1、Designation: E 1508 98 (Reapproved 2003)Standard Guide forQuantitative Analysis by Energy-Dispersive Spectroscopy1This standard is issued under the fixed designation E 1508; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide is intended to assist those using energy-dispersive spectroscopy (EDS) for quantitative analysis ofmate
3、rials with a scanning electron microscope (SEM) orelectron probe microanalyzer (EPMA). It is not intended tosubstitute for a formal course of instruction, but rather toprovide a guide to the capabilities and limitations of thetechnique and to its use. For a more detailed treatment of thesubject, see
4、 Goldstein, et al.2This guide does not cover EDSwith a transmission electron microscope (TEM).1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health pra
5、ctices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3E 3 Methods of Preparation of Metallographic SpecimensE 7 Terminology Relating to Metallography3E 673 Terminology Relating to Surface AnalysisE 691 Practice for Conducting an Int
6、erlaboratory Study toDetermine the Precision of a Test Method3. Terminology3.1 DefinitionsFor definitions of terms used in this guide,see Terminologies E 7 and E 673.3.2 Definitions of Terms Specific to This Standard:3.2.1 accelerating voltagethe high voltage between thecathode and the anode in the
7、electron gun of an electron beaminstrument, such as an SEM or EPMA.3.2.2 beam currentthe current of the electron beam mea-sured with a Faraday cup positioned near the specimen.3.2.3 Bremsstrahlungbackground X rays produced byinelastic scattering (loss of energy) of the primary electronbeam in the sp
8、ecimen. It covers a range of energies up to theenergy of the electron beam.3.2.4 critical excitation voltagethe minimum voltage re-quired to ionize an atom by ejecting an electron from a specificelectron shell.3.2.5 dead timethe time during which the system will notprocess incoming X rays (real time
9、 less live time).3.2.6 k-ratiothe ratio of background-subtracted X-rayintensity in the unknown specimen to that of the standard.3.2.7 live timethe time that the system is available todetect incoming X rays.3.2.8 overvoltagethe ratio of accelerating voltage to thecritical excitation voltage for a par
10、ticular X-ray line.3.2.9 shaping timea measure of the time it takes theamplifier to integrate the incoming charge; it depends on thetime constant of the circuitry.3.2.10 spectrumthe energy range of electromagnetic ra-diation produced by the method and, when graphically dis-played, is the relationshi
11、p of X-ray counts detected to X-rayenergy.4. Summary of Practice4.1 As high-energy electrons produced with an SEM orEPMA interact with the atoms within the top few micrometresof a specimen surface, X rays are generated with an energycharacteristic of the atom that produced them. The intensity ofsuch
12、 X rays is proportional to the mass fraction of that elementin the specimen. In energy-dispersive spectroscopy, X raysfrom the specimen are detected by a solid-state spectrometerthat converts them to electrical pulses proportional to thecharacteristic X-ray energies. If the X-ray intensity of eachel
13、ement is compared to that of a standard of known composi-tion and suitably corrected for the effects of other elementspresent, then the mass fraction of each element can becalculated.5. Significance and Use5.1 This guide covers procedures for quantifying the el-emental composition of phases in a mic
14、rostructure. It includesboth methods that use standards as well as standardlessmethods, and it discusses the precision and accuracy that one1This guide is under the jurisdiction of ASTM Committee E04 on Metallographyand is the direct responsibility of Subcommittee E04.11 on X-Ray and ElectronMetallo
15、graphy.Current edition approved Nov. 1, 2003. Published December 2003. Originallyapproved in 1993. Last previous edition approved in 1998 as E 1508 98.2Goldstein, J. I., Newbury, D. E., Echlin, P., Joy, D. C., Romig, A. D., Jr., Lyman,C. D., Fiori, C., and Lifshin, E., Scanning Electron Microscopy a
16、nd X-rayMicroanalysis, 2nd ed., Plenum Press, New York, 1992.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.
