8/26/2023 0 Comments Absorption spectra for hydrogen![]() Clothianidin and thiamethoxam are neonicotinoid pesticides. All are transparent in the soft UV and visible range. 1 for six substances relevant to water quality. UV absorption spectra were measured for a range of contaminants in water (concentration 10 mg/L, optical path 10 mm in silica glass cuvettes). It is shown that the spectral discrimination of even such a simple apparatus could be sufficient to usefully distinguish certain contaminant substances. We combine absorption spectra obtained using a conventional UV–Vis spectrometer and selected-wavelength transmittance measurements using LEDs and a wavelength-thresholding UV detector whose photocathode operates in inert gas 30. Here we present UV absorption data for several water contaminants of practical interest. The choice of light detector then becomes important: good responsivity in the chosen wavelength ranges is important but too broad a detection band, particularly one extending into the visible range, could increase background signal and hence limit sensitivity. ![]() The advent of efficient and increasingly affordable light emitting diodes (LEDs) spanning the range 250 nm to 350 nm suggests that multi-wavelength UV measurements could be made without resorting to dispersing spectrometry. However, as inputs for fuzzy logic or similar real-time analysis of water systems, “naive” absorbance measurements at multiple wavelengths could be extremely useful. The complexity of natural water means that such measurements should ideally account for both scattering and absorption at different wavelengths. In the low-absorption regime such effects can be exploited in the optical detection of water contamination. Absorption and scattering are strongly influenced by dissolved substances, suspended particulates, bubbles and other impurities 28, 29, making inherent optical properties difficult to extract reliably. Pure water has a scattering-independent absorption coefficient below 0.1 m \(^\) at the upper end of this range 27. The standard UV-254 parameter relates to absorbance at 254 nm only but water is reasonably transparent over a wide range of wavelengths into the visible. While field-deployable spectrometers are available, they are relatively expensive and so simpler single-wavelength absorbance devices could be attractive for widespread sensing networks. Fluorescence and Raman scattering are also used, again with a dispersing spectrometer 15, 26. UV–Vis spectrometry uses a dispersing spectrometer to measure absorbance as a function of wavelength 14, 25. The data can be used as early-warning alerts 24, on water network or process management “dashboards” or to manage water systems via fuzzy logic 16 or machine learning. Such devices can provide rapidly sampled information on the behaviour of contaminants and biochemical processes in habitats, distribution networks, catchment areas and industrial settings 20, 21, 22, 23. There is increasing interest in distributed and continuous water quality monitoring using remote sensors for parameters such as resistivity, temperature, pH, total suspended solids (TSS), absorbance at 254 nm (UV-254) and total organic content (TOC) 17, 18, 19. Simpler methods can also be very useful, particularly where there are practical or economic constraints on sampling, transport to a laboratory setting, etc. However, laboratory data are not necessarily needed to provide valuable inputs to water quality monitoring and control schemes 16. Detection methods for pollutants in water include sophisticated analytical approaches such as liquid chromatography 10, electrode potentiometry 11, 12, mass spectrometry 13, UV–Vis spectrometry 14 and Raman scattering 15. Significant worry has arisen over the adverse effects of neonicotinoid pesticides such as clothianidin and thiamethoxam on bees and other pollinators, and these are included in EU-wide watch-lists for monitoring in surface water 9. Contaminants include pesticides 2, 3, heavy metals 4 and sewage 5 while many other emerging pollutants are of concern 6, 7, such as pharmaceuticals and personal care products 8. The contamination of water by human activity is a major environmental and health concern 1.
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