ISO 19463:2018

INTERNATIONAL ISO
STANDARD 19463
First edition
2018-07
Microbeam analysis — Electron probe
microanalyser (EPMA) — Guidelines
for performing quality assurance
procedures
Analyse par microfaisceaux — Analyse par microsonde électronique
(microsonde de Castaing) — Lignes directrices pour la mise en œuvre
des procédures d'assurance qualité
Reference number
©
ISO 2018
© ISO 2018
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Published in Switzerland
ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General principles of electron probe microanalyser quality assurance (EPMA QA) .6
4.1 Objective . 6
4.2 Selection of challenge materials. 6
4.2.1 General. 6
4.2.2 General characteristics of analysed materials . 6
4.2.3 Specific characteristics of challenge materials . 7
4.3 QA measurement parameters . 7
4.3.1 General. 7
4.3.2 Laboratory environment preparation . 7
4.3.3 Instrument parameters . 8
4.4 Data acquisition .11
4.5 Frequency of QA diagnostic testing .12
5 Test report .12
6 Data analysis and performance tracking .13
6.1 General .13
6.2 Quantitative analysis of the challenge material .13
6.3 Calculation of means and standard deviations .13
6.4 Statistical tests performed on data .13
6.4.1 General.13
6.4.2 Normality test . .14
6.4.3 Variance test .14
Annex A (informative) Examples of challenge materials and reference materials for EPMA
WDS QA .15
Annex B (informative) Distinguishing specimen preparation effects from instrument
malfunction .17
Annex C (informative) Graphical rendering of data and control charting .21
Annex D (informative) Failure modes indicated by test results .26
Bibliography .30
Foreword
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This document was prepared by Technical Committee ISO/TC 202, Microbeam analysis, Subcommittee
SC 2, Electron probe microanalysis.
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iv © ISO 2018 – All rights reserved

Introduction
This document was developed to provide a general method for operators of electron probe
microanalysers (EPMA) to perform the most complete and reliable instrument diagnostic routine
possible in the smallest amount of operator time, instrument time and analysis time. Performing this
procedure on their instruments at regularly scheduled intervals will allow the operator to track the
quality of an instrument’s elemental qualitative and quantitative performance, and alert the operator of
the need for instrument service and calibration shortly after it fails to meet its operating specifications
for measurement uncertainty. With equal application of this document to the diagnostics procedure
of multiple instruments in a single laboratory, or even multiple instruments managed by different
operators in separate laboratories, analysis results can be normalized between instruments using the
performance comparison, facilitating analytical reproducibility.
The chief product of an analytical laboratory quality assurance (QA) program, ultimately, is confidence
– confidence that the analysis of any specimen sent to any laboratory participating in the program
will be consistent, correct within tolerance and interchangeable with equivalent analyses of related
specimens performed by any other laboratory in the program. In order to maximize confidence, the QA
tests and test materials chosen should evaluate the broadest possible range of instrument functionality.
In the context of EPMA, this means testing not only the stability of the electron gun and the function of
the photon counters, but also the functionality of every component of each wavelength spectrometer
mounted to the system. This includes the numerous types of diffracting crystals that disperse the
X-rays, the mechanical components that switch the spectrometer from one crystal to another, and the
drive mechanisms that scan the crystal through a spectral region of interest. Since these spectrometer
components can fail independently of the others, and many such failures will not be noticeable in all
measurements, a complete QA test will include materials that generate X-ray lines that span the range of
any diffracting crystal and methods to properly analyse them. It will therefore generate the maximum
possible information on the instrument’s functional integrity. From this information, instrument
performance can be optimized, thereby obtaining maximum analytical confidence. The procedures and
reference material attributes outlined in this document are designed to achieve these goals.
INTERNATIONAL STANDARD ISO 19463:2018(E)
Microbeam analysis — Electron probe microanalyser
(EPMA) — Guidelines for performing quality assurance
procedures
1 Scope
This document provides guidelines for performing routine diagnostics and quality assurance procedures
on electron probe microanalysers (EPMA). It is intended to be used periodically by an instrument’s
operator to confirm that the instrument is performing optimally, and to aid in troubleshooting if it is
not. It covers the properties of reference materials required and the analysis procedures necessary to
independently test and fully evaluate the functionality of the main components of an EPMA system.
