Electron energy loss spectroscopic imaging in biology

One of the goals in biology is to relate the ultrastructure with the movement of elements to understand better physiological and pathophysiological mechanisms. Electron energy loss spectroscopy (EELS) imaging, which was developed in the last decade, appears to be an ideal technique to make such correlation. EELS takes advantage of the energy distribution of transmitted electrons which interacted with the specimen. All these electrons are collected and can be displayed as an energy loss spectrum for analytical purposes. Images can be produced from selected regions from the energy distribution allowing the mapping of specific elements. The main advantage of EELS imaging in biology is its spatial resolution of 0.5 nm or less and its great sensitivity allowing nearly a single atom detectability. The limitations reside essentially in specimen preparation. In order to obtain optimal results with EELS imaging, only very thin specimens can be used. This restricts the way biological specimens can be prepared. This is a real challenge for the analysis of diffusible elements. Other limitations reside in the difficulty of quantifying the results obtained. This is greatly due to the fact that theoretical considerations still have to be experimentally validated. The purpose of this review is not to repeat in length the principle of EELS but to emphasize its achievement in biology and to assess the present advantages and limitations. Also, as EELS imaging is still in its development phase, results already obtained are a strong indication that this technique has a great prospect in the analysis of dynamic biological processes.