pFI Method for Nitrate
The method is based on reduction of nitrate into nitrite on Cd column, and subsequent conversion of nitrite by Gries reaction in a red azo dye monitored at 620nm. Flow programming (A) allows to extend the range from 1 ppB N to 400 ppB N, while the sample throughput falls from 180 to 60 samples/hour. The calibration in these ranges is linear (B,C). Sample volume 300 mcrL, 10 cm flow cell . The flow rate at 100% is 4mL /min per channel. For mid ppB level, (B) no dwell period, for low ppB level (C) 2% flow rate for dwell period of 25 sec was applied. All data were collected at 50% flow rate for all range, and raw data of duplicate runs are shown overlaid. Compare this with the traditional (cFI) method 1.3.2.
Since formation of the azo dye is very rapid and since also the reduction of nitrate on 5cm long Cd column is fast, slowing the flow rate during sample passage through the column amplified the response only about 5 times. The advantage of flow programing is, in that it allows to cover a very wide range of concentrations by using a simple software command that slows the flow rate, rather then physically changing manifold configuration ( sample volume, flow cell), as done in traditional cFI design.
1.2.33.
