Wavelength Technology Forum: X-Ray Technology

July 9, 2009

Joining us for this discussion are Drew Hession-Kunz, i-Nalysis; Anton Kleyn, Thermo Fisher Scientific; Dale E. Newbury, NIST Fellow (National Institute of Standards and Technology).

X-ray techniques are finding increased application in numerous fields, from the pharmaceutical industry to homeland security. And as X-ray technology becomes more sophisticated and the instrumentation more sensitive, this older technique will find even more uses in modern materials analysis.

Joining us for this discussion are Drew Hession-Kunz, i-Nalysis; Anton Kleyn, Thermo Fisher Scientific; Dale E. Newbury, NIST Fellow (National Institute of Standards and Technology).

What advances or developments have been seen recently in the application of X-ray techniques to the pharmaceutical industry? Will it always find widespread use there?

Hession-Kunz: Yes, I think XRF will continue to find widespread use in the pharmaceutical industry, due to quality requirements and the ever-increasing global dispersion of the supply chain. Recently, some companies have been using portable XRF at raw materials inspection to instantly confirm incoming materials, and some have been doing more spot-checking as XRF becomes more flexible.

We are seeing XRF used in confirming organometallics, which are used as catalysts in some pharma processes and other areas. Also, certain enzymes “crater” in the presence of metals such as silver, etc., which are easily visible with XRF at many points in the process.

Kleyn: X-ray techniques include X-ray fluorescence, as well as X-ray diffraction. The application of the latter in the pharmaceutical industry has certainly become more widespread over the years for measurement of such things as protein formation and structure. However, the applications of X-ray fluorescence in the pharmaceutical industry have always been more limited due to the fact that XRF is basically an inorganic analysis technique, whereas most substances in pharma are organic, obviously. The rather thin area in-between often pushes XRF equipment to its limits of sensitivity, which has limited the opportunities for growth. If the technology were to achieve a significant improvement in detection limits, certainly pharma would be more than happy to incorporate more of these instruments in their R & D and quality control process.

What are the advantages and disadvantages of using X-ray technology in the field of forensics?

Hession-Kunz: XRF is very useful in forensics – in fact, CSI, that notoriously “reliable” source of facts about forensic work, has featured several handheld XRF instruments. There is very often a question both of “What is this material we are looking at?” as well as the interesting question “What contamination is present?” XRF is very good at answering the inorganic portion of both questions.

The downside of XRF use in forensics is that for contamination work, the instruments only cover a small area at a time, and so an exhaustive test can be, well, exhausting. For example, finding gunshot residue (a perfect XRF application) is straightforward in many situations. You check the clothing of suspects, and areas near the source of the shot. However, checking an entire crime scene, if there is no other evidence suggesting positioning of a weapon when it was fired, could be a challenging and time-consuming project.

Kleyn: Let's look at the advantages first. Whether it's XRF or XRD, both techniques may be used with minimal if any effect on the sample, whether through preparation, handling, or the analysis itself. In other words, the analyzed sample remains effectively unchanged, which has both legal, as well as analytical consequences: the sample can always be retrieved from the archive for additional analysis or examination, perhaps following a break in the case or some advance in technology. The other benefit is that the relatively quick analysis time, with initial results often available within minutes - and with portable instruments, even right there, on-site - may make all the difference in solving a case or serving justice.

The disadvantage of XRF, specifically, in forensics is the same as its drawbacks for pretty much any application: without proper and thorough sample preparation, XRF may not tell the whole story about the sample. We always have to remind our own users that X-rays typically penetrate the surface of the material only microns deep, so any difference between the surface and bulk of the material - i.e., homogeneity, particle size, and mineralogy - may skew the final result, especially in quantitative analysis. The other disadvantage or just an inherent limitation of the technique as a whole is that it isn't generally used for analysis of organic elements (C, H, N, O). Unfortunately, many samples in forensics and other areas are organic in nature, which complicates the analysis. Both of these limitations can generally be overcome with proper sample preparation or the use of a complementary organic analyzer, but that would mean modification or possibly destruction of the sample, which somewhat ironically takes away one of the key advantages of the technique. That being said, XRF is indispensable and irreplaceable in forensics as a rapid, nondestructive, semi-quantitative or qualitative analyzer that typically provides enough information to "fingerprint" the material, so to speak.

What impact has the silicon drift detector had on classic energy dispersive X-ray? Will it change the field of X-ray dramatically in your opinion?

Hession-Kunz: Yes and no, which I suppose is a typical physicist's answer. Resolution in most XRF instruments is highly dependent on the detector, and the silicon drift detector (SDD) offers much higher resolution.

However, in many applications, there is no need for more resolution. There is need for more integration into processes, more need for integration into vendor's supply chain, better, faster reporting, and the like. There are certainly applications where the SDD will allow XRF to do more. I think there are also many where its improvement will be only marginal in reality.

