Detection of phenolic acids in beverages by capillary electrophoresis with electrochemical detection

Capillary electrophoresis using amperometric detection is used to detect phenolic acids in beer samples. Both the detection and injection conditions were optimized. First, the electrophoretic separation requires that the phenolic acids be charged and therefore the pH be above their pK(a)s, However, electrochemical detection is optimal when the pH is low so that the phenolic acids are neutral and not repulsed by negative charges on the electrode surface. These divergent conditions were met by using a pH 7.2 run buffer and lowering the pH after the separation by using nitric acid in the detection reservoir. Cationic and neutral compounds in the beer samples interfered with electrochemical detection by passivating the electrode surface. These compounds were removed using a reversed-polarity injection technique to elute them from the separation capillary into the sample reservoir prior to the electrophoretic separation. These techniques were demonstrated by detecting several phenolic acids in various types of beer. Electrophoretic peaks in the samples were identified by both matching their elution time and electrochemical properties with standards. The use of voltammetric characterization provided improved peak identification for complex samples.