High temporal resolution field monitoring of phosphate in the River Frome using flow injection with diode array detection

The design and deployment of an in situ flow injection (FI) monitor for high temporal resolution monitoring of phosphate in the River Frome, Dorset, UK, is described. The monitor incorporates solenoid, self-priming micropumps for propulsion, solenoid-operated switching valves for controlling the fluidics and a miniature CCD spectrometer for full spectrum (200-1000nm) acquisition and operates in a graphical programming environment. A tangential filtration unit is attached to the sample inlet line to remove suspended particulate matter and prevent blockage of the micropumps and valves. Detection (at 710nm) is based on molybdenum blue chemistry with tin(II) chloride reduction. The detection limit is 0.67 μM PO₄ and the linear range can be adjusted by using different wavelengths for detection. Pump noise is eliminated by subtraction of the signal at a non-absorbing wavelength (447nm). Data from an intensive (sample every 30 min) field trial on the River Frome performed in October 2000 are presented, and the implications of the data for refining an export coefficient model for phosphorus from the catchment are discussed.