An x-ray fluorescence technique for detecting the level of arsenic in a sample of water or body fluid. Arsenic and lead are expected to fluoresce in a first energy band and lead is expected to also fluoresce separately in a second energy band. An excitation path directs x-rays toward the sample; a first detection path detects x-ray fluorescence of the first energy band from the sample; and a second detection path detects x-ray fluorescence of the second energy band from the sample. The level of arsenic can be obtained by analyzing the x-ray fluorescence from both detection paths, and using a constant which relates the level of lead in the second energy band to the level of lead in the first energy band. The excitation path and each detection path may each include a monochromating optic to transmit the desired x-ray energy band, e.g., a doubly curved optic.