Energy-Dispersive X-ray Fluorescence Spectrometry: A Long Overdue

May 17, 2011 - Peter Vandenabeele , Mary Kate Donais ... Margaret West , Andrew T. Ellis , Philip J. Potts , Christina Streli , Christine Vanhoof , Da...
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Energy-Dispersive X-ray Fluorescence Spectrometry: A Long Overdue Addition to the Chemistry Curriculum Peter T. Palmer* Department of Chemistry and Biochemistry, San Francisco State University, San Francisco, California 94132, United States ABSTRACT: Portable Energy-Dispersive X-Ray Fluorescence (XRF) analyzers have undergone significant improvements over the past decade. Salient advantages of XRF for elemental analysis include minimal sample preparation, multielement analysis capabilities, detection limits in the low parts per million (ppm) range, and analysis times on the order of 1 min. This article aims to stimulate interest in incorporating XRF into undergraduate chemistry lecture and lab settings. KEYWORDS: Analytical Chemistry, Environmental Chemistry, Forensic Chemistry, Laboratory Equipment/Apparatus, Qualitative Analysis FEATURE: Instrumentation Topics for the Teaching Laboratory

X

-ray science has a rich and important history. In the early 1900s, six different Nobel Prizes were awarded for the discovery of X-rays, development of X-ray diffraction (XRD) and X-ray spectrometry, and related work. Current X-ray methods include general techniques such as XRD and wavelength- and energy-dispersive X-ray fluorescence (WDXRF and EDXRF), as well as more specialized techniques such as total reflectance XRF (TXRF), proton-induced X-ray emission (PIXI), extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge fine structure (XANES). Although EDXRF (hereafter referred to as XRF) is currently used for environmental, regulatory, forensic, and many other diverse applications, it is largely overlooked in most chemistry programs in the United States. Indeed, most chemistry and biochemistry students graduate with no knowledge of these techniques and their qualitative and quantitative capabilities. Possible reasons for this include the lack of faculty expertise or training in this area, resistance to change, and the cost of the equipment. Recent advances in portable XRF analyzers and their impressive analytical performance have been ignored or misunderstood by most chemistry instructors. The purpose of this article is to bring attention to XRF, its capabilities, and applications, and to encourage its integration into the undergraduate chemistry curriculum.

portable XRF for determination of heavy metals in soils and sediments,1 which further promulgated the use of this technology for environmental screening and remediation. Over the past two decades, continual development of improved X-ray sources, detectors, electronics, and software has rendered these devices ever more powerful and easier to use. Innov-X (now part of Olympus) and Niton (now part of Thermo Fisher) introduced XRF analyzers with an X-ray tube source in 2001 and 2002, respectively. This was a major advance that obviated the need for replacement of short-lived radioisotopebased sources such as 109Cd, annual leak testing, and formal safety training for all users. Other manufacturers were quick to follow suit, and today, a majority of all new XRF analyzers are based on X-ray tube sources. Bruker was the first to replace silicon photodiode (Si-PIN) and lithium-drifted silicon [Si(Li)] detectors with the Silicon Drift Detector (SDD) in 2008, thereby improving the spectral resolution from 0.2 to 0.15 keV (measured at the full width at half-maximum, fwhm) in 2008. Innov-X was the first to manufacture an XRF analyzer with an integrated radiation shield and touch-screen PC running the Windows operating system. Collectively, these advances and the concomitant improvement in various analytical figures of merit have rendered XRF increasingly relevant for an ever-growing number of applications. The most common applications in XRF are outside of “mainstream” chemistry and represent an amazingly diverse range of disciplines. Geoscientists use XRF for rock, mineral, and soil analysis. Various health professionals use it for environmental screening. The U.S. Food and Drug Administration (FDA) has begun using XRF devices for a number of regulatory applications

’ COMMERCIALIZATION OF PORTABLE XRF ANALYZERS The development of small XRF analyzers was stimulated by the U.S. Environmental Protection Agency (EPA) Small Business Innovation Research and Environmental Technology Verification grant programs in the 1990s. This led to the commercialization of these analyzers and their primary initial applications, which were focused predominantly on field screening of lead in paint. EPA developed a new method based on Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.

