EELS instrumentation and experimental aspects
Electron Energy Loss Spectroscopy (EELS) conducted in transmission mode essentially probes bulk electronic and chemical properties of sample regions typically 100 nano meter (nm) thick. The coils are symmetrically arranged about the plane midway between the pole pieces. Since the spectrometer is not rotationally symmetrical, its second-order aberration coefficients are not necessarily zero. However, by suitably curving the edges of the pole pieces the aberration coefficients can be changed and some of them made zero. The EELS system is normally computer controlled so that the computer scans the spectrum in a series of equal steps, holds the spectrum at each position for a given dwell time and measures the intensity at each step. The choice of energy step for the spectrum depends on the energy resolution required. It is sensible to allot several channels per resolution element to observe detailed features.