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ASTM B r A number in parentheses indicates the year of last reapproval. A superscript epsilon e indicates an editorial change since the last revision or reapproval. It is not intended to substitute for a formal course of instruction, but rather to provide a guide to the capabilities and limitations of the technique and to its use.
For a more detailed treatment of the subject, see Goldstein, et al. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Current edition approved Nov. Originally approved in Last previous edition approved in as E — It covers a range of energies up to the energy of the electron beam.
Summary of Practice 4. The intensity of such X rays is proportional to the mass fraction of that element in the specimen. In energy-dispersive spectroscopy, X rays from the specimen are detected by a solid-state spectrometer that converts 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 effects of other elements present, then the mass fraction of each element can be calculated.
It includes both methods that use standards as well as standardless methods, and it discusses the precision and accuracy that one Copyright? 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. If the specimen is inhomogeneous at the micrometre level, then a truly quantitative analysis is not possible, and a bulk technique such as X-ray?
Lower limits of detection are possible with longer counting times, but the precision of trace element analysis is poorer than when the element is present at the percent level. Guidelines are given in Methods E 3. The specimen must be?
Six Sigma: Scanning Electron Microscopy & Energy Dispersive Spectroscopy
This requirement does not preclude scratches; however, any scratches in the immediate vicinity of the analyzed region must be insigni? The operator must also be aware of the possibility of spurious X rays from parts of the chamber, polishing compound elements, or from adjacent phases or a combination thereof. Note that these requirements for surface preparation preclude the quantitative analysis of casual samples, such as unpolished surfaces like fracture surfaces. Although data can be generated on these casual surfaces, the results would be of signi?
If they are etched, the operator must make sure that the composition in the region to be analyzed has not been altered and that the region to be analyzed is? Lowering the accelerating voltage may reduce or eliminate the effect of charging in some samples, but applying a conductive coating is still the most common method. Evaporated carbon is usually the most suitable coating material. Heavy metals such as gold that are often used for SEM imaging are less suitable because they heavily absorb X rays; if the coating is thick enough, X-ray lines from those metals will be seen in the spectrum.
If one is analyzing carbon in the specimen, then aluminum makes a good coating. The coatings are usually applied in thicknesses of several tens of nanometres. Carbon that appears to be tan in color on the specimen surface, or on a piece of? For the most accurate analysis, standards and unknowns should be coated at the same time to assure equal coating thicknesses. Specimens mounted in a nonconducting medium must make electrical contact with the microscope stage.
This is often accomplished by painting a stripe of carbon or silver paint from the specimen to the specimen holder. Often aluminum and copper are used, and sometimes both the K and L lines of copper are used. The two elements need not be in the same specimen. A spectrum from pure aluminum could be collected followed by pure copper in the same spectrum.
Software is usually available to calibrate the EDS system, and one should consult the system manual for the details of operation. To ensure reproducible results, calibration should be checked periodically. An overvoltage that is too low will not sufficiently excite X rays; one that is too high yields low spatial resolution and causes absorption as X rays escape from deep within the specimen.
An overvoltage of at least 1. When analyzing hard and soft X rays in the same specimen, analyses at two voltages may be necessary.
For materials such as minerals and ceramics, which contain light elements that is, of low atomic number15 kV is usually a good compromise.
For many metals containing medium atomic number elements, 20 to 30 kV is a good choice. Heavy elements those of higher atomic number may be analyzed using L or M lines, and so higher voltages are not necessary. The actual accelerating voltage of the electron beam does not always correspond with the voltage selected on the instrument. It can be determined by expanding the vertical scale of the EDS spectrum and observing the energy above which continuum X rays do not occur.
This range contains K lines of the? Accordingly, most operators choose a 0—10 keV display at higher display resolution rather than a 0—20 keV display at lower resolution.
