Understanding XRF Spectrometry and How It Works

X-ray fluorescence (XRF) analysis once was limited to testing geological samples; however, today, it has become an ideal analytical technology for a wide range of applications and industries such as metals, foods, pharmaceuticals, cosmetics, wastes, biofuels, and more. XRF offers an elemental analysis that is fast, efficient, and affordable. Moreover, XRF technology helps these industries ensure the materials they use and produce meet many national and international regulations, such as ASTM, EN, DIN, IEC, and ISO.

However, understanding XRF spectrometry and how it works can be complex. Below, we break it down by describing x-ray technology and how it’s used in XRF spectrometry. We also detail the types and applications of x-ray spectrometry to provide you with a better understanding of its value, especially in the metal industry.

What Are X-Rays?

X-rays are produced by the deceleration of high-energy electrons and the transition of electrons in the inner orbital of atoms. The invisible rays travel in a straight line and can be transmitted, reflected, diffracted, and scattered upon striking another matter. Interaction with matters can also produce and emit photoelectrons, auger electrons, spectral lines, and continuum radiation. The x-ray is part of the electromagnetic spectrum, and the range of x-ray used for XRF applications closely matches gamma rays and the high-energy UV range.

Understanding the XRF Spectrometry Working Principle

An XRF spectrometer begins its analysis by sending a beam of primary x-rays onto the sample. When these x-rays hit the sample, they excite or ionize the material. As a result, the atom, which consists of a nucleus (proton + neutron) surrounded by electrons in different orbitals that correspond to different energy levels, inside the material ejects an electron from its inner shell. The innermost shell has the lowest energy level, and the outermost shell has the highest. Thus, it is easier to remove the inner shell electron through small energy radiation.

To fill in the gap left by the removal of the inner shell electron, an outer shell electron drops into the vacant position. This transition results in fluorescent radiation (secondary x-rays) with an energy equivalent to the energy difference between the two electron shells.

Each element in the sample produces x-rays unique to that element, similar to a fingerprint. Thus, analyzing that emitted energy helps you identify the exact element.

Types of XRF Spectrometers

There are two types of XRF spectrometers, energy-dispersive and wavelength-dispersive. Both types are described in more detail below.

Energy-Dispersive (EDXRF)

In an energy-dispersive XRF instrument, the fluorescent radiation is collected by a semiconductor detector. Different unique energy signatures received from different materials are sorted and then processed at a high speed in a multi-channel analyzer.

Wavelength-Dispersive (WDXRF)

A wavelength-dispersive XRF instrument can be either sequential, simultaneous, or a combination of both. The sequential WDXRF analyzes only one element at a time, while the simultaneous WDXRF can measure all elements at the same time; however, it requires separate measurement channels for each element.

Reading XRF Results

Understanding XRF spectrometry results is the key to the correct analysis. The XRF spectrometer displays the number of x-rays counted on the y-axis and the x-ray energy on the x-axis as shown below.

If the analyzer reads only a few spectrums of a specific element, it indicates a low concentration in the sample and vice versa. An ideal XRF device with high sensitivity and a high count rate ensures high precision and fast measurement.

When it comes to quantification, modern spectrometers can calculate results from several parts per million to one part per million. For a quantitative test, you have to mix pulverized solids and sample liquid into a pressed pallet or fused bead for analysis inside benchtop analyzers. This will take a few more minutes than the handheld device but will produce more accuracy. However, correct calibration is necessary to compensate for the matrix effects for high accuracy.

Unique Applications of XRF Analyzers

XRF technology is useful for delivering qualitative data and analyzing different materials, along with their concentrations, present within a sample. The sample to be analyzed should be representative of the material to be analyzed and should have the same uniform concentration of elements throughout (homogeneous).

XRF analyzers are available in different sizes, starting from handheld portable guns to high-performance benchtop analyzers. Depending on the type, XRF analyzers can be used in the following industries:

• Glass analysis: XRF technology is used for analyzing raw materials including sand, limestone, dolomite, as well as the finished product.
• Refractory and slag analysis: XRF spectrometers are used for analyzing major oxides and trace elements.
• Vintage paintings: XRF technology can be useful for gaining insight into the distribution of individual pigments in vintage paintings.
• Rocks and minerals analysis: XRF instruments are useful for analyzing most types of rocks, such as granites, granodiorite, basalt, schists, clays, and carbonates.
• Cement and mortar analysis: XRF technology can provide an elementary analysis of raw materials for cement and determine the phase clinker composition.
• Soil sediments and sludges analysis: XRF instruments can provide rapid and potentially accurate soil elemental analyses in contexts of agriculture and pollution.

However, XRF technology is also often used in the metal industry, so understanding XRF spectrometry and how it’s used for verifying the composition of metals is important for fabricators and machinists particularly.

The Importance of XRF Spectrometry in the Metal Industry

XRF analyzers support the end-to-end metal production process to ensure accurate quality and composition and to help industries meet regulatory compliances.

If you cannot be assured that the metal you are purchasing meets your required specifications, the entire production process could be jeopardized. However, it is difficult to identify metals solely by their appearances. For instance, aluminum or stainless alloys can look the same, but if not correctly identified correctly, can have a disastrous outcome. Also, identifying two different alloy grades of the same metal comes with the same challenge. Fortunately, XRF technology can help you easily distinguish between different types and grades of metal alloys.

XRF technology is also helpful while analyzing scrap metals for valuation. Identifying oxidized, coated, or painted scrap metals through their colors can be misleading. A handheld XRF device can easily read the composition of metals within a few seconds to help you separate each by quality.

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