Portable flow-injection instrument incorporating a miniature spectrometer for the real-time monitoring of nitrate in rivers

A field-deployable, flow-injection-based instrument for monitoring nitrate in rivers is described. It incorporates a miniature spectrometer that gives full spectral acquisition (200-700 nm) in real time. This allows the dynamic range to be adjusted in the field to meet local requirements by monitoring at different wavelengths. For pristine environments, a linear range of 0-1 mg L-1 NO3-N (r2 = 0.9998, n = 6) and a detection limit of 0.007 mg L1 NO3-N (based on blank plus 3s) were achieved by monitoring at 597 nm. For impacted environments, a linear range of 0-9 mg L-1 NO3-N (r2 = 0.9998, n = 6) was achieved at 510 nm. Conventional peristaltic pumps were replaced by solenoid-operated, self-priming micropumps, and injection of sample was achieved by a series of electronic switching valves. The pulsations of the micropumps were effectively removed by subtracting the response at a nonabsorbing wavelength (444 nm). Instrument control and data acquisition and processing were done within a graphical programming environment. The monitor was successfully deployed for 3 days at the River Frome in Dorset, United Kingdom.