The analytical procedure described herein is distinct from single-element diagnostic procedures, which
can be performed more rapidly. Such procedures are valid for the diffractor position and conditions
under which the test is performed, whereas the procedure described herein is intended to qualify an
instrument’s capabilities for exploratory analysis of unknowns, trace analysis and non-routine work
(such as peak interferences).
This document is applicable to EPMA and other wavelength dispersive spectrometer (WDS) systems in
which elemental identification and quantification are performed by analysis of the energy and intensity
of the characteristic X-ray lines observed in wavelength-dispersed X-ray spectra. It is not directly
applicable to elemental analysis using energy dispersive spectrometry (EDS).
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 3534-2, Statistics — Vocabulary and symbols — Part 2: Applied statistics
ISO 14595, Microbeam analysis — Electron probe microanalysis — Guidelines for the specification of
certified reference materials (CRMs)
ISO 22489, Microbeam analysis — Electron probe microanalysis — Quantitative point analysis for bulk
specimens using wavelength dispersive X-ray spectroscopy
ISO 23833, Microbeam analysis — Electron probe microanalysis (EPMA) — Vocabulary
ISO/IEC Guide 99, International vocabulary of metrology — Basic and general concepts and associated
terms (VIM)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/IEC Guide 99, ISO 3534-2,
ISO 23833 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
electron probe microanalyser
EPMA
instrument for carrying out electron-excited X-ray microanalysis
Note 1 to entry: This instrument is usually equipped with more than one wavelength spectrometer and an optical
microscope for precise specimen placement.
[SOURCE: ISO 23833:2013, 3.2]
3.1.1
electron probe microanalysis
EPMA
technique of spatially-resolved elemental analysis based upon electron-excited X-ray spectrometry
with a focussed electron probe and an electron interaction volume with micrometer to sub-micrometer
dimensions
[SOURCE: ISO 23833:2013, 3.1]
3.2
wavelength dispersive spectrometer
WDS
device for determining X-ray intensity as a function of the wavelength of the radiation, where separation
is based upon Bragg's law, nλ = 2dsinθ, where n is an integer, λ is the X-ray wavelength, d is the spacing
of the atom planes of the diffracting crystal or the repeated layers of a synthetic diffractor and θ is the
angle at which constructive interference takes place
Note 1 to entry: This definition excludes the recent technological development of WDS spectrometers based on
diffraction at gratings, which are not as yet in widespread use.
[SOURCE: ISO 23833:2013, 4.6.14, modified — Note 1 to entry replaced.]
3.3
diffracting crystal
dispersion element
X-ray scattering element in a wavelength-dispersive X-ray spectrometer, consisting of a periodic array
of atoms obtained either in a natural crystal or in a synthetic multilayer
Note 1 to entry: For the purposes of this document, the term “diffracting crystal” is used rather than the term
“dispersion element” in order to avoid confusion when discussion of components of X-ray energy analysers is
intermingled with discussion of chemical elements from the periodic table.
[SOURCE: ISO 23833:2013, 4.6.14.3, modified — Note 1 to entry has been added.]
3.3.1
lithium fluoride
LiF
[4]
diffracting crystal featuring 2d spacing of 0,402 8 nm used in WDS for dispersion of X-rays
Note 1 to entry: This can also sometimes be written as LiF(200) to denote the most common crystallographic
orientation of LiF used. However, it is also available in other less commonly used orientations that feature
different 2d spacings; for example, the [220] orientation has a 2d spacing of 0,284 8 nm. Additionally, some
instruments could utilize LiF in the [422] or the [420] orientation. If the orientation is not stated, the [200]
orientation is assumed. All orientations are typically used to disperse short wavelength/high energy X-rays.
3.3.2
pentaerythritol
PET
[4]
diffracting crystal featuring 2d spacing of 0,874 2 nm used in WDS for dispersion of X-rays
Note 1 to entry: PET is typically used to disperse intermediate wavelength/intermediate energy X-rays.
