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Please check out our Portable XRF Analyzer / Handheld XRF Analyzer section to find manufacturers that sell these products
We are all familiar with X-rays, the short, highly energetic waves used by dentists to check for cavities, and by airport screeners to look for contraband. We experience fluorescence—the phenomenon by which something absorbs energy and in turn releases it at a different wavelength—from almost every light bulb. It turns out that elements can absorb very specific frequencies of X-rays and, in turn, through a reshuffling of their electrons, kick out other, characteristic bands of X-rays. Xray fluorescence spectrometry utilizes this process to identify and quantify the elements found in a sample.
According to Ted Shields, Portable Products Manager at Boston-based Olympus Innov-X, a leading supplier of X-ray fluorescence (XRF) instruments, there are four measurement steps. First, the X-rays are produced and sent to the sample. Fluorescent X-rays are then sent back to a detector, and the detector counts them. The algorithms then turn these counts into a concentration.
While laboratory XRF instrumentation can be large, powerful, and imbued with complex and sophisticated competencies, the breed of portable instruments is designed to be taken to a work or research site. For the most popular of these—the handheld radar gun-like instruments—taking a measurement is essentially a matter of pointing at a sample and pulling the trigger. A bigger issue than learning how to operate the equipment is learning how to make sure a sample is representative, pointed out Alexander Seyfarth, Senior Global Product Manager for Hand Held XRF for BRUKER Elemental in Kennewick, WA.
XRF applications
XRF is a nondestructive technology, making it ideal for examining artifacts, geological formations, works of art, archeological sites, and other samples where harming the sample is not an option. “You can’t take away a few grams and grind it up and do a normal destructive test,” said Shields. In a laboratory setting, he added, “the reason you’d want to use handheld is if the samples are either large or if the geometry of them is such that they don’t easily fit into the cabinet of laboratory equipment.”
Industrial uses of handheld XRF
By far the largest industrial applications of handheld XRF revolve around alloy identification, whether for the scrap metals industry or for the positive materials identification market (which may involve the manufacture and repair of refinery equipment and pipelines, for example), i.e., anywhere it is important to know the grade and chemistry of an alloy, explained Seyfarth. XRF is also a highly prized technique for restricted materials analysis—testing for toxic and/or banned substances such as arsenic, cadmium, and mercury in manufactured goods, for example—and for detection of lead-based paint in remodeling.
The speed and portability of XRF allows it to be used for research or mineral exploration in cases in which thousands of samples need to be queried in real time. In the case of environmental testing, an entire region can be assayed meter-by-meter to establish a grid map, which would then perhaps be the basis of more intensive scrutiny in areas in which an issue is indicated.
Types of portable XRF systems
There are many dichotomies in portable, battery-powered XRF equipment. Handheld units are radar gun-like, while the portable boxlike instruments are more similar to laboratory units in that they tend to be higher powered and generally have a closed compartment into which the sample is placed. Handheld units can also be mounted on a special stand or a tripod to allow for longer, steadier measurements.
Some handheld units are designed to simultaneously examine only the heavier metals—from titanium to uranium—while others are capable of reliably querying down to magnesium (atomic number 12) as well. In general, the former will utilize the older technology, less expensive, silicon-PIN detectors. However, “if you want to get down to the lowest parts-per-million that you can test in environmental soil samples, and also care about aluminum and silicon, then you need a silicon drift detector [SDD],” explained Shields.
Nearly all portable XRFs use an X-ray tube to generate X-rays. Both the current and the voltage can be adjusted in order to change the X-rays that come out, to make the appropriate source for the sample you are shooting, explained Shields. Isotope-sourced devices, which can be less expensive and achieve higher energies, are still made for niche applications, such as lead paint in homes and rare-earth elements. Because of safety and regulatory concerns, as well as the fact that isotopes decay and thus the instruments need to be resourced every couple of years, though, devices using radioactive elements have largely been made obsolete.
Seyfarth distinguished between what he considers analyzers versus spectrometers. “You buy an alloy analyzer, a soil analyzer, a precious metal analyzer knowing what application you need and it’s literally bought for that application.” Such instruments tend to be precalibrated, drop-down-menu-driven, and operated by users with no scientific training. On the other hand, he said, “a spectrometer is very flexible, and is designed to be fairly universal and to be calibrated by the user.”
The spot size—the area of which a measurement is taken—usually ranges from 3 to 10 mm, with some units offering both a larger and smaller spot, explained Seyfarth. For certain applications, such as examining the solder strips on integrated circuit boards, smaller spots are needed. But, said Seyfarth, spot size is inversely proportional to the power required to generate enough signal to acquire a reliable measurement, “so the spot size is correlated to the price of the instrument. And usually the heavier they get, the more portable/benchtop they become, because you need a larger X-ray generation device” and thus larger battery capacity.
Use of a handheld XRF analyzer is generally straightforward. A worker in a scrap yard, for example, may point the instrument at a sample, pull the trigger, and see a readout telling the percentages of iron, chromium, and manganese (among other metals), and whether they are within tolerance for 303-grade stainless steel, while an environmental abatement specialist might be given a list of banned substances and the percentages of those contained in the sample and whether they are above the threshold. What requires more training is knowing to scrape away corrosion, for example, or to find spots away from potentially interfering material.
The instruments will have an onboard computer into which location and sample identification data can be input and stored. Some may be equipped with RFID sensors, barcode readers, or GPS receivers, and feature ways to manually input data (often by drop-down menus and virtual keyboard). Data can be retrieved and copied to an external computer by a variety of means—USB stick, memory card, or perhaps WiFi or Bluetooth—depending on the model.
For those samples that are more complicated—i.e., nonstandard, inhomogeneous samples, as may be dealt with by art conservators or geoscientists, for example—a more open platform is often required, according to Seyfarth. Such users will often have a spectrometry background, and they see the XRF as an X-ray spectral imager, analogous to orthogonal techniques like Fourier transform-infrared (FTIR) and mass spectrometry. The instrument should have maximal software control, allow for manual calibration, and be able to deliver the raw data as a spectrum.
Recent innovations in XRF instruments
In many ways, XRF instruments mirror their visible light cousins, the camera: Larger instruments are more power and tend to allow for many more user-controlled options. Most handheld instruments are point-and-shoot, in which nearly foolproof (shallow learning curve) operation is gained at the expense of flexibility. Yet even in the handheld space, several vendors offer XRF spectrometers designed for the researcher to maintain maximum control, in which different profiles can be created for different applications, custom compensation can be programmed in, and the raw data file can be examined. GPS and WiFi or Bluetooth are available as options on both analyzers and spectrometers to make location tracking and sending (nearly) automatic.
Table 1 – Purchasing considerations for portable XRF units
For the laboratory or field scientist who does XRF analysis day in and day out, “soft features” such as ergonomics, price, and software interface may become the deciding factors, implied Seyfarth. Researchers, on the other hand, may want to ask (in addition to what has been discussed above) about features like elemental range, excitation range, detector capabilities like resolution and total count rate, and beam modifiers. Table 1 lists some purchasing considerations for portable XRF instruments.
Josh P. Roberts has been a full-time biomedical science writer for more than a decade. After earning an M.A. in the history and philosophy of science, he went through the Ph.D. program in molecular, cellular, developmental biology, and genetics at the University of Minnesota, with dissertation research in ocular immunology; e-mail: [email protected]
Please check out our Portable XRF Analyzer / Handheld XRF Analyzer section to find manufacturers that sell these products