ASTM E1508-98(2003)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:E1508–98 (Reapproved 2003)
Standard Guide for
Quantitative Analysis by Energy-Dispersive Spectroscopy
This standard is issued under the fixed designation E 1508; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3.2.3 Bremsstrahlung—background X rays produced by
inelastic scattering (loss of energy) of the primary electron
1.1 This guide is intended to assist those using energy-
beam in the specimen. It covers a range of energies up to the
dispersive spectroscopy (EDS) for quantitative analysis of
energy of the electron beam.
materials with a scanning electron microscope (SEM) or
3.2.4 critical excitation voltage—the minimum voltage re-
electron probe microanalyzer (EPMA). It is not intended to
quired to ionize an atom by ejecting an electron from a specific
substitute for a formal course of instruction, but rather to
electron shell.
provide a guide to the capabilities and limitations of the
3.2.5 dead time—the time during which the system will not
technique and to its use. For a more detailed treatment of the
process incoming X rays (real time less live time).
subject, see Goldstein, et al. This guide does not cover EDS
3.2.6 k-ratio—the ratio of background-subtracted X-ray
with a transmission electron microscope (TEM).
intensity in the unknown specimen to that of the standard.
1.2 This standard does not purport to address all of the
3.2.7 live time—the time that the system is available to
safety concerns, if any, associated with its use. It is the
detect incoming X rays.
responsibility of the user of this standard to establish appro-
3.2.8 overvoltage—the ratio of accelerating voltage to the
priate safety and health practices and determine the applica-
critical excitation voltage for a particular X-ray line.
bility of regulatory limitations prior to use.
3.2.9 shaping time—a measure of the time it takes the
2. Referenced Documents
amplifier to integrate the incoming charge; it depends on the
time constant of the circuitry.
2.1 ASTM Standards:
3.2.10 spectrum—the energy range of electromagnetic ra-
E 3 Methods of Preparation of Metallographic Specimens
diation produced by the method and, when graphically dis-
E 7 Terminology Relating to Metallography
played, is the relationship of X-ray counts detected to X-ray
E 673 Terminology Relating to Surface Analysis
energy.
E 691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
4. Summary of Practice
3. Terminology
4.1 As high-energy electrons produced with an SEM or
EPMAinteract with the atoms within the top few micrometres
3.1 Definitions—For definitions of terms used in this guide,
of a specimen surface, X rays are generated with an energy
see Terminologies E 7 and E 673.
characteristic of the atom that produced them. The intensity of
3.2 Definitions of Terms Specific to This Standard:
such X rays is proportional to the mass fraction of that element
3.2.1 accelerating voltage—the high voltage between the
in the specimen. In energy-dispersive spectroscopy, X rays
cathode and the anode in the electron gun of an electron beam
from the specimen are detected by a solid-state spectrometer
instrument, such as an SEM or EPMA.
that converts them to electrical pulses proportional to the
3.2.2 beam current—the current of the electron beam mea-
characteristic X-ray energies. If the X-ray intensity of each
sured with a Faraday cup positioned near the specimen.
element is compared to that of a standard of known composi-
tion and suitably corrected for the effects of other elements
ThisguideisunderthejurisdictionofASTMCommitteeE04onMetallography
present, then the mass fraction of each element can be
and is the direct responsibility of Subcommittee E04.11 on X-Ray and Electron
calculated.
Metallography.
Current edition approved Nov. 1, 2003. Published December 2003. Originally
5. Significance and Use
approved in 1993. Last previous edition approved in 1998 as E 1508 – 98.
