Analysis of Total Caffeine and Other Xanthines in Specialty Coffees Using Mixed Mode Solid-Phase Extraction and Liquid Chromatography-Diode-Array Detection After Microwave Digestion

In this study, solid-phase extraction (SPE) in mixed mode operation was employed to isolate xanthines including caffeine and theobromine from milled caffeinated and decaffeinated coffee samples after microwave digestion. 8-Chlorotheophylline was used as an internal standard. SPE was performed in hydrophobic mode using ethyl acetate/methanol (90:10, 2 mL) as the first elution solvent and in ionic exchange mode using ethyl acetate/acetonitrile/ammonium hydroxide (78:20:2, 3 mL) as the second elution solvent. The eluates were combined, evaporated to dryness and dissolved in aqueous formic acid for analysis. Liquid chromatography with photodiode-array detection was used in isocratic mode employing a C18 column and a mobile phase consisting of acetonitrile/formic acid (0.1% aqueous). The limits of quantitation and detection for this method were 1 and 0.1 mg/L, respectively. The method was linear from 1 to 200 mg/L (r2 > 0.999) with recoveries of the individual xanthines greater than 95%. The decaffeinated coffees contained caffeine at levels less than 0.5 mg/g (range 0.23 to 0.49 mg/g) and caffeinated samples had wide range of levels of caffeine (5.18 to 12.21 mg/g).