In the context of biological specimens, it is in principle desirable to quantitatively map, rather than just point analyze, the distribution of physiologically important elements, and to do so at subcellular resolution. Presently, this can be accomplished by electron energy loss spectrum-imaging (EELSI) in both the scanning transmission electron microscope (STEM) and the energy-filtering transmission electron microscope (EFTEM). Until recently, this approach has been of limited value for mapping the particularly important element Ca, mainly because intracellular total Ca concentrations are normally quite low (<5 mmol/kg dry weight) and because the background in the vicinity of the Ca L23 edge is complex and requires precise background modeling to extract the very weak Ca signals. As a result, the Ca signal is usually not high enough to reach detection threshold during a practical EELSI acquisition time.