Kleyn: Energy-dispersive XRF has always been a very wide and varied field and more recently even more so as it expanded into portable, handheld configurations, thus extending the price range from $20K all the way up to $100K and above. Instruments were quite competent and/or affordable even before the SDD (silicon drift detector) came onto the scene, and its arrival has certainly affected some parts of the EDX market, but I would say it has not fundamentally changed the applications of EDX for the users of the technique or the market positions of the instrument suppliers. Certainly, the PIN detector that was for years the only choice for low-end EDX has now been effectively replaced by the SDD, but the high end of the EDX market - the one closest to WDX in terms of sensitivity - remains dominated by the Si(Li) detectors. Moreover, and perhaps most significantly, the SDD has done little to encroach onto WD performance or applications. Although the detector is a critical component, nevertheless it's only one of several components defining the overall performance the instrument. The growth of EDX in performance and application is limited by the pulse-processor technology as much as the detector itself, and it's hard to imagine any order-of-magnitude improvements in count-rate or instrumental stability in the near future that would allow EDX to start taking share from WDX.

Newbury:The SDD-EDS will continue to sweep Si(Li)-EDS away until Si(Li)-EDS becomes available only with a special order because SDD-EDS is equal to or superior to Si(Li)-EDS in all but one aspect of analytical performance:

(1) SDD-EDS has demonstrated the best limiting resolution performance (122 eV at MnKa) despite operating with Peltier cooling only (-25 °C) compared to the best Si(Li)-EDS which requires liquid nitrogen cooling (-190 °C). (Independence from liquid nitrogen cooling with SDD-EDS is becoming critically important in many laboratories because of hazardous material operating restrictions now being widely applied to the handling of liquid nitrogen.)

(2) For any given active area, SDD-EDS has superior resolution.

(3) SDD-EDS has superior peak channel stability and resolution stability with increasing input count rate.

(4) For a given resolution, SDD-EDS operates with a much faster time constant, e.g., 500 nanoseconds for 127.5 eV (MnKa) for SDD-EDS rather than 25 microseconds for Si(Li)-EDS at 129 eV.

(5) Maximum throughput for SDD-EDS is typically 50 times higher than that of Si(Li)-EDS for the same resolution. Thus, if the specimen can withstand increasing the primary irradiation (electrons, photons, or ions) necessary to increase the rate of X-ray production, SDD-EDS can provide 50 times more counts (at the same deadtime) in the same amount of clock time. Since having more counts in the spectrum makes everything better in all applications of analytical X-ray spectrometry, e.g., limits of detection, peak deconvolution, background fitting, etc., this is the critical contribution of SDD-EDS that will inevitably lead to its dominance.

The one aspect of detector performance for which Si(Li)-EDS must inevitably be superior to SDD-EDS is the efficiency of collecting high-energy photons above 10 keV in energy. X-ray absoprtion depends on the mass thickness of the detector. SDD-EDS is constructed on a thin Si wafer 450 micrometers thick while Si(Li)-EDS is constructed from a thick crystal 3 mm thick. Considering the absorption of U La (13.61 keV) within the detector, the Si(Li)-EDS is still 100% efficient while SDD-EDS has decreased to 70%. The efficiency continues to decrease as the photon energy increases. If measuring photons above 25 keV is the analytical objective, then SDD-EDS is not the optimum choice.

How has the advent of portable XRD and XRF devices affected geological and archaeological applications? Has it impacted any other applications? What are the disadvantages of using these instruments?

Hession-Kunz:As the handheld instruments have improved, they have worked through many applications, but not a lot was done to address the geological marketplace. That is changing, and now quite a few of the portable makers have– in part due to the SDD we already discussed, and in part just due to lots of hard work on the specifics of these applications– made excellent products available to the public. The portable instruments have not affected geological and archaeological applications as much as you might expect, but that area is about to explode.

In terms of other applications that will need XRF testing in the near future, the market is wide open, especially given the CPSIA regulations passed down from the Consumer Product Safety Commission and Congress. Businesses will need to protect themselves, know their inventory, and will be held accountable by customers. For some companies, particularly those smaller businesses that haven’t had to deal with this issue historically, this can be a real challenge because the standard X-ray fluorescence tools that are used to measure lead content can be very expensive. Luckily, new XRF devices are emerging that make it affordable and an easy process to manage for these companies.

Kleyn:Portable or more precisely "handheld" XRF has certainly affected the efficiency of in-situ sampling. Ten years ago, a field geologist or archaeologist would have had to look for a robust bench-top instrument and install it in a mobile laboratory, whereas now he can spend a whole day with a similar capability in his hand. Surprisingly, field applications in geology and archaeology are actually not the biggest markets for handheld XRF. Mining and exploration - geology on an industrial scale - is a field that has benefited tremendously from a small, easy analyzer that helps avoid cutting into less valuable rock. One field that was revolutionized by portable XRF is the sorting and identification of metals and alloys in the scrap/recycling industry as well as industrial QC. Metal scrap is the final phase of the giant metals industry, where all the metals get reprocessed into something else.

Despite their growth and popularity in many fields and industries, portable instruments lack some features and capabilities that are usually expected in equipment designed for laboratory use. On the hardware side, they have lower light-element sensitivity and less stability/repeatability than the larger EDX instruments. After all, something has to be taken out when things are compressed. But even more importantly may be the reduced software capability. Since handheld devices were designed for ease-of-use in strictly defined, fixed applications (such as metals or soil analysis), their software was not designed to work (easily) with user-constructed method or calibration or those ubiquitous, truly "unknown" samples that always come through the lab.

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