# Analytical Methods for Exploring Nutraceuticalticals Based on Phenolic Acids and Polypphenols
## 4 Analytical Methods for the Determination of Active Compounds in Nutraceutical Products
### Voltammetric Methods  
Electroanalytical techniques such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV) have been widely used for the detection of polyphenols. A direct relationship between the concentration of electroactive functional groups and the current intensity generated is often found when they are either oxidized or reduced at the working electrode surface [55; 56; 57].  
The techniques commented above are especially recommended for the analysis of polyphenols as their antioxidant properties are often related to the ability to donate electrons. Voltammetric signals at low oxidation potentials suggest the presence of polyphenols of high antioxidant activity, whereas those compounds with low antioxidant power show electrochemical activity at more positive potentials [55; 57]. Regarding electrodes, recent studies to determine polyphenols in nutraceuticals and related samples have relied on a wide variety of working electrodes, such as the pencil-graphite electrode (PGE), glassy carbon electrode (GCE), platinum (Pt), and different types of biosensors. In general, Pt has been used as an auxiliary electrode and Ag/AgCl (3 M KCl) as the reference electrode [55; 57; 58; 59].  
Some representative examples are depicted in Figure 3, which shows the differential pulse voltammograms recorded in the range from \(-0.2\) to \(+0.9\) V vs. an Ag \(|\)AgCl \(|\) KCl reference electrode. In the case of catechin, with two independent dihydroxymethyl moieties, two oxidation peaks corresponding to each side of the molecule have been detected.  
These techniques result in powerful alternatives to traditional spectrophotometric measurements due to the high sampling throughput, simplicity, sensitivity, reduced sample, and reagent consumption. However, one of the main issues of voltammetric studies is related to the electrode fouling during the measurements due to the adsorption of several nutraceutical sample components. In such circumstances, a cleaning step may be necessary before each voltammetric measurement to clean or renew the electrode surface [55; 57; 58; 60].  
Figure 3: Differential pulse voltammograms recorded in the range from \(-0.2\) to \(+0.9\) V vs. an Ag \(|\)AgCl \(|\) KCl reference electrode for three representative examples: (**a**) 3/4-dihydroxybenzoic acid; (**b**) 2/4-dihydroxybenzoic acid; (**c**) catechin (adapted from reference [48]).  
Some representative examples are mentioned as follows. For instance, grape extracts analyzed by CV with GCE showed a first oxidation peak at 420 mV corresponding to the catechol group on the B-ring of flavanols, and a second oxidation peak at 800-860 mV belonging to the resorcinol group on the A-ring [56]. In another study, the determination of TPC in Turkish green, black, and white tea extracts using DPV allowed a sensitive, rapid, and inexpensive evaluation of samples, with recoveries of caffe acid around 95% [59]. In addition, DPV and CV were demonstrated to be appropriate to express the antioxidant activity of coffee extracts, presenting good correlations with DPPH spectrophotometric results [60].