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Table 1. Selected Figures of Merit for Several Commercially Available XRF Analyzers Vendor/Model (References)a

Excitation

Detector

Control

Price, USDb

Bruker Tracer III (ref 15)

Tube (Rh), 45 kV, 4 filters

SDD

PDA or PC

$38,000

Olympus Innov-X Delta (ref 16)

Tube (Ag, Rh, or Ta), 40 kV, 8 filters

SDD

Embedded processor and display screen

$30,000

Olympus Innov-X X-5000 (ref 16)

Tube (Ag, Rh, or Ta), 50 kV, 5 filters

SDD

Embedded touch screen PC

$52,500

QSX (Skyray) Pocket-III (ref 17)

Tube (W), 40 kV, 1 filter

Si(PiN)

iPAQ

$25,000

Oxford X-MET5000 (ref 18)

Tube (Rh), 45 kV, 4 filters

SDD

iPAQ

N/A

Spectro Ametek xSORT (ref 19)

Tube (Ag), 40 kV, 2 filters

SDD

PDA or PC

N/A

Thermo Scientific Niton XL3t GOLDDþ (ref 20)

Tube (Au), 50 kV, 6 filters

SDD

Embedded processor and display screen

$42,000

a

A detailed description of each analyzer is beyond the scope of this article; consult the cited Web sites and vendors for more specific information. b These represent approximate base prices of the XRF analyzer; contact vendors regarding options and academic pricing.

involving toxic elements in consumer products.2,3 Museum professionals utilize XRF as a routine tool to authenticate artifacts; determine the composition of items such as paints, jewelry, and coins; and identify toxic elements such as arsenic and mercury in natural history collections.4,5 The sophistication of some of these users is evidenced by their use of principal component analysis to classify obsidian based on XRF data.6 The tremendous growth in sales of XRF analyzers has led to their perhaps unavoidable misuse and brought attention to the obvious lack of understanding of this analytical measurement device, the results, and their implications. A significant number of users buy into the “point and shoot” mentality promoted by overzealous marketing literature, treat their XRF analyzer like a “tricorder”, and place blind faith in the list of detected elements and their concentrations. While this may be appropriate for wellcharacterized samples and metal recycling-type applications, this attitude has contributed to a number of instances where XRF data has been misinterpreted and misused. To date, vendor software on most portable XRF analyzers are still compromised by unsophisticated software that gives a false positive for Pb when high levels of Fe are present. Newbury pointed out a number of flaws in automated spectral analysis in commercial XRF software and noted that this threatens the credibility of the analytical community.79 In 2010, a massive recall of Shrek glasses was initiated because of the presence of Cd on the outside of the glasses despite the fact that “the amount of cadmium is too low to cause harm ...and the glasses would not be recalled today”.10 Although the phrase caveat emptor applies here, one cannot ignore the fact that these devices are being used by people who often have little or no understanding of the subtleties and nuances of spectral interpretation, fitness of purpose for an analytical method, the difference between screening and confirmation, and how to do accurate quantitative analysis. We in academia need to do our part to educate our peers and students about the advantages and limitations of XRF, become involved in generating consensus on acceptable methods for qualitative and quantitative methods via XRF, and make a more concerted effort in including XRF in the undergraduate chemistry curriculum.

Figure 1. (A) Thermo Scientific Niton XL3t handheld XRF analyzer;20 (B) Bruker Tracer III handheld XRF analyzer with optional vacuum pump and portable PC;15 (C) Olympus Innov-X X-5000 portable XRF analyzer with integrated radiation shield and touch screen PC.16. Images reproduced with permission.

more common XRF analyzers that may be appropriate for an undergraduate chemistry laboratory class are provided in Table 1. Note that the information in this table represents base model XRF analyzers. Interested parties should contact the manufacturers about potential options, which include a radiation shield or test stand, mounting tripod, custom X-ray tubes, secondary filters, different types of detectors, a vacuum or helium purge option to improve detection limits for light elements, beam collimators, and an integrated CCD camera for analyzing a small spot or specific location on heterogeneous samples. Figure 1 shows photographs of three different portable XRF analyzers. These can be configured for use as a true handheld device for field work, connected to a portable PC and test stand for use in the laboratory, or an integrated system that includes a touch-screen PC, test stand, and radiation shield. Each analyzer includes a source of X-rays and an energy-dispersive detector that detects single photon events. A sample is placed as close to the source and detector as possible, and the resulting spectrum that is acquired plots the intensity of X-ray fluorescence as a function of energy (keV). As given by Moseley’s law, the energy of X-ray fluorescence is proportional to square of the atomic number, Z, which provides relatively unique fluorescent energies

