What is XRF?
XRF stands for X-Ray Fluorescence and it works very similarly to the way your eyes work, just using a high energy spectrum than visible light.
Visible light is between 1 and 3 Electron Volts (eV), which is what your eyes are sensitive to. XRF is sensitive to X-ray light between 1,000 and 54,000 eV (or 1keV and 54keV).
An X-ray tube (in the case of handheld XRF spectrometers, a miniature tube) shines these X-rays on the sample being analysed and induces an X-ray Fluorescence from the individual atoms in the sample, which is then detected by the XRF detector.
Each atom type (or element) has its own unique fluorescence and so this is how XRF can determine which elements are in the sample. The amount of each type of fluorescence detected is then used to work out how much of each element is in the sample.
This whole process happens very quickly (in a matter of seconds) and is the same process whether you are considering a large laboratory XRF or a small field portable XRF spectrometer.
What elements can XRF measure?
Handheld X-Ray Fluorescent Spectrometers, also known as a pXRF, handheld XRF, or even an XRF gun, are capable of analysis of up to 48 elements in the one scan. These elements are typically in the range on the Periodic Table between Magnesium (Mg) and Uranium (U).
Some elements are excluded from the analysis because of a number of reasons:
- Unavailability of suitable standards to calibrate the instrument to an element.
- Interferences between elements (normally, the most commonly requested element is given preference in this case).
- The incompatibility between the type of X-ray tube being used and the element (e.g. Chlorine (Cl) is not possible when using a Rhodium (Rh) anode X-ray tube). This is why manufacturers offer different X-ray tube types depending on the requirements of the application (e.g. Tungsten (W), Gold (Au), Silver (Ag) and Rhodium (Rh) tubes may be selected).
Which applications can you use handheld XRF for?
Handheld XRFs can be configured for different applications. Each application requires the instrument to be trained to recognise elements (at the instrument build stage) in different sample types.
Here are some examples of the various applications that a pXRF can be calibrated for:
- Soils – the instrument is calibrated to detect low concentrations (parts per million, or ppm) of elements in a soil-type sample.
- Mining – the instrument is calibrated to detect commonly sought after elements in a mineral sample, from low ppm values to high % concentrations.
- Environmental – similar to Soils, but with a preference for heavy metals that may be required to be identified, such as Mercury (Hg), Lead (Pb) or Arsenic (As).
- Scrap Metal – this calibration shows common alloy-forming elements (such as Iron (Fe), Nickel (Ni) and Chromium (Cr)) and then matches a mix of these elements to a known alloy composition, such as Stainless Steel 316 or Brass CDA 360. The library of alloys are those most commonly processed at scrap metal yards.
- Alloys Positive Materials Identification (PMI) – similar to the Scrap Metal calibration, but library of alloys is expanded to those commonly seen in industry, such as Nickel based alloys in aircraft engine parts, or 718 Steel for Oil and Gas pipelines. The list of elements is usually expanded from the Scrap Metal calibration to include Magnesium (Mg), Aluminium (Al), Silicon (Si), Phosphorus (P) and Sulphur (S).
- Precious Metals Alloys – again, similar to the above Alloys calibrations, but considering mostly those elements used in precious metals alloys, such as Gold (Au), Silver) (Ag), Platinum (Pt), Rhodium (Rh) and others. Users of this application normally call the XRF a gold tester gun or similar.
- Coatings – usually calibrated for alloy-type samples but considering a layer of one (or more) metals over another. For example, Zinc (Zn) over Iron (Fe) as used in the galvanising process. The output of the instrument is normally shown to display the thickness of the top layer in micrometres (µm).
- Restriction of Hazardous Substances (RoHS) / Waste Electrical and Electronic Equipment (WEEE) – Focussing on the 10 hazardous elements, as identified by the European Union, that may be present in electronic and electrical equipment, that are hazardous to human health. These elements include Cadmium (Cd), Mercury (Hg), Lead (Pb), Chromium (Cr) and Bromine (Br). The application is usually on samples such as silicon chip circuit boards.
- Car Catalytic Converters – Many scrap metal yards receive catalytic converters, which contain high concentrations of the valuable elements, Platinum (Pt), Palladium (Pd) or Rhodium (Rh). The samples are normally low-density honeycomb structures and the XRF operator is required to cut open the catalytic converter to access the material for analysis.
How do field portable XRFs compare to laboratory?
Field Portable XRFs have the ability to produce results as accurate as laboratory analysers, provided the user employs sound data collecting techniques, including considerations for sample selection, preparation and presentation to the XRF.
It is a little-known fact that the components that make up most Field Portable X-Ray Fluorescent Spectrometers are every bit as capable of producing high quality results as those that make up laboratory and benchtop XRFs.
The reason why results of pXRFs can differ from laboratory-obtained assays is mostly due to considerations for sample selection, preparation and presentation to the XRF. In the laboratory, operators are highly trained in identifying and overcoming sample variations that affect results, whereas users of handheld X-ray guns are generally not considerate of such things.
Obtaining good repeatable results can be quite simple for some applications, including alloys identification, where perhaps a little surface cleaning is required to get a highly accurate result.
However, for most geochemical applications, most users of pXRFs are not interested in high degrees of accuracy; they simply want to get an indicative result for their elements of interest so they can make immediate decisions on their field activities, or even flag possible high-grade results for further laboratory analysis.
SciAps recommends all users undertake manufacturer provided training on their XRF so that high quality results are achievable.
For more help or information on XRF analysers, phone us on 0409707816
Or email us at [email protected]
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