Focused-microwave-assisted reaction in flow injection spectrophotometry: a new liquid–vapor separation chamber for determination of reducing sugars in wine

The usefulness of an unsegmented liquid/vapor phases flow injection system for determining reducing sugars in wines using a focalized poly(tetrafluoroethylene) (PTFE) coiled reactor positioned at the output antenna of a domestic microwave oven at 700 W is demonstrated. In this system, sample or reference solution is mixed to 2.5 mol l−1 sodium hydroxide and 6.8 mmol l−1 potassium hexacyanoferrate(III) solutions previously merged in a confluence point and the reducing sugars (glucose plus fructose) were oxidized by potassium hexacyanoferrate(III) solution into the PTFE coiled reactor positioned in the microwave oven cavity. After phases separation at a proposed separation chamber (SC), the decrease of K3Fe(CN)6 concentration in the liquid phase was measured at 420 nm. Using this SC, a favorable dispersion profile and a decrease of hydrodynamic pressure were observed. The flow procedure presented similar sensitivities to glucose and fructose in the experimental conditions adopted, allowing the determination of the reducing sugars content (fructose plus glucose concentrations) in the concentration range from 40 to 400 μmol l−1 with a detection limit of 9 μmol l−1. The RSD was 1.9% for a 120 μmol l−1 fructose solution (n=12) and the analytical frequency was 54 h−1. The results obtained for reducing sugars in several wines using the proposed flow system and those obtained using the Somogyi–Nelson batch procedure were in agreement at the 95% confidence level.