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Please check out our XRF Analyzer section for more information or to find manufacturers that sell these products.
Most biologists are intimately familiar with fluorescence: Hit certain highly conjugated organic molecules with light in the ultraviolet-to-visible range, and they will kick that light right back at a characteristic, lower-energy wavelength. The phenomenon enables such studies as sequencing DNA, imaging the spatial arrangement of proteins within tissues, and capturing biomolecular interactions.
But there is another kind of fluorescence, too. If a material is bombarded with highly energetic x-ray light, and it too will absorb and then release that light, emitting x-ray radiation with properties characteristic of the material's elemental (rather than molecular) composition.
The technique is called x-ray fluorescence (XRF), and it is, says Laura Oelofse, XRF Product Marketing Manager at Rigaku Americas Corp., "ubiquitous." From scrap metal yards and electronics manufacturing, to mines and building inspection firms, the technique is used "whenever you want an elemental analysis," Oelofse says.
In XRF, when an x-ray beam is directed at a sample, it dislodges electrons in the inner orbitals of the atoms that comprise it. Because that ionized state is so unstable, lower-energy electrons from outer electron shells (that is, farther from the nucleus) rapidly collapse to fill the gap, releasing energy as they do so in the form of secondary x-rays.
In other words, the atoms fluoresce. But, unlike biology fluorophores, most elements fluoresce at more than one "color," because the initial chemical transition event leaves yet another hole which must also be filled. An even lower-energy electron falls into the new hole to compensate, releasing yet another quantum of energy, and so on.
"If you have a very populated atom with a large number of electrons, they likely have more than one transition," says Oelofse. These electronic transitions, or "lines," are characteristic of each atom's electronic structure, and thus act as fingerprints to identify the elemental composition of the material under study.
Remarkably, for all the electronic musical chairs that causes it, that fingerprinting technique is relatively non-destructive, Oelofse says, because it "doesn't intrinsically change the atomic structure" of the analyzed material. That makes XRF an ideal choice for analysis of rare or valuable objects, for assessing the metallurgy of pipes in a manufacturing plant, for quality control assurance, and so on.
Commercial XRF instruments come in two basic forms, which differ in their detection method. Wavelength-dispersive (WD) XRF uses a crystal filter to allow only specific x-ray wavelengths to hit the detector; by rotating the crystal, the entire spectrum of possible signatures can be scanned – an approach reminiscent of the operation of a quadrupole mass spectrometer. Energy-dispersive (ED) XRF devices, in contrast, use an electrified semiconductor (usually a silicon diode) to count the x-rays en masse, using the resulting semiconductor current to approximate the energy of the incident beams.
Of the two, ED is sometimes less expensive – but at a price. ED suffers from considerably poorer spectral resolution than does WD. The resolution of a typical WD detector is around 3 eV, compared to about 130 eV for ED detectors. As a result, some transition lines cannot definitively be assigned, leading to potential ambiguity in the final elemental analyses.
"The actual mechanics to do wavelength-dispersive XRF requires extremely precise engineering," Oelofse says. "However, the resolution, the ability to discriminate similar energies is much better using a crystal than using a PIN [positive intrinsic negative] diode [semiconductor]."
XRF instruments are available in both handheld and benchtop configurations. Benchtop devices generally provide higher sensitivity and resolution, as well as a wider elemental range, because they utilize more powerful x-ray sources. "The more x-rays going in, the more fluorescent x-rays coming out," says Jon Shein, Director of Marketing Communications for the Thermo Scientific Niton Analyzer Product Line at Thermo Fisher Scientific, which sells handheld XRF devices.
Handheld instruments offer the ability to analyze materials on site or in the field, without having to take samples back to the lab. Looking somewhat like an electric drill or pistol, these handhelds can be used, for instance, by geologists, who might wish to perform on-site mineral analyses, or facilities managers, who might want to assess the metallurgy of steel pipe welds.
Between these two are "transportable" systems (such as Oxford Instruments' LAB-X 3500), providing intermediate capabilities: small and rugged enough to be transported on a cart or van to the analysis site, but with resolution to match many benchtop systems.
Also available are microXRF systems, such as Horiba Jobin Yvon's XGT-7000 X-ray Analytical Microscope, which use collimated x-ray beams for more targeted analyses. Bulk instruments use x-ray beams with spot sizes in the millimeter-to-centimeter range. But microXRF systems can produce beams between 10 and 500 micrometers in diameter, small enough to probe the thickness of fine gold wires on printed circuit boards.
