ASTM E1508-2012 Standard Guide for Quantitative Analysis by Energy-Dispersive Spectroscopy 《用能量分散能谱学作定量分析的标准指南》.pdf

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1、Designation:E150898 (Reapproved 2008) Designation: E1508 12Standard Guide forQuantitative Analysis by Energy-Dispersive Spectroscopy1This standard is issued under the fixed designation E1508; the number immediately following the designation indicates the year oforiginal adoption or, in the case of r

2、evision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () 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 quantitativ

3、e analysis of materials witha scanning electron microscope (SEM) or electron probe microanalyzer (EPMA). It is not intended to substitute for a formal courseof instruction, but rather to provide a guide to the capabilities and limitations of the technique and to its use. For a more detailedtreatment

4、 of the subject, see Goldstein, et al.2This guide does not cover EDS with a transmission electron microscope (TEM).1.2 UnitsThe values stated in SI units are to be regarded as standard. No other units of measurement are included in thisstandard.1.3 This standard does not purport to address all of th

5、e safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2.1 ASTM Standards:3E3 Guide for Preparation of M

6、etallographic SpecimensE7 Terminology Relating to MetallographyE673 Terminology Relating to Surface AnalysisE691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method3. Terminology3.1 DefinitionsFor definitions of terms used in this guide, see Terminologies E7

7、and E673.3.2 Definitions of Terms Specific to This Standard:3.2.1 accelerating voltagethe high voltage between the cathode and the anode in the electron gun of an electron beaminstrument, such as an SEM or EPMA.3.2.2 beam currentthe current of the electron beam measured with a Faraday cup positioned

8、 near the specimen.3.2.3 Bremsstrahlungbackground X rays produced by inelastic scattering (loss of energy) of the primary electron beam in thespecimen. It covers a range of energies up to the energy of the electron beam.3.2.4 critical excitation voltagethe minimum voltage required to ionize an atom

9、by ejecting an electron from a specificelectron shell.3.2.5 dead timethe time during which the system will not process incoming X rays (real time less live time).3.2.6 k-ratiothe ratio of background-subtracted X-ray intensity in the unknown specimen to that of the standard.3.2.7 live timethe time th

10、at the system is available to detect incoming X rays.3.2.8 overvoltagethe ratio of accelerating voltage to the critical excitation voltage for a particular X-ray line.3.2.9 SDD (silicon drift detector)An x-ray detector characterized by a pattern in the biasing electrodes which inducesgenerated elect

11、rons to move laterally (drift) to a small-area anode for collection, resulting in greatly reduced capacitance whichto a first approximation does not depend on the active area, in contrast to conventional detectors using flat-plate electrodes.41This guide is under the jurisdiction of ASTM Committee E

12、04 on Metallography and is the direct responsibility of Subcommittee E04.11 on X-Ray and ElectronMetallography.Current edition approved JuneMay 1, 2008.2012. Published September 2008.November 2012. Originally approved in 1993. Last previous edition approved in 20032008as E1508 98(20038). DOI: 10.152

13、0/E1508-98R08. 10.1520/E1508-12.2Goldstein, 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 and X-rayMicroanalysis, 3rd ed., Plenum Press, New York, 2003.3For referencedASTM standards, visit theASTM website, www

14、.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.4Johnson, G. G., Jr., and White, E. W., X-Ray Emission Wavelengths and KeV Tables for Nondiffractive Analysis, ASTM Dat

15、a Series DS 46, ASTM, Philadelphia, 1970.4Gatti, E. and Rehak, P Semiconductor drift chamber an application of a novel charge transport scheme. NIM-A 225:608-621, (1984).1This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes

16、have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official

17、document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2.10 shaping timea measure of the time it takes the amplifier to integrate the incoming charge; it depends on the timeconstant of the circuitry.3.2.103.2.11 spectrumthe energy

18、 range of electromagnetic radiation produced by the method and, when graphically displayed, isthe relationship of X-ray counts detected to X-ray energy.4. Summary of Practice4.1 As high-energy electrons produced with an SEM or EPMA interact with the atoms within the top few micrometres of aspecimen

19、surface, X rays are generated with an energy characteristic of the atom that produced them. The intensity of such X raysis proportional to the mass fraction of that element in the specimen. In energy-dispersive spectroscopy, X rays from the specimenare detected by a solid-state spectrometer that con

20、verts them to electrical pulses proportional to the characteristic X-ray energies.If the X-ray intensity of each element is compared to that of a standard of known composition and suitably corrected for the effectsof other elements present, then the mass fraction of each element can be calculated.5.

