SEMs image the sample to be analyzed using the electrons elastically scattering from the surface. Thus, the surface of thick layers can be studied. The surface of the sample has to be covered by a conductive layer, such as metal or graphite. The surface is scanned by an electron beam. The diameter of this electron beam (<1 pm) determines the horizontal resolution of the SEM.

A portion of the electrons scatters inelastically, ejecting electrons from the K or L electron orbitals. Similar to XRF, the following characteristic X-ray emission provides qualitative and quantitative analytical possibilities. These instruments are called “electron microprobes.”

In electron microprobes, the characteristic X-ray photons are detected in two ways: in an energy-dispersive or a wavelength-dispersive manner. Similar to X-ray fluorescence spectrometers, energy-dispersive systems are equipped with semicon­ductor detectors, the resolution of which is about 130 eV. In wavelength-dispersive systems, the detector is a single crystal at a precise angle. The structure, including the characteristic spacing between the planes of the crystal lattice, is known. The wavelength of the X-ray photons is measured using the Bragg formula (Eq. (10.14)). The resolution of the wavelength-dispersive detectors is about (10 eV). All elements, except hydrogen, helium, and lithium, can be measured.

Electron microprobes have 0.01% relative and 1014g absolute detection limits with 3% relative accuracy.

In Figure 10.22, the SEM picture of montmorillonite clay treated with lead ions is shown. The left picture shows the image (morphology) obtained by the elastically scattered electrons. The right picture is the lead map obtained by the energy — dispersive spectrum of the characteristic X-ray photons.

In Figure 10.23, the wavelength-dispersive X-ray spectrum obtained at a certain area of a clay sample treated by manganese ions is shown.

The scanning of a sample surface provides the possibility of analyzing the ele­mentary composition along a straight line. The concentration profiles of different elements of montmorillonite clay treated with lead ions are shown in Figure 10.24.