On line coupling of a liquid-liquid extraction flow-reversal system to a spectrophotometric flow-through sensor for the determination of polyphenols in olive oil

The coupling of a liquid-liquid extraction approach based on flow-reversal to a flow-through sensor is proposed. The basis of the sensor is the use of Folin-Ciocalteau reagent immobilized on an anionic exchange resin packed in a flow cell located in a spectrophotometer, where the reaction product (molybdenum blue) is formed, retained and detected on the resin surface, thus integrating the reaction, retention and detection. The regeneration of the sensor is accomplished by passing an acidic Ce(IV) solution through it. This coupled system allows the extracted analytes (polyphenols) to be monitored continuously, and it was used to develop a method for the determination of polyphenols in oil. The proposed method features two linear response ranges, from 15 to 30 and from 30 to 90 μg ml−1, expressed as caffeic acid, with a relative standard deviation < 3.1%. The 3σ detection limit was 10 μg ml−1 caffeic acid. It was applied to the determination of polyphenols in olive oil samples with results in excellent agreement with those provided by a conventional method.