21、 Significance and Use5.1 This guide covers procedures for quantifying the elemental composition of phases in a microstructure. It includes bothmethods that use standards as well as standardless methods, and it discusses the precision and accuracy that one can expect fromthe technique. The guide appl

22、ies to EDS with a solid-state X-ray detector used on an SEM or EPMA.5.2 EDS is a suitable technique for routine quantitative analysis of elements that are 1) heavier than or equal to sodium in atomicweight, 2) present in tenths of a percent or greater by weight, and 3) occupying a few cubic micromet

23、res, or more, of the specimen.Elements of lower atomic number than sodium can be analyzed with either ultra-thin-window or windowless spectrometers,generally with less precision than is possible for heavier elements. Trace elements, defined as 100 %. For quantitative analysis using standards, the be

24、am current(not specimen current) must be the same for both the specimen and the standards or one must be normalized to the other.8.2.6 The geometric configuration of the sample and detector, shown schematically in Fig. 1, also affects the analysis. Thenumber of X-ray photons that reach the detector

25、is a function of the solid angle and take-off angle, including the effect of specimenand detector tilt. The count rate incident on an X-ray detector is directly proportional to the size of the solid angle defined asfollows for a detector normal to the line of sight to the specimen:E1508-12_2where: =

26、 solid angle in steradians,A = active area of the detector crystal; for example, 30 mm2, andr = sample-to-detector distance, mm.The larger the active area of the detector, the more counts will be collected, but at the expense of spectral resolution. Mostdetectors have a movable slide and can be brou

27、ght closer to the sample if a higher count rate at a given beam current is needed.The take-off angle is defined as the angle between the surface of the sample and a line to the X-ray detector. If the sample is nottilted, the take-off angle is defined as follows:E1508-12_3where: = take-off angle,W =

28、working distance,V = vertical distance, andS = spectrometer distance.Working distance is measured in the microscope; its accuracy depends on the method used to measure it and the specimenposition. Vertical distance is the distance from the bottom of the pole piece of the final lens to the centerline

29、 of the detector; itusually can be measured within the microscope with a ruler. Spectrometer distance is the horizontal distance from the spectrometerto the beam; it is measured using the scale provided by the manufacturer on the spectrometer slide. All distances must be in thesame units. The take-o

30、ff angle should be as high as possible to minimize absorption of X rays within the specimen and maximizethe accuracy of quantitative analysis. If the specimen is tilted such that the beam is not perpendicular to the specimen surface, aneffective take-off angle is used. There are several expressions

31、in use by commercial manufacturers to calculate this, and all producesimilar results if the tilt angle is not extreme. When analysis is performed on a tilted specimen, the azimuthal angle between theline from the analysis point to the EDS detector and the line perpendicular to the stage tilt axis mu

32、st be known. If standards areused, they must be collected under the identical geometrical conditions as the unknowns.8.3 Spectral Artifacts:8.3.1 There are a number of artifacts possible with EDS, and these are discussed by Fiori, et al.Most of them are related todetector electronics and are rarely

33、seen in a properly functioning system. However, two artifacts that are commonly seen are pulsepileup peaks and silicon escape peaks. Pileup peaks occur when several X-ray photons reach the detector at the same time, andthe pulse processing electronics erroneously record the sum of their energies rat

34、her than each one individually. Lowering the beamcurrent to lower the count rate usually eliminates the problem. Alternatively, the amplifier shaping time can be decreased; thisFIG. 1 Schematic Diagram of Electron Microscope SystemE1508 124action will allow pulses to be processed faster, but at the

