Understanding XRF (X-Ray Fluorescence) testing
XRF (X-ray fluorescence) is a powerful analytical technique that is widely used in various industries to determine the elemental composition of materials. It is a non-destructive method that measures the fluorescent X-rays emitted from a sample when it is excited by a primary X-ray source. Each element present in the sample produces a unique set of fluorescent X-rays, which acts as a fingerprint for that specific element. XRF spectroscopy is thus an excellent technology for qualitative and quantitative analysis of material composition.
The XRF process involves irradiating a solid or liquid sample with high-energy X-rays from a controlled X-ray tube. When an atom in the sample is struck with an X-ray of sufficient energy, an electron from one of the atom's inner orbital shells is dislodged, and the atom regains stability by filling the vacancy with an electron from one of the atom's higher energy orbital shells. The electron drops to the lower energy state by releasing a fluorescent X-ray, whose energy is equal to the specific difference in energy between two quantum states of the electron. This energy measurement is the basis of XRF analysis.
Most atoms have several electron orbitals (K shell, L shell, M shell, etc.), and X-ray energy causes electrons to transfer in and out of these shell levels. This process creates XRF peaks with varying intensities, which are present in the spectrum, a graphical representation of X-ray intensity peaks as a function of energy. The peak energy identifies the element, and the peak height/intensity is generally indicative of its concentration.
Which industries use XRF testing?
XRF is widely used in various industries for different applications. Handheld XRF analyzers, for example, identify alloys, detect tramp elements, deliver geochemical data, analyze precious metals, and determine coating weight and plating thickness, ensuring material chemistry specifications are met.
Oil and gas industry: XRF is used for positive material identification (PMI) of piping material, which is critical where flow accelerated corrosion or sulfidic corrosion is a concern
Metal fabricating: XRF is used for non-destructive elemental analysis to ensure that no incorrect or out-of-specification metals or alloys enter the manufacturing process.
Automotive and aerospace industries: XRF is used for incoming inspection and quality control of metallic and coated parts.
Scrap metal recycling: XRF is used for fast and accurate sorting of scrap metals, enhancing both workflow efficiency and profitability.
Precious metal recycling: XRF is used to determine the grade of precious metals and prevent deleterious metals from entering the recycling process.
Mining and exploration: XRF is used to quickly identify and recover the most economically viable resources.
Construction and environmental engineering: XRF is used for screening risk assessment, hazardous site modeling, and remediation quality control.
Although XRF is a safe technique, reasonable effort should be made to maintain exposures to radiation as far below dose limits as is practical, following the ALARA principle (As Low as Reasonably Achievable). During analysis, the analyzer emits a directed radiation beam when the tube is energized, and care must be taken to always point a handheld XRF analyzer directly at the sample and never at a person or a body part. Radiation safety training should be provided to operators, and primary beam indicator lights should be observed. Samples should not be held during analysis, and the analyzer should be handled and used with respect. Finally, XRF analyzers should be stored securely, following local storage requirements, and any safety emergencies should be reported to the Radiation Safety Officer (RSO) and analyzer vendor.
6 ways to prepare samples for XRF testing
X-ray Fluorescence Spectrometry (XRF) is a powerful technique for the analysis of various materials. However, accurate results depend on proper sample preparation. There are various ways to prepare a sample for XRF analysis, each with its own advantages and disadvantages. We will discuss the six most common methods of sample preparation and what you need to consider for each method.
Fused Bead Sample Preparation:
To perform sample preparation using fusion, a gas or electric fusion machine can be used. The process involves mixing a finely powdered sample with a flux and heating the mixture to temperatures ranging from 1000oC to 1200oC in a platinum crucible. The sample dissolves in the flux and is then cast into a mold to form a fused bead that is free of any mineral structures.
Using a fusion machine for sample preparation has several benefits, including reducing the matrix effects that can cause less accurate analysis and allowing for the combination of multiple different samples in the same calibration process.
Solid Sample Preparation:
Solid samples are typically ground to a fine powder, which is then pressed into a pellet or fused into a glass bead. The method of choice depends on the type of material and the required analysis. Pellet pressing is suitable for analyzing low-density materials, whereas glass beads are preferable for high-density samples. This method can be time-consuming, but it provides consistent and reliable results.
Liquid Sample Preparation:
For liquid samples, XRF analysis requires the sample to be dried and transformed into a solid form. The most common way to do this is to evaporate the liquid and then mix the residue with a binding agent, such as wax or boric acid. The resulting mixture is then compressed into a pellet. Alternatively, the sample can be mixed with a solid matrix, such as quartz or alumina, to form a bead. This method is particularly useful for analyzing solutions and suspensions.
Loose Powder Sample Preparation:
In this method, the sample is simply placed into a sample cup, and the cup is inserted into the XRF instrument. This method is quick and easy, but it can lead to inaccuracies due to inhomogeneity of the sample. Loose powder sample preparation is useful for screening or semi-quantitative analysis.
Thin Film Sample Preparation:
Thin film samples are typically analyzed in reflection mode, where the X-rays penetrate the film and reflect back to the detector. In this case, the sample must be a thin and uniform layer on a flat substrate. The most common method for preparing thin films is sputtering or evaporating the sample onto a substrate, such as a glass slide or quartz disc.
Bulk Analysis:
Bulk analysis is often used for large objects or materials that cannot be prepared by any of the above methods. In this method, the XRF instrument is used to analyze the bulk material directly. However, this method is less accurate and precise than other sample preparation methods.