ISO 18227:2014
(Main)Soil quality — Determination of elemental composition by X-ray fluorescence
Soil quality — Determination of elemental composition by X-ray fluorescence
ISO 18227:2014 specifies the procedure for a quantitative determination of major and trace element concentrations in homogeneous solid waste, soil, and soil-like material by energy dispersive X-ray fluorescence (EDXRF) spectrometry or wavelength dispersive X-ray fluorescence (WDXRF) spectrometry using a calibration with matrix-matched standards. ISO 18227:2014 is applicable for the following elements: Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Ag, Cd, Sn, Sb, Te, I, Cs, Ba, Ta, W, Hg, Tl, Pb, Bi, Th, and U. Concentration levels between approximately 0,000 1 % and 100 % can be determined depending on the element and the instrument used.
Qualité du sol — Détermination de la composition élémentaire par fluorescence X
General Information
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 18227
First edition
2014-03-01
Soil quality — Determination of
elemental composition by X-ray
fluorescence
Qualité du sol — Détermination de la composition élémentaire par
fluorescence X
Reference number
©
ISO 2014
© ISO 2014
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
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ii © ISO 2014 – All rights reserved
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Safety remarks . 3
5 Principle . 3
6 Apparatus . 3
7 Reagents . 4
8 Interferences and sources of error . 5
9 Sample preparation . 5
9.1 General . 5
9.2 Drying and determination of dry mass . 6
9.3 Preparation of pressed pellet . 6
9.4 Preparation of fused beads . 6
10 Procedure. 7
10.1 Analytical measurement conditions . 7
10.2 Calibration . 8
11 Quality control .13
11.1 Drift correction procedure .13
11.2 Blank test .14
11.3 Reference materials .14
12 Calculation of the result .14
13 Test report .14
Annex A (informative) Semi-quantitative screening analysis of waste, sludge, and soil samples .15
Annex B (informative) Examples for operational steps of the sample preparation for soil and
waste samples .18
Annex C (informative) Suggested analytical lines, crystals, and operating conditions .23
Annex D (informative) List of reference materials applicable for XRF analysis .26
Annex E (informative) Validation .28
Bibliography .37
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
committee has been established has the right to be represented on that committee. International
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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
constitute an endorsement.
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 WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 190, Soil quality, Subcommittee SC 3, Chemical
methods and soil characteristics.
iv © ISO 2014 – All rights reserved
Introduction
X-ray fluorescence spectrometry is a fast and reliable method for the quantitative analysis of the total
content of certain elements within different matrices.
The quality of the results obtained depends very closely on the type of instrument used, e.g. bench top or
high performance, energy dispersive or wavelength dispersive instruments. When selecting a specific
instrument, several factors have to be considered, such as the matrices to be analysed, the elements
to be determined, the detection limits required, and the measuring times. The quality of the results
depends on the element to be determined and on the surrounding matrix.
Due to the wide range of matrix compositions and the lack of suitable reference materials in the case
of inhomogeneous matrices such as waste, it is generally difficult to set up a calibration with matrix-
matched reference materials.
Therefore, this International Standard describes two different procedures:
— a quantitative analytical procedure for homogeneous solid waste, soil, and soil-like material in the
normative part. The calibration is based on matrix-matched standards;
— an XRF screening method for solid and liquid materials as waste, sludge, and soil in Annex A which
provides a total element characterization at a semi-quantitative level. The calibration is based on
matrix-independent calibration curves, previously set up by the manufacturer.
The technical content of this International Standard is identical with the European Standard
EN 15309:2007.
INTERNATIONAL STANDARD ISO 18227:2014(E)
Soil quality — Determination of elemental composition by
X-ray fluorescence
1 Scope
This International Standard specifies the procedure for a quantitative determination of major and trace
element concentrations in homogeneous solid waste, soil, and soil-like material by energy dispersive
X-ray fluorescence (EDXRF) spectrometry or wavelength dispersive X-ray fluorescence (WDXRF)
spectrometry using a calibration with matrix-matched standards.