2 © ISO 2018 – All rights reserved

3.3.3
thallium acid phthalate
TAP
[4]
diffracting crystal featuring 2d spacing of 2,59 nm used in WDS for dispersion of X-rays
Note 1 to entry: TAP is typically used to disperse long wavelength/low energy X-rays.
3.3.4
layered synthetic microstructure
multilayer diffracting element engineered to feature an arbitrary 2d spacing used in WDS for dispersion
of X-rays
Note 1 to entry: layered synthetic microstructure is typically used to disperse long wavelength/low energy
X-rays in the light element region of the spectrum inaccessible by TAP.
3.4
peak energy
peak wavelength
spectrometer position or channel at which the characteristic peak intensity is measured
Note 1 to entry: Due to X-ray counts originating from higher-order Bragg reflections, both of these terms
describe the measurand but not the actual measurement; an EPMA instrument counts X-rays from the higher-
order reflections and the principle first-order reflection simultaneously. Pulse filtering electronics can be used
to preferentially distinguish X-rays at the wavelength or energy of interest; in practice, such strategies reduce
but do not eliminate spurious counts.
3.5
peak counting time
time spent measuring X-ray emission at a given characteristic peak energy
3.6
peak counting rate
mean rate at which characteristic peak X-rays are collected by the detector at the peak energy
3.7
background reference
background reference energy
background reference wavelength
spectrometer position or channel at which the continuous background radiation is measured so that
an estimate can be made of what portion of the measured intensity at a characteristic peak originates
from characteristic photoemission
Note 1 to entry: Multiple background positions are typically chosen to improve the estimate; often, at least one
on each side of the characteristic peak of interest.
Note 2 to entry: Due to X-ray counts originating from higher-order Bragg reflections, both of these terms
describe the measurand but not the actual measurement; an EPMA instrument counts X-rays from the higher-
order reflections and the principle first-order reflection simultaneously. Pulse-filtering electronics can be used
to preferentially distinguish X-rays at the wavelength or energy of interest; in practice, such strategies reduce
but do not eliminate spurious counts.
3.8
background counting time
time spent measuring X-ray emission at a given background energy
3.9
background counting rate
mean rate at which continuum X-rays are collected by the detector at the background energy
Note 1 to entry: The background counting rate is used to estimate the portion of peak counts due to continuum
X-rays; this estimate may be derived by interpolation, extrapolation, or comparison to the background rate
generated by a selection of materials characterized by a range of mean atomic numbers.
3.10
beam defocus
condition in which the objective lens of the electron optical column is set such that the size of the
incidence of the electron beam on the surface of the specimen (the “beam spot”) is expanded to a
diameter greater than the diameter of the focal point
Note 1 to entry: Increasing the spot size is a technique used to compensate for specimen heterogeneity when
performing a quantitative analysis or to reduce the damage caused to a beam-sensitive specimen by distributing
the electron dose over a greater volume.
3.11
quality assurance
QA
procedure by which standard measurements of model materials are
performed on a periodic basis to confirm that each component of the electron probe microanalyser is
functioning such that the instrument’s uncertainty specification is attainable
3.12
confidence interval
range of analytical error expected to contain the true value with a stated uncertainty as estimated
from a statistical model of the measurement process
[SOURCE: ISO 23833:2013, 5.4.2.1]
3.13
error
natural deviation from the true value in a measured quantity arising from (1) random counting
fluctuations in a time-distributed phenomenon (e.g. X-ray photons) and (2) systematic deviations
from the true value introduced during application of calculated correction factors (e.g. ZAF matrix
correction factors) to convert the measured quantity (e.g. X-ray photons) to a different dimension (e.g.
concentration)
[SOURCE: ISO 23833:2013, 5.4.2]
3.14
uncertainty
quantitative statement that provides a value for the expected deviation of a measurement from an
estimate of the value of the specific measured quantity
[SOURCE: ISO 23833:2013, 5.5.13]
3.15
detection limit
smallest amount of an element or compound that can be measured under specific analysis conditions
Note 1 to entry: By convention, the detection limit is often taken to correspond to the amount of material for
which the total signal for that material minus the background signal is three times the standard deviation of the
signal above the background signal. This convention might not be applicable to all measurements and, for a fuller
discussion of detection limits, Reference [11] should be consulted.
