Fluorescence spectroscopy techniques have been gaining in popularity since the late 1990s and originate based on the intrinsic fluorescent properties of different groups of organic matter (OM); that is, organic matter that fluoresces naturally. Over time, groups of organic matter have been associated with different fluorescence excitation and emission wavelengths. When an OM particle is excited with UV light, the electrons are elevated to higher energy levels surrounding the atom. When the excitation ends, the electrons can return to their original energy level or 'ground-state'- via a number of pathways, some of which are radiative (involving the release of light). For example, tryptophan-like fluorescence is known to emit 350-360nm when excited 230nm(280nm). Based on this knowledge, laboratory desk-top spectrophotometers were used to scan extensive ranges of wavelengths to analyse liquids and identify their OM content.
In the last decade, technology has been developed to take techniques that were previously only achievable in a laboratory setting and integrating them into a standalone sensor, whereby particular wavelengths can be targeted; from this came the patented Proteus. The Proteus utilises UV sensors which target specific known wavelengths (TLF and Peak C/fulvic-like fluorescence) and then combines the reading with an algorithm and other sensor outputs to translate to BOD as well as other parameters such as Total Coliforms (TC). The exact sensor set-up is related to the client's individual requirements but the number of UV fluorometers required is flexible.
The only similarity of UV-Vis (sometimes referred to as UV 254) to fluorescence spectroscopy is that it utilizes wavelengths on the UV portion of the EM spectrum. The approach is different and the results, when applied to BOD, are vastly different. The principles of UV-Vis surround absorbance of UV light on a wavelength 200-750nm, where the absorption of light is related to the chemical structure of the compound of interest; the absorption is then related to the concentration of substances in-line with the Beer-Lambert law.
UV-Vis is generally considered as a 'first pass' for investigating water quality, as it is a nonspecific measure that is unsuitable if the target chemical compounds are unknown e.g. with UV 254 as not all organic compounds absorb at 254nm. Generally, the UV wavelengths scanned are a much broader spectrum and are unable to target specific wavelengths unlike fluorescence spectroscopy methods.
|Specific Wavelength targeting?||Yes||No|
|High Sensitivity with single molecule detection possible?||Yes||No|
|Discrimination between organic matter that absorbs on similar wavelengths?||Yes||No|
|Complex considerations for the environment e.g. path length?||No||Yes|
|In-built temperature correction?||Yes||No|
|In-built sensor health check?||Yes||No|
|Complicated mounting and install requirements?||No||Yes|
|High ongoing maintenance costs?||No||Yes|
|Integrated self-cleaning as standard?||Yes||No|
|Includes non-fluorescence/UV sensors for more detailed water quality analysis?||Yes||No|
|Long-life LEDS used as standard?||Yes||Not all models|
|Internal battery option that supports flexible logging?||Yes||No|