’ HANDHELD AND PORTABLE XRF ANALYZERS A number of texts, articles, and Web sites provide a good general introduction to XRF theory, instrumentation, and applications.1114 Vendors of portable XRF analyzers include Bruker,15 Olympus Innov-X,16 Oxford,17 Quick Shot XRF (QSX) Instruments (manufactured in China by Skyray),18 Spectro Ametek,19 and Thermo Fisher (Niton).20 Specifications on the 869

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away from the X-ray tube source and never pointing an operating handheld XRF analyzer at someone nearby). Most XRF analyzers are equipped with a “fail-safe” feature that turns off the X-ray source when no sample is detected in front of the beam. When analyzing samples, most of the X-rays are absorbed by either the sample or air. Any X-rays that escape will be rapidly attenuated by air and their intensity will fall off exponentially as a function of distance from the source. When used properly, radiation exposure from portable XRF analyzers is not detectable above background levels and operators can (presumably) be assured that these analyzers are indeed safe to use. Faculty who are interested in using XRF in a teaching lab are urged to consult with their campus radiation safety officer for specific requirements for their location. Figure 2. Partial XRF spectra of several consecutive elements in the periodic table. Note ∼0.2 keV resolution (measured at fwhm), presence of two fluorescence peaks for each element (KR and Kβ), and potential for spectral overlaps. (Image adapted from ref 21, p 20.)

for different elements. The intensity of the fluorescence is proportional to the concentration of that element. Figure 2 shows XRF spectra of several 1000 ppm ICPMS standards analyzed in their original plastic bottles. Note that line overlaps are indeed possible, as the width of each peak is finite owing to the limited resolution of the detector. Such spectral overlaps can be anticipated and in some cases avoided by choosing another line or by using more sophisticated spectral deconvolution algorithms. Commercial XRF analyzers typically include several different algorithms that are designed for specific applications. Fundamental parameters mode (also known as alloy mode) is typically used for analyzing alloy, metallic, and rock samples. Compton normalization mode (i.e., soil mode) is used for analyzing parts per million (ppm) levels of metals in soils and samples of differing densities. Other algorithms include thin-film mode for filter and wipe samples and passfail mode for Regulation of Hazardous Substances/Wastes in Electronic and Electrical Equipment (RoHS/WEEE) applications. While the XRF spectra “never lie”, the output of these algorithms (i.e., elements present and their concentrations) may include erroneous conclusions (false positives, false negatives, and incorrect concentrations). New users are well advised to learn the advantages and limitations of these algorithms and how to best derive accurate and reliable qualitative and quantitative results. A few comments on radiation safety are appropriate for any XRF application and even more so when considering its use in an instructional laboratory class. XRF analyzers represent a source of ionizing X-rays, but the power of the X-ray tube sources in these devices is orders of magnitude lower than those commonly used for medical and dental X-rays. For instructional use of XRF, a lead-lined radiation shield or test stand is strongly recommended in keeping with the concepts of ALARA (keeping radiation exposure “as low as reasonably achievable”). Note that many manufacturers’ designs for this mode of operation can be considered to be a “closed beam” system, insofar that the X-ray source will not turn on unless the test stand is closed. A handheld XRF analyzer is required for field applications such as soil analysis and when analyzing large samples that will not fit into the test stand. In this situation, the analyzer must be used in “open beam” mode and some common sense is required (i.e., keeping one's extremities