Note 2 to entry: The detection limit may be expressed in many ways depending on the purpose. Examples of
expressions are mass or weight fraction, atomic fraction, concentration, number of atoms, and mass or weight.
Note 3 to entry: The detection limit will generally be different for different materials.
[SOURCE: ISO 23833:2013, 5.2]
3.16
instrument uncertainty specification
manufacturer’s estimate of the lowest uncertainty attainable by a
given instrument based upon physical limitations and construction
4 © ISO 2018 – All rights reserved

3.17
control chart
chart on which some statistical measure of a series of samples is plotted in a particular order to steer
the process with respect to that measure and to control and reduce variation
[SOURCE: ISO 3534-2:2006, 2.3.1, modified — Notes 1 and 2 to entry have been removed.]
3.17.1
mean and standard deviation plot
mean and range plot
x̅ and R plot
graphical representation of a set of measurements that plots the data means in relation to a certified or
targeted value and also plots the standard deviations in relation to a control limit
Note 1 to entry: Mean and standard deviation plots can be used as an aid in determining when and how the data
no longer attains the instrument uncertainty specification.
3.17.2
box-and-whisker plot
box plot
graphical representation of a set of measurements that plots the data along with the data mean, median,
inner quartiles (“box”) and a chosen outlier delimiter (e.g. standard deviation, outer quartiles or other
“whiskers”)
Note 1 to entry: Box plots can be used as an aid in determining when and how the data no longer attains the
instrument uncertainty specification.
3.17.3
bean plot
density plot
graphical representation of a set of measurements that plots the data along with the data mean and a
density function
Note 1 to entry: Bean plots can be used as an aid in determining when and how the data no longer conforms to
the instrument uncertainty specification.
3.18
failure mode
observable deviation from a normal data distribution within the
instrument uncertainty specification that is symptomatic of a specific instrument malfunction
3.19
reference material
RM
material, sufficiently homogeneous and stable with reference to specified properties, which has been
established to be fit for its intended use in measurement or in examination of nominal properties
Note 1 to entry: For electron probe microanalysis, a material whose overall composition is known from
independent, ideally absolute, measurements (e.g. separations and gravimetric analysis) and that is
microscopically homogeneous on a sufficiently fine scale that any location measured with an electron probe
microanalyser produces the same X-ray intensities, within counting statistics.
[SOURCE: ISO/IEC Guide 99:2007, 5.13, modified — Note 1 to entry has been added.]
3.19.1
certified reference material
CRM
reference material (3.19)accompanied by documentation issued by an authoritative body and providing
one or more specified property values with associated uncertainties and traceabilities, using valid
procedures
Note 1 to entry: For certified reference materials for electron probe microanalysis, the microscopic heterogeneity
at the micrometer scale is certified as well as the composition.
[SOURCE: ISO/IEC Guide 99:2007, 5.14, modified — Note 1 to entry has been added.]
3.19.1.1
challenge material
certified reference material (3.19.1) of known composition that is measured as if it were an unknown
sample in the EPMA QA procedure
Note 1 to entry: Challenge materials are selected by the analyst to present an analytical challenge to specific
components of an EPMA instrument. Ideally, challenge materials that present an analytical challenge to as many
components of a given instrument as possible should be selected.
4 General principles of electron probe microanalyser quality assurance
(EPMA QA)
4.1 Objective
When performing analysis of unknown specimens in EPMA, it is crucial for the analyst to know that
their instrument is working properly. Herein is described a procedure that should be performed
periodically to ensure that analyses performed using EPMA are reliable. The procedure is built upon
the analysis of challenge materials.
4.2 Selection of challenge materials
4.2.1 General
Challenge materials and their associated reference materials shall be selected such that they conform
to the criteria outlined in the following subsections.
4.2.2 General characteristics of analysed materials
Challenge materials and their associated reference materials shall meet the requirements for certified
reference materials as described in ISO 14595. The materials shall:
a) be stable in vacuum;
b) not degrade under interrogation by the electron beam incidence;
c) be characterized by heterogeneity sufficiently less than the instrument’s repeatability specification
so as to be indistinguishable from the instrument uncertainty specification;
d) be suitably conductive to eliminate electrostatic charging under electron beam interrogation (or be
coated with conductive material with a path to instrument ground);
Many types of solids meet these criteria, including a number of pure elemental solids, single-phase
alloys, vitreous solids such as glass, natural or synthetic minerals, and pure compounds.
