Three-way analysis-based pH-UV-Vis spectroscopy for quantifying allura red in an energy drink and determining colorant's pKa

Abstract

Three-way analysis-based pH-UV-Vis spectroscopy was proposed for quantifying allura red in an energy drink product without the need for chromatographic analysis, and determining the colorant's pKa without using any titration technique. In this study, UV-Vis spectroscopic data matrices were obtained from absorbance measurements at five different pH levels from pH 8 to pH 12 and arranged as a three-way array (wavelength × sample × pH). In the three-way analysis procedure, parallel factor analysis (PARAFAC) was implemented to decompose the three-way array into a set of trilinear components. Each set of three components relates to spectral, pH and relative concentration profiles of allura red and sample matrix in the energy drink. First, UV-Vis spectra of the colorant's acid-base pair and sample's matrix were characterized by using the estimated spectral profile. Then, from the pH profile the pKa value was found to be 11.28 for the related colorant. Finally, allura red in energy drink samples was determined using the estimated concentration curve in the relative concentration profile. In the quantitation procedure, the working concentration range was 0.8-19.2 μg/mL. PARAFAC approach was tested in terms of selectivity, precision, and accuracy of the method. Added recovery results obtained by applying the proposed method to spiked samples were between 101.5% and 103.5%. In the application of the method to the analysis of real samples, successful results were reported. For a comparison, an ultra-performance liquid chromatographic method was developed for the quantitation of the colorant. Compared to the chromatographic method, we observed that PARAFAC model was simple and less expensive without requiring separation.

Fig. 1

Chemical structure of allura red AC (E129).

Fig. 2

UV/VIS spectra of a) allura red and b) real sample containing allura red at five different pH levels.

Fig. 3

Representative illustration of going from a matrix to a three-way array for UV-VIS spectra of samples at different pH media (pH8, pH9, pH 10, pH 11 and pH 12). Here we showed a) data matrix with dimensions 1033 × 52 (wavelength × sample), b) collection of data matrices and c) three-way array.

Fig. 4

Acid-base forms of allura red with (a–c) and without (d–f) the interference. (RF: relative fraction of acid-base species of allura red on the dissociation equilibrium).

Fig. 5

UPLC chromatogram of (a) standard solution of allura red and (b) commercial energy drink sample.