Bead injection spectroscopic flow-through renewable surface sensors with commercial flow cells as an alternative to reusable flow-through sensors Original Research Article

For the first time, a bead injection spectroscopy–flow injection analysis (BIS–FIA) system with spectrophotometric detection in a commercially available flow cell (Hellma 138-OS) is developed. The flow cell is filled by injecting in the flow system a homogeneous bead suspension of an appropriate solid support previously loaded with the chromogenic reagent. The solid beads work as a flow-through chemical sensing microzone integrating on-line separation/reaction/detection, developing the analytical signal when the injected sample containing the analyte reaches the beads. At the end of the analysis, the beads are discarded by reversing the flow and instantaneously transported out of the system. These systems can be specially recommended in those flow-through optosensors in which the species of interest is so strongly retained on the solid sensing microzone that the regeneration of the solid beads becomes extraordinarily difficult. They can be considered as belonging to the third generation of FI microanalytical techniques which use renewable sensing solid surface, so avoiding the need for a reversible sensing mechanism. To demonstrate the utility of this technique, a single BIS–FIA system is developed to determine: (a) directly Fe(II); (b) Fe(III) or total iron (Fe(II)+Fe(III)), using a previous reduction; and (c) indirectly ascorbic acid. Ferrozine (Fz) is the bead loaded chromogenic reagent, the analytical signal corresponding to the absorbance of the complex [Fe(II)Fz3]4−. Sampling frequencies of 16 and 13 h−1, and R.S.D.(s) (n=10) of 4.02 and 4.19% were obtained with 100 and 1000 μl of Fe(II) solution, respectively. The system was applied to the determination of (a) iron in wine; water and pharmaceuticals; and (b) ascorbic acid in fruit juices, pharmaceuticals and conservative liquids, obtaining satisfactory results.