X-ray Fluorescence

X-ray Fluorescence (XRF) Spectroscopy involves measuring the intensity of x-rays emitted from a specimen as a function of energy or wavelength. The energies of large intensity 'lines' are characteristic of atoms of the specimen. The intensities of observed lines for a given atom vary as the amount of that atom present in the specimen. Qualitative analysis involves identifying atoms present in a specimen by associating observed characteristic lines with their atoms. Quantitative analysis involves determining the amount of each atom present in the specimen from the intensity of measured characteristic x-ray lines.

Figure 1.1. WDS pattern of characteristic lines of silver, palladium, and cadmium.

The emission of characteristic atomic x-ray photons occur when a vacancy in an inner electron state is formed, and an outer orbit electron makes a transition to that vacant state. The energy of the emitted photon is equal to the difference in electron energy levels of the transition. As the electron energy levels are characteristic of the atom, the energy of the emitted photon is characteristic of the atom. Molecular bonds generally occur between outer electrons of a molecule leaving inner electron states unperturbed. As x-ray fluorescence involves transitions to inner electron states, the energy of characteristic x-ray radiation is usually unaffected by molecular chemistry. This makes XRF a powerful tool of chemical analysis in all kinds of materials.

In a gas, fluoresced x-rays are usually little affected by other atoms in the gas and line intensities are usually directly proportional to the amount of that atom present in the gas. In a solid, atoms of the specimen both absorb and enhance characteristic x-ray radiation. These interactions are termed 'matrix effects' and much of quantitative analysis with XRF spectroscopy is concerned with correcting for these effects. See the page Theory of XRF for details.

While the principles are the same, a variety of instrumentation is used for performing x-ray fluorescence spectroscopy. There are two basic classes of instruments: Wavelength dispersive and energy dispersive. Wavelength dispersive spectrometers measure x-ray intensity as a function of wavelength while energy dispersive spectrometers measure x-ray intensity as a function of energy.

An extremely important aspect of x-ray fluorescence spectroscopy is the method by which the inner orbital vacancy is created. Bombarding the sample with high energy x-rays is one method. Bombarding with high-energy electrons and protons are other approaches. An incident photon beam experiences a photon absorption interaction with the specimen while electron and proton beams primarily experience a Coulomb interaction with the specimen.

X-ray tubes accelerate high-energy electrons at a target within the tube that is then caused to fluoresce x-rays. The resulting x-ray beam includes a continuum and characteristic lines of the tube target. Radioactive sources can also be used to generate x-ray, electron (beta emitters), and proton (alpha emitters) beams. X-ray tubes can generate a high power x-ray beam, but the radiation is not monochromatic. Radioactive sources produce monochromatic beams, but of comparatively lower power. Proton-Induced X-ray Emission (PIXE) utilises a beam of protons. Wavelength Dispersive Spectrometry (WDS) generally utilises an x-ray tube as does Energy Dispersive X-ray Spectrometry (EDX). Instruments such as the electron microprobe and electron microscope directly bombard the sample with high-energy electrons to eject inner orbital electrons (EDS). Note that the charged particle beam approaches require the specimen to be electrically conductive.

Wavelength dispersive spectrometers use Bragg's Law and a standard analysing crystal to select the wavelength that enters a detector. A discriminator is used to reject pulses from undesired orders of Bragg reflections through the analysing crystal. By selecting analysing crystals with different Bragg d-spacings, the wavelength sensitivity of the instrument can be selected.

Energy dispersive spectrometers use a solid state detector and a multi-channel analyser to measure x-ray intensity as a function of energy. The detector measures all energies simultaneously, the sensitivity determined primarily by the detector and selected with the multi-channel analyser.

Please see the following links for a discussion of the various forms of XRF spectroscopy and their instrumentation


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