Integrated FIA for the Determination of Ascorbic Acid and Dehydroascorbic Acid in a Microfabricated Glass-Channel by Thermal-Lens Microscopy

An on-chip chemical micro-determination device, integrating the FIA system, was applied to L-ascorbic acid (AA) and dehydroascorbic acid (DHAA) determination. Microchannels having a 250×100 μm cross-section were fabricated in a quarts glass chip as a component in an integrated FIA system. They were put to use for flow, mixing, reaction, and detection. The reaction system was a coupled redox-complexation reaction between AA and a 1,10-phenantroline-Fe(III) mixture. Sample solutions were introduced into micro-channels by micro-syringes, the flow rate was controlled by a micro-syringe pump, and the reagents, were mixed by molecular diffusion. A photothermal microscope was used for the ultra-sensitive detection of the non-fluorescent reaction product (ferroin). For DHAA determination, dithiothreithol was used as a reducing agent and the total AA was determined. The DHAA quantity was calculated by subtraction. This method allows for the determination of AA with a linear range of up to 40 μM. The RSD for 25 μM AA (n=5) is 1%, and the limit of detection (LOD) is 0.1 μM. The procedure was successfully applied to determining the AA and DHAA content of urine and AA in vitamin C tablets. The results agree well with those obtained by reference methods.