ISO 22489:2016
(Main)Microbeam analysis — Electron probe microanalysis — Quantitative point analysis for bulk specimens using wavelength dispersive X-ray spectroscopy
Microbeam analysis — Electron probe microanalysis — Quantitative point analysis for bulk specimens using wavelength dispersive X-ray spectroscopy
ISO 22489:2016 specifies requirements for the quantification of elements in a micrometre-sized volume of a specimen identified through analysis of the X-rays generated by an electron beam using a wavelength dispersive spectrometer (WDS) fitted either to an electron probe microanalyser or to a scanning electron microscope (SEM). ISO 22489:2016 also describes the following: - the principle of the quantitative analysis; - the general coverage of this technique in terms of elements, mass fractions and reference specimens; - the general requirements for the instrument; - the fundamental procedures involved such as specimen preparation, selection of experimental conditions, the measurements, the analysis of these and the report. ISO 22489:2016 is intended for the quantitative analysis of a flat and homogeneous bulk specimen using a normal incidence beam. It does not specify detailed requirements for either the instruments or the data reduction software. Operators should obtain information such as installation conditions, detailed procedures for operation and specification of the instrument from the makers of any products used.
Analyse par microfaisceaux — Microsonde de Castaing — Analyse quantitative ponctuelle d'échantillons massifs par spectrométrie à dispersion de longueur d'onde
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 22489
Second edition
2016-10-15
Microbeam analysis — Electron
probe microanalysis — Quantitative
point analysis for bulk specimens
using wavelength dispersive X-ray
spectroscopy
Analyse par microfaisceaux — Microsonde de Castaing — Analyse
quantitative ponctuelle d’échantillons massifs par spectrométrie à
dispersion de longueur d’onde
Reference number
ISO 22489:2016(E)
©
ISO 2016
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ISO 22489:2016(E)
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ISO 22489:2016(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Abbreviated terms . 1
4 Procedure for quantification . 2
4.1 General procedure for quantitative microanalysis . 2
4.1.1 Principle and procedure of quantitative microanalysis . 2
4.1.2 Coverage of the quantitative analysis . 2
4.1.3 Selection of reference materials . 3
4.2 Specimen preparation . 3
4.3 Calibration of the instrument . 3
4.3.1 Accelerating voltage . 3
4.3.2 Probe current . 3
4.3.3 X-ray spectrometer . 3
4.3.4 Dead time . 4
4.4 Analysis conditions . 4
4.4.1 Accelerating voltage . 4
4.4.2 Probe current . 4
4.4.3 Analysis position . 4
4.4.4 Probe diameter . 5
4.4.5 Scanning the focused electron beam . 5
4.4.6 Specimen surface . 5
4.4.7 Selection of X-ray line . 5
4.4.8 Spectrometer . 5
4.4.9 Method for measurement of X-ray peak intensity . 6
4.4.10 Method for measurement of background intensity . 6
4.5 Correction method based on analytical models . 6
4.5.1 Principles . 6
4.5.2 Correction models . 7
4.6 Calibration curve method . 7
4.6.1 Principle . 7
4.6.2 Selection of reference materials . 8
4.6.3 Procedure . 8
4.7 Uncertainty . 8
5 Test report . 8
Annex A (informative) Physical effects and correction .10
Annex B (informative) Outline of various correction techniques .12
Annex C (informative) Measurement of the k-ratios in case of “chemical effects” .14
Bibliography .15
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ISO 22489:2016(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
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For an explanation on the meaning of ISO specific terms and expressions related to conformity assessment,
as well as information about ISO’s adherence to the World Trade Organization (WTO) principles in the
Technical Barriers to Trade (TBT) see the following URL: www.iso.org/iso/foreword.html.
The committee responsible for this document is ISO/TC 202, Microbeam analysis, Subcommittee SC 2,
Electron probe microanalysis.
This second edition cancels and replaces the first edition (ISO 22489:2006), of which it constitutes a
minor revision to update the references and to revise text in 4.4.1 and 4.4.8.
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ISO 22489:2016(E)
Introduction
Electron probe microanalysis is widely used for the quantitative analysis of elemental composition in
materials. It is a typical instrumental analysis and the electron probe microanalyser has been greatly
improved to be user friendly. Obtaining accurate results with this powerful tool requires that it be
properly used. In order to obtain reliable data, however, optimum procedures must be followed. These
procedures, such as preparation of specimens, measurement of intensities of characteristic X-rays
and calculations of concentrations calculated from X-ray intensities, are given for use as standard
procedures in this International Standard.
