X-ray Fluorescence (XRF) is a technique widely used in elemental analysis. It is based on the principle that an atom's inner electrons, when bombarded with high energy radiation such as X-rays, are ejected. The atom relaxes by emitting photons of characteristic wavelengths, which are used to identify the element.
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How does XRF work?
1) A stable atom
A stable atom is made up of a nucleus which is orbited by electrons. Different energy levels 'bind' different numbers of electrons in their respective ‘shells’. A carbon atom, showing the 'K', 'L', 'M' and 'N' shells.
2) An XRF-excited atom
When an X-ray wave exceeds the binding energy of an inner electron shell, an electron is dislodged and ejected. The atom as a whole regains stability by replacing this empty space with another electron from a higher-energy orbital shell. A fluorescent X-ray is released as this electron lowers its energy level, in order to migrate to the inner shell. We detect this x-ray fluorescence (XRF).
An XRF spectrometer (or XRF 'analyzer') is designed around an X-ray source (or X-ray 'tube') and a detector. X-rays are generated by the source and directed at the sample. Sometimes, a filter is used to modify the X-ray beam.
After the X-ray hits the sample, secondary X-rays are created as the atoms react. These secondary waves are received and processed by the detector. A spectrum is then generated, showing the amount of various elements in the sample according to the strength of various peaks.
The range of elements analyzed using XRF usually spans from sodium (Na) to Uranium (U) and each of them will have different detection levels depending on the availability of orbitals to which excited electrons can move to.
Most XRF spectrometers fall into 2 general types, Energy Dispersive XRF spectrometers (ED-XRF) or Wavelength Dispersive XRF spectrometers (WD-XRF):
- ED-XRF spectrometers are simple and easy to use, can simultaneously collect the signals from several elements and offer resolution from 150 eV – 600 eV.
- WD-XRF spectrometers collect one signal at a time at different angles with help of a goniometer. These instruments are normally more complex and expensive, however resolution is considerably higher, from 5 eV to 20 eV.
Popular uses of XRF can be found in cement, metal ores, mineral ores, oil & gas, environmental and geological applications. However, virtually any laboratory with the right expertise may make use of it.
XRF sample preparation equipment
One important aspect, in order to obtain high quality results, is the sample preparation of XRF samples for analysis. For XRF, samples can be analyzed as liquid or solids.
1) Liquid XRF sample preparation
Liquid samples have only one mode of preparation in which the liquid is poured in a cup and a film is used as a seal. The trick here is to choose the most suitable film that will provide enough support and transmission while keeping the sample free of contaminants.
2) Solid XRF sample preparation
Solid samples have various preparation options, the most common are pressed pellets and fused beads. Pressed pellets are produced by employing a press and a die set, in this case, the sample is usually ground to a grain size of
If the sample will not bind during pressing, a wax binder can be used to assist, which is normally added in proportion of 20-30% to the sample. Different loads are required depending on what the sample is and how easy it will bind together.
- A food sample may only require 2-4 tons.
- A pharmaceutical product may require 20 tons – an ideal application for the Manual XRF Press.
- A mineral ore may require up to 40 tons – a suitable tonnage for the 'Autotouch' automatic XRF Press.
On the other hand, fused beads are used when a better homogenisation of the sample is required. In this technique the sample, grinded toA downside to the fused bead XRF technique is the inability to detect trace elements as the sample has to be diluted.
Specac can provide all the pressing tools needed to produce high-quality XRF pellets. Our XRF pellet dies are manufactured with highly durable hardened 440 C stainless steel pressing faces. For XRF spectroscopic measurements where iron is a particular element to study, internal pressing face pellets made of Tungsten Carbide in place of stainless steel can be used with our XRF pellet dies. All pressing faces have a mirror finish to help with consistencey and repeatability between samples