Determination of lime juice adulteration, using PTR -ToF/MS and chemometrics approach

Introduction: As the lime juice adulteration is an ongoing problem, suitable analytical methods are needed for rapid detection of lime juice adulteration. Aroma profile of the volatile fraction of lime juice is a useful marker for its quality control. As far as we know, there is no method reported in the literature for evaluation of VOCs of lime juice using PTR-ToF-MS. Therefore, the aim of this study is to develop for the first time a PTR-ToF-MS method combined with chemometrics approach as a non-targeted technique for detection of adulterated lime juice samples.Methods: Fifteen Iranian genuine lime juice samples and 28 adulterated samples with the citric acid to iso-citric acid ratio over 300 (based on our previous study applying LC-MS/MS) were used in this study. For PTR-ToF-MS measurement, 500 μl of each sample was diluted with 9500 μl of purified water, and 500 μl of the mixture was introduced to PTR-ToF-MS machine. For each sample, mass spectral data was collected in duplicate over 70 cycles in the mass range from 15 to 506 m/z. Principal component analysis (PCA) was performed on 290 variables in the whole averaged spectrum of genuine and adulterated samples. Partial Least Squares Discriminant Analysis (PLS-DA) following different data pre-processing was performed to make a predictive model.Results: In all genuine lime juice samples, ions with the m/z values of 81 and 137 had the highest concentrations, and were identified as cyclohexadiene and limonene, respectively. By performing PCA on log10 transformed and then mean-centered data, almost all genuine samples were separated from adulterated ones. Regarding PLS-DA analysis, the best result was achieved with mean-centering and then log10 transformation. A predictive model of 10 genuine and 20 adulterated samples (training set; 70% of samples) delivered an accuracy of 100% on hold-out samples (test set; 30% of samples) using 4 factors for each group. Furthermore, propionic acid/methyl acetate, cycloheptan, furfural, p-alpha-Dimethyl styrene, 2-Isopropyl-N, 2,3-trimethylbutyramide, and 2-Octylfuran were identified as classifier mass peaks in adulterated samples.Conclusion: This is the first study focusing on the application of PTR-ToF-MS technique as a rapid technique for detection of lime juice adulteration based on the VOCs profile. The result of this study holds a great promise for future application of PTR-ToF-MS and classification methodology in detection of lime juice adulteration. Further model validation using lime juice samples from a wider range of variability is required to confirm the promising results of this study.

Keywords:

PTR-ToF/MS , Lime juice , Adulteration , Chemometrics , PLS-DA