Introduction to X-ray Fluorescence (XRF) Technology

X-ray fluorescence (XRF) is a commonly used technique for chemical analysis. The word ‘fluorescent’ simply means radiating light or other energy. Typical fluorescent lamps use electricity to excite argon and mercury atoms to produce white light radiation, while incandescent lamps produce electrons as a light source. XRF instruments use X-rays to ‘excite’ a material in order to characterise its composition by identifying the elements in the sample (qualitative analysis) or by determining the intensity of an element in the sample (quantitative analysis).

1. X-ray Spectroscopy So what's the deal with the term ‘X-ray’? We mentioned that X-rays are the excitation source in an XRF instrument, but in our definition, this is not its primary purpose. Characteristic X-rays are a by-product of the excitation process. For this we need a little science, X-rays are a subset of the electromagnetic spectrum, which covers everything from radio waves to visible light to X-rays and gamma rays. All materials are made up of atoms, and different atoms are represented as different elements in the periodic table.

Atoms are made up of subatomic particles, including: neutrons, protons and electrons. Protons and electrons are electrically charged, while neutrons are ‘neutral’. As the true workhorses of chemistry, electrons are bound to the atom by the proton charge of the nucleus. When an atom loses an electron, it is ionised, and the charge of the atom usually attracts the nuclei of other atoms to form chemical bonds. Most elements, especially metals, tend to bond with oxygen and get oxidised - like rust on iron. They can bond with themselves to form an elemental material, or find highly reactive atoms like sodium to bond with, causing all sorts of damage. In common compounds, most elements behave stably with other elements.

2. Classifying elements with characteristic X-ray energies

How do we obtain and classify characteristic X-rays?

Electrons orbit the nucleus of an atom in a series of shell layers labelled K, L, M, N, and so on.

When the inner shell layers (lower energy levels) become unbalanced by the loss of an electron due to radiation, an electron in the outer shell layers (higher energy levels) leaps to the inner shell layers to maintain stability. This process of electron jumping emits a specific amount of energy in the X-ray range. Put a detector in your instrument to measure these characteristic X-ray energies and you will know what elements are in your material. If you look at the periodic table of elements in an XRF, you will find a series of numbers that represent the energy of the characteristic X-rays for each leap from the outer shell layer to the inner shell layer, with different elements having uniquely different combinations of numbers. The energies represented by these combinations of numbers are all described in kiloelectronvolts (Kev).

3. Resolving conflicting spectral peaks If you look at the characteristic X-ray energies of the elements in the XRF Periodic Table, you will notice that when exceeding the element zinc, the numbers in the L-shell layer have similar energies to the k-shell layer of the element sodium, a phenomenon that will continue to be repeated for the elements with lower atomic numbers. For example, barium-L (4.467 keV) is almost identical to titanium-K (4.508 keV). Geologists and agronomists looking at minerals in the soil are likely to get a lot of indistinguishable L-line energy in their samples.

4. Energy dispersive X-ray fluorescence spectroscopy

Energy dispersive XRF (EDXRF) instruments excite and detect all elements from fluorine to uranium, providing energy spectra with energy peaks that characterise the material. If the user has some knowledge of the material, they can adjust the different excitation energies to select a more specific spectral range or use a filter assembly to screen out conflicting energies.The XRF software can also use spectral fitting algorithms to help calibrate the instrument for greater accuracy.

5. Wavelength dispersive X-ray fluorescence spectroscopy

In some cases, however, overlapping energies simply cannot be resolved in an energy dispersive instrument. For this, we need to use Bragg's law and specialised equipment. Bragg's law describes how X-rays pass through parallel atomic planes in a crystal. After excitation of the sample material, the characteristic X-rays produced are confined and diffracted as parallel light through a slit (collimator) and then through a crystal, which acts as a highly sensitive X-ray filter. In EDXRF, samples with Barium and Titanium will show a broad peak with an energy of about 4.5 keV, masking the presence of both elements, whereas the WDXRF spectrometer is able to show two different peaks from these two elements.

Due to the non-destructive nature of XRF technology and its sensitivity, X-ray fluorescence has become the technology of choice for scientists and regulatory agencies for elemental detection in the environmental, chemical, glass, cement, metal and mineral, and automotive aerospace sectors.