte (pH similar to 2). The spectral changes we re calculated and the region between 1400 and 1180 cm(-1) used to set-up a partial least squares (PLS) calibration model. For the PLS model only stand ards containing the analytes but no matrix molecules were used. By evaluati on of the spectral changes induced by the pH modulation the PLS model could successfully be applied to test samples containing sugars as well as to na tural soft drinks. (C) 2000 Elsevier Science B.V. All rights reserved.