’ BUILDING XRF INTO THE CURRICULUM Many chemistry programs in Europe and Asia include X-ray techniques in their curricula. In Canada, Charles Wu teaches an annual short course in XRF at the University of Western Ontario and the Pittsburgh Conference.22 The Denver X-Ray Conference offers a number of very worthwhile tutorial sessions on XRF and related techniques.23 A search of all ACS journals for the keyword XRF in the title turned up a surprisingly few number of hits: 15. Some potentially useful articles on XRF for chemical educators include ones by Bachofer on sampling techniques and XRF methods for analysis of lead in soil,24 and Perring and Audrey on use of XRF for determination of a number of different elements in milk-based products,25 interelement absorption effects on chlorine peak ratios,26 and the use of XRF and other instrumental methods for the analysis of works of art.27,28 Surprisingly, none of the major quantitative analysis or introductory analytical chemistry textbooks include any mention of XRF. Other than the few examples cited above, it is clear that XRF is not being taught as a viable analytical technique, much less used as part of the laboratory curriculum. All of this can be seen as an opportunity for chemistry educators. We already devote significant time and effort to teaching our students the fundamental concepts of spectral interpretation and quantitative analysis. In the classroom, XRF offers the instructor the chance to discuss the “forgotten portion” of the electromagnetic spectrum (X-rays), the effect that highenergy photons have on atoms and molecules (removing inner shell electrons and causing fluorescence), and instrumentation (i.e., X-ray sources and optics, energy dispersive and pulse counting detectors). Unlike IR, MS, and NMR spectra of complex mixtures, XRF spectra are much simple and easier to interpret. XRF spectra of metallic mercury, mercuric sulfide, and dimethyl mercury will always show the characteristic Hg peaks at 9.99 (LR) and 11.82 keV (Lβ). Indeed, using the aforementioned Innov-X X-5000 XRF analyzer, the author has frequently demonstrated the technique of XRF in the classroom, with results available in seconds and the output (i.e., raw spectra or results) displayed on screen. Without question, giving our students the opportunity to see a portable analyzer used for near real-time elemental analysis of various samples in a classroom setting is a rare and powerful learning experience. In the lab, XRF offers a number of significant advantages that make it eminently suitable as an instrumental technique. Compared to other spectroscopic instrumentation such as IR, Raman, and MS, XRF analyzers are usually less expensive to purchase and maintain. Typical supplies are limited to SRMs, XRF sample 870

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Journal of Chemical Education cups, and film to seal them. Unlike many techniques, XRF requires minimal sample preparation. For screening, samples can often be analyzed with little or no sample preparation. For more accurate quantitative analysis, homogenization is required to obtain a more representative sample, but there is no need for time-consuming digestion and filtration required for most FAAS, ICPAES, and ICPMS methods. Finally, XRF is fast and informative: a typical 1-min measurement can provide data on most of the elements in the periodic table (typically from S to U), often down to single parts per million (ppm) levels. Clearly, XRF is well suited for both nonmajor (i.e., forensic chemistry, environmental chemistry) and major courses (i.e., general chemistry, quantitative analysis, instrumental analysis, environmental analysis). For the past five semesters, all of the students in San Francisco State University’s quantitative analysis class have been introduced to XRF for individual experiments and a variety of group experiments. Each student is required to bring in a sample that contains one or more elements that are detectable via XRF, acquire a spectrum on their own, and identify what elements are present. Rather than having the students use the results provided by the built-in software algorithms (i.e., elements present and their concentrations), the major learning objective here is to have students confirm detection of an element by evaluating their spectrum to ensure it shows two peaks at energies within (0.05 keV of their tabulated reference energies and at the proper intensity ratios. Students quickly learn that samples containing only low Z materials that are mostly organic elements are inappropriate. Some are happy to verify that their rings actually contain precious metals such as Au, Ag, or Pt, but every once in a while, someone is disappointed to find out that their diamond is actually Zr. Some interesting samples that have been analyzed include a silver pendant from India that contained percent levels of Cd, an Ayurvedic medicine product that had percent levels of Hg, and some samples from a hazmat facility that contained lead arsenate, Th, and U. In group experiments, students have determined Pb in soil samples from a local remediation site, leachable Pb and Cu in imported tableware, and Ca in powdered milk. These experiments provide the opportunity to teach our students about more advanced calibration methods2,1113 such as fundamental parameters (i.e., modeling the experimental spectrum using physical constants and XRF-determined composition data), internal standards (i.e., use of Compton normalization to correct for varying sample densities and thicknesses), and standard additions. Indeed, determination of Ca in powdered milk via externalstandard-based calibration shows a determinate error due to attenuation of Ca fluorescence by the concomitant presence of K in the sample, whereas standard-addition-based quantitation gives very accurate results. The simplicity, speed, and nature of these experiments are such that the instructor can focus the students on the entire analytical process, including defining the problem, devising an appropriate method and XRF excitation conditions, and obtaining a representative sample. This is eminently more satisfying than the somewhat narrow focus on data acquisition and interpretation that is unfortunately all too common in most quantitative analysis experiments. Use of XRF and comparison to traditional quantitative analysis experiments such as gravimetric determination of chloride, determination of calcium via EDTA titration, and determination of lead via FAAS provide the opportunity for students to critically assess and compare different methods with respect to their analytical figures of merit. Finally,

