Quantitative X-ray microanalysis is used to obtain the elemental compo
sitions of tissues and cells. Concentrations are calculated on the bas
is of volume (mmo1L(-1) of packed cells) and mass (mmolkg(-1) dry weig
ht). Elemental maps are obtained by using a computer to control the po
sition of a beam in an electron microscope and to record the signals f
rom the scanning transmission electron microscope (STEM) and energy di
spersive X-ray spectrometer (EDS) X-ray detectors. These X-ray images
provide better visualization of elemental distributions than ''spot''
mode analysis by analyzing thousands of spots sequentially. When conce
ntrations are determined on the basis of volume, it is assumed that th
e section thickness created during cryosectioning, and shrinkage durin
g freeze-drying of a frozen section, are uniform. These assumptions ha
ve been examined with a nucleated red blood cell model. Even distribut
ions of cytoplasmic Fe and It, which we observed, can only occur if cr
yomicrotomy produces sections with smooth surfaces and uniform thickne
ss. In addition, by using bone marrow cryosections we have found that
the relative shrinkage between nucleated and non-nucleated cells is si
milar. Therefore, the assumptions made about volume calculations do ap
pear to be reasonable under the conditions used in this study.