Energy dispersive X-ray spectroscopy (EDX or EDS or EDXS) in transmission electron microscopy (STEM) can contribute valuable data in life science; for example for cell and tissue imaging. Silicon drift detectors, used for EDS, have become so sensitive in the lower energy region, that the detection of small amounts of light elements relevant for life science, such as calcium, oxygen, nitrogen and sulfur, is routinely possible now. Furthermore, EDS allows mapping the whereabouts of nearly all the elements of the periodic table within one single experiment, taking just a few minutes. This makes EDS in electron microscopy predestined for the investigation of complex biomaterials. Biomineralization is a suitable object immediately, but also mapping of elements such as oxygen or Ca is feasible. Additionally, immunolabels can be distinguished by EDX easily from their surroundings if they contain or are composed of a suitable element.
As an example, resin embedded human erythrocytes infected with Plasmodium falciparum, the Malaria causing parasite, were investigated using EDX in a standard STEM. The element distribution was mapped and analyzed quantitatively (Fig. 1). Peak overlaps such as of osmium, used for staining, and the lighter phosphorous can be easily separated using the versatile and very open EDS analysis software ESPRIT. Fig. 2 shows that the inclusion of phosphorous overlapping with osmium is essential for the correct representation of the measured spectrum. Furthermore, the calcium peak, used for mapping clearly appears in the background subtracted spectrum.
A second example is an element map of a resin embedded yeast cell (Fig. 3). The whereabouts of light and heavy elements can be clearly retrieved. The nitrogen distribution is shown quantitatively with an accuracy of a few mass percent. Most interesting is the mapping of the silver beads used as immunolabels and the sulfur signal associated with them.