Simultaneous determination of thiamine and pyridoxine in pharmaceuticals by using a single flow-through biparameter...

For the first time, an UV-photometric flow-through sensing device has been developed for the simultaneous determination of two cationic species (thiamine and pyridoxine). The sensor is based on the retention of the analytes on a cationic ion-exchanger gel placed in the detection zone itself into a quartz flow-cell. A double discrimination is used for detecting the analytes: (a) a double and simultaneous working wavelength, performed by the use of a diode array detector; and (b) a temporary sequentiation in the arrival of the analytes to the sensing zone by on line separation using a cationic ion-exchanger (the same used in the sensing zone) placed into a minicolumn just before the flow cell. Pyridoxine is determined the first (by measuring its intrinsic absorbance at 293 nm) because it passes through the minicolumn while thiamine is strongly retained on it. Then, thiamine is conveniently eluted from the precolumn and its intrinsic UV absorbance measured at 255 nm. In both cases, transitory signals were obtained because both the carrier (in the case of the pyridoxine) and the eluting (in the case of thiamine) solutions used also eluted the respective analyte from the sensing zone. Using 1000 microl of sample, the analytical signal showed a very good linearity in the range 2-30 microg ml(-1) for both analytes with detection limits of 0.10 and 0.084 microg ml(-1) for thiamine and pyridoxine, respectively. The optosensor was satisfactorily applied to the determination of these two analytes in pharmaceuticals.