Distributed water pollution source localization with mobile UV-visible spectrometer probes in wireless sensor networks

UV-visible Spectroscopy has advantages of fast response, in-situ multi-parameter analysis, no secondary pollution, and low maintenance costs, receiving for these reasons widespread attention, especially in the field of surface water quality monitoring. For traditional UV-visible spectrometer probes, the fixed optical path may lead to loss of accuracy or even invalid results at high or low pollutant concentrations, due to the instrumental factors and the sample properties. Also, the traditional dilution approach makes the systems more complex and cannot achieve in-situ analysis, which is not suitable for our application. However, reliable sensing at pollutant concentrations varying in a wide range is essential here. Thus, the UV-visible spectrometer probes are not only designed to analyze multiple parameters, including TOC, nitrate nitrogen, turbidity, etc., but also improved to adjust the optical path dynamically with an adaptive optical path mechanism.

The structure of an UV-visible spectrometer probe with an adaptive optical path is shown in Figure 1. With compact design, it mainly comprises a xenon flash lamp, a collimating lens, a condensing lens, a slit, a flat-field holographic concave grating, a complementary metal oxide semiconductor (CMOS) linear image detector, a motorized linear stage and a slider. The xenon flash lamp light source, which delivers high stability and long service life, produces a broad spectral output from 185 nm to 2000 nm with a rated power of 5 W. The flat-field holographic concave grating works between 200 nm and 800 nm with reduced aberrations, bringing benefits of low light losses and simplified optical system. The CMOS linear image detector has 512 pixels, a spectral response range from 200 nm to 1000 nm, and on-chip charge amplifiers. During the operation of the UV-visible spectrometer probe, the light from the xenon flash lamp is collimated by the collimating lens and the beam passes through the water in the open flow cell. Then, the beam after absorption is condensed by the condensing lens and emitted from the slit. Finally, the flat-field holographic concave grating acts as a spectroscopic element and also an imaging element, while the CMOS linear image detector records the UV-visible absorption spectra. The UV-visible absorption spectra has an effective wavelength range from 200 nm to 800 nm and a resolution of 3 nm. Especially, the slider is driven by the motorized linear stage with a maximum speed of 10 mm/s, which makes the optical path in the open flow cell adjustable from 2 mm to 30 mm. The power and data cable, which supports power supply and data transmission, as well as the probe encapsulation are waterproof, so the whole probe can be placed in the water for in-situ analysis, the UV-visible absorption spectra can be acquired by a processor module for further process, and the optical path can be adaptive to internal and external information. As shown in Figure 2, the prototype of the UV-visible spectrometer probe is developed, which is used to analyze multiple parameters. Figure 2a shows the outside view of the prototype, Figure 2b shows the internal structure of the prototype, and Figure 2c shows the typical raw spectra of the blank and the multi-component mixture with TOC at 16 mg/L, nitrate nitrogen at 8 mg/L, nitrite nitrogen at 2 mg/L and turbidity at 20 NTU.