SEM but delivering analysis results which areorders of magnitude better. Newcomers maybe fooled by those superficial similarities.

ANALYS IS OF TRACE QUANTIT IESThe reason for using EPMA, in a nutshell, istrace analysis and reliable quantification.EPMA makes possible accurate quantificationof minor and trace elements, especially with-out detailed a priori knowledge of the speci-men. However, although like SEM-EDS it usesan electron-beam to stimulate x-ray emissionfrom the sample, from that point there is astrong divergence between the two tech-niques. The ability to routinely and accurately mea-

sure and quantify trace levels of a few tens ofpermils without any special efforts is one ofthe most evident capabilities of EPMA. A typical example is shown in Figure 1 which

is a linescan of the concentration of Y203 ingarnet, where the maximum trace quantity is800 ppm and the minimum around 100 ppm.These are routine levels for EPMA.These data were acquired at 20 keV and 200

nA over 12 hours on a CAMECA SX100 micro-probe with a thermal source. The accuracy ofthe trace Y analysis is unaffected by the chang-

B I O G R A P H YIan Holton obtained hisBSc and PhD in physicsfrom the University ofLiverpool. Ian previouslyran a SEM design andmanufacture company inCambridge, and has over30 years experience in design and develop-ment of electron spectroscopy, X-ray andelectron source instrumentation, in the VGgroup (bought out by Thermo) and OmicronNanotechnology (now part of Oxford Instru-ments), mostly designing, developing, run-ning R&D and product management. Roleshave included Managing Director, andDirector of Development, Production Man-ager and R&D Physicist/Engineer in differentnanotech instrument companies. Ian Holtonstarted Acutance Scientific Ltd in January2011.

A B S T R A C TWith improvements in energy-dispersivespectroscopy (EDS) there has been a grow-ing misunderstanding that scanning elec-tron microscope EDS is an adequate substi-tute for electron probe microanalysis(EPMA) and some blurring of the under-standing of the limits of quantification ofSEM-EDS, especially amongst newcomers.This introductory article reviews the reasonswhy, although SEM-EDS is perfectly ade-quate in many cases, EPMA is essential incertain classes of work for both reliablequantification and trace analysis.

K E Y W O R D Sscanning electron microscopy, energy-dis-persive X-ray spectroscopy, wavelength dis-persive X-ray spectroscopy, microprobe,microanalysis, materials science

A C K N O W L E D G E M E N T SThe author would like to thank Jon Wade ofOxford University for the inspiration, MichelOutrequin, Marion Chopin and DavidSnoeyenbos of CAMECA for supporting dataand useful discussions, and Nicholas Ritchieof NIST for a preprint.

A U T H O R D E TA I L SDr Ian Holton, Acutance Scientific Ltd, 142 Stephens Road,Tunbridge Wells, Kent TN4 9QA, UKTel: +44 (0) 845 479 6989Email: ian@acutance.co.uk

Microscopy and Analysis 26(4):S4-S7 (AM), 2012

SEM-EDS VS EPMA

I N T R O D U C T I O NWhat possible reasons might there be for theburgeoning misconception that “SEM-EDS isan adequate substitute for EPMA”? Of course,there has to be a demonstrably critical need inorder to succeed in finding the sort of moneythat EPMA costs, rather than just buyinganother SEM. Those, though, who use EPMA,invariably know only too well why they reallydo need it. Unfortunately the uninitiated whoreally do need it, often do not know that theirSEM-EDS results are wrong. Overconfidence in automatic EDS results has

led to some highly embarrassing misidentifica-tions being made by auto-ID EDS and pub-lished. Ease of use of EDS has advanced a lot inrecent years and it is tempting to gloss overthe hidden limitations. Also, pressing a buttonfor the results to fall out unquestioned, maybe tempting where there are financial con-straints on the availability of suitably qualifiedstaff. Further, there is always an aura of mys-tique around expertise, and there are not somany EPMA experts. But it is hardly as thoughEPMA is a technique that cannot be learned,especially by the SEM-literate.EPMA is a dedicated and specialist technique

bearing a strong superficial resemblance to

IsEnergy-DispersiveSpectroscopy in theSEMaSubstitute forElectronProbeMicroanalysis?Ian Holton, Acutance Scientific Ltd, Tunbridge Wells, Kent, UK

Figure 1: Linescan of Y203 in garnet where maximum trace quantity is 800 ppm. These data were acquired at 20 keV and 200 nA over 12 hours on a CAMECASX100 (thermal source). Data courtesy of Michel Outrequin, CAMECA SAS; sample courtesy of Dr Norman Pearson, MacQuarie University.

