Colourimetric solid‐phase extraction coupled with fibre optic reflectance spectroscopy for determination of ascorbic acid in pharmaceutical formulations

A redox colourimetric solid‐phase extraction (C‐SPE) procedure for the determination of ascorbic acid (AA) in pharmaceutical formulations was proposed. Iron (III)‐2,2′‐dipyridyl (Fe(III)‐Bpy) reagent solution was used as a colouring reagent for AA and the immobilization of the redox product onto Amberlite XAD‐16 resin was achieved. The analyte in the sample reacted with a solid sorbent loaded with the colourimetric reagent (Fe(III)‐Bpy) and then quantified directly on the sorbent surface by using a fibre optic reflectance spectrometer (FORS). The amount of AA was reflectometrically determined in a few seconds with a total sample workup and readout time of ∼10 min using only 10‐ml sample volumes. The limit of detection (LOD) and quantification (LOQ) values were 0.18 and 0.6 mg L −1 , respectively, and the linear dynamic range for AA extended up to 8.8 mg L −1 . The C‐SPE for different extractions (n = 5) gave a relative standard deviation (RSD) of 2.9% at 5.28 mg L −1 AA level. Copyright © 2011 John Wiley & Sons, Ltd.