Laser- and UV-LED-induced fluorescence detection of dissolved organic compounds in water

We have developed a laser-induced fluorescence (LIF) system to detect and continuously observe in real time the levels of colored dissolved organic matter (CDOM) or Dissolved Organic Compounds (DOCs) in water from various sources, such as tap water and reverse osmosis processed water. At the same time, we have studied deep-UV light emitting diodes (LEDs) as alternative light sources for our system, which would make the apparatus cheaper and more compact. Our portable LIF system had two interchangeable microchip Nd:YAG lasers, operating at 266 nm and 355 nm, as UV sources, and fluorescence was measured over the range of 260-680 nm. The fluorescence was collected at 90 degrees to the laser beam. We have also studied deep-UV LEDs emitting between 265 nm and 355 nm as alternative sources of fluorescence excitation. The average laser power was approximately 30 times that of the LED. Fluorescence spectra from sea water, tap water, and reverse osmosis water for both excitation sources were also measured, and similar spectra were observed. Differences in the signal intensity due to the difference in the laser and LED excitation intensity were consistent with theory.