Spectrophotometric flow injection determination of caffeine in solid and slurry coffee and tea samples using supported liquid membranes

A method for the determination of caffeine in coffee and tea samples based on the use of supported liquid membranes coupled to a flow system has been developed. The sample may be analysed both as solid and slurry. In the case of solid sample, this is directly placed in the membrane unit, and when the sample is slurry, this is continuously pumped to the membrane unit. In both cases, the caffeine released from the sample passes through the membrane (PTFE/n-undecane:hexyl ether) into an acidic acceptor stream. This stream flows through a spectrophotometric detector allowing the measurement of the absorbance of caffeine at 274 nm. The method shows a linear range between 0.5 and 15 g l⁻¹, with a relative standard deviation of ±3.7% and a sample throughput of 7–8 samples h⁻¹.