摘要：Guaraná is a tropical fruit that grows in the Amazon basin and the northeastern part of Brazil, from which a popular soft drink is manufactured. The main methylxanthine (MX) in the guaraná powder is caffeine (CF); theobromine (TB) and theophylline (TP) are also present in much lower concentrations. Because of the popularity of this soft drink, and the stimulating effects of MXs, we introduced an undergraduate chemistry project on their extraction from guaraná powder and subsequent analysis. A literature survey showed that there is some variation in the values of λmax and εmax of the three MXs. Using purified authentic samples, we showed that values of λmax and εmax of CF, TB and TP are very close. Consequently, their concentrations in guaraná extract cannot be calculated using a single technique (UV-Vis). After discussing this problem with the students, the following project was carried out: (i) guaraná powder was extracted with acidified aqueous ethanol under different experimental conditions (alcohol volume fraction in the binary solvent mixture, ; powder extraction time, t); (ii) the apparent CF concentration was calculated from the absorbance of the extract at λmax = 272.5 nm; (iii) the extracts were analyzed by high performance liquid chromatography; the three MXs were identified in the chromatograms using internal standards, and the true [CF] calculated. Stage (i) involved use of chemometrics to optimize powder extraction, and to get information on the relative importance of the experimental variables ( is more important than t). Stage (ii) was a straightforward application of Beer’s law. Stage (iii) demonstrated the power of using tandem techniques to solve complex, everyday situation, namely the separation and analysis of biologically active components in consumer products (soft- and energy drinks).
