Flow system exploiting multicommutation to increase sample residence time for improved sensitivity. Simultaneous determination of ammonium and ortho-phosphate in natural water

摘要

A flow system for the simultaneous determination of ammonium and phosphate in river water at the μg ml~(-1) level employing a low expensive LED-based photometer is described. The manifold of the flow system comprised four analytical pathways containg a set of three-way solenoid values and an automatic injector commutator. The signal measurements of both analytes were carried out using two LED-based photometers attached to the flow cells. A microcomputer running a programme written in quickbasic 4.5 provided facilities to control the system and to carry out simultaneously two analytical procedures also performing data acquisition. For the determination of orthophosphate the method based on reaction with molybdate and ascorbic acid was employed, while for ammonium the method based on reaction with hypochlorite and salicylic acid was selected. The four-pathway structure of the manifold allowed the sample incubation time to be increased to 130 s to permit the reaction to occur without a decrease in sample throughput. The usefulness of the system was ascertained by analyzing a set of water samples. Applying the paired t-test to results obtained employing reference methods, no significant difference at the 95% confidence level was observed for both analytes. Other profitable features such as an analytical throughput of 112 analyte determination per hour; relative standard deviations of 1.1 and 0.7% (n = 6) ammonium and phosphate, respectively, reagent consumption of 0.3 mg ammonium molybdate, 0.75 mg salicylic acid, 3.3 mg ascorbic acid and sodium hypochlorite per determination; detection limits of 7.0 μg l~(-1) NH_4~+ and 17.0 μg l~(-1) PO_4~(3-) were also achieved.