The Changing Role of XRF
September 30, 2009
During the past few decades, energy dispersive X-ray fluorescence analyzers have gone through a number of advancements. As a result of component and software developments, the instruments have become easier to use, lower in cost and more compact. As advancements were made, the instruments took on a larger role in quality control testing processes-and that role continues to expand.
The concept of X-ray spectrometry was first developed in the early 1900s and since the 1950s has been used in commercial elemental analysis. However, nearly all of these early X-ray spectrometers used wavelength dispersive technology. It was not until the 1970s that energy dispersive X-ray fluorescence (EDXRF) systems became available. EDXRF offered many benefits, including cost efficiency, ease of use and the ability to measure elements that were very close in atomic number.
XRF analyzers initially found a place in quality control practices as a measurement device providing fast and accurate plating thickness measurements. The newer systems provide solutions for a continuously growing number of quality control needs, including alloy verification and hazardous substance detection. These additional applications have become available because of a number of advancements in EDXRF hardware build and software design.
Hardware AdvancementXRF is based on having sufficient energy light waves (X-rays) absorbed by an atom so that the inner shell electrons are excited to an outer shell or removed completely. The empty inner shell that remains is “filled” by electrons from an outer shell of the atom. The difference in energies between the two shells involved is excess energy, which is emitted as radiation (fluorescence) when generated in this process.
In a given element the energy difference between two specific orbital shells is characteristic of that element and always the same; therefore, the emitted light wave will always have the same energy. By determining the energy emitted by a particular sample, operators are able to identify the element involved.
Energy dispersive X-ray fluorescence is built around two major components and the accompanying computer system. Those components include the X-ray tube, which is the source of light waves, and the detection system, which collects the emitted radiation. The detection system sends the information it collects to the computer system that, through a series of algorithms, provides information specific to that sample piece.
During the past two decades, detection systems have seen great developments, and many options are available in EDXRF systems that have led to the technology’s increased role in quality control. A critical number to consider when reviewing different detection systems is the energy resolution it provides, reported as electron volts (eV). The lower the resolution number, the better the system’s ability to clearly identify elements by differentiating spectrums.
HistoryThe first EDXRF systems that were-and still are-popular in quality control used proportional counter detector tubes. These detector tubes use photo-ionization gases within the counter to detect emitted X-rays. The resolutions on these tubes are not as good as newer systems, but do provide fast and accurate results for specific tasks such as PCB quality control and plating thickness measurements.
The other early EDXRF systems used silicon-lithium (SiLi) detection systems in order to offer the widest possible elemental range and still provide some of the best resolutions; early versions were typically at 160 eV to 200 eV with new models providing a range of 145 eV to 170 eV. The SiLi detectors are dependable; however, they are cooled by using liquid nitrogen (LN2), which is costly and can be dangerous. For these reasons, other detectors were developed and used in EDXRF.
In the early 1990s, a U.S. organization developed the silicon pin semiconductor detector. This detector features an ultra-thin window design and some have a signal-to-noise enhancer (SNE) for increased accuracy of the instrument. Si-PIN detectors are electronically cooled so that LN2 is not required and instruments can be used at normal room temperatures. While the resolution of these systems was greater than others, initially as high as 280 eV, the benefits-most notably liquid nitrogen-free use-made them a common choice for use in many XRF analyzers.
The most recent development in detection systems is the silicon-drift detector (SDD). Developed in Germany in the last decade, this system is electrically cooled, uses an ultra-thin beryllium window and can be kept at normal temperatures providing high-efficiency and results for light elements. The resolution of these systems typically lands in the 120 eV to 139 eV range.
While many of the detector technologies discussed here are still in use today, the advancements have allowed EDXRF to increase its range of applications and market. Advancements in other areas of EDXRF also have played a role in its expanded use, including the many software developments.
Software AdvancementsLike many technologies, EDXRF has advanced because of the developments in computers and operating systems. Initial versions of X-ray fluorescence systems used firmware that was permanently written on e-prompts. However, it was clear that this needed to change because of numerous problems, including the fact that firmware could not easily be updated and lacked the ability to electronically store results, leaving operators to print results for paper records.
In the mid 1980s, DOS-based EDXRF became available as IBM systems grew in popularity. DOS provided a number of drastic improvements in EDXRF, including the ability to store results on a floppy disc or hard drive for easy recall. However, the major negative of DOS was that it was not simple to operate. An EDXRF operator required knowledge of the key commands, limiting the user base.
In the early 1990s, XRF manufacturers switched to Windows operating systems. The fact that anybody with basic computer knowledge could learn how to effectively use XRF analyzers increased the interest in EDXRF for quality control purposes. In addition to being simple to learn and navigate, Windows OS offered the ability to store results using Excel and Word. This gave operators better data management opportunities, including greater statistical ability and report generation options.
With these software versions, an operator required some basic knowledge of the makeup in a given sample. The operator would then reference a previously inputted calibration curve so that he could accurately determine a thickness measurement. This continues to be a popular approach because use of calibration curves and reference standards provides increased accuracy in a measurement, but new developments in software would give operators another option.
The latest software development for desktop XRF came in the form of fundamental parameter (FP) software in the mid to late 1990s. This update had the greatest impact on the advancement of EDXRF’s role in quality control. With FP software, a system does not require an entire calibration curve for an application. Based on the fundamental parameters, which saves all of the data including count rates and calibration curves, it is possible to obtain accurate results with very limited or no preparation. However, it is still advisable to have known and traceable reference standards to verify true accuracies. With FP software, an operator now can obtain positive material identification of a previously unknown sample. This software shortened set-up time, limited the need for reference standards and resulted in improved process control for multi-layer and ultra-thin coatings among other applications.
Regardless of whether a software package is fundamental parameter or reference standard based, improvements in software have helped increase the accuracy of XRF and expand the number of people that can easily use the instruments for quality control purposes.
Get SmallAs with other technologies, including cellular phones and personal computers, the size of EDXRF analyzers has moved toward smaller, more compact systems because of decreased size in printed circuits boards and other components. While instruments for every application cannot be reduced in size because of stage requirements, there is a general trend toward limiting the footprint of these systems.
As sizes have decreased, one of the most popular XRF devices has evolved in the past decade-the handheld XRF analyzer. These instruments evolved from the isotope-based systems and use micro-components in a lightweight, less than 4 pounds, gun-like device that has allowed quality control testing to be accurate, versatile and now, taken on the road.
The handheld device also has benefited from software advancements. Using software on PDA devices, the system can provide real-time analysis of product for hazardous substance detection or alloy verification. With no limit to a sample’s size or location, quality control experts can go to suppliers’ facilities, easily moving the testing instrument from one warehouse to the next, screening their inventory.