Goldstein,J.I.,Newbury,D.E.,Echlin,P.,Joy,D.C.,Romig,A.D.,Jr.,Lyman,
5.1 This guide covers procedures for quantifying the el-
C. D., Fiori, C., and Lifshin, E., Scanning Electron Microscopy and X-ray
emental composition of phases in a microstructure. It includes
Microanalysis, 2nd ed., Plenum Press, New York, 1992.
both methods that use standards as well as standardless
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
methods, and it discusses the precision and accuracy that one
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1508–98 (2003)
can expect from the technique. The guide applies to EDS with theevaporator, isprobablythickenough. For themostaccurate
a solid-state X-ray detector used on an SEM or EPMA. analysis,standardsandunknownsshouldbecoatedatthesame
5.2 EDS is a suitable technique for routine quantitative time to assure equal coating thicknesses. Specimens mounted
analysisofelementsthatare 1)heavierthanorequaltosodium in a nonconducting medium must make electrical contact with
in atomic weight, 2) present in tenths of a percent or greater by the microscope stage. This is often accomplished by painting a
weight, and 3) occupying a few cubic micrometres, or more, of stripe of carbon or silver paint from the specimen to the
the specimen. Elements of lower atomic number than sodium specimen holder.
can be analyzed with either ultra-thin-window or windowless
spectrometers,generallywithlessprecisionthanispossiblefor
8. Spectrum Collection
heavier elements. Trace elements, defined as <1.0 %, can be
8.1 Calibration—The analyzer shall be calibrated on two
analyzed but with lower precision compared with analyses of
X-ray peaks or other methods implemented by the equipment
elements present in greater concentration.
manufacturer in software to set the amplifier gain and offset.
Often aluminum and copper are used, and sometimes both the
6. Test Specimens
K and L lines of copper are used. The two elements need not
6.1 Suitable specimens are those that are normally stable
be in the same specimen. A spectrum from pure aluminum
under an electron beam and vacuum and are homogeneous
could be collected followed by pure copper in the same
throughout the volume of X-ray production. If the specimen is
spectrum. Software is usually available to calibrate the EDS
inhomogeneous at the micrometre level, then a truly quantita-
system, and one should consult the system manual for the
tiveanalysisisnotpossible,andabulktechniquesuchasX-ray
details of operation. To ensure reproducible results, calibration
fluorescence should be used.
should be checked periodically.
6.2 The concentration of each element to be analyzed
8.2 Operating Parameters:
should equal or exceed about 0.1 wt %. Lower limits of
8.2.1 The accelerating voltage of the SEM must be chosen
detection are possible with longer counting times, but the
to provide an adequate overvoltage to excite the X-ray lines of
precision of trace element analysis is poorer than when the
interest. An overvoltage that is too low will not sufficiently
element is present at the percent level.
excite X rays; one that is too high yields low spatial resolution
and causes absorption as X rays escape from deep within the
7. Specimen Preparation
specimen. An overvoltage of at least 1.5 times the critical
7.1 Specimens for quantitative EDS analysis should be
excitationpotentialofthehighestenergyX-raylineanalyzedis
prepared in accordance with standard metallographic or petro-
recommended. When analyzing hard and soft X rays in the
graphic techniques. Guidelines are given in Methods E 3. The
samespecimen,analysesattwovoltagesmaybenecessary.For
specimen must be flat in the region to be analyzed. This
materials such as minerals and ceramics, which contain light
requirement does not preclude scratches; however, any
elements (that is, of low atomic number), 15 kV is usually a
scratchesintheimmediatevicinityoftheanalyzedregionmust
goodcompromise.Formanymetalscontainingmediumatomic
be insignificant with respect to the X-ray volume.The operator
number elements, 20 to 30 kV is a good choice. Heavy
must also be aware of the possibility of spurious X rays from
elements (those of higher atomic number) may be analyzed
parts of the chamber, polishing compound elements, or from
using L or M lines, and so higher voltages are not necessary.
adjacent phases or a combination thereof. Note that these
The actual accelerating voltage of the electron beam does not
requirements for surface preparation preclude the quantitative
always correspond with the voltage selected on the instrument.
analysis of casual samples, such as unpolished surfaces like
ItcanbedeterminedbyexpandingtheverticalscaleoftheEDS
fracture surfaces. Although data can be generated on these
spectrum and observing the energy above which continuum X
casual surfaces, the results would be of significantly lower
rays do not occur.
precision with unpredictable variations.