17、1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.can expect from the technique. The guide applies to EDS witha solid-state X-ray detector used on an SEM or EPMA.5.2 EDS is a suitable technique for routine quantitativeanalysis of eleme
18、nts that are 1) heavier than or equal to sodiumin atomic weight, 2) present in tenths of a percent or greater byweight, and 3) occupying a few cubic micrometres, or more, ofthe specimen. Elements of lower atomic number than sodiumcan be analyzed with either ultra-thin-window or windowlessspectromete
19、rs, generally with less precision than is possible forheavier elements. Trace elements, defined as 100 %. For quantitative analysisusing standards, the beam current (not specimen current) mustbe the same for both the specimen and the standards or onemust be normalized to the other.8.2.6 The geometri
20、c configuration of the sample and detec-tor, shown schematically in Fig. 1, also affects the analysis. Thenumber of X-ray photons that reach the detector is a functionof the solid angle and take-off angle, including the effect ofspecimen and detector tilt. The count rate incident on an X-raydetector
21、 is directly proportional to the size of the solid angledefined as follows for a detector normal to the line of sight tothe specimen:V5A/r2(2)where:V = solid angle in steradians,A = active area of the detector crystal; for example, 30mm2, andr = sample-to-detector distance, mm.The larger the active
22、area of the detector, the more countswill be collected, but at the expense of spectral resolution.Most detectors have a movable slide and can be brought closerto the sample if a higher count rate at a given beam current isneeded. The take-off angle is defined as the angle between thesurface of the s
23、ample and a line to the X-ray detector. If thesample is not tilted, the take-off angle is defined as follows:c5arctan W 2 V!/S (3)where:c = take-off angle,W = working distance,V = vertical distance, andS = spectrometer distance.Working distance is measured in the microscope; its accu-racy depends on
24、 the method used to measure it and thespecimen position. Vertical distance is the distance from thebottom of the pole piece of the final lens to the centerline of thedetector; it usually can be measured within the microscopewith a ruler. Spectrometer distance is the horizontal distancefrom the spect
25、rometer to the beam; it is measured using thescale provided by the manufacturer on the spectrometer slide.All distances must be in the same units. The take-off angleshould be as high as possible to minimize absorption of X rayswithin the specimen and maximize the accuracy of quantitativeanalysis. If
26、 the specimen is tilted such that the beam is notperpendicular to the specimen surface, an effective take-offangle is used. There are several expressions in use by com-mercial manufacturers to calculate this, and all produce similarresults if the tilt angle is not extreme. When analysis isperformed
27、on a tilted specimen, the azimuthal angle betweenthe line from the analysis point to the EDS detector and the line5Andersen, C. A., and Hasler, M. F., X-Ray Optics and Microanalysis, 4th Intl.Cong. on X-Ray Optics and Microanalysis, Hermann, Paris, 1966, p. 310.FIG. 1 Schematic Diagram of Electron M
28、icroscope SystemE 1508 98 (2003)3perpendicular to the stage tilt axis must be known. If standardsare used, they must be collected under the identical geometricalconditions as the unknowns.8.3 Spectral Artifacts:8.3.1 There are a number of artifacts possible with EDS, andthese are discussed by Fiori,
29、 et al.6Most of them are related todetector electronics and are rarely seen in a properly function-ing system. However, two artifacts that are commonly seen arepulse pileup peaks and silicon escape peaks. Pileup peaksoccur when several X-ray photons reach the detector at thesame time, and the pulse
30、processing electronics erroneouslyrecord the sum of their energies rather than each one individu-ally. Lowering the beam current to lower the count rate usuallyeliminates the problem. Alternatively, the amplifier shapingtime can be decreased; this action will allow pulses to beprocessed faster, but
31、at the expense of degraded spectralresolution.8.3.2 A silicon escape peak occurs when an ionized atom ofsilicon in the detector generates an X ray. If that X ray escapesfrom the detector, its energy that would ordinarily have beenmeasured is lost. The result is a peak at 1.74 keV (Si Ka) belowthe pr
32、oper peak. This artifact is greatest at about 2 keV, near thePKaor Zr Lapeaks. The artifact cannot occur at energiesbelow the absorption edge of the Si K line, and it becomesnegligible at higher energies such as the Cu Kaline.9. Quantification9.1 Background Subtraction and Peak Deconvolution:9.1.1 B
33、efore the proportionality between X-ray intensityand elemental concentration can be calculated, several stepsare required to obtain the intensity ratio (k-ratio) betweenunknown and standard. Or, if the standardless technique isused, then a pure net intensity is required. A spectrum of X raysgenerate
