A low power ultra violet spectrophotometer for measurement of nitrate in seawater: introduction calibration and initial sea trials

A low power UV spectroabsorptiometer has been developed to measure the concentration of dissolved nitrate in seawater, in situ, with a rapid 1 Hz response. Measurements at a number of wavelengths in the region of 220 nm determine the concentrations of dissolved nitrate, sea salt and, possibly, dissolved organics, whilst a third channel at 300 nm acts as a reference channel to correct for light intensity changes related to lamp output and scattering by particles in seawater. This paper describes the first prototype UV nitrate sensor designed to evaluate and compare options for the light source, spectral filter and detector. The choice of components was based on a compromise between performance, cost, power consumption and convenience of use. The eventual instrument uses a xenon flashlamp light source, fused silica windows and lenses, a sample cavity, a grating spectrometer and UV enhanced silicon photodiode detectors. The power consumption of the sensor was typically from 3 to 4 W, depending on the selected repetition rate of the flashlamp source. Initial laboratory trials have shown the sensor to be unresponsive to changes in pressure allowing for continuous use to depths of up to 5000 m. Initial responses to changes in temperature have been eliminated by the adjustment of the sensor's power supply and lamp flash rate. Calibration of the sensor has been performed under laboratory conditions and resulted in a second order linear fit (r2 > 0.99, p<0.05) with respect to nitrate concentration. Calibrations performed to assess the response of the instrument to changes in salinity also resulted in second order linear fits (r2>0.99, p<0.05). Salinity calibrations indicated that a change of 1 psu resulted in a change in absorbance equivalent to 0.52 μmol l−1 NO3. The detection limit of the sensor is a function of the signal/noise ratio. The minimum nitrate concentration change resolved by the sensor was found to be >0.21 µmol l-1 N03. Measurements made in situ were compared to concentrations of nitrate determined in water samples using an AAII type autoanalyser. These demonstrated the capability of the instrument to precisely determine nitrate concentrations in seawater