This International Standard is applicable for the following elements: Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr,
Mn, Fe, Co, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Ag, Cd, Sn, Sb, Te, I, Cs, Ba, Ta, W, Hg, Tl, Pb, Bi, Th,
and U. Concentration levels between approximately 0,000 1 % and 100 % can be determined depending
on the element and the instrument 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 11464, Soil quality — Pretreatment of samples for physico-chemical analysis
ISO 11465, Soil quality — Determination of dry matter and water content on a mass basis — Gravimetric
method
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
EN 14346:2006, Characterization of waste — Calculation of dry matter by determination of dry residue or
water content
EN 15002:2006, Characterization of waste — Preparation of test portions from the laboratory sample
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
NOTE See References [11] and [14] for non-specified terms.
3.1
absorption edge
jump of the mass absorption coefficient at a specific wavelength or energy
3.2
absorption of X-rays
loss of intensity of X-rays by an isotropic and homogenous material as described by the Bouger-Lambert
law
3.3
analytical line
specific characteristic X-ray spectral line of the atom or ion of the analyte used for the determination of
the analyte content
3.4
continuous radiation
electromagnetic radiation produced by the acceleration of a charged particle, such as an electron, when
deflected by another charged particle, such as an atomic nucleus
3.5
Compton-line
spectral line due to incoherent scattering (Compton-effect) occurring when the incident X-ray photon
strike an atom without promoting fluorescence
Note 1 to entry: Energy is lost in the collision and therefore the resulting scattered X-ray photon is of lower energy
than the incident X-ray photon.
3.6
drift correction monitor
physically stable sample used to correct for instrumental drift
3.7
emitted sample X-ray
radiation emitted by a sample consisting of X-ray fluorescence radiation and scattered primary X-rays
3.8
fused bead
analyte sample prepared by dissolution in a flux
3.9
liquid sample
analyte sample submitted as a solution for direct measurement in the sample cup
3.10
mass absorption coefficient
constant describing the fractional decrease in the intensity of a beam of X-radiation as it passes through
an absorbing medium
Note 1 to entry: This is expressed in cm /g.
Note 2 to entry: The mass absorption coefficient is a function of the wavelength of the absorbed radiation and the
atomic number of the absorbing element.
3.11
polarized excitation X-ray spectrometer
energy dispersive X-ray spectrometer where the excitation is performed by polarized radiation and the
emitted X-ray fluorescence radiation is detected along the direction of polarization
3.12
powder sample
analyte sample submitted as a powder for direct measurement in the sample cup
3.13
precision
closeness of agreement of results obtained by applying the method several times under prescribed
conditions
[SOURCE: ISO 5725-2:1994]
3.14
pressed pellet
analyte sample prepared by pressing milled material into a disk
3.15
primary X-ray
X-ray by which the sample is radiated
2 © ISO 2014 – All rights reserved
3.16
quality control sample
stable sample with known contents, e.g. certified reference material (CRM), used to monitor instrument
and calibration performance
3.17
X-ray fluorescence radiation
emission of characteristic X-rays from a sample that has been bombarded by high-energy X-rays or
gamma rays
4 Safety remarks
Anyone dealing with waste and sludge analysis has to be aware of the typical risks that this kind of
material presents irrespective of the parameter to be determined. Waste and sludge samples can
contain hazardous e.g. toxic, reactive, flammable, and infectious substances, which could potentially
undergo biological and/or chemical reaction. Consequently, it is recommended that these samples
should be handled with special care. The gases that can be produced by microbiological or chemical
activity are potentially flammable and pressurize sealed bottles. Bursting bottles are likely to result in
hazardous shrapnel, dust, and/or aerosol. National regulations should be followed with respect to all
hazards associated with this method.
The X-ray fluorescence spectrometer shall comply with international and national regulations relevant
to radiation protection.
The person responsible for managing or supervising the operation of X-ray equipment shall provide
evidence of his knowledge of radiation protection according to national regulations.
