A flow injection-fluorometric method for the determination of ammonium in fresh and saline waters with a view to in situ analyses

A version of the orthophthaldialdehyde-fluorescence ammonium determination for flow injection analysis (FIA) is presented here, with a view to its use for in situ, low-power consumption systems, Thus, the reaction temperature was limited to 30 degreesC and FIA was used in stop-Bow mode (3 min stop). The calibration is linear up to 50 mu mol l(-1), but concentrations up to 100 mu mol l(-1) can be measured. Repeatability is around 1% in the range o f 0.5-4 mu mol l(-1) and the detection limit is about 0.03 mu mol l(-1) Ove r the salinity range of 5-35 (seawater practical salinity scale) the salt e ffect is almost negligible (within +/- 2%); and below salinity of 5 it incr eases to a maximum of -9% in fresh water compared to seawater. Hydrogen car bonate, dissolved oxygen and turbidity (either suspended sediments or phyto plankton cells) have almost no adverse effect in a wide range of concentrat ions, covering most natural water conditions. Relative interference of prim ary amines is negligible and mercury (a common sample preservative) does no t depress the signal up to 20 mg l(-1) Hg II. Sulfide, that may be present in areas with anoxic waters, depresses the signal only slightly and linearl y (-9% at 100 mu mol l(-1) S2-). The method appears to be convenient for th e determination of ammonium in most coastal, estuarine and fresh waters. Sa mple throughput is 9 h(-1). The performance of the method can be improved, either by increasing the reaction time (low throughput) or, if enough energ y is available, by increasing the reaction temperature (non-stop-flow mode, high throughput). Combining FIA and fluorometry appears to be interesting for in situ determination (submersible devices) of dissolved compounds in e nvironments with variable salinity and turbidity (especially coastal and es tuarine waters). (C) 2001 Elsevier Science Ltd. All rights reserved.