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ing major element composition because ofbackground measurements and matrix correc-tion at each measured point. Regarding the power of EPMA in trace

analysis, for instance, it has been reported [3]that EPMA can routinely achieve confidencedetection of ~100 ppm with fairly minimaleffort in many cases. Donovan et al. used analgorithm which results in detection limits of 2to 3 ppm for Ti and 6 to 7 ppm of Al in quartzat 99% t-test confidence and with similar lev-els for absolute accuracy, by means of applyinga correction to the emitted X-ray intensities.Major element analysis is mainly a matter of

measuring an intense peak against relativelylow background (suited to EDS) but trace ele-ment analysis depends far more on back-ground characterisation than upon peakcounts. Jerkinovic et al [4] demonstrate howmere accumulation of counts is insufficient to

Figure 4: Standard calibration sample of Rare Earth Elements with 5% of La, Sm, Gd and Yb, run using EDS (top) and WDS(bottom). Again, the value of the order-of-magnitude higher energy resolution of WDS in making unambiguous and accurate quantifications is immediately apparent. Data courtesy of Michel Outrequin, CAMECA SAS.

Figure 2 (left): There is a need for higher resolution(WDS) in a number of critical cases. The graph shows X-ray spectra ofthe same kinetic energy regiontaken with EDS and with WDSrespectively. The order-of-magni-tude difference in resolution isapparent, as is the value of higherresolution WDS over EDS in certaincases in making unambiguous andaccurate quantification.Data courtesy of Michel Outrequin,CAMECA SAS.

Figure 3 (right): A typical electron microprobe. Thereis a need for higher resolution (WDS)in a number of critical cases. Thisinstrument is fitted with five WDSanalysers. Courtesy of Claude Guignes,CAMECA SAS.

Figure 5: Effect of source type on a commercial EPMA. Here, FEG gives an order of magnitude higher sample-currentinto the same spot-size under these conditions, although thermal emission enjoys a higher maximum currentoverall. Data by courtesy Michel Outrequin, CAMECA SAS.

obtain accuracy in trace element analysis andthat accuracy becomes dominated by the abil-ity to accurately determine the backgroundshape and interferences at high precision,which cannot be done by EDS. That character-isation generally must consider slope or evencurvature, known anomalies and interferingpeaks. This also cannot be done by EDS.Such trace element analysis can only be done

with beam currents which are much higherthan in SEM, which are very considerably morestable than in SEM, using analysers with peak-to-background ratios far below those obtain-able with EDS and with vastly greater sensitiv-ity and resolution. The data in Fig. 1 were takenwith a beam current of 200 nA over 12 hours. This sensitivity requirement is one reason for

EPMA instruments usually having five WDSanalysers (Figure 3). For speed, an EPMA simul-taneously acquires spectra from as many WDS

as are needed, each covering a different spec-trum and each having different crystals forthat purpose. Even with such powerful capa-bility it is not uncommon for an EPMA data-acquisition to be run overnight for fine traceanalysis and high-accuracy mapping. However, there is still room for EDS for

broad scans of major elements. In a shortly to-be-published paper, Ritchie et al. [5] have stud-ied advances in EDS due to the increased countrate of silicon drift detectors (SDD) detectors.They include consideration of coincidencespectral artefacts of SDD, the worse resolutionand the dominance of EDS count rate byunwanted Bremsstrahlung in EDS. The authorsconclude that in some measurement cases SDDcan replace WDS with improved accuracy andprecision, and propose that manufacturersconsider a future generation of EPMA in whichEDS and WDS are more equal partners.

Figure 6:A low energy (6 kV) beam generates a lower interaction volume in the sample and therefore a higher spatial resolution – in this case about 300 nm.This is a benefit of a FEG source.

