X-Ray Microanalysis of Epithelial Cells in Culture
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X-ray microanalysis is a technique of elemental analysis that is carried out in an electron microscope. The technique is based on the generation of characteristic X-rays in atoms of the specimen by the incident beam electrons. These X-rays are characteristic for the element from which they originate, and hence contain information on which elements are present in the specimen (and how much of each). In addition to characteristic X-rays, the beam electrons generate background or continuum X-rays that do not carry information on the elemental composition of the sample, but the amount of background radiation is related to the total mass of the analyzed volume in the specimen and is used in quantitative analysis (1,1) Fig. 1 ). The lowest concentration of an element that can be detected is in the order of a few m
M
/kg, and the smallest amount is in the order of 10-
18
g. The spatial resolution of the analysis depends on the thickness of the specimen. The best spatial resolution is obtained in (thin) sections, where as a rule of thumb, the diameter of the analyzed volume is about half the section thickness. If bulk specimens are analyzed, as is done in the scanning electron microscope, the spatial resolution depends on the accelerating voltage and the composition of the specimen. A typical value for analysis of freeze-dried biological material at an accelerating voltage of 20 kV is about 10 μ
M
(
1
).
Fig. 1.
Energy-dispersive X-ray spectrum from a cultured bronchial epithelial cells showing peaks for endogenous elements, as well as for Ti from the titanium grid on which the cells were grown. The peak at 0 keV is an artificially generated peak used to calibrate the energy scale.
