Elemental maps, that is images showing the concentration of different elements in a sample, can be obtained in an electron microscope equipped with an electron energy-loss spectrometer (EELS) by acquiring and processing data in three dimensions: spatial coordinates x and y, and the energy loss ΔE. Since the electron detector is necessarily at most a two-dimensional one, acquiring all the required data at the same time is not possible. Instead, one can either use an imaging electron spectrometer and acquire a series of whole images at one energy at a time, or use a small probe in a scanning-transmission electron microscope (STEM), and acquire the data image-point by image-point. With a serial-detection spectrometer the data at each image-point must be recorded sequentially, while with a parallel-detection spectrometer a whole spectrum can be recorded at the same time.The two approaches are illustrated schematically in figure 1. The individual sampling points in the three- dimensional volume have been called voxels (by analogy with two-dimensional pixels).
