Automated solid-phase extraction of phenolic acids using layered double hydroxide–alumina–polymer disks

The application of layered double hydroxide-Al₂ O₃ -polymer mixed-matrix disks for solid-phase extraction is reported for the first time. Al₂ O₃ is embedded in a polymer matrix followed by an in situ metal-exchange process to obtain a layered double hydroxide-Al₂ O₃ -polymer mixed-matrix disk with excellent flow-through properties. The extraction performance of the prepared disks is evaluated as a proof of concept for the automated extraction using sequential injection analysis of organic acids (p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, gallic acid) following an anion-exchange mechanism. After the solid-phase extraction, phenolic acids were quantified by reversed-phase high-performance liquid chromatography with diode-array detection using a core-shell silica-C18 stationary phase and isocratic elution (acetonitrile/0.5% acetic acid in pure water, 5:95, v/v). High sensitivity and reproducibility were obtained with limits of detection in the range of 0.12-0.25 μg/L (sample volume, 4 mL), and relative standard deviations between 2.9 and 3.4% (10 μg/L, n = 6). Enrichment factors of 34-39 were obtained. Layered double hydroxide-Al₂ O₃ -polymer mixed-matrix disks had an average lifetime of 50 extractions. Analyte recoveries ranged from 93 to 96% for grape juice and nonalcoholic beer samples.