Measuring the absolute position of EELS ionisation edges in a TEM.

Measurements of absolute positions of electron energy loss spectroscopy (EELS) core-loss edges in a transmission electron microscopy (TEM) are hampered by noticeable errors caused by instabilities of the primary energy of the incident electrons. These instabilities originate from a continuous drift and random ripple of the high tension and are unavoidable in the present generation of TEM and scanning TEM microscopes. However, more precise measurements are desired, for instance, to study the shift of the edge onset between atoms of different valency or chemical environment, the so-called chemical shift. A solution to this problem is presented by collecting a series of short low-loss acquisitions immediately followed by core-loss ones. To ensure a minimal time lapse between core-loss and low-loss acquisitions, all operations must be computer controlled. Accumulation of a number of acquisitions and their summation corrected for energy drift allows to cancel the energy instabilities and to relate the core-loss EELS spectra to the absolute energy scale. A practical algorithm is presented as well as the necessary calibrations for such a procedure. Also, examples of spectra collected using this principle and the resulting measured chemical shifts in several metal-oxides are presented.