Fourier transform infrared determination of caffeine in roasted coffee samples

Abstract

A new procedure has been developed for the FT-IR determination of caffeine in roasted coffee samples. The method involves wetting the coffee samples with a 0.25 M aqueous NH₃ solution, extracting the caffeine with CHCl₃, and measuring absorbance at 1659 cm^–1 using a baseline established between 1900 and 830 cm^–1. The procedure proposed is fast, only requiring a total extraction time of 16 min for each sample, and provides a drastic reduction of the organic solvent consumed, from the 200 mL diethyl ether and 50 mL CHCl₃ required for each sample by the reference chromatographic UV-spectrometric determination to only 5 mL CHCl₃. The method provides a limit of detection of the order of 3 mg L^–1 caffeine and a relative standard deviation of 0.4% for 3 independent analyses of a sample containing 18.6%mg/g caffeine. The accuracy of the FT-IR procedure was evaluated from recovery experiments on spiked samples providing values from 94.4 to 100.1% and from the comparison of results found for a series of commercial samples, by both FT-IR and the official reference procedure.