Neil Dagger, Global Marketing Communications Manager for the Industrial Analysis Group at Oxford Instruments, which specializes in XRF instruments, says handheld devices are particularly useful for such field applications as mining, scrap metal sorting, and ‘in home’ inspection, where its advantageous to take the instrument to the sample.
For instance, as a result of the recent controversy surrounding Chinese drywall (much of which had to be replaced because it was contaminated with sulfur compounds), XRF has found a place in drywall analysis. "XRF is a fantastic non-destructive way of doing that testing," Dagger says. Instead of requiring the inspector to remove samples of individual drywall panels for testing in the lab – that is, to knock holes in what may be perfectly good walls -- the inspector could point a handheld XRF device (such as Oxford's X-MET5000 or the Thermo Scientific Niton XL3 GOLDD) at the wall, pull the trigger, and get a result.
The resulting test, indicating the amount of sulfur and strontium in the drywall material, is more attractive to the inspector, Dagger says, as he can do more analyses per day, and do them more quickly. But, "it's [also] more attractive to the customer," he adds, "who doesn't have to have their house ruined to do the test."
Another XRF application: Ensuring compliance with environmental regulations in electronics manufacturing. Donna Guarrera of JEOL USA says her company's benchtop EDXRF instruments (the JSX-3100RII and JSX-3400RII), in addition to general purpose XRF analysis, are specifically designed to help ensure compliance with the European Union RoHS/WEEE and ELV standards, which govern, among other things, the amount of lead, hexavalent chromium, and cadmium in consumer products.
As Assistant Product Manager for scanning electron microscopes at JEOL USA, Guarrera notes that XRF is similar to a technique often used in scanning electron microscopy called energy-dispersive x-ray spectrometry. In that technique, the instrument's electron (rather than x-ray) beam is used to induce x-ray emission from the sample, which also is characteristic of its atomic composition. Such SEM-based testing is popular, among other places, in museum science laboratories, where it helps determine the chemical makeup of pigments in priceless works of art without the need for first taking a sample.
Dagger says the biggest market for his company by far, is "PMI," or positive material identification – an application in which a quality control manager ensures that material composition of key elements meets required specifications. "That's one of the most popular and successful parts of the business," he says.
Choosing an XRF Analyzer
Whatever your application, when making a purchasing decision, it all comes down to application. Elemental composition (both the range of elements in your samples and their variability), sample size and format, throughput and speed requirements, sensitivity, budget – all these factors must be taken into account, Dagger says.
Compared to benchtop units handheld instruments are relatively inexpensive (ranging from $17,000 to about $50,000 in the case of the Thermo Scientific Niton analyzer product line, for instance) and insensitive but also are quite flexible. They are specifically programmed (via associated software) to detect particular elements or classes of materials; an instrument tuned to sort steel in a scrap metal recycling yard, for instance, probably wouldn't be calibrated for detecting cadmium contamination in children's toys, nor environmental contaminants in soil. On the other hand, these battery-powered devices can typically run for hours without calibration, storing readings until they are downloaded onto a computer at the end of the shift.
Benchtop units cost considerably more (the JEOL JSX instruments cost around $100,000), yet can typically detect a wider range of elements at higher sensitivity than can more portable units. For instance, Shein notes that his company's Niton XL3 GOLDD handheld can detect elements as light as magnesium or aluminum, "but to go lower [that is, sodium or smaller], a benchtop system is going to be more appropriate," he says.
If precision, as opposed to bulk analysis is required, consider a microXRF instrument such as an X-Strata980, where a built-in camera allows pin-point analysis accuracy on a tiny part of a sample to assess both coatings thickness and materials composition. Or, if throughput is an issue, consider walkway automation, for instance using the auto-sampler 10 position sample carousel available on Oxford Instrument's X-Supreme.
Benchtop wavelength dispersive units such as those offered by Rigaku have opened up a new genre of technology bringing the extreme precision of this technique to a small bench top format. These are extremely useful for working with low atomic number elements F(9) – Al(13) as well as analyzing element rich matrices and generally have the advantage of greater detectable x-ray flux and intensity than ED, while now utilizing the same design components as the Benchtop EDX systems. Generally very efficient for complex matrix materials where a high degree of reproducibility is desired to control a manufacturing process. WDXRF has become the method of choice for truly quantitative analysis where a high degree of precision is required whereas EDXRF has found a niche as a screening tool where ballpark analysis figures or positive identification of an elements presence or absence above a certain minimum threshold is all that is required.
Please check out our XRF Analyzer section for more information or to find manufacturers that sell these products.