ISO 11726:2004
(Main)Solid mineral fuels — Guidelines for the validation of alternative methods of analysis
Solid mineral fuels — Guidelines for the validation of alternative methods of analysis
ISO 11726:2004 describes procedures for validating alternative methods of analysis for coal and coke either directly by comparison with the relevant International Standard method or indirectly by comparison with reference materials that have been exhaustively analysed using the relevant International Standard method. The statistical analysis methods used are parametric, i.e. their use is possible only when the characteristic is expressed as a simple number on an approximately linear scale. The results from some methods, for example the Gray-King Swelling Index, are not so expressed and the methods given here should be used only if the data are converted to a parametric scale.
Combustibles minéraux solides — Lignes directrices pour la validation de variