Flow Injection Fourier Transform Infrared Determination of Caffeine in Soft Drinks

A fully automated procedure has been developed for FT-IR determination of caffeine in soft drinks. Samples are first degasified by filtration and then directly injected into a flow manifold and passed through a 100 mg C 18 SPE cartridge, conditioned with methanol and water. After the cartridge has been cleaned with water, the caffeine is eluted with CHCl 3 and stabilized with ethanol. The flow injection (FI) recording is obtained by measuring the absorbance at 1658 cm -1 with a baseline established at 1800 cm -1 . Area values for the FI recording obtained between 0.4 and 1.4 min after sample injection are interpolated in the calibration graph of a series of aqueous standards of caffeine, which were loaded and eluted in the same way as the samples. Preliminary studies were carried out using off-line SPE and off-line elution and on-line extraction and on-line elution, and the parameters involved in both the retention and elution of caffeine in solid phase extraction cartridges were evaluated, together with the measurement conditions for FT-IR determination of caffeine in CHCl 3 solutions and the flow injection parameters. The procedure developed provides a limit of detection of 10 μg mL -1 of caffeine for a sample injection of 1 mL, a sampling frequency of 30 h -1, and a relative standard deviation of 3.5% for eight independent measurements at a concentration level of 100 μg mL -1 .