Determination of ultratraces of nitrite by solid-phase preconcentration using a novel flow-through spectrophotometric optrode

A novel sorbent-packed planar wall microcolumn-based optosensor coupled to a flow injection analysis system (FIA) for the determination of traces of nitrite by solid-phase spectrophotometry is presented. The methodology is based on the on-line nitrite derivatization with a modified Griess reagent to form an azo-dye whose retention onto octadecyl-covalently bonded silica gel by physical adsorption and partitioning is continuously monitored. After the analytical signal has been recorded, the sorbed compound is eluted with 80% methanol/water (v/v), rendering the system ready for the next extraction. Selection of the solid support and the suitable flow-through cell configuration are discussed in detail. By minimization of the blank signal, a concentration of nitrite as low as 1 ng/ml can be easily determined using 2.5 ml of sample. Detection limit (3sigmablank) of 0.46 ng/ml of nitrite, relative standard deviation better than 4%, sample throughput of 12 h-1 and an enrichment factor of 93 in relation to the conventional spectrophotometric method were obtained. The utility of the present optosensing system, wherein a mass calibration is also feasible, was testified by the satisfactory results obtained in its application to tap, rain and aquarium water samples.