Preconcentration and GC-MS determination of caffeine in tea and coffee using homogeneous liquid-liquid microextraction based on solvents volume ratio alteration

A simple, fast and green homogeneous liquid-liquid microextraction (HLLME) method based on solvents volume ratio alteration (SVRA) combined with gas chromatography-mass spectrometry (GC-MS) was developed for the preconcentration and determination of caffeine in tea and coffee samples. In the proposed HLLME-SVRA method, the primary extraction from solid samples was achieved by a 2:1 ethanol-water mixture. A micro-volume of dichloromethane (DCM) formed a homogeneous solvent with this mixture after choosing an appropriate volume ratio between the three solvents. After vigorous shaking, an extra volume of water was added that resulted in phase separation due to the solvents volume ratio alteration. As a result, complete extraction of caffeine was achieved after centrifugation. The sedimented dichloromethane phase was then injected into GC-MS for the analysis. The influence of a number of parameters influencing the efficiency of the extraction was investigated and optimized. Under the optimal conditions, an enrichment factor of 11, a limit of detection of 0.05 μg mL^-1 and a limit of quantification of 0.16 μg mL^-1 were obtained for caffeine. A linear dynamic range of 0.16 to 50 μg mL^-1 and a determination coefficient (R²) of 0.9980 were achieved. The precision of the method, expressed as relative standard deviation, was 4.8% for six replicated measurements. The method was successfully applied to the determination of caffeine in tea and coffee samples.