35、expense of degraded spectral resolution.7Most of them are related todetector electronics and are rarely seen in a properly functioning system. However, two artifacts that are commonly seen are pulsepileup peaks and silicon escape peaks. Pileup peaks occur when several X-ray photons reach the detecto

36、r at the same time, andthe pulse processing electronics erroneously record the sum of their energies rather than each one individually. Lowering the beamcurrent to lower the count rate usually eliminates the problem. The ratio of pileup peak to parent peak(s) is proportional to the inputcount rate o

37、f the parent peak(s). For the same total input count rate, a complex spectrum will have smaller relative pileup peaksthan a simple spectrum having a few dominant peaks. The pileup peak for two parent peaks of different energies is larger than fora single peak with the same count rate as the two-peak

38、 pair. Relative pileup peak height is inversely related to energy for the samecount rate. Systems with SDDs generally have smaller pileup peaks under the same conditions, because the reduced capacitanceof the SDD makes it easier for the pulse processing electronics to recognize close coincidences as

39、 separate events. EDS systemsoften have software to model pileup peaks and correct for them.8.3.2 Asilicon escape peak occurs when an ionized atom of silicon in the detector generates an X ray. If that X ray escapes fromthe detector, its energy that would ordinarily have been measured is lost. The r

40、esult is a peak at 1.74 keV (Si K) below the properpeak. This artifact is greatest at about 2 keV, near the P Kor Zr Lpeaks. The artifact cannot occur at energies below theabsorption edge of the Si K line, and it becomes negligible at higher energies such as the Cu Kline. line. SDDs have larger esca

41、pepeaks relative to the parent peak because they are thinner (typically 0.4 to 0.5 mm compared to several mm for Si(Li) detectors)and thus have a higher surface-to-volume ratio.9. Quantification9.1 Background Subtraction and Peak Deconvolution:9.1.1 Before the proportionality between X-ray intensity

42、 and elemental concentration can be calculated, several steps arerequired to obtain the intensity ratio (k-ratio) between unknown and standard. Or, if the standardless technique is used, then a purenet intensity is required. A spectrum of X rays generated by electrons interacting with the specimen c

43、ontains a backgroundconsisting of continuum X rays, often called Bremsstrahlung. Observing the high-energy cutoff of the continuum, as noted in 8.2.1,gives the most accurate determination of the beam voltage, and this is the value that should be used for quantitative analysis. Ifthe voltage measured

44、 in this manner is much lower than the voltage setting, it may be an indication that the specimen is charging.The background in the spectrum is not linear and simple interpolation is inadequate. Two approaches to this problem commonlyused in commercial systems are background modeling and digital fil

45、tering. The background models are based on known physicsplus a suitable correction for the real world. This method lets the user pass judgment on the quality of the model by comparingthe model with the actual spectrum. The digital filter method treats the background as a low frequency component of t

46、he spectrumand mathematically sets it to zero. This method is not based on any model and, therefore, is more general. It is also useful for thelight element region of the spectrum where the models were never intended to be used; however, it does not take into accountabsorption edges. Some software a

47、lso allows the operator to fit his own background.9.1.2 The other step that must be accomplished before an intensity ratio can be measured is peak deconvolution. EDS detectorsdo not resolve all peaks. For example, the S K,MoL, and Pb Mlines are all within about 50 eV of each other and thereforeare s

48、everely overlapped. Even though one cannot see the individual components of a peak envelope in a spectrum, there arecomputer methods of deconvolution. Two methods in common use are 1) the method of overlap factors and 2) the method ofmultiple least squares. Both methods work well, and they are usual

49、ly combined with one of the background subtraction methodsin the manufacturers software. One should consult the manufacturers instructions for their use.9.1.3 Although in most cases these computer methods handle spectra well, the operator should be aware of conditions that aredifficult. For example, trace element analysis is sensitive to background subtraction because the computer is looking for a smallpeak above the continuum. Accordingly the spectrum must be collected long enough to provide enough statistics to discern smallpeaks. In like manner, deconvo