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the addition of XRF to the chemistry curriculum might encourage more on-campus interactions between chemists and people in other disciplines who might make good use of this technique, such as those in geosciences, museum studies, and environmental studies. Despite the advances and its obvious utility, XRF receives little respect as a viable analytical method. Common opinions heard from analytical chemistry colleagues, reviewers, and even editors are “XRF cannot be used for quantitation” and that “conventional atomic spectrometry techniques are a better option”. Clearly, XRF provides a number of significant advantages, including multielement analysis, limits of detection in the low parts per million (ppm) range, minimal sample prep, and measurement times on the order of a minute. It is time to recognize that XRF is a powerful tool for elemental analysis, and that XRF has a place within the chemistry curriculum. The potential audience for this includes students in forensic science, introductory, general, analytical, and inorganic chemistry, and materials science courses. In a laboratory setting, portable XRF allows for handson and inquiry-based learning, and reinforcement of important concepts such as periodicity, qualitative analysis, quantitative analysis, and even community service, service learning, and environmental analytical applications such as surveying toxic elements at a remediation site.29 To address the fact that little information is available on XRF in conventional textbooks, a set of PowerPoint slides has been made available for noncommercial use on the Analytical Science Digital Library (ASDL) Web site.21 These are suitable for lectures and provide an introduction to the technique, instrumentation, and guidelines for reliable qualitative and quantitative analysis. Interested educators are urged to review the citations included in this article for more details on various XRF applications that are suitable for consideration as potential laboratory experiments. Interested faculty can also contact the author for more information.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The author welcomes comments or suggestions and can be contacted at [email protected]. First and foremost, he thanks Richard Jacobs (FDA) and David Anderson (CFSAN/FDA) for introducing him to the “joy of XRF”. He also thanks Jack Hanson (Olympus Innov-X) and Bruce Kaiser (Bruker) for freely sharing their knowledge and expertise in XRF. He gratefully acknowledges Peter Greenland (Thermo Scientific Niton), Jack Hanson and Kim Russell (Innov-X), and Bruce Kaiser (Bruker) for loans of portable XRF analyzers for use in San Francisco State University’s (SFSU) quantitative and instrumental analysis lab classes. Finally, he acknowledges the hundreds of SFSU students for their involvement in XRF applications and their feedback on the use of XRF in the chemistry curriculum. ’ REFERENCES (1) EPA Method 6200: Field Portable X-Ray Fluorescence Spectrometry for the Determination of Elemental Concentrations in Soils and Sediments. http://www.epa.gov/wastes/hazard/testmethods/sw846/ pdfs/6200.pdf (accessed Apr 2011). 871