34、d by electrons interacting with the specimen containsa background consisting of continuum X rays, often calledBremsstrahlung. Observing the high-energy cutoff of the con-tinuum, as noted in 8.2.1, gives the most accurate determina-tion of the beam voltage, and this is the value that should beused fo
35、r quantitative analysis. If the voltage measured in thismanner is much lower than the voltage setting, it may be anindication that the specimen is charging. The background in thespectrum is not linear and simple interpolation is inadequate.Two approaches to this problem commonly used in commercialsy
36、stems are background modeling and digital filtering. Thebackground models are based on known physics plus a suitablecorrection for the real world. This method lets the user passjudgment on the quality of the model by comparing the modelwith the actual spectrum. The digital filter method treats theba
37、ckground as a low frequency component of the spectrum andmathematically sets it to zero. This method is not based on anymodel and, therefore, is more general. It is also useful for thelight element region of the spectrum where the models werenever intended to be used; however, it does not take intoa
38、ccount absorption edges. Some software also allows theoperator to fit his own background.9.1.2 The other step that must be accomplished before anintensity ratio can be measured is peak deconvolution. EDSdetectors do not resolve all peaks. For example, the S Ka,MoLa, and Pb Malines are all within abo
39、ut 50 eV of each otherand therefore are severely overlapped. Even though one cannotsee the individual components of a peak envelope in aspectrum, there are computer methods of deconvolution. Twomethods in common use are 1) the method of overlap factorsand 2) the method of multiple least squares. Bot
40、h methodswork well, and they are usually combined with one of thebackground subtraction methods in the manufacturers soft-ware. One should consult the manufacturers instructions fortheir use.9.1.3 Although in most cases these computer methodshandle spectra well, the operator should be aware of condi
41、tionsthat are difficult. For example, trace element analysis issensitive to background subtraction because the computer islooking for a small peak above the continuum. Accordingly thespectrum must be collected long enough to provide enoughstatistics to discern small peaks. In like manner, deconvolut
42、ionroutines work well in most cases, but not when the overlappedlines arise from elements present in widely different concen-trations. For example, if one element constitutes 90 % of thespecimen and the other element 10 %, precision will be greatlydegraded. In this situation use of a different analy
43、tical line maybe possible, or if not, a technique with higher spectralresolution such as wavelength dispersive spectrometry isindicated.9.1.4 Once the background is subtracted and the peaks arestripped of interferences, one can calculate their ratio to thoseof similarly background-subtracted, deconv
44、oluted standardspectra. The unknowns and standards must have been collected1) under the same geometrical configuration, 2) at the sameaccelerating voltage, 3) at the same count rate per current unit,and 4) with the same processing algorithm.9.1.5 Even standardless analysis requires background sub-tr
45、action and peak deconvolution, but the intensity is calculatedfrom pure intensity curves and the ratio of peak integrals in theunknown spectrum. Standardless analyses always total 100 %,or some other value specified by the analyst. In normalizing thetotal concentrations to 100 %, important informati
46、on is lost. Atrue mass total, as in analysis against standards, providesinformation about the quality of the analysis. It calls attentionto problems such as elements not specified for analysis oranalysis of more than one phase under the beam. Analysestotaling exactly 100 % should always be viewed wi
47、th skepti-cism, whether they be standardless or normalized standardsanalyses. Whichever method is used, all elements present mustbe specified even if some need not be analyzed. This is becausea correction is necessary to account for the effect of otherelements (the matrix) present in the specimen.9.
48、2 Matrix Corrections:9.2.1 The k-ratio of an element is a starting estimate of thatelements concentration. There are, however, effects of atomicnumber, absorption, and fluorescence between the unknownsand the standards. The atomic number or “Z” factor corrects for6Fiori, C. E., Newbury, D. E., and M
49、yklebust, R. L., “Artifacts Observed inEnergy Dispersive X-ray Spectrometry in Electron Beam InstrumentsA Caution-ary Guide,” NIST Special Publication 604, Proceedings of the Workshop on EnergyDispersive Spectrometry, National Institute of Standards and Technology, Gaithers-burg, Maryland, 1981.E 1508 98 (2003)4differences in the number of X rays generated. The absorptionor “A” factor corrects for differences in the number of X raysthat escape the sample to be detected. The fluorescence or “F”factor corrects for non-electron generated X rays, that is, thosefluoresce