8.2.2 Almost all elements can be analyzed using character-
7.2 Unetched or lightly etched specimens are preferred. If
istic X-ray lines in the range of 0–10 keV. This range contains
they are etched, the operator must make sure that the compo-
K lines of the first transition series (scandium–zinc (Sc-Zn)), L
sition in the region to be analyzed has not been altered and that
linesofthesecondtransitionseriesplusthelanthanides,andM
the region to be analyzed is flat.
lines of the third transition series plus the actinides. Accord-
7.3 Nonconducting specimens should be coated with a
ingly, most operators choose a 0–10 keV display at higher
conductive material to prevent charging. Lowering the accel-
display resolution rather than a 0–20 keV display at lower
erating voltage may reduce or eliminate the effect of charging
resolution. Tables of X-ray energies can be found in various
in some samples, but applying a conductive coating is still the
2 4
texts, such as Goldstein, et al or Johnson and White.
most common method. Evaporated carbon is usually the most
8.2.3 X-ray spatial resolution degrades with overvoltage,
suitable coating material. Heavy metals such as gold that are
because as the electrons penetrate deeper into the specimen, X
often used for SEM imaging are less suitable because they
rays are generated from a larger volume.An approximation of
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
Johnson, G. G., Jr., and White, E. W., X-Ray Emission Wavelengths and KeV
of several tens of nanometres. Carbon that appears to be tan in
Tables for Nondiffractive Analysis,ASTM Data Series DS 46,ASTM, Philadelphia,
color on the specimen surface, or on a piece of filter paper in 1970.
E1508–98 (2003)
the diameter of this tear-drop-shaped excitation volume, re-
ferred to as the X-ray range, can be obtained using the
following equation.
1.68 1.68
R 5 0.064~E 2 E !/r (1)
o c
where:
R = the range in µm,
E = the accelerating voltage in kV,
o
E = the critical excitation potential in keV, and
c
r = the density in g/cm .
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.
8.2.4 The beam can be placed in the spot mode to form a
probe to analyze the minimum volume, or it can be scanned
over a homogeneous region to lower the electron dose at any
FIG. 1 Schematic Diagram of Electron Microscope System
one point. 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
where:
themselves a function of composition.
V = solid angle in steradians,
8.2.5 Thecurrentintheelectronbeamdeterminesthefluxof A = active area of the detector crystal; for example, 30
mm , and
X rays that are generated. It does not affect spatial resolution
r = sample-to-detector distance, mm.
for X-ray analysis in the same way it detracts from electron
The larger the active area of the detector, the more counts
image resolution. Typically it is adjusted to keep the dead time
will be collected, but at the expense of spectral resolution.
in the EDS system below 40 %. Dead times of 20 to 30 %
Most detectors have a movable slide and can be brought closer
producegoodspectra,whereasdeadtimesabove40 %canlead
to the sample if a higher count rate at a given beam current is
to spectra containing artifacts, such as those discussed in 8.3.1.
needed. The take-off angle is defined as the angle between the
Maximum throughput, that is, the most X rays/real time, is
surface of the sample and a line to the X-ray detector. If the
achieved at about 40 % dead time. Higher count rates can be
sample is not tilted, the take-off angle is defined as follows:
achieved by lowering the shaping time on the system amplifier
from about 10 µs, but spectral resolution will be lost. For c5 arctan ~W 2 V!/S (3)
quantitative analysis, a shaping time of about 10 µs or greater
where:
is used. The beam current must remain stable throughout the
c = take-off angle,
analysis, because the counts collected are directly proportional
W = working distance,
to the beam current. Thus, a 1 % upward drift in beam current
V = vertical distance, and
will produce a 1 % increase in all the reported mass fractions,
S = spectrometer distance.
resulting in a reported total >100 %. For quantitative analysis
Working distance is measured in the microscope; its accu-
using standards, the beam current (not specimen current) must
racy depends on the method used to measure it and the
be the same for both the specimen and the standards or one
specimen position. Vertical distance is the distance from the
must be normalized to the other.
bottomofthepolepieceofthefinallenstothecenterlineofthe
8.2.6 The geometric configuration of the sample and detec-
detector; it usually can be measured within the microscope
tor,shownschematicallyinFig.1,alsoaffectstheanalysis.The
with a ruler. Spectrometer distance is the horizontal distance
number of X-ray photons that reach the detector is a function
from the spectrometer to the beam; it is measure
...