A Green Liquid Chromatography Method for Simultaneous Quantification of Caffeine and its Three Major Metabolites in Urine, Drinks, and Herbal Products

A green reversed-phase liquid chromatography method was developed to determine caffeine (CF), theobromine (TB), theophylline (TP), and paraxanthine (PX). The stationary phase was a silica-based C18 monolithic column (100 x 3.0 mm2 i.d.), eluted with a gradient from 6% to 10% (v/v) of ethanol in water, at a flow rate of 0.75 mL/min (35 degrees C) and UV detection at 272 nm. The compounds were separated in less than 6 min, with a total time of 10 min for column reconditioning. The effect of gradient composition, flow rate, injection volume, and column temperature was evaluated in the separation efficiency, especially regarding the critical pair PX/TP, for which the best resolution was 1.37. The linear range was between 0.5 and 100 mu g/mL. The LODs were 0.065, 0.057, 0.040, and 0.044 mg/mL for TP, TB, PX, and CF, respectively. The method was applied to green tea and yerba mate infusions, soft drinks, energy drinks, and soluble coffee. Urine samples were analyzed before and after solid-phase extraction. The accuracy was evaluated by the spiking/recovery strategy standing in the 79.6% to 116.4% range. The separation performance and the mobile phase consumption were compared to those of another method using a gradient of methanol in water.