5 Principle
After a suitable preparation, if necessary, the sample is introduced into an XRF spectrometer and excited
by primary X-rays. The intensities of the secondary fluorescent energy lines specific for each element
are measured and the elemental composition of the sample is determined by reference to previously
established calibration graphs or equations and applying corrections for inter-element effects. The
calibration equations and inter-element corrections are established using pure reagents and/or series
of internal or reference materials providing they meet all the requirements of the relevant preparation
technique.
6 Apparatus
6.1 X-ray fluorescence spectrometer, shall be able to analyse the elements according to the scope of
this International Standard.
The following types of X-ray fluorescence spectrometers are applicable:
— energy dispersive X-ray fluorescence (EDXRF) spectrometer that achieves the dispersion of the
emitted X-ray fluorescence radiation by an energy dispersive detector;
— wavelength dispersive X-ray fluorescence (WDXRF) spectrometer that achieves the dispersion of
the emitted X-ray fluorescence radiation by diffraction by a crystal or a synthetic multilayer.
The spectrometer consists of a number of components:
— primary X-ray source, an X-ray tube with a high-voltage generator;
— a sample holder;
— detector unit including electronic equipment;
— source modifiers to modify the shape or intensity of the source spectrum or the beam shape (such
as source filters, secondary targets, polarizing targets, collimators, focusing optics, etc.).
The detector unit is different for WDXRF and for EDXRF spectrometers. WDXRF spectrometers take
advantage of the dispersion of the emitted radiation by scattering by a crystal or a synthetic multilayer.
The detector does not need to be capable of energy discrimination. EDXRF spectrometers use an
energy dispersive detector. Pulses of current from the detector, which are a measure of the energy of
the incoming X-rays, are segregated into channels according to energy using a multi-channel analyser
(MCA).
NOTE 1 The use of a high-energy X-ray tube increases the potential for losses of volatile analytes from samples
by heating in the spectrometer during analysis.
NOTE 2 The new generation of EDXRF spectrometers takes advantage of the polarizing target theory resulting
in a significant decrease of the background scattering, and therefore lower limits of detection can be achieved
(comparable to WDXRF).
6.2 Mill, preferable with walls made of agate, corundum, or zircon.
6.3 Pellet preparation equipment, manual or automatic pellet press, capable of providing a pressure
of at least 100 kN.
6.4 Aluminium cup, supporting backing cup for pressed pellets.
6.5 Fusion apparatus, electric, gas, or high-frequency induction furnace that can be heated up to a
fixed temperature of between 1 050 °C and 1 250 °C.
6.6 Fusion crucibles, crucibles made of non-wetting platinum alloy (Pt 95 %; Au 5 % is suitable).
Lids, if used, shall be made from platinum alloy.
NOTE Certain metal sulphides (so called platinum poisons) affect the platinum crucibles in which the sample
is melted.
6.7 Casting moulds, non-wetting platinum alloy (Pt 95 %; Au 5 % is suitable).
7 Reagents
The reagents mentioned are used as carrier material.
7.1 Binder, liquid or solid binder free of analytes of interest.
Solid materials can contain a certain amount of moisture, which shall be compensated for.
NOTE Different type of binders can be used. A binder commonly used is wax.
7.2 Flux, solid flux free of analytes of interest.
Solid materials can contain a certain amount of moisture, which shall be compensated for (see ISO 12677
for compensation for moisture in flux).
NOTE Different type of fluxes can be used. Fluxes commonly used are lithium metaborate, lithium tetraborate,
or mixtures of both.
4 © ISO 2014 – All rights reserved
8 Interferences and sources of error
The container in which the sample is delivered and stored can be a source of error. Its material shall be
chosen according to the elements to be determined.
NOTE Elemental Hg can penetrate polyethylene walls very rapidly in both directions. In the case of glass
containers, contamination can be observed for some elements, e.g. Al, As, Ba, Ce, K, Na, and Pb.
Interferences in X-ray fluorescence spectrometry are due to spectral line overlaps, matrix effects,
spectral artefacts, and particle size or mineralogical effects.
