On-line solid–phase extraction using a C18 minicolumn coupled to a flow injection system for determination of caffeine in green and roasted coffee beans

•Quantification of caffeine coupling solid phase extraction to a flow injection manifold.•Aqueous extractant to extract caffeine from coffee beans.•Separation of caffeine of the aqueous extracts using a C18 phase-reverse column. A novel and fast method based on the solid phase extraction (SPE) coupled to a flow injection system for the determination of caffeine in coffee beans was developed. The caffeine extraction of coffee beans was carried out with hot water. A C18 reverse-phase mini-column was coupled to a continuous flow manifold to carry out the on-line SPE and the quantification of caffeine from aqueous extracts. Column length, retention time, elution volume, extracting solution and injection volume were evaluated. The retention time was of 90s and the elution was carried out with 400μL of a methanol:water mixture (25:75). The proposed on-line SPE was compared against a chloroform extraction from aqueous extracts. With the proposed method the sample preparation was minimised and the sample throughput was increased (10determination/h) because no dilution was required. Green coffee beans and beans with different roasting degree were analyzed.