A sensor for measuring a concentration of a particular ion, molecule or atom in a fluid includes a sample handling portion for providing at least some of the fluid, a first photo-detection device, and a first light source. The first photo-detection device is configured to measure a power of light incident thereon, and the first light source includes a solid-state light emitting device. The first light source is configured to emit light having a wavelength less than 240 nanometers incident on the fluid provided by the sample handling portion, and the first photo-detection device is configured to receive light having passed through the fluid.