Spectral line overlaps occur when an analytical line cannot be resolved from the line of a different
element. Corrections for these interferences are made using the algorithms provided with the software.
Matrix effects occur when the X-ray fluorescence radiation from the analyte element is absorbed or
enhanced by other elements in the sample before it reaches the detector. In the case of complex matrices,
these effects generally have to be corrected.
Spectral artefacts, e.g. escape peaks, sum peaks, pulse pile up lines, dead time, and Bremsstrahlung
correction, are accounted for by the provided software. Spectral artefacts differ for energy dispersive
and wavelength dispersive XRF spectrometry.
Particle size effects can be reduced by milling the sample, and both particle size and mineralogical
effects can be eliminated by preparing bead samples. It is vital for quantitative analysis that the same
sample preparation procedure is applied to both the standards and the samples to be analysed.
9 Sample preparation
9.1 General
In analysis by XRF spectrometry, the sample preparation step is crucial as the quality of the sample
preparation strongly influences the accuracy of the results.
For quantitative analysis of solid samples, pressed pellets or fused beads have to be prepared. The
application of the pressed pellet method is recommended for the quantification of trace elements and
mandatory for the quantification of volatile elements, and the fused bead method for the determination
of non-volatile major and minor elements.
NOTE 1 The preparation of fused beads eliminates effects due to particle size and mineralogy.
The conditions of the preparation of fused beads shall be adapted to the matrix properties. Otherwise,
the preparation of fused beads can be difficult or can cause problems in case of waste-like matrices such
as sludges.
For a given calibration, the same preparation method shall be used throughout, for both samples and
standards.
NOTE 2 Depending on the sample type, other sample preparation methods can be applied according to Annex B.
For precise quantitative measurements, homogeneous and representative test portions are necessary.
Pre-treatment and preparation of test portions shall be carried out according to the appropriate clauses
of ISO 11464 and EN 15002. The particle size of the sample can strongly affect the precision of the
measurement. The particle size should preferably be smaller than 150 µm.
NOTE 3 Particle size smaller than 80 µm is recommended for the analysis of low atomic mass elements when
using the pressed pellet method.
9.2 Drying and determination of dry mass
Prepare and dry the sample according to ISO 11464 or EN 15002. Determine the dry mass according to
ISO 11465 or prEN 14346.
9.3 Preparation of pressed pellet
After drying and milling or grinding the sample, a pellet is prepared in the pellet press (6.3). Before
pressing, the sample shall be mixed and homogenized with a binder (7.1) in a ratio of sample:binder of
10:1 by weight. For the preparation of 40 mm in diameter pellets, about 10,0 g of sample is taken; for
32 mm in diameter pellets, about 4,5 g of sample is required. The amount of binder in the pellet shall be
taken into account for the dilution factor. It is recommended to press the sample in an aluminium cup
(6.4) as support.
NOTE 1 Different types of binders can be used. A binder commonly used is wax. In the case of a liquid binder,
the pellet is placed in an oven to evaporate organic solvent.
NOTE 2 Different dilution factors can be used.
9.4 Preparation of fused beads
After drying and milling or grinding the sample, a fused bead is prepared using the fusion apparatus
(6.5).
Ignite the sample at 1 025 °C ± 25 °C until constant mass is reached. Determine the loss on ignition at the
chosen temperature to correct for volatile elements and/or compounds being released during ignition
of the sample.
NOTE 1 The ignition temperature can vary depending on the sample matrix.
Because of the wide applicability of the fused bead technique, various fluxes and modes of calibration are
permitted providing they have been demonstrated to be able to meet certain criteria of reproducibility,
sensitivity, and accuracy.
For application of alkaline fusion technique (e.g. selection of flux, fusion temperature, and additives),
ISO 14869-2 or CEN/TR 15018 should be used.
NOTE 2 Fluxes commonly used are lithium metaborate, lithium tetraborate, or mixtures of both.
NOTE 3 Loss of volatile elements, e.g. As, Br, Cd, Cl, Hg, I, S, Sb, Se, and Tl, can occur during the fusion process.
