Sequential injection spectrophotometric analysis of nitrite in natural waters using an on-line solid-phase extraction and preconcentration method

An automatic sequential injection analysis (SIA) set-up for the isolation, preconcentration and spectrophotometric determination of nitrite in waters based on the Shinn reaction was designed and evaluated. The system performs the on-line azo dye formation and its subsequent extraction on a solid phase (monofunctional C-18), which is held inside a glass column incorporated into the system. A large sample volume (maximum, 10 ml) is sequentially segmented with sulfanilamide and N-(1-naphthyl)ethylenediamine dihydrochloride by using an iterative method. The azo dye collected is eluted with a small volume of 80% methanol and conducted to a diode-array spectrophotometer for quantitative analysis. It has been proved that the retention efficiency is maintained for up to 45 sample injections of 10 ml in spite of the high chromogenic reagent acidity. Nitrite has been determined within the 13.4-160 ng ml(-1) and 0.83-20 ng ml (-1) ranges for 1 and 10 ml of sample, respectively. For these volumes, the detection limits are 5.9 and 0.32 ng ml (-1), the enhancement factors 17 and 170 and the sample throughput 15 and 3 h(-1), respectively. A maximum RSD of 4.0% was achieved in all determinations. It is an advantage of this approach that it is possible to use the same mass calibration graph for any sample volume.