X-ray fluorescence analysis

(spectroscopy) A nondestructive physical method used for chemical elemental analysis in which a material is irradiated by photons or charged particles of sufficient energy to cause its elements to emit (fluoresce) characteristic x-ray line spectra.

A nondestructive physical method used for chemical elemental analysis of materials in the solid or liquid state. The specimen is irradiated by photons or charged particles of sufficient energy to cause its elements to emit (fluoresce) their characteristic x-ray line spectra. The detection system allows the determination of the energies of the emitted lines and their intensities. Elements in the specimen are identified by their spectral line energies or wavelengths for qualitative analysis, and the intensities are related to their concentrations for quantitative analysis. Computers are widely used in this field, both for automated data collection and for reducing the x-ray data to weight-percent and atomic-percent chemical composition or area-related mass (of films). See also Fluorescence.

The materials to be analyzed may be solids, powders, liquids, or thin foils and films. The crystalline state normally has no effect on the analysis, nor has the state of chemical bonding, except for very light elements. All elements above atomic number 12 can be routinely analyzed in a concentration range from 0.1 to 100 wt %. Special techniques are required for the analysis of elements with lower atomic numbers (4–11) or of lower concentrations, and for trace analysis. The counting times required for analysis range from a few seconds to several minutes per element, depending upon specimen characteristics and required accuracy; but they may be much longer for trace analysis and thin films. The results are in good agreement with wet chemical and other methods of analysis. The method is generally nondestructive for most inorganic materials in that a suitably prepared specimen is not altered by the analytical process.

There are two general methods for producing x-ray spectra for fluorescence analysis excitation by photons and excitation by charged particles. The most common method is to expose the specimen to the entire spectrum emitted from a standard x-ray tube. It is sometimes modified by using a secondary target material (or monochromator) outside the x-ray tube to excite fluorescence. This has the advantage of selecting the most efficient energy close to the absorption edge of the element to be analyzed and reducing or not exciting other interfering elements, but the intensity is reduced by two or three orders of magnitude. Further alternatives are radioactive sources and synchrotron radiation.

The other method, used in electron microscopes and the electron microprobe, uses an electron beam directly on the specimen, and each element generates its own x-ray spectrum, under electron bombardment, as in an x-ray tube. See also Electron microscope.

The output signals from a detector are fed into the analyzer, where the photon counts are stored in memory locations (1024–8192 channels are generally used) that are related to the energies of these photons. This also allows visual observation on a cathode-ray-tube screen of the accumulated spectrum and of the simultaneous counting process. Analyzers are usually provided with cursor markers to easily identify the peaks in the spectrum. Computer memories can be used for storage of the spectral counts, thus providing efficient access to computer routines for further data evaluation.

The electron microprobe is widely used for elemental analysis of small areas. An electron beam of 1 micrometer (or smaller) is used, and the x-ray spectrum is analyzed with a focusing (curved) crystal spectrometer or with an energy dispersive solid-state detector. Usually two or three spectrometers are used to cover different spectral regions. Light elements down to beryllium, boron, and carbon can be detected. An important use of the method is in point-to-point analysis with a few cubic micrometers of spatial resolution. X-Y plots of any element can be made by moving the specimen to determine the elemental distribution. See also Electron-probe microanalysis.