Figure 8: EPMA instruments must be extremely stable over long periods in order to be useful. In this example a steel sample is run repeatedly over a period offour months without re-calibration of the WDS analysers. These data courtesy of Michel Outrequin, CAMECA SAS.

Figure 7:Mapping light elements. Here maps of B, Cr and Ni are taken alongside BSE data. The high peak to background ratio achieved with WDS in the low-Zregion is critical to such a map of low atomic number compounds.

E N E R G Y R E S O L U T I O N A N D E L E M E N T C O N F U S I O NWhere the characteristic X-ray energies of ele-ments in a given sample are known to be con-veniently well-separated energetically, EDS(which is limited to resolutions greater than120 eV) is useful. There are many practical cases in which the

presence of some elements is missed by use ofEDS alone because many lines overlap. Thisoften happens for instance for top-row transi-tion elements. In the case of low beam energy(Figure 3), it is necessary to measure transitionelements by means of the characteristic L lines,which are well-resolved with WDS. Withoutsuch high energy resolution, any attempt tomeasure light elements, such as carbon andoxygen, together with the L lines of some 3dtransition elements like V, Cr, Mn is at leastproblematic and in many cases unreliable. Inthe case of the data of Figure 7, although Blines do not overlap with Cr and Ni lines, thesedata demonstrate the high peak to back-ground ratio achieved with WDS in the low-Zregion.Figure 4 shows a standard calibration sample

of Rare Earth Elements run using EDS andWDS. WDS has a resolution of around 10 eV,an order of magnitude better than EDS and sopermitting resolution of overlapping lines.

B E A M C U R R E N TGenerally in SEM specifications, beam currentsare not mentioned concurrent with beamenergy and spot-size because the SEM image iseither good enough or it is not. Beam currentat a given spot-size must of necessity becomea key figure of merit where the object is traceelement analysis. So beam energy, spot-sizeand beam-current are always concurrent inany EPMA specification in order for it to meananything. Typical beam-currents for analysismight be a few tens of nA, although where thebest possible accuracy on trace elements isrequired (such as in the garnet case shown inFigure 1) it is possible in EPMA to run hundredsof nA into the sample, trading off spatial resolution.Further, for small spot-sizes the analytical

resolution is not limited by beam diameter butby the well-known interaction volume, inwhich multiple scattering of the electronscauses X-ray emissions from a much larger areathan the spot-size at the surface, so limitingthe analytical spatial resolution. So low beamenergy is often desirable because it reducesthe volume of interaction of the beam withthe sample. Producing a high beam current at a low

beam energy for a given analytical spatial res-olution is a challenge, but is an absoluterequirement for EPMA. This is greatly helpedby the optional use of a field-emission source. Figure 5 shows the effect of choice of source

type on a commercial EPMA. Here, FEG givesan order of magnitude higher sample-currentinto the same spot-size under these condi-tions, although thermal emission enjoys ahigher maximum current overall. Figure 6shows a benefit of high current at low beamenergy: because the interaction volume is

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Figure 9: EPMA needs greatly superior light optics to SEM for sample-positioning, which is critical to quantification accuracy and reliability, and also particularlyfor Earth Sciences work. SEM light optics simply won’t do. Data courtesy of Michel Outrequin, CAMECA SAS.

reduced, the spatial resolution of the chemicalmap is correspondingly improved. This is wherefield emission has a particular advantage.

S TA B I L I T YBecause EPMA is in a different category ofquantification to SEM, considerable stability isrequired of it. For this reason the condenseraperturing is differently designed, and veryaccurate and rapid feedback of the beam cur-rent is essential. Beam stability is a key figureof merit of EPMA design, and in a well-designed EPMA, beam-stability might be0.05%per hour for a tungsten filament. Such adesign constraint applies equally to the WDSanalysers. If there were unnoticed drift thiswould lead to loss of quantification, one of themost important reasons for having an EPMA.Or if such a defect were ameliorated by meansof frequent re-calibration of WDS, this wouldlead to significant down-time (and cost!) A sta-ble instrument design is essential for reliablemeasurement not only of trace quantities butof major components of a sample, if those fewdecimal places are to mean anything.By way of example, Figure 8 shows the sta-

bility of an EPMA over a period of fourmonths, demonstrating the lengths thatdesigners go to in order to achieve extremestability. Here, a steel sample is run repeatedlyover a period of four months without re-cali-bration of the WDS analysers.

