A novel flow-through disk-based solid-phase extraction diffuse reflectanceoptrode. Application to preconcentration and determination of trace levelsof nitrite

A novel, versatile and sensitive flow-through optical fiber diffuse reflect ance sensor to implement disk-based solid-phase extraction in a flow inject ion analysis (FIA) system is presented. Nitrite optosensing at trace levels is chosen as a model of chemistry to demonstrate its applicability. The me thodology is based on on-line nitrite derivatization with Shinn reagent to form a moderately polar azo dye, whose preconcentration oil to octadecyl co valently bonded silica gel particles tightly bound to an inert matrix (C-18 disk) is continuously monitored using a plug-in diode-array spectrophotome ter. After the analytical signal has been recorded, fast sensor regeneratio n 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 o f nitrite as low as 1 ng ml(-1) are easily determined using 2.5 ml of sampl e. A detection limit (3 sigma (blank)) of 0.1 ng ml(-1) of nitrite, repeata bility 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 prop osed optrode. The utility of the flow-through optosensing system, wherein m inimization of additive matrix interferences is feasible, was testified by the satisfactory results obtained in its application to tap, ground, harbor and aquarium water samples.