Also, Cu can be volatile if a bromide-releasing agent is used.
The flux (7.2) is added to the ignited material in a dilution ratio of sample:flux of 1:5 by weight. For the
preparation of 40 mm in diameter beads, about 1,6 g of ignited sample is taken; for 32 mm in diameter
beads, about 0,8 g of ignited sample is required. The amount of flux in the bead shall be taken into account
for the dilution factor. The same sample preparation procedure and ratio of sample to flux shall be used
for samples and standards. The beads produced should be visually homogeneous and transparent.
NOTE 4 Non-ignited material can be used to prepare beads but, nevertheless, loss of ignition needs to be
determined and needs to be taken into account in the calculation of the results. It should be noted that non-ignited
material can contain compounds that can damage the platinum crucibles during fusion.
NOTE 5 Different dilution factors can be used.
After fusion in a platinum-gold crucible (6.6) the melt is poured into a casting mould (6.7) to make a
bead.
Beads can deteriorate because of adverse temperature and humidity conditions, so it is recommended
that beads are stored in desiccators.
6 © ISO 2014 – All rights reserved
10 Procedure
10.1 Analytical measurement conditions
10.1.1 Wavelength dispersive instruments
The analytical lines to be used and the suggested operating conditions are given in Table C.1. The
settings are strongly dependent on the spectrometer configuration, e.g. the type of X-ray tube (Rh, Cr),
tube power, available crystals, and type of collimators.
10.1.1.1 Intensities and background corrections
For the determination of trace elements, the measured intensities have to be background-corrected.
The measured background positions should be free of spectral line interferences. The net peak intensity
I, expressed as the number of counts per second of the element of interest, is calculated as the difference
between the measured peak intensity of the element and the background intensity:
II=−I (1)
pb
where
is the count rate of the element i, expressed as the number of counts per second;
I
p
I
is the background count rate of the element i, expressed as the number of counts per second.
b
10.1.1.2 Counting time
The minimum counting time is the time necessary to achieve an uncertainty (2σ ), which is less than
%
the desired precision of the measurement. Choose a reference material with a concentration level in the
middle of the working range and measure the count rate. The counting time for each element can be
calculated according to Formula (2):
100 1
t = . (2)
2σ
II−
%
pb
where
t is the total counting time for the peaks and background, in seconds;
2σ
is the relative target precision at a confidence level of 95 %, expressed as percentage.
%
10.1.2 Energy dispersive instruments
The analytical lines to be used and the suggested operating conditions are given in Table C.2. The
settings are strongly dependent on the spectrometer configuration, e.g. type of X-ray tube (Rh, Pd), tube
power, available targets, and type of filters.
Intensities and background corrections
Deconvolution of the spectra and background correction are needed when analysing the samples with
overlapping lines. Usually, XRF instruments are supplied with a specific software module for that
purpose.
10.2 Calibration
10.2.1 General
The calibration procedure is similar for energy dispersive and wavelength dispersive techniques.
In general, calibration is established by using matrix-adapted reference materials. The calibration
equations and inter-element corrections are calculated by the software of the instrument. An accuracy
check is performed with CRMs or samples with known composition.
Different procedures for correcting matrix effects can be used according to the analytical accuracy
required:
— the scattered radiation method is based on the principle that the intensities of the analyte line
and of the Compton-line are affected in the same proportion due to the overall mass absorption
coefficient of the sample. This linear relationship holds when all analytes are at low concentrations
(trace elements) and their absorption coefficients are not affected by an adjacent absorption edge.
In this case, an internal Compton correction can be used. Aside from that, a correction method using
the Compton intensity with mass absorption coefficients (MAC) is also applicable. In this method,
the intensities of the major elements are measured to apply a jump edge correction for the analysed
trace elements;
— correction using the fundamental parameter approach;
— correction using theoretical correction coefficients (alphas) taking basic physical principles,
instrumental geometry, etc. into account;
— correction using empirical correction coefficients (alphas) based on regression analysis of standards
with known elemental concentrations.
