Automatic selective determination of caffeine in coffee and tea samples by using a supported liquid membrane-modified piezoelectric flow sensor with molecularly imprinted polymer

An on-line supported liquid membrane-piezoelectric detection based on molecularly imprinted polymer (SLM-PZ-MIP) manifold was developed and applied to the quantitative determination of caffeine in coffee and tea samples. The proposed assembly provides all the advantages of an on-line system as regards automation, in addition to good selectivity, acceptable sensitivity and precision. The slurry sample was directly placed in SLM unit located in the loop of an injection valve, and the caffeine released from the sample passed through the membrane (n-undecane/hexyl ether) into an acidic acceptor stream. The caffeine is quantified by using a quartz crystal modified with a molecularly imprinted polymer (MIP). The method shows a linear range between 10 and 1000 ng ml−1, with a relative standard deviation of ±5% (at 50 ng ml−1). Satisfactory results were found by analysing real coffee and tea samples.