Spectrophotometric flow analysis techniques for the determination of total phosphorus and total nitrogen in natural waters

Abstract

Eutrophication, the over enrichment of nutrients in an aquatic system, is associated with harmful algal blooms, and as such is a serious environmental issue. Consequently, interest in the monitoring of nutrient concentrations in aquatic systems has increased in tandem with a burgeoning public and scientific awareness of environmental problems. This thesis describes the development of a number of flow analysis techniques for the monitoring of nutrient concentrations in natural waters; namely total phosphorus and total nitrogen, as well as the design and construction of a total internal reflective flow-cell for use in flow analysis systems. The portable flow analysis system for the determination of total phosphorus in natural waters was designed with rapid underway monitoring in mind. The digestion module consisted of a ultra-violet photo-reactor, thermal heating unit, in-line filter and debubbler, with sample being merged with an acidic peroxodisulfate digestion reagent. A multi-commutational flow analysis unit was used to introduce gaspressurised molybdenum blue chromogenic reagents using two miniaturised solenoid valves, followed by spectrophotometric detection using a multi-reflective flow cell with a light emitting diode source and photo-diode detector. The fully automated system has a throughput of 115 measurements per hour, a detection limit of 1.3 μgPL-1, is highly linear over the calibration range of 0 - 200 μgPL-1 (r2 = 0.9998), and a precision of 4.6 %RSD at 100 μgPL-1 (n=10). Shipboard field validation of the instrument and method was performed in Port Philip and Western Port Bays in Victoria, SE Australia, where 2499 analyses were performed over a 25 hour period, over a cruise path of 285 kilometres. Good agreement was observed between determinations of samples taken manually and analysed in the laboratory and those measured in situ with the flow analysis system. Historically, total nitrogen has been determined by Kjeldahl digestion or oxidative digestion to nitrate followed by reduction of the generated nitrate to nitrate by cadmium with spectrophotometric detection via the Griess assay. The Kjeldahl digestion does not measure nitrate and nitrite, and the reduction of nitrate to nitrite involves the use of a toxic cadmium reagent that degrades rapidly in the presence of residual oxidant. The flow analysis system developed for the measurement of total nitrogen involves photo-oxidation of all nitrogenous compounds in the presence of alkaline peroxodisulfate, with in-line filtration and debubbling, followed by ultraviolet second derivative spectrophotometric detection of the nitrate generated. A ten minute stop flow period in the photo-reactor removes a substantial amount of residual oxidant, which is a spectral interferent in the 220 nm range used to quantify nitrate. Second derivative spectroscopy is used to minimise interference from any residual oxidant, as well as other species such as sulfate. The fully automated system has a throughput of 5 measurements per hour taken in triplicate, has a detection limit of 0.05 mgNL-1, is highly linear over the calibration range of 0 - 2 mgNL-1 (r2 = 0.9989), and features a precision of 1.2 %RSD for 1 mgNL-1 as ammonia (n = 10). Excellent agreement was found between storm water samples measured using the flow analysis system in comparison to those obtained using a reference method. The design and construction of a total internal reflective photometric flow-through cell is described. This cell consists of a tubular length of fused silica quartz capillary, where light is introduced and collected from the cell using quartz optical fibres. Incident light undergoes total internal reflection at the air-quartz external wall interface, and thus undergoes multiple reflections as it propagates through the capillary. This cell was found to have several desirable features in common with liquid core waveguides (efficient light throughput that leads to high signal to noise ratio, versatile choice of irradiant light wavelength) and coated capillary multireflective cells (low hydrodynamic dispersion, no entrapment of bubbles, high tolerance to refractive index effects).