Dynamic microwave-assisted extraction coupled on-line with clean-up for determination of caffeine in tea

Abstract

A rapid method based on dynamic microwave-assisted extraction (DMAE) coupled on-line with clean-up was developed for the determination of caffeine in tea samples. A TM 010 microwave resonance cavity was applied to concentrate the microwave energy. An extraction vessel was placed in microwave irradiation zone of the TM 010 microwave resonance cavity. A silica gel column connected with the extraction vessel was used to remove chlorophyll in tea. The extraction and clean-up procedures were carried out simultaneously in a single step. The DMAE parameters were optimized by the Box–Behnken design. The maximum extraction efficiency was achieved using 70 W of microwave power, 3.5 mL of extraction solvent and 1.0 mL min −1 of extraction solvent flow rate. The limit of detection obtained is 0.012 mg g −1. The RSDs of intra- and inter-day of eight kinds of tea samples are 2.7%–5.4% and 5.1%–7.3%, respectively. The recovery of caffeine in the tea samples is in the range of 88.2%–99.3%.The proposed method has selectivity and sensitivity and considerably less labor and time for determination of caffeine in tea compared with conventional extraction methods, such assoxhlet extractionand liquid–liquid extraction.