The colorimetric assay for nitrite is a single reagent assay based on Gries reaction whereby
nitrite is converted into red azo dye monitored at 520nm. Reagent (blue) and sample (red) are aspirated into holding coil (Rv 25μL+Sv 50μL) forming a single interface (A) within the channel filled with carrier stream of distilled water. Since sample zone is aspirated as second, its leading edge penetrates trough reagent zone and upon flow reversal the reaction product (yellow) is carried into the flow cell.
The assay protocol comprises five steps designed to execute wash, reagent aspiration, sample aspiration, monitoring, and washout. The optimized flow reversal volume was found to be 50μL, determined by an experiment (2.2.12.) described in the previous section.
Next, reaction rate was investigated by increasing the time period, within which the reacting mixture is held in the flow cell (C), by using 500 ppB N nitrite standard and stop of 3, 9, 19 and 39 sec. Since the azo dye is formed already on the way to flow cell, the response curve comprises two parts, the second part representing reaction rate. Therefore, by selecting delay time the sensitivity of the assay can be conveniently adjusted. In order to achieve a high sampling frequency the shortest stop flow period of 3 seconds was selected yielding a linear calibration in the range 100 to 1000 ppB N. (C).
Nitrite Assay: Accelerated SI
2.2.14.
