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

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 induced by the pH change of the sample. The pH modulation was carried out in a novel sheath-flow cell which was connected to a sequential injection analysis system. The sheath-flow cell comprised three stream lines flowing adjacent to each other in a strongly laminar fashion. The sample (pH∼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 maintaining the flow, hence allowing the measurement of the sample spectrum at alkaline pH (∼9). Upon stopping the flow, diffusion of protons from the outer stream lines into the central line occurred resulting in a complete protonation of the analyte (pH∼2). The spectral changes were 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 standards containing the analytes but no matrix molecules were used. By evaluation 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 natural soft drinks.