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INTERNATIONAL STANDARD ISO 22489:2016(E)
Microbeam analysis — Electron probe microanalysis
— Quantitative point analysis for bulk specimens using
wavelength dispersive X-ray spectroscopy
1 Scope
This International Standard specifies requirements for the quantification of elements in a micrometre-
sized volume of a specimen identified through analysis of the X-rays generated by an electron beam
using a wavelength dispersive spectrometer (WDS) fitted either to an electron probe microanalyser or
to a scanning electron microscope (SEM).
This International Standard also describes the following:
— the principle of the quantitative analysis;
— the general coverage of this technique in terms of elements, mass fractions and reference specimens;
— the general requirements for the instrument;
— the fundamental procedures involved such as specimen preparation, selection of experimental
conditions, the measurements, the analysis of these and the report.
This International Standard is intended for the quantitative analysis of a flat and homogeneous bulk
specimen using a normal incidence beam. It does not specify detailed requirements for either the
instruments or the data reduction software. Operators should obtain information such as installation
conditions, detailed procedures for operation and specification of the instrument from the makers of
any products used.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 14594, Microbeam analysis — Electron probe microanalysis — Guidelines for the determination of
experimental parameters for wavelength dispersive spectroscopy
ISO 14595, Microbeam analysis — Electron probe microanalysis — Guidelines for the specification of
certified reference materials (CRMs)
ISO/IEC 17025:2005, General requirements for the competence of testing and calibration laboratories
3 Abbreviated terms
EPMA electron probe microanalyser
SEM scanning electron microscope
EDS energy dispersive spectrometer
PHA pulse height analyser
P/B peak-to-background ratio
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ISO 22489:2016(E)
4 Procedure for quantification
4.1 General procedure for quantitative microanalysis
4.1.1 Principle and procedure of quantitative microanalysis
The characteristic X-ray intensities from electron beam interactions with a solid are approximately
proportional to the mass fraction of the elements contained within the interaction volume. By
measurement of characteristic X-ray intensities, the mass fractions of the elements that compose a
specimen can be determined.
Quantitative analysis is performed by comparing the intensity of a characteristic X-ray line of an element
in the specimen with that from a reference material containing a known mass fraction of the element,
the measurements being performed under identical experimental conditions. The relationship between
intensity and mass fraction is not linear over a wide mass fraction range; correction calculations for
both specimen and reference material are therefore required.
X-ray absorption within the specimen and the reference material results in the emitted intensities being
less than the generated intensities; therefore, a correction is made for this. A correction is also made for
characteristic X-ray fluorescence in the analytical volume, and the effect of loss of X-ray production due
to electron backscattering. When electrons enter the specimen, they lose energy due to the interactions
with the constituent atoms. As well as being dependent on electron energy, the rate of energy loss is a
function of the mean atomic number. The matrix correction procedure, thus, has three components,
corresponding to the atomic number (Z), the absorption (A) and the characteristic fluorescence (F).
The accuracy of the quantitative analysis depends upon the selection of the reference materials, the
specimen preparation process, the measurement conditions/method, the stability and calibration of
the instrument, and the use of models for quantitative correction.
4.1.2 Coverage of the quantitative analysis
Reference materials and unknown specimens shall fulfil the following conditions:
— be stable under the action of the electron beam and stable in vacuum;
— have a flat surface perpendicular to the electron beam;
— be homogenous over the analysis volume;
— have no magnetic domains.
For the analysis volume, see ISO 14594 (analysis area and depth and volume).
It is possible to perform quantitative elemental analysis for elements with an atomic number greater
than or equal to 4 (beryllium).
The detection limit for quantitative analysis depends on many parameters, such as the X-ray line
selected, the matrix and the operating conditions (beam intensity, accelerating voltage and counting
parameters). It varies from a few parts per million (ppm) to a few hundred ppm.
NOTE 1 Detection limits are covered in ISO 17470.
NOTE 2 For light-element analysis or strong X-ray absorption conditions, the detection limit may be above 1 %
(i.e. B Kα in silicon matrix).
The accuracy obtainable is governed by the mass fraction of the element, the measurement conditions
and the correction calculation. It is generally considered that the relative precision and relative
accuracy for major elements can be better than 1 % and 2 %, respectively.
NOTE 3 For analysis of elements in a strongly absorbing matrix with a reference material not matched to the
specimen in composition, accuracy may be significantly worse than 2 %.
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ISO 22489:2016(E)
4.1.3 Selection of reference materials
The reference materials shall be in accordance with the specifications of ISO 14595.
In general, pure elements are used, but corrections for matrix effects are minimized when the
composition of the reference material is close to that of the unknown specimen.
When coating of the specimen is required (see 4.2), the reference material shall be coated under the
same conditions.
4.2 Specimen preparation
The specimens (reference specimen and unknown specimen) shall be clean and free of dust.
