X-ray Microanalysis Lecture Notes

Elemental analysis is one of the many services an Electron Microscopy lab can offer. Whenever electron beams are impinged upon the surface of a sample, a wide variety of signlas are released. X-rays are one such signal. These x-rays can be collected by a detector to yield information about which atomic species are present and in what relationship. The accuracy of the results are dependant upon the technology employed. The most common technologies used fall into two types: Energy Dispersive Spectrometry and Wavelength Dispersive Spectrometry.

Elemental Line Nomenclature

One of the scattering events that happens when an electron beam interacts with a sample is the generation of characteristic x-rays. A beam electron may strike an electron within an atom of the specimen, knocking it free from it's orbital. This can ionize the atom. A higher order electron of the atom may replace the lost electron, and a x-ray photon might be released. The name of the detected energy (k-alpha) refers to the orbital vacated, and the transition that occurred to fill the vacancy. The energy of characteristic x-rays varies with the atomic number of the parent atom. For a given line type (k-alpha), the x-ray energy increases with atomic number (defined in Moseley's law). The electron microscope accelerating voltage is usually set at 2.5-3 times the energy of the elemental line of interest (referred to as overvoltage).

Energy Dispersive Spectrometry

Energy dispersive spectrometers (EDS) utilize lithium-drifted silicon solid-state detectors to absorb x-rays given off by the sample. To minimize noise in these systems, the detectors are held at cryogenic temperatures. Unfotunately, this can make the detectro act as a cold trap to the vacuum environment and thus, the detectors are usually shielded from the microscope by a thin window. The material used to manufacture this window will impact the sensitivity of the detector. Low energy x-ray photons released from the sample might not have enough energy to penetrate the window and be absorbed by the detector. Windowless detectors are available for increased sensitivity, at the risk of contamination of the detector crystal.

As an x-ray photon released from a sample enters a solid state detector, it deposits its energy via photoelectric absorption. The energy that is collected can be used to form a histogram, with energy plotted on the x-axis vs. the number of x-ray events (counts) on the y-axis. Plotted over time, a spectrum of elemental peaks can be generated. The number of counts within the peaks can be used to determine the ratio of the elements present in the sample. In the best case, the minimunm detection limit for an element is 0.1%. EDS quickly shows all elements present in the sample. A few disadvantages of EDS are as follows: quantitative accuracy falls off at low concentrations, resolution is limited to roughly 100eV, low sensitivity to light elements, and the need for a cryogen.

Wavelength Dispersive Spectrometry

In wavelength dispersive spectrometry, x-ray photons released from a sample are focused by a crystal onto a gas flow proportional counter. Bragg's law defines the interaction of the x-ray with the crystal. For crystal planes of spacing d and x-rays of wavelength w, there exists an angle theta.

nw=2d sin theta

Crystals used in WDS are mounted on a focusing circle. Also, the crystals are manufactured with a curved face. This allows x-rays of similar wavelength coming from a point on the sample to be diffracted (focused) and detected by a gas filled proportional counter. Movement of the crystal along the focusing circle allows x-rays of different wavelengths to be focused. To allow the system to focus x-rays of varying lengths, crystals with different d spacings can be purchased. Three to five crystals in one spectrometer unit is fairly common

WDS systems have excellent peak to background sensitivity, with accuracy in the ppm (parts per million) range. Data acquisition is slow compared to EDS. In 60 seconds, the EDS system will give the investigator a good idea of what elements are in the sample. Since the crystals in the WDS system are moved along a focusing circle (Rowland circle), acquisition times can be 5 to 10 minutes. If every crystal in the unit is scanned, acquisition time could approach 1 hour. Many investigators will perform EDS to find out what elements of interest are present, then perform WDS on this element list, speeding WDS acquisition.

To research these techniques at greater length, see: Scanning Electron Microscopy and X-ray Microanalysis: A Text for Biologists, Material Scientists and Geologists, Goldstein et al, ISBN 0-306-44175-6.