26 Jun 2017

What is XRF? | Spectroscopy Guides

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’. 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
K
L
M
N

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).

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
K
L
M
N



 
 
The energy released in this transition equates to the difference in energy levels between these two quantum states of the electron, old shell vs new shell. Measuring this energy is the foundation of XRF analysis.

XRF spectrometers

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.

 
Sample
 
 
 
 
 
(Filter)
 
Detector
 
X-ray source

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):

  1. 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.
  2. 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.

Read our guide on 'How to prepare liquids for XRF analysis' here.

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 <75 µm.

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.

On the other hand, fused beads are used when a better homogenisation of the sample is required. In this technique the sample, grinded to <75 µm particle size, is mixed with a flux (usually a lithium tetraborate or tetraborate/metaborate mixture) in ratios of flux/sample 5:1 to 10:1 and heated in a platinum crucible to high temperatures (potentially up to 1,600 °C). A 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

Check out our 6 tips on how to make an XRF pellet and look up #SpectroscopyGuides for more analysis tips and advice.