Flow-through sorptive preconcentration with direct optosensing at solid surfaces for trace-ion analysis

This article highlights the potential of the solid-phase optosensing scheme as a microanalytical technique for the monitoring of ionic species at the low ng/ml level. We describe the background of solid-phase optosensing, critically comparing the performance of its applications in batch and flowing stream systems. We discuss in detail the current state of the art of flow-through solid-phase extraction (SPE)-based sensors, which are regarded as being a mature technique, and we outline its intrinsic and unique analytical features. We give special emphasis to the criteria to be fulfilled by both bead and membrane sorbent materials if used in a reversible fashion. We also thoroughly address the potentialities and limitations of different optrode cell designs devoted to absorbance/reflectance detection. We present the concept of renewable-bead injection as a powerful scheme that can be exploited with a wide variety of reagent-based assays, including those relying on irreversible reactions. As a practical example to compare the analytical performance of the variety of approaches and cell configurations described in the bulk of the text, we selected on-column monitoring of ultratrace levels of nitrite, which involved the reversed-phase extraction of the derivative compound resulting from the Shinn reaction. (C) 2003 Published by Elsevier B.V.