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(2) Palmer, P. T.; Jacobs, R.; Baker, P. E.; Ferguson, K.; Webber, S. J. Agric. Food Chem. 2009, 57, 2605–2613. (3) Anderson, D. J. Assoc. Official Anal. Chem. 2003, 86, 583–597. (4) Sirois, P. J.; Sansoucy, G. Collect. Forum 2001, 17, 49–66. (5) Janssens, K.; Vittiglio, G.; Deraedt, I.; Vekemans, B.; Vincze, L.; Wei, F.; Deryck, I.; Schalm, O.; Adams, F.; Rindby, A.; Knochel, A.; Simionovici, A.; Snigirev, A. X-Ray Spectrom. 2000, 29, 73–91. (6) Shackley, M. S. X-Ray Fluorescence Spectrometry (XRF) in Geoarcheology; Springer: New York, 2010. (7) Newbury, D. Microsc. Microanal. 2005, 11, 1–17. (8) Newbury, D. Scanning 2007, 29, 137–151. (9) Newbury, D. Scanning 2009, 31, 1–11. (10) Szabo, L. USA Today, Dec. 6, 2010, D12. An extended version of this article with the quote included is hosted at http:// www.greenchange.org/article.php?id=6333 (accessed Apr 2011). (11) Potts, P. J.; West, M. Portable X-Ray Spectrometry: Capabilities for In-Situ Analysis; Royal Society of Chemistry: London, 2008. (12) Grieken, R.; Markowicz, A. Handbook of X-Ray Spectrometry, 2nd ed.; CRC Press: Boca Raton, FL, 2002. (13) Jenkins, R.; Windfordner, R. J. X-Ray Fluorescence Spectrometry, 2nd ed.; Wiley: New York, 1999. (14) Spectro XRF Fundamentals. http://www.spectro.com/pages/ e/p08050001.htm (accessed Apr 2011; N.B. that log in is required to access this Web site). (15) Bruker-AXS Handheld X-Ray Spectrometry Web Page. http:// www.bruker-axs.com/handheldproducts.html?utm_id=18&utm_term=portable%20XRF (accessed Apr 2011). (16) Olympus (Innov-X) Handheld XRF Overview Page. http:// www.innovx.com/products/handheld (accessed Apr 2011). (17) Oxford Handheld Portable XRF Analysers Page. http://www. oxinst.com/products/xrf-analysers/hand-held/Pages/hand-held.aspx (accessed Apr 2011). (18) QSX Instruments XRF Analyzers Web Page. http://www. quickshotxrf.com/xrf-analyzers-overview (accessed Apr 2011). (19) SpectroAmetek Portable and Mobile On-Site Metal Analyzers Web Page. http://www.spectro.com/pages/e/p0101.htm (accessed Apr 2011). (20) Thermo Scientific Niton XRF Analyzers Web Page. http://www. niton.com/Niton-Analyzers-Products.aspx?sflang=en&_kk=portable% 20XRF%20analyzers&_kt=a8bbc234-da73-4b08-b127-dfebd9691c8e&gclid=COGPlLrA2KcCFQoTbAodFn609Q (accessed Apr 2011). (21) Palmer, P. T. Introduction to Energy-Dispersive X-ray Fluorescence (XRF)—An Analytical Chemistry Perspective. http://www.asdlib.org/ onlineArticles/ecourseware/Palmer/ASDL%20Intro%20to%20XRF. pdf (accessed Apr 2011). (22) Unknown. Total Reflectance X-Ray Fluorescence Spectroscopy. Spectroscopy 2011, Feb. 8. http://spectroscopyonline.findanalytichem.com/spectroscopy/X-ray-Fluorescence-Spectrometry-Ready-forthe-Clas/ArticleStandard/Article/detail/706816 (accessed Apr 2011). (23) 60th Annual Denver X-Ray Conference Home Page. http:// www.dxcicdd.com/ (accessed Apr 2011). (24) Bachofer, S. J. J. Chem. Educ. 2008, 85, 980–982. (25) Perring, L.; Andrey, D. J. Agric. Food Chem. 2003, 51, 4207–4212. (26) Durham, C. R.; Chase, J. M.; Nivens, D. A.; Baird, W. H.; Padgett, C. W. J. Chem. Educ. 2011, 88, 819–821. (27) Nivens, D. A.; Padgett, C. W.; Chase, J. M.; Verges, K. J.; Jamieson, D. S. J. Chem. Educ. 2010, 87, 1089–1093. (28) Uffelman, E. S. J. Chem. Educ. 2007, 84, 1617–1624. (29) Gardella, J. A.; Milillo, T. M.; Oh, G; Manns, D. C.; Coffey, E. Anal. Chem. 2007, 79, 810–818.

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