10.2.2 General calibration procedure
For calibration purposes, the measurement of analyte lines of samples of known composition is needed.
Formula (3) implies a linear relationship between the intensity and the concentration.
Ca=+aI. (3)
ii,0 i,1i
where
is the concentration of the element of interest, expressed in mg/kg or percentage dry matter;
C
i
a
is the offset of the calibration curve;
i,0
a
is the slope of the calibration curve;
i,1
I
is the net intensity of the element of interest, expressed as counts per second.
i
Matrix effects have to be taken into account in X-ray spectrometry according to Formula (4):
Ca=+(.aI ).M (4)
ii,0 i,1i
where
M is the correction term due to the matrix effects.
The matrix effect correction term can consist of an internal standard Compton correction method or
can be calculated from mathematical models.
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10.2.3 Internal standard correction using Compton (incoherent) scattering method
The measured intensity of incoherent scattering can be used directly to compensate for matrix effects
or indirectly for the determination of the effective mass absorption coefficient μ to correct for matrix
effects. The compensation for matrix effects is based on a combination of sample preparation and
experimental intensity data but not on fundamental and experimental parameters.
The Compton scatter method can be expressed as:
I I
inc,r i,u
CC=(. ).() (5)
i,ui,r
I I
i,r inc,u
where
is the concentration of the element i of the sample, expressed in mg/kg or percentage dry mat-
C
i,u
ter;
C
is the concentration of the element i of the calibration reference material, expressed in mg/kg
i,r
or percentage dry matter;
I
is the intensity of the incoherent Compton-line of the sample, expressed in counts per second;
inc,u
I
is the intensity of the incoherent Compton-line element of the calibration reference material,
inc,r
expressed in counts per second;
I
is the intensity of the element i of the sample, expressed in counts per second;
i,u
I
is the intensity of the element i of the calibration reference material, expressed in counts per
i,r
second.
10.2.4 Fundamental parameter approach
The fundamental parameter approach uses the physical processes forming the basis of X-ray fluorescence
emission and scattering to construct a theoretical model for the correction of matrix effects in practice.
The correction term M is calculated from first principle expressions. These are derived from basic
X-ray physics and contain physical constants and parameters that include absorption and scattering
coefficients, fluorescence yield, primary spectral distributions, and spectrometry geometry. The use
of scattered radiation (Compton and/or Rayleigh) allows the determination of matrix effects caused by
sample elements that cannot be measured directly. The calculation of analyte concentrations in samples
is based on making successively better estimates of composition by an iteration procedure. These
iteration cycles are performed until the difference between the compared results is below a defined
value.
NOTE The algorithm used for the procedure is usually implemented in the manufacturer’s software.
10.2.5 Fundamental or theoretical influence coefficient method
The fundamental influence coefficient method encompasses any mathematical expression relating
emitted intensities and concentrations in which the influence coefficients are defined and derived
explicitly in terms of fundamental parameters.
The calculation of the concentration from the intensities is performed by linear regression, whereby
the net intensities are corrected for the present matrix effects. For each element, the concentration is
calculated according to Formulae (6) and (7):
C
i,r
C = .IM (6)
i,u i,u
IC()1+ α
i,ri∑ jjr
j
C
i,r
C = .IC1+ α (7)
i,u i,ui∑ jju
IC()1+ α
j
i,ri∑ jjr
j
where
is the concentration of the element i of the sample, expressed in mg/kg or percentage dry mat-
C
i,u
ter;
C
is the concentration of the element i of the calibration reference material, expressed in mg/kg
i,r
or percentage dry matter;
I
is the intensity of the element i of the calibration reference material, expressed in counts per
i,r
second;
I
is the intensity of the element i of the sample, expressed in counts per second;
i,u
C
is the concentration of the matrix element j of the calibration reference material, expressed in
j,r
mg/kg or percentage dry matter;
C
is the concentration of the matrix element j of the sample, expressed in mg/kg or percentage
j,u
dry matter;
M is the matrix correction term;
α
is the correction coefficient (called alphas) calculated from theory, although some approxima-
ij
tions are involved.
