The Dynamic World of X-ray Fluorescence

Jul 01, 2011
Volume 26, Issue 7

Many people think X-ray analysis, including X-ray fluorescence (XRF) spectrometry, is a mature technology with limited sensitivity, useful primarily for low-resolution industry process applications. They should think again. Over the past two decades, XRF has evolved and matured, but not into a boring old age. Rather, its current capabilities, including sharper optics and ever-improving handheld devices, have led to use in new application areas and expanded its use in areas where it previously penetrated only nominally. At the same time, cutting-edge research is ongoing in areas such as two-dimensional (2D) and three-dimensional (3D) X-ray mapping using micro X-ray fluorescence (µXRF), as well as in synchrotron radiation–based µXRF and micro X-ray absorption near edge structure (XANES). That work at synchrotrons is enabling fascinating research in fields such as cellular biology, nanotechnology, and cultural heritage, and at the same time is prompting further investigation into what can be achieved with XRF in the laboratory and the field.

Instrument Developments: Detectors, Optics, and Computing Power

XRF's current higher status in the field of spectroscopy results from important developments in several enabling technologies over the past two decades that led to critical improvements in the capabilities of modern XRF instruments.

First, the development in the late 1980s of the room-temperature silicon p-i-n diode detector made it possible to replace the large, gas-filled proportional counter with a thimble-sized device with excellent energy resolution. This led to wider use of sophisticated quantification algorithms and at the same time started a gradual change in the form factors and the miniaturization of energy-dispersive X-ray fluorescence (EDXRF) systems.

In addition, broadband polycapillary optics, developed in the mid-to-late 1990s by XOS (East Greenbush, New York), greatly improved the spatial resolution and signal-to-noise ratio of XRF generally, and enabled the field of µXRF more specifically. More recently, XOS has introduced narrowband doubly curved crystal (DCC) optics, which are designed to greatly reduce background noise and are useful for applications examining a narrow range of elements.

The other major change in this period was the development of the miniature X-ray tube for use in EDXRF instruments. Earlier EDXRF instruments designed for field use relied on radioactive isotopes as the source of exciting radiation. Although such sources were small, they were not as efficient as X-ray tubes. Also, the radioisotopes could not be turned on and off, and strict safety regulations made transporting radioisotope-based instruments complicated.

With the availability of miniature, battery powered X-ray tubes, XRF units suddenly became much more mobile, and over the past decade the use of mobile and specifically of handheld units has skyrocketed. Today, the ongoing adaptation of handheld units for specialized applications is the fastest growing area in XRF.

Lastly, XRF analysis, just like most aspects of science today, has benefited greatly from increased computational capability. Calculations and calibration are better and faster, and the modeling that underlies the interpretation of XRF spectra has improved significantly.

"Recent improvements in computational capability are translating into new ways to analyze data and new ways to use instrumentation," says Tim Elam, senior physicist at the Applied Physics Laboratory at the University of Washington (Seattle, Washington) and co-chair of the Denver X-ray Conference. "I think we've just scratched the surface of what those capabilities will enable us to do."

As a result of all of these developments in XRF technology, the use of XRF is expanding, although the growth areas for industrial applications vary from one part of the world to another. Quality control process applications for heavy industries like cement and steel manufacturing are growing the most in developing countries. In industrialized nations, meanwhile, XRF is used increasingly for the analysis of more-complex samples.

"U.S. factories increasingly have to make materials that are more specialized and more demanding to make," says Alexander Seyfarth, XRF product manager for North America at Bruker AXS (Madison, Wisconsin). "That means that XRF has to be able to do more complicated analysis, such as analyzing new battery types, solar cells, or high performance polymers that have low tolerances."

Another growth area in industrialized countries is analytical testing done to comply with environmental and safety regulations. Regulations such as Europe's Regulations of Hazardous Substances (RoHS) Directive have generated a lot of XRF use for analyses such as detecting lead in plastics, toys, and other materials.

Testing for lead also highlights the effectiveness of modern XRF systems as a screening tool, says Stan Piorek, director of applied research for portable XRF analyzers at Thermo Fisher Scientific (Billerica, Massachusetts).

"By using XRF instead of a destructive laboratory technique, you can test 100% of the product instead of doing random sampling," he points out. "As a result, your compliance with safety regulations is better overall, and you spend less time and money doing it."

And in a screening application, the fact that XRF detection limits are often not as low as those of inductively coupled plasma (ICP) or other atomic spectroscopy techniques is not a problem. In cases where the sensitivity of XRF is insufficient to be sure if a limit has been passed, one sends those samples off to the laboratory. "XRF is great for sorting things so you can spend your effort on those that need it," Piorek concludes.

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