6 © ISO 2018 – All rights reserved

4.2.3 Specific characteristics of challenge materials
The purpose of a challenge material is to provide an analytical challenge for an instrument that
requires the proper function of as many instrument components as possible. Therefore, a challenge
material should be sufficiently complex such that each diffracting crystal on every WDS in a given
EPMA can be used to quantify at least one of the elements of which it is composed. For multi-crystal
WDS spectrometers, diffracting crystal switching should be required.
The challenge material should also be sufficiently simple to analyse such that deconvolution of peak
overlaps and large absorption or fluorescence corrections are not required to calculate the composition.
Secondary standard reference materials should not be necessary to achieve an accurate result. Finally,
the elements of interest for evaluating the performance of a given spectrometer should be present in
sufficient concentration that uncertainties associated with concentrations approaching the detection
limit are not a factor.
In summary, challenge materials should possess the following characteristics, in addition to those
identified in 4.2.2:
a) For evaluation of a given diffracting crystal, the challenge material shall contain at least two
elements that emit characteristic X-rays that fall within the diffracting crystal’s analytical range.
Alternatively, a single element is allowable if it emits two well-separated characteristic X-rays that
can be independently measured using a single diffracting crystal during the same analysis (e.g. Kα
and Kβ transitions for certain high-Z elements), after which the composition of the element shall
be evaluated for each X-ray line separately. A single element is also allowable if it emits a single
characteristic X-ray that can be measured using at least two different Bragg reflections (e.g. first
order and second order reflections) on the same diffracting element (see Annex A for examples).
b) The characteristic X-rays analysed for each element in the challenge material should overlap
minimally with any other characteristic X-rays emitted by any other matrix element and with any
absorption edge of any matrix element.
c) The composition of each analysed element in the challenge material should be greater than 1 %.
d) Analysis of X-ray lines whose emission energies depend upon chemical bonding effects shall
be avoided; for example, the Lα lines of third row transition elements. For further details, see
Reference [10].
4.3 QA measurement parameters
4.3.1 General
Upon specification of the challenge material best suited to use in the evaluation of a given instrument’s
array of spectrometers, and the selection of reference materials best suited to quantitatively analyse
the challenge material, a procedure for performing the analysis can be specified. This procedure should
be optimized such that a maximum of diagnostic information about the instrument is collected in a
minimum amount of time.
4.3.2 Laboratory environment preparation
The long-term stability of the laboratory environment affects instrument stability. In particular,
temperature fluctuations can affect the beam current stability and the diffracting crystal lattice
constants, thereby contributing to measurement uncertainty. Ideally, room temperature should be
held constant for the duration of the QA diagnostic, and should be unchanged between diagnostic tests.
The room temperature shall be monitored during the QA diagnostic; at minimum, it may be recorded
both before and after each QA diagnostic is performed. If neither the temperature nor its fluctuation
conforms to the limits allowed by the instrument’s installation specification, the test results shall be
deemed invalid and the laboratory environment shall be stabilized before repeating the test.
4.3.3 Instrument parameters
4.3.3.1 General
The instrument parameters identified below are required to perform a diagnostic evaluation of the
EPMA electron gun and WDS X-ray spectrometers. Upon selection of the characteristic X-ray lines to
be analysed and the spectrometers with which the analysis is to be performed, the greatest value can
be extracted from a QA diagnostic experiment if the instrument is finely tuned ahead of time to make
the best possible measurement. This fine-tuning includes confirming and readjusting the X-ray peak
centres on the diffracting crystals (frequently referred to as “peaking” the spectrometers) and fine-
tuning the X-ray counter electronics discriminator settings to maximize noise rejection and higher-
order coincidence peak rejection while still collecting the entirety of the signal X-rays. See ISO 14594
for complete details of these procedures.