Different types of alpha coefficient exist, but all of them are calculated without reference to experimental
data; they are calculated using intensity data resulting from a fundamental parameter expression. The
alpha coefficients vary as a function of sample composition and are calculated by an iterative process.
10.2.6 Empirical alpha correction
Empirical alphas are obtained experimentally using the regression analysis of data from reference
materials in which the elements to be measured are known and the total concentration range is covered.
Best results are achieved when the samples and reference materials are of similar composition. Thus,
empirical alphas are based strictly on experimental data and do not take fundamental and instrumental
parameters into account. Different models can be applied, but generally they are based on Formulae (6)
and (7) where the correction term for matrix effects is a function of concentrations.
The empirical alphas are only applicable for a limited concentration range and a well-defined analytical
method where the matrices of samples and standards are similar. The reference materials used should
contain each analyte together with fairly wide concentration ranges of each matrix element. Poor
analytical results are obtained when inappropriate combinations of analytes are chosen. A large number
of reference materials have to be analysed to define the alphas (rule of thumb: minimum of 3 times the
number of parameters to be calculated).
10.2.7 Calibration procedure for trace elements using the pressed pellet method
The pressed pellet method is used to determine the concentrations of trace elements.
Select calibration standards with a similar composition as the samples under investigation containing
the elements of interest and covering the concentration range of interest. The use of reference materials
from different recognized producers is recommended (see Annex D) or synthetic mixtures of oxides can
be prepared. The element concentrations shall vary independently in the standards. If the calibration
covers many elements in a wide range of concentrations, a large number of calibration samples can be
necessary.
10 © ISO 2014 – All rights reserved
Prepare pressed pellets from the selected calibration standards according to 9.3.
Define the analytical measurement method for EDXRF or WDXRF as described in 10.1.
Start up the XRF equipment according to the instrument manufacturer’s manual and measure the
calibration standards using the defined measurement method. All measurements shall be performed
under vacuum.
NOTE It is important to note that the pressed pellet method is not ideal for the determination of major
elements, but these elements are measured so that alpha corrections can be applied to some elements of interest.
Follow the guidelines in the instrument manufacturer’s manual to perform the regression, the
background correction, the line overlap correction, and the matrix corrections for all elements under
consideration. In Table 1, the possible spectral line overlaps are indicated (dependent on the configuration
of the instrument) and also the matrix correction method that can be applied. For trace elements with
an absorption edge above the absorption edge of iron, a Compton internal standard correction can
be applied. Otherwise, a theoretical alpha correction or correction for the absorption edge should be
performed (for these corrections, all elements in the sample have to be analysed).
Depending on the type of instrument and the software programs available, alternative correction
methods can be applied. Validation of the final calibration curves shall demonstrate the accuracy of the
method.
Perform the regression calculation and verify that the correlation factors are within the limits of
accuracy required.