The specimen surface shall be flat. If necessary, the specimen shall be embedded in a conducting
medium and metallographically polished.
The specimen must have good electrical conductivity. Charging under electron beam irradiation can be
avoided by coating the specimen with a very thin conductive layer of a suitable material. A conducting
path shall be established between the specimen surface and the metallic specimen holder.
Carbon coating is generally used but, in particular cases (e.g. light-element analysis), other materials
should be considered (Au, Al, etc.). Carbon to a thickness of about 20 nm can be used.
It is recommended that both the reference material and unknown specimen be coated with the same
element at the same thickness.
4.3 Calibration of the instrument
4.3.1 Accelerating voltage
It is important to check that the accelerating voltage is correct for the quantitative analysis to be
accurate.
Quantification errors will occur if the accelerating voltage is not known accurately and if it is not stable.
The accelerating voltage shall therefore be calibrated and stable.
NOTE If an EDS system is attached to the EPMA, the true voltage may be determined through measurement
[15]
of the Duane-Hunt limit. If an EDS system is not attached, there is no generally available calibration method. It
is advisable to request that the manufacturer periodically checks the voltage values.
4.3.2 Probe current
Quantification errors will occur if the probe current is not known accurately and if its stability is low.
The probe current shall therefore be accurately monitored and stable.
The probe current is normally measured using a Faraday cup.
4.3.3 X-ray spectrometer
It is necessary to confirm the accurate adjustment of the X-ray spectrometer prior to its use for
measurement. This should be done for all spectrometers and all crystals by following the instructions
given by the manufacturer of the instrument.
The proportionality of the X-ray detector shall be checked.
NOTE The proportionality of the X-ray detector is covered in ISO 14594.
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ISO 22489:2016(E)
4.3.4 Dead time
It is necessary to correct for the loss of X-ray counts due to the counting-chain dead time. A dead-time
calibration curve shall be determined as specified in ISO 14594.
4.4 Analysis conditions
4.4.1 Accelerating voltage
The accelerating voltage, typically between 5 kV and 30 kV, shall be selected to meet the following
criteria:
— the accelerating voltage shall exceed 1,5 times the critical ionization energy of the most energetic
X-ray line used in the analysis;
— the volume to be analysed should be homogenous over a volume larger than that of the ionization
volume;
— the accelerating voltage shall not be so high as to induce heat or electrostatic damage or make large
absorption corrections necessary.
For every element, the measurements on the reference and unknown specimen should be performed
at the same accelerating voltage. In particular cases, however, it is possible to carry out quantitative
analysis using different accelerating voltages to optimize the X-ray intensities of elements in the same
energy range.
4.4.2 Probe current
The probe current shall be selected to meet the following criteria:
— the X-ray intensity shall be high enough for an accurate result to be obtained;
— the X-ray intensity shall not be so high that it saturates the X-ray detector;
— contamination and thermal and electrostatic damage shall be minimized.
The stability of the probe current shall be checked before making a measurement.
Glasses and some minerals (e.g. plagioclases) contain alkali metals such as Na, K, etc., which migrate
under a focused beam and they should therefore be analysed using a defocused beam.
4.4.3 Analysis position
If the instrument has an optical microscope, the feature requiring analysis should be positioned in the
centre of the optical field and the height of the specimen adjusted until it is in focus. In addition, the
operator shall ensure that the position of the probe is stable.
The focal point of the spectrometer shall be adjusted to be the same as the focal point of the optical
microscope, at the centre of the optical microscope and the centre of the electron image.
With vertically mounted spectrometers, the spectrometer sensitivity falls rapidly if the specimen
height is incorrect. Therefore, it is essential to use the instrument’s optical microscope because its
small depth of focus ensures that, when a sharp image is obtained, the specimen is correctly positioned.
With inclined spectrometers usually fitted to SEMs, the sensitivity is much less dependent upon vertical
variations and it is sufficient to locate the specimen to within 100 μm.
In an SEM/WDS having no optical microscope, one can proceed as follows. First, select a place in the
reference specimen (specimen holder) that is known to be at the focal point of the WDS at the analysis
working distance, then drive the holder to that working distance, select the secondary or back-scattered
electron imaging mode and bring the image into focus at fairly high magnification. Then, bring the
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ISO 22489:2016(E)
unknown specimen under the electron beam and focus the electron image by adjusting the height of the
specimen only.
4.4.4 Probe diameter
The probe diameter shall be as small as possible for accurate results while being consistent with the
aim of the analysis. The same probe diameter shall be used during the measurement on the reference
and the unknown specimen. If necessary, the probe diameter can be enlarged to prevent specimen
damage and to reduce contamination.
NOTE The probe diameter and analysis volume are covered in ISO 14594.
