What Is XRF ?
XRF or X-ray Fluorescence spectrometry, is a chemical analysis technique used to determine the chemical composition of a wide range of bulk solids, liquids and slurries. The technique is most commonly used for the quality control of materials in industrial applications in the cement, steel and mining and similar industries. It is also often used in the petrochemical industry to monitor the sulfur content of fuels and the quality control of plastics. XRF is a versatile tool and is also found in many university geology departments where its ability to measure a wide range of elements and simple operation make it an extremely useful tool.
What Can it Measure?
XRF instruments are capable of measuring most of the periodic table from elements as light as Beryllium (Be) to the Uranium (U) and beyond. The range of elements that can be measured depend on the type of XRF and the material being measured. The elemental range for liquid samples for example is limited to Sodium to Uranium. It's most often used to measure bulk solid materials, but can also be adapted to measure liquid and slurry samples. Solid samples can be presented for analysis as is, but powdered samples such as cement are most often mixed with a binder and pressed into a pellet or steel ring for support. Powdered samples can also be mixed with a flux and fused into a glass bead. The advantage of this approach is to create a perfectly homogenous sample for analysis. Liquid and slurry samples are placed into a plastic cells with a plastic support film such as Mylar and analyzed in a helium environment. The plastic film absorbs element lighter than sodium limiting the analytical range for liquids.
Samples prepared as fused beads Sample pressed into a steel ring
How Does it work ?
XRF analysis works by first generating x-rays in a source or X-ray tube and then using these X-rays to irradiate samples which then produce characteristic X-rays for each element present in the sample. These "secondary" X-rays are measured by a detector. XRF is a comparative technique so samples with known concentrations for each element of interest must be measured first and then a relationship is developed between the counts measured on the detector and the concentration of the element in the standard to produce a calibration curve. Unknown samples can then be measured by the instrument and the counts measured are converted to the concentration for each element using the stored calibration curves.
What is it used for?
XRF is an inherently stable technique and the advent of modern X-ray generators and counting electronics have made it even more reliable. This is a huge advantage over many analytical techniques in that once calibrated an instrument can be used for years without needing to be recalibrated. Modern software has made this powerful analysis technique more accessible to novices and experts alike. Once a method is calibrated almost anyone can successfully analyze a sample and obtain the same results. The stability, wide range elemental range and ease of use have made this technique attractive to many industrial applications. Many ASTM (American Society for Testing and Materials) methods have been developed for industrial applications including:
ASTM C-114 for the chemical analysis of hydraulic cement,
D 2622:Standard Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry
ASTM D7085: Standard Guide for Determination of Chemical Elements in Fluid Catalytic Cracking Catalysts by X-ray Fluorescence Spectrometry (XRF)
Graph showing the reproducibility of XRF measurements of Na2O in raw meal for a cement industry application.
The XRF Instruments and the Future
XRF instruments come in many shapes and sizes from basic handheld units to full size laboratory quality control instruments and have been commercially available since the 1950's. The myriad of XRF instrumentation available is another topic for a future blog post.
Despite being one of the most venerable analytical techniques, XRF has stood the test of time because of its versatility, stability and ease of use. Improvements in electronics and software continue to drive innovation XRF development and expand its range of applications for the future.
- If you're interested in learning more about XRF and its applications you can join our LinkedIn technical discussion group. XRF Applications and Sample Preparation
- You might also be interested in our application note on the reproducibility of raw meal analysis by XRF.