Quantitative X-ray mapping of biological cryosections

The potential for applying X-ray mapping to the elemental microanalysis of biological cryosections is discussed. Methods are described for acquiring and processing data, including use of the top-hat digital filter to remove the average effects of the background contribution. Practical considerations for X-ray mapping are discussed in terms of typical counts per pixel and minimum detectability which depends on the number of pixels chosen to integrate the signal. These aspects are illustrated with elemental maps (Na, P, K, Ca and Fe) from freeze-dried cryosections of mouse cerebellar cortex. A calcium sensitivity in the range 0.5 to 2.5 mmol/kg wet weight of tissue is demonstrated. The correction for overlap of potassium K beta and calcium K alpha is demonstrated with X-ray maps from cryosectioned synaptosomes of squid optic lobe. Quantitative results obtained using internal standards to determine wet weight concentrations are in reasonable agreement with expected values. Alternate schemes applicable to X-ray maps for determining the dry mass concentration, such as the peak/continuum (Hall method), are also discussed.