A novel flow-through disk-based

Abstract

A novel, versatile and sensitive flow-through optical fiber diffuse reflectance sensor to implement disk-based solid-phase extraction in a flow injection analysis (FIA) system is presented. Nitrite optosensing at trace levels is chosen as a model of chemistry to demonstrate its applicability. The methodology is based on on-line nitrite derivatization with Shinn reagent to form a moderately polar azo dye, whose preconcentration on to octadecyl covalently bonded silica gel particles tightly bound to an inert matrix (C₁₈ disk) is continuously monitored using a plug-in diode-array spectrophotometer. After the analytical signal has been recorded, fast sensor regeneration is achieved with a methanolic eluent, rendering the system ready for the next extraction. Selection of the solid disk support and the suitable flow-through cell configuration to reduce back-pressure effects are discussed in detail. By matching the illumination and retention zones, concentrations of nitrite as low as 1 ng ml⁻¹ are easily determined using 2.5 ml of sample. A detection limit (3σ_blank) of 0.1 ng ml⁻¹ of nitrite, repeatability and reproducibility better than 3.2%, an analytical throughput of 11 h⁻¹ and an enrichment factor of 140 are the figures of merit of the proposed optrode. The utility of the flow-through optosensing system, wherein minimization of additive matrix interferences is feasible, was testified by the satisfactory results obtained in its application to tap, ground, harbor and aquarium water samples.