Spectrophotometry is a well-established analytical method, which has been used for decades in chemistry, physics, biochemistry, and chemical engineering for quantitative analyses. A light source and sensor are used to measure the intensity of light passing through a water sample in a known path length cell. Most commonly, ultraviolet and visible (UV-VIS) light sources are used. The principle is that substances in the water sample absorb or transmit light over a given wavelength range. According to Beer’s Law the measured absorbance of substances in the water sample is directly proportional to their concentration.