Flow injection Fourier transform infrared determination of caffeine in coffee

Abstract

A fully automatized procedure has been developed for the Fourier transform infrared (FT-IR) spectroscopic determination of caffeine in coffee samples. The method involves the on-line extraction of caffeine with CHCl 3. Samples, weighed inside empty PTFE cartridges of 0.5 cm internal diameter (i.d.) and 1.5 ml volume, were humidified with four drops of 0.25 M NH 3. The cartridge was installed in a flow manifold, in which samples were extracted in a closed-flow system with 1 ml CHCl 3 during 6 min. Four hundred microliters of the extract were introduced in a microflow cell and absorbance measured as a function of time at 1659 cm −1, with a baseline established between 1900 and 830 cm −1, thus providing a fiagram. Peak height values of the FI recordings, obtained for samples, were interpolated in an external calibration line established from standard solutions of caffeine in CHCl 3. The method provided a limit of detection (LOD) of 9 mg l −1 caffeine, a relative standard deviation of 0.6% for five independent measurements of a solution containing 1 mg ml −1 and a sampling frequency of the whole procedure of 6 h −1. Results obtained for market samples agree well with those found by the official chromatographic–spectrometric method, but involving a drastic reduction of solvents, from the 200 ml ether and 50 ml CHCl 3 required for each sample preparation by the reference procedure to less than 30 ml CHCl 3 necessaries for the whole determination of caffeine in a sample, also including standards and carrier solution.