A low power ultra violet spectrophotometer for measurement of nitrate in seawater: introduction, calibration and initial sea trials
Introduction
The main objectives of this sensor programme were to produce a self contained monitoring system with a low power consumption which would allow for towed or lowered oceanographic deployment in addition to long term unattended deployment using a battery power source. The sensor would have to have a rapid response time (<1 s) to enable a resolution of 1 m when lowered through seawater on vertical hydrographic casts and a resolution of ∼5 m on an undulating towed vehicle.
Several other methods have been developed to measure the concentration of nitrate in seawater. These include selective colour indicator tests, nitrate sensitive electrodes 1, 2, and nitrate optrodes 3, 4, 5as well as the use of a fibre optic coupled UV photometer [6]. However, none of these methods meet all the requirements for long term monitoring. Recently, a submersible osmotically pumped analyser has also been reported [7]but this does not have a sufficiently rapid response time for towed applications.
The determination of nitrate using ultra violet spectroscopy is well known 8, 9. Several investigations have been carried out 10, 11, 12to assess the absorption of UV light by natural waters including seawater. A number of problems have been found with this method of analysis [10], but it has shown that a simple measurement of UV absorbance at 220 nm could be used as an indicator of dissolved nitrate distribution in open ocean waters 9, 13. Methods for freshwater analysis of dissolved nitrate have been reported 14, 15, 16but the application of a self-contained electronic/optical instrument utilising UV measurement has never been widely applied in the marine environment due to the reliable chemical based analysis involving the copper–cadmium reduction method. Recent developments have combined UV spectroscopy with flow injection analysis or have used 2nd derivative spectroscopic techniques [17].
Nitrate absorbs significantly at wavelengths up to 230 nm. The main ionic species found in seawater exhibiting similar characteristics include Cl− and Br− plus low concentrations of . Except for these anions have their main absorption band well below 210 nm. All other ions, except chloride and bromide, are present in such low concentrations that the interference they produce is negligible [8]. Molar extinction coefficients (ϵ) for halides are about 106 times less than nitrate but their concentration in seawater is typically 5×104 times higher resulting in relative absorbance 5×10−2 times that of nitrate. Under the conditions used in this study, ϵCl−=0.01 M−1 cm−1, at 220 nm at 220 nm, at seawater concentrations with a path length of 15 cm and ϵNO3=644 M−1 cm−1, at 220 nm, at a concentration of 10 μM with a path length of 15 cm.
The ions HCO−3 and CO2−3 are present in the open ocean in a ratio of approximately from 97% to 2%. Since HCO−3 has its main absorption band below 210 nm and CO2−3 is present in such low concentrations, they should not contribute significantly to any interference effect. However, in estuaries, particularly fed by two chalk rivers, the effects of will become more significant due to the higher concentrations of CO2−3 ions resulting from a shift in the carbonate equilibrium. These effects are currently being investigated.
Fig. 1 illustrates the UV absorption spectra of nitrate, nitrite, sodium chloride and seawater solutions in distilled water. In seawater, chloride and bromide are present in a nearly constant ratio, both to one another and to the total salt content (salinity). Salinity is routinely determined to a high precision in marine waters so a simple correction can be applied to absorbance measurements made to determine nitrate.
Significant problems may be associated with UV analysis of nitrate in seawater due to absorption of UV light by dissolved organic compounds as their concentrations are not predictable. Aliphatic and aromatic compounds, for example, mineral oils, petrol, alcohols and biologically derived compounds (as well as several anionic species in addition to Cl− and Br−) exhibit broad band absorption within the wavelength range from 150 to 400 nm. However, previous observations suggest that the interference may be relatively small in open ocean water [8].
Section snippets
Experimental development of in situ device
The design of a spectrophotometric instrument for monitoring nitrate in seawater followed a three-stage process. Firstly, an optical bench mounted configuration was used to evaluate potential components such as light sources, filters and detectors. Secondly, after the preferred components had been selected, a boxed sensor was designed to perform measurements at three wavelengths in the laboratory. Thirdly, a fully submersible unit was manufactured to allow tests to be carried out in the field.
Methods
To examine the feasibility of using ultra violet spectroscopy to determine nitrate concentrations in the marine environment, a study was carried out at sea during the Charles Darwin 84 cruise. A continuous system was set up using an AAII type autoanalyser 18, 19. In addition to the normal NED/sulphanilamide method for nitrate analysis, a second parallel channel was set up on the autoanalyser using a deuterium light source and an appropriate filter (220±5 nm) to measure ultra violet absorption
Nitrate
Following the elimination of the effects of temperature, calibrations were carried out to produce a series of equations to derive nitrate concentration from output voltage. The output voltage of the sensor was first converted to absorbance using the Beer–Lambert law. Nitrate concentration was then derived from absorbance by measuring the changes in absorbance produced by known concentrations of nitrate and using the method of least squares to derive a line of best fit. The line of best fit was
Conclusions
This investigation into the feasibility of producing a self-contained, low power, high resolution instrument has shown that it is possible to measure dissolved nitrate in situ in seawater using the ultra violet absorbing properties of the nitrate ion. Measurements made at sea during Charles Darwin cruise CD94 using the fully engineered sensor showed good agreement with those obtained from chemical analysis, thus proving the ability of the instrument in the open ocean environment. However, there
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