T H E I M P O R TA N C E O F T H E L I G H TM I C R O S C O P E I N E P M AIn EPMA, the light/optical performance of theinstrument is not something that is tackedonto the end of the design process, as is gen-erally the case in a SEM, where a camera ispopped onto a side-port. It is a dominant fac-tor in the integrated design of the instrument,along with the electron optics and WDS design(Figure 9). In a good EPMA the designer goesto considerable lengths to ensure that thelight performance is extraordinary for the elec-tron beam circumstances. This is critical inEPMA because achieving correct sample-height is critical for accurate and reliably quan-tified operation of the WDS. This sample-height criticality results from the object posi-tion of the WDS being critical, as the WDS toois, of course, an optical device. SEMs have dif-ferent working distances for analytical andhigh resolution imaging modes. Because theEPMA has optics that permit high resolutionSEM and microprobe analysis to be carried outon the specimen without changing its distancefrom the final lens pole-piece this places addi-tional design constraints on the light opticaldesign. EPMA optics permit high-resolutionimages and X-ray analysis to be carried out atthe same working distance.

W D S C O N F I G U R AT I O NIf 12 hours seems a long run for multiple-trace-element analysis with several WDS, perhapsone or even two WDS is not such an attractiveidea! Further, as discussed above, it is a simplemisconception to believe that reduced sensi-tivity can be countered merely by longer acqui-sition. Quite apart from this obvious benefit of

configuring typically five WDS, a low take-offangle results in increased background andcomplicates the quantitative analysis of theresults. For this reason WDS take-off angle inEPMA is generally set to an angle of 40°, whichis often not possible on a SEM, even for justone WDS. Rough sample surfaces can result in appar-

ent variation in element concentration whichin fact is due to the height-variation being inthe energy-dispersion direction of the WDS.This is generally not a problem in Earth Sci-ences, where samples are polished flat beforeanalysis. (See example image in Figure 7.) Inmaterials analysis it is overcome by turning theWDS on its side such that variation occurs inthe non-dispersive direction of the WDS. Thisconfiguration is of course often possible on aSEM, but may be restricted by the multi-tech-nique configuration of SEM, where EPMA cantake one horizontal and three vertical WDS atthe same time with no problem.

C O N C L U S I O N SAccuracy and reproducibility in quantitativeanalysis are the key characteristics of electronprobe microanalysis instruments. Where tracequantities of <0.1% concentration are to beanalysed, EPMA must be used rather thanSEM-EDS. Trace quantities of 100 ppm are rou-

tinely measurable and with care a few ppmhave been measured in some samples. Whenhighly accurate quantification is needed (forinstance major elements to a reliable two ormore decimal places) EPMA is essential.

R E F E R E N C E S1. Sample courtesy of Dr Norman Pearson, Department of

Earth and Planetary Sciences, Faculty of Science, MacQuarieUniversity, NSW 2109, Australia. Data courtesy of Dr MichelOutrequin, CAMECA.

2. Stephen J. B. Reed. In: “Electron Probe MicroAnalysis”,Chapter 2, from Microprobe Techniques in the EarthSciences, Ed. Philip J Potts, John F W Bowles, Stephen J BReed and Mark R Cave. Chapman & Hall, 1995.

3. Improved electron probe microanalysis of trace elements inquartz, John J. Donovan, Heather A. Lowers, and Brian G.Rusk, American Mineralogist, Vol 96, pp 274-282, 2011.

4. Trace Analysis in EPMA, by Michael J Jercinovic, M. L.Williams, J. Allaz and J. J. Donovan, http://emas-web.net/Content/EMAS2011%20presentations/Jercinovic_ppt.pdf

5. EDS Measurements of X-Ray Intensity at WDS Precision andAccuracy using a Silicon Drift Detector. Nicholas W. M.Ritchie, Dale E. Newbury, Jeffrey M. Davis. MaterialsMeasurement Laboratory, National Institute of Standardsan Technology. Unpublished Preprint courtesy of NicholasRitchie, National Institute of Standards and Technology.

©2012 John Wiley & Sons, Ltd