Table 1 — Suggested analytical lines, spectral line overlaps, and correction methods
Element Line Spectral line overlap Type of matrix correction method
Na Kα ZnLβ Alpha or FP
Mg Kα AsLα Alpha or FP
Al Kα BrLα Alpha or FP
Si Kα Alpha or FP
P Kα Alpha or FP
S Kα CoKα PbMα NbLβ Alpha or FP or MAC
Cl Kα Alpha or FP or MAC
K Kα Alpha or FP
Ca Kα Alpha or FP
Ti Kα BaLα ΙLβ Alpha or FP
V Kα Ti Kβ Alpha or FP or MAC
Cr Kα VKβ PbLα Alpha or FP or MAC
Mn Kα CrKβ Alpha or FP
Fe Kα MnKβ Alpha or FP
Co Kα FeKβ Alpha or FP or MAC
Ni Kα CoKβ Compton or FP or MAC
Cu Kα TaLα ThLβ Compton or FP or MAC
Zn Kα WLα Compton or FP or MAC
Kα
PbLα
As Compton or FP or MAC
BrKα
Kβ
Se Kα Compton or FP or MAC
Br Kα AsKβ Compton or FP or MAC
Table 1 (continued)
Element Line Spectral line overlap Type of matrix correction method
Rb Kα ULα BrKβ Compton or FP or MAC
Sr Kα ULα Compton or FP or MAC
Y Kα RbKβ Compton or FP or MAC
Zr Kα SrKβ Compton or FP or MAC
Nb Kα YKβ ULβ Compton or FP or MAC
Mo Kα ZrKβ ULβ Compton or FP or MAC
Kα Compton or FP or MAC
Ag CrKβ
Lα Alpha or FP
Kα Compton or FP or MAC
Cd AgLβ
Lα Alpha or FP
Kα Compton or FP or MAC
Sn CoKα
Lα Alpha or FP or MAC
Kα Compton or FP or MAC
Sb CoKβ
Lβ Alpha or FP or MAC
Kα Compton or FP or MAC
Te SnLβ
Lα Alpha or FP or MAC
Kα Compton or FP or MAC
I
Lα Alpha or FP or MAC
Kα Compton or FP or MAC
Cs ZnKα ILβ
Lα Alpha or FP or MAC
Kα Compton or FP or MAC
Ba TiKα ILβ CuKβ
Lα Alpha or FP or MAC
Ta Lα CuKα NiKβ Compton or FP or MAC
W Lα TaLn Compton or FP or MAC
Hg Lα WLβ Compton or FP or MAC
Tl Lβ PbLβ Compton or FP or MAC
Pb Lβ ThLα BiLβ SnKα Compton or FP or MAC
Bi Lα TaLγ Compton or FP or MAC
Th Lα BiLβ PbLβ Compton or FP or MAC
U Lα BrKβ RbKα Compton or FP or MAC
10.2.8 Calibration procedure for major and minor oxides using the fused bead method
The fused bead method is used to determine the concentrations of major and minor elements.
Select calibration standards with a similar composition as the samples under investigation containing
the elements of interest and covering the total concentration range of interest. The use of reference
materials from different recognized producers is recommended (see Annex D) or synthetic mixtures
of oxides can be prepared. The element concentrations shall vary independently in the samples. If the
calibration covers many elements in a wide range of concentrations, a large number of calibration
samples can be necessary.
Prepare fused beads from the selected calibration standards according to 9.4.
NOTE Due to a higher dilution factor for fused beads, the limit of detection of the different elements is higher
than those for pressed pellets.
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Define the analytical measurement method for EDXRF or WDXRF as described in 10.1.
Start up the XRF equipment according to the instrument manufacturer’s manual and measure the
calibration standards using the defined measurement method. All measurements shall be performed
under vacuum.
In the calibration program, all the elements of the reference materials have to be defined as oxides and
the concentrations are reported as oxides.
Follow the guidelines in the instrument manufacturer’s manual how to perform the regression, the
background correction, the line overlap correction, and the matrix corrections for all elements under
consideration. In Table 1, the possible spectral line overlaps are indicated (dependent on the configuration
of the instrument). For all elements, an alpha correction method using theoretical alphas should be
applied.
Depending on the type of instrument and the software programs available, alternative correction
methods can be applied. Validation of the final calibration curves shall demonstrate the accuracy of the
method.
Perform the regression calculation and verify that the correlation factors are within the limits of
accuracy required.
10.2.9 Analysis of the samples
Follow the instrument manufacturer’s instructions for set up, conditioning, preparation, and maintenance
of the XRF spectrometer.
Select the required preparation method and prepare the samples. For the quantification of trace
elements, the pressed pellet method is recommended and for the determination of major and minor
elements, the fused bead method should be used.
To analyse the prepared samples, an analytical measurement method has to be defined. The measurement
method describes the analytical lines to be measured and the measurement parameters, e.g. the XRF
generator settings (tube voltage and current), selection of primary beam filters, targets and crystals,
detector to be used, and measurement time.
The sa
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