Automated Sequential Injection Method for Determination of Caffeine in Coffee Drinks

An automated sequential injection analysis spectrophotometric assay for the determination of purine alkaloids in coffee drinks was developed. The sample was treated with a carrez reagent for matrix suppression followed by filtration; subsequently, alkaloids were separated from organic acids using a short C18 monolithic column (10 × 4.6 mm). The flow rate of the separation step was 10 μL s−1 with 10% v/v of methanol as the mobile phase. The sum of alkaloids evaluated as caffeine was detected at 274 nm. The influence of the main parameters affecting the quantification of purine alkaloids was optimized. One sample analysis lasted 15 min when aspirated in triplicate. The linear range was 1–15 mg L−1, and the determination coefficient (r2) was 0.9969. The limit of detection and limit of quantitation were 0.128 and 0.425 mg L−1, respectively. The repeatability evaluated as the relative standard deviation (RSD) was 3.58% (n = 12, 10 mg L−1). Under optimal conditions, the method was successfully applied to determine purine alkaloids in different real samples including soluble coffee, coffee from an espresso machine, and brewed coffee drinks.