ASTM E1508 – 12a
Tables of X-ray energies can be found in various texts, such as Goldstein, et al2 or Johnson and White. An approximation of 4 Johnson, G. More accurate interaction volumes can be computed by Monte Carlo computer methods to generate random electron trajectories, but Eq 1 provides a reasonable estimate for most purposes.
Defocusing the beam or scanning it over an area of varying composition does not provide an average composition, because the correction factors applied to the intensity ratio are themselves a function of composition. It does not affect spatial resolution for X-ray analysis in the same way it detracts from electron image resolution.
Higher count rates can be achieved by lowering the shaping time on the system ampli? For quantitative analysis, a shaping time of about 10? The beam current must remain stable throughout the analysis, because the counts collected are directly proportional to the beam current.
For quantitative analysis using standards, the beam current not specimen aetm must asrm the same for both the specimen and the standards or one must be normalized to the other. The number of X-ray photons that reach the detector is a function of the solid angle and take-off angle, including the effect of specimen and detector tilt.
The count rate incident on an X-ray detector is directly proportional to the size of the solid angle de? The larger the active area of the detector, the more counts will be collected, but at the expense of spectral resolution. Most detectors have a movable slide and can be brought closer to the sample if a higher count rate at a given beam d1508 is needed. The take-off angle is de? If the sample is not awtm, the take-off angle is de? Working distance is measured in the microscope; its accuracy depends on the method used to measure it and the specimen position.
Vertical distance is the distance from the bottom of the pole piece of the? Spectrometer distance is the horizontal distance from the spectrometer to the ashm it is measured using the scale provided by the manufacturer on the spectrometer slide. All distances must be in the same units. The take-off angle should be as high as possible to minimize absorption of X rays within the specimen and maximize the accuracy of quantitative analysis. If the specimen is tilted such that the beam is not perpendicular to the specimen surface, an effective take-off angle is used.
There are several expressions in use by commercial manufacturers to calculate this, and all produce similar results if the tilt angle is not extreme. If standards are used, they must be collected under the identical geometrical conditions as the unknowns.
However, two artifacts that are commonly seen are pulse pileup peaks and silicon escape peaks. Pileup peaks occur when several X-ray photons reach the detector at the same time, and the pulse processing electronics erroneously record the sum of their energies rather than each one individually.
Lowering the beam current to lower the count rate usually eliminates the problem. If that X ray escapes from the detector, its energy that would ordinarily have been measured is lost. The result is a peak at 1.
The artifact cannot occur at energies below the absorption edge of the Si K line, and it becomes negligible at higher energies such as the Cu Ka line. Or, if the standardless technique is used, then a pure net intensity is required. A spectrum of X rays generated by electrons interacting with the specimen contains a background consisting of continuum X rays, often called Bremsstrahlung.
Observing the high-energy cutoff of the continuum, as noted in 8. If the voltage measured 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 commonly used in commercial systems are background modeling and digital?
The background models are based on known physics plus a suitable correction for the real world. This method lets the user pass judgment on the quality of the model by comparing the model with the actual spectrum.
This method is not based on astmm model and, therefore, is more general. It is also useful for the light element region of the spectrum where the models were never intended to be used; however, it does not take into 6 Fiori, C. Some software also allows the operator to?
EDS detectors do not resolve all peaks. Even though one cannot see the individual components of a peak envelope in a spectrum, there are computer methods of deconvolution. Two methods in common use are 1 the method of overlap factors and 2 the method of multiple least squares. For s1508, trace element analysis is sensitive to background subtraction because the computer aetm looking for a small peak above the continuum.
Accordingly the spectrum must be collected long enough to provide enough statistics to discern small peaks. In like manner, deconvolution routines work well in most cases, but not when the overlapped lines arise astn elements present in widely different concentrations.
In 1e508 situation use of a different analytical line may be possible, or if not, a technique with higher spectral resolution such as wavelength dispersive spectrometry is indicated. The unknowns and standards must have been collected 1 under the same geometrical con?
A true mass total, as in analysis against standards, provides information about the quality of the analysis. It calls attention to problems such as elements not speci?