A method used for determining the elemental composition of materials, based on the xrays emitted by different elements when bombarded with high-energy electrons. It is a micro method that can detect xray photons emitted by the atoms within a small volume excited by an electron beam focused to 10 nanometers diameter or less. In biology electron-probe microanalysis can be used to determine the composition of cell organelles without isolating them and therefore altering the distribution of diffusible elements Studies with electron-probe analysis have thrown light on the major problems of excitation-contraction coupling in muscle, visual transduction in retinal rods, and the functions of the endoplasmic reticulum and mitochondria in the regulation of cytoplasmic calcium.