As alkali metals such as Na, K, etc., migrate under a focused electron beam, a defocused electron beam
should be used for analysis of these elements.
4.4.5 Scanning the focused electron beam
When wishing to analyse an area larger than the normal spot size, either enlarge the spot or use the
microscope in the scanning mode. If using the latter, the same procedures for spot analysis shall be
considered with the same limitations.
The area analysed should fall within the area of maximum sensitivity of the spectrometer. If the
scanning raster is too large, the spectrometer sensitivity will fall off at the extremes of the raster. Thus,
spot mode analysis is preferable for high accuracy.
4.4.6 Specimen surface
In quantitative microanalysis, the specimen surface shall be planar and perpendicular to the axis of
the electron beam. The specimen shall be polished so that it is as flat and scratch-free as possible. The
specimens (reference specimen and unknown specimen) shall be clean and free of dust. The specimen
shall be analysed in the unetched condition so as not to alter its topography or surface chemistry.
NOTE It is possible to perform a quantitative analysis on tilted specimens, if the correction model is
dedicated to this application and the tilt angle is accurately known.
4.4.7 Selection of X-ray line
In selecting the X-ray line to be used for the analysis, the instructions given hereafter shall be followed:
a) a peak with a high intensity and high P/B ratio shall be chosen;
b) a background shall be selected with which measurement of the continuum is possible;
c) whenever possible, the peak selected should be free of overlapping peaks.
If overlapping peaks cannot be avoided, the following instructions are useful:
— when the overlapping peaks are of higher order, the pulse height analyser should be operated to
eliminate these overlaps (for PHA operation, see ISO 17470 and ISO 14594);
— in the event of first-order overlap, a specific programme can be used for peak deconvolution; this
[9]
procedure can, however, influence the accuracy of the results.
4.4.8 Spectrometer
The spectrometer, the analysing crystal and the detector shall be selected according to the elements
and X-ray lines of analytical interest. This shall be done in accordance with the manufacturer’s
specifications unless extraordinary conditions make following those specifications inappropriate.
The analysing crystal should be selected by making use of data supplied by the instrument manufacturer
or from that obtainable from textbooks.
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ISO 22489:2016(E)
If the X-ray detector is equipped with an adjustable entrance slit, it should be set to the slit size
appropriate to the analytical problem. Light-element measurement may require the use of a wide slit,
whereas high-resolution X-ray spectroscopy measurements require the use of the narrowest slit.
4.4.9 Method for measurement of X-ray peak intensity
4.4.9.1 Wavelength position
For the measurement of X-ray peak intensity with an unknown specimen, the spectrometer shall be
positioned at the maximum intensity of the peak measured with the reference material.
In analysis of low-energy peaks (<1 keV), the position and the shape of the peak can be different with
an unknown specimen and with a reference material. In this case, an appropriate procedure shall be
applied (see Annex C).
4.4.9.2 Counting time
The counting times for the peak and the background are determined by the sensitivity, detection limit
and statistical accuracy required. All three can be improved by increasing the counting time. However,
there are restrictions due to beam damage and contamination.
For minor- and trace-element analysis, the counting time needs to be longer than that for major
elements, and the counting time for the peak and the background should be as close to equal as possible.
The counting time can be selected in order to reduce the statistical error in the counting process. Since
the distribution of the X-ray counts is Poissonian, the standard deviation (σ) of the distribution of N
counts is N .
Since N, the net counts, is derived from the number of counts for the peak (P) and that for the background
(B), the standard deviation for N counts (N = P − B) is
22
σσ=+() ()σ =+()PB
NP B
The term “beam damage” refers to element migration and carbon contamination. Counting time should
be adjusted to limit the consequences of element migration and contamination. For this reason, the
peak measurement should be done prior to the background measurement.
4.4.10 Method for measurement of background intensity
The procedures for background measurement given in ISO 14594 shall be followed.
4.5 Correction method based on analytical models
4.5.1 Principles
To a first approximation, the measured X-ray intensities are roughly proportional to the mass fractions
of the emitting elements as shown by Formula (1):
unk std unk std
kI=≅IC /C (1)
AA A A A
unk std
where I and I are the measured intensities of the characteristic X-ray emission of element A
A A
unk std
emitted from the unknown and the reference specimen, respectively. I and I correspond to the
A A
unk std
difference between the peak intensity and the background intensity. C and C are the mass
A A
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ISO 22489:2016(E)
fraction of element A in the unknown and in the reference specimens, respectively. k is the X-ray
A
intensity ratio usually called the k-ratio.
Formula (1) is only valid when both the unknown and the reference specimen have mass fractions and
atomic numbers close to each other. For common applications where these mass fractions may be quite
different, the measured intensities shall be corrected for matrix effects by terms ca
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