4.3.3.2 Accelerating voltage
The electron beam energy, or accelerating voltage, shall be set such that it exceeds the excitation energy
for the highest characteristic X-ray to be measured during the QA diagnostic by an overvoltage factor
of at least 1,5. While higher overvoltage is acceptable, there is usually little benefit to exceeding the
highest excitation energy by a factor of three. The beam energy should be held constant for the duration
of the diagnostic. Ideally, it should be set where future measurements are expected to be made using
this instrument.
The operator may choose to optimize accelerating voltage for analysis of the system by plotting the
measured X-ray intensity versus accelerating voltage for both the highest energy characteristic X-ray
line and the selected characteristic line emitted by the element with the lowest concentration in the
challenge material. The accelerating voltages corresponding to the highest X-ray intensity for each of
the curves establish the limits for the range in which the ideal accelerating voltage for analysing this
system lies. It might not, however, necessarily correspond to the voltage that is expected to be used for
typical measurements on a given instrument.
Should the operator desire performance data for multiple accelerating voltages, a separate QA diagnostic
should be planned for each. While using multiple accelerating voltages to perform a single analysis can
be advantageous in some circumstances, maintaining a single accelerating voltage is necessary for this
exercise in order to assess electron beam stability.
If electron beam damage at higher beam energy is a problem for either the challenge material or for the
selected reference materials, this should also be considered when selecting the accelerating voltage.
For low overvoltage or soft X-rays, carbon contamination and surface oxidation should be avoided as
much as possible.
4.3.3.3 Probe current
The probe, or electron beam, current shall be set such that the resulting X-ray count rate falls within the
proportional counting range for every characteristic X-ray to be analysed from the challenge material
and any reference material during the QA diagnostic. Ideally, this current setting will be consistent
with the setting at which future measurements are expected to be made. The beam current should be
held constant for the duration of the diagnostic measurement. Failure to confirm that the count rate is
within proportional counting range for every specimen to be analysed risks the possibility of errors
due to detector dead time discrepancies or to counts falling outside of the pulse height analyser (PHA)
acceptance window.
If electron beam damage at higher current is a problem for either the challenge material or for the
selected reference materials, this fact should also be considered when setting the probe current.
Damage caused by high current dose may also be mitigated by defocusing the beam (and thus the beam
dose per unit volume) if reducing the beam current itself reduces the X-ray count rate to unacceptable
levels. Beam defocus can also mitigate uncertainties arising from specimen heterogeneity on the scale
8 © ISO 2018 – All rights reserved

of the size of the beam incidence. See Annex B for further details. The beam defocus shall not exceed the
acceptance angle of the spectrometer.
The operator might find it convenient to collect a full spectrum wave scan using each spectrometer
and diffracting crystal selected (see 4.3.3.4) on all reference material and challenge material, and then
adjust the probe current to optimize the count rate for the most intense line to be analysed.
Should the operator desire performance data at multiple probe currents, a separate QA diagnostic
should be planned for each. While using multiple probe currents to perform a single analysis can be
advantageous in some circumstances, maintaining a single probe current is necessary for this QA
diagnostic exercise to properly assess electron beam stability.
4.3.3.4 Spectrometer and diffracting crystal selection
Many electron probe microanalysis instruments feature considerable built-in spectrometer redundancy,
i.e. multiple spectrometers equipped with identical diffracting crystals. Although this feature affords
the analyst considerable versatility in approaches to a given analysis, it also makes it nearly impossible
for most configurations to be tested in their entirety during a single diagnostic QA.
EXAMPLE 1 An EPMA instrument configured with five spectrometers each equipped with two diffracting
crystals would require a challenge material composed of at least 20 elements each at greater than 1 % mass
fraction in order to test each diffracting crystal, each Rowland circle mechanical translation and each diffracting
crystal exchange motion.
Given this limitation, the operator should select diffracting crystals and spectrometers for the QA
diagnostic that are most likely to be used in future measurements. If the diffracting crystals and
spectrometers to be tested exceed what is required to properly analyse the challenge material, the
operator should schedule multiple iterations of the QA diagnostic that sequentially tests a subset of the
most necessary components.
EXAMPLE 2 An operator wishes to perform a QA diagnostic on an EPMA instrument configured with two
spectrometers each equipped with TAP and lead stearate diffracting crystals, and two spectrometers each
equipped with PET and LiF diffracting crystals. The challenge material selected for the diagnostic is an alloy
composed of six elements: three pairs that can be analysed using TAP, PET and LiF, respectively. The operator
determines that for upcoming analyses all spectrometers will be required, but it is unlikely that any unknown
will necessitate a diffracting crystal change on any spectrometer. Given the operator’s expectations, it is decided
that the QA diagnostic will be run twice – once for each TAP spectrometer, with the PET and the LiF on the
remaining spectrometers alternated between runs. Using such a procedure, the functionality of all four X-ray
counters, all diffracting crystals except the lead stearate, and all diffracting crystal translation mechanisms
on each Rowland circle are tested, while none of the mechanisms responsible for changing between diffracting
crystals on the same spectrometer are diagnosed.
4.3.3.5 Peak and background energy selection
The measured energy for each analysed X-ray line shall be selected to coincide with the most probable
maximum intensity of that line as determined by the spectrometer peaking procedure employed.
For linear interpolation of the background intensity at the peak energy, the background reference
energies should be selected such that one energy is chosen on the higher and one on the lower energy
sides of each analysed X-ray line. The selected background energies shall be chosen such that they are not
coincident (within the specified uncertainty of the instrument) with potential sources of interference,
such as other characteristic X-ray lines or photoabsorption edges of the specimen or detector. To
minimize the uncertainty in the background intensity, the background energies should be selected such
that they are as far as possible from the peak energy, yet not so far that the continuum intensity profile
between the background energies no longer approximates a straight line. Furthermore, the product of
the background intensity and the distance, in spectrometer units, from the peak energy should ideally
be equal for both background energies chosen.
Performing a wave scan of an energy range from the low background through the peak to the high
background energy is suggested to best evaluate the background energy choices.
4.3.3.6 Selecting the analysis positions on the challenge material and the reference material
The analysis positions on all specimens measured during the diagnostic shall be located on polished,
clean, conductively coated (if necessary) and large (more than two electron beam interaction volume
diameters, and more than the mean free path length of fluorescing X-rays, away from any edge of a
single-phase homogeneous region of the specimen) regions of the material. At least seven randomly
selected positions separated by at least twice the electron beam interaction volume diameter shall be
measured on all reference material and challenge material to ensure that the 90 % confidence interval
is subsumed as in Formula (1).
I ±2 σ (1)
where
I is the mean measured X-ray intensity expressed in total counts;
σ is the standard deviation of the measurements.
More positions may be selected if more rigorous statistics are desired; consult ISO/IEC Guide 98-
3:2008, Annex G, Table G.2 to estimate the number of measurements required to achieve the desired
precision.
4.3.3.7 Selecting the optimum X-ray counting times
4.3.3.7.1 General
Once the specimens have been selected, the instrument parameters appropriate for their complete
analysis have been determined, and the instrument spectrometers have been properly peaked and
tuned, optimized X-ray counting times will enable the operator to obtain the maximum actionable
diagnostic information from the EPMA instrument in a minimum of instrument and analysis time.
4.3.3.7.2 Peak counting time
In 4.3.3.3, an electron probe current setting was selected to generate an acceptable X-ray count rate
for all analysed lines to be measured during a given QA diagnostic. The optimum X-ray peak counting
time, in turn, is governed by this selected beam current and by the repeatability specification of the
instrument component to be tested. The peak counting time shall equal or exceed the time required
to collect a total number of counts above background that equals or exceeds the total counts needed
to match the instrument’s repeatability uncertainty specification. Assuming that Poisson counting
statistics govern the measurement uncertainty, the target peak counting time, in seconds, is as in
Formula (2).
rr+
PB
t ≥ (2)
P
ur×−r
()
spec PB
where
t is the target peak counting time, in seconds;
P
u is the instrument’s uncertainty specification expressed as a decimal (e.g. 1 % is expressed
spec
as 0,01);
r is the expected mean peak counting rate in counts/second;
P
r is the interpolated background counting rate in counts/second.
B
The higher the peak counting time, the greater the likelihood of introducing uncertainties unrelated
to Poisson counting statistics, such as beam damage or beam-induced contamination, which would
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invalidate this procedure for estimating it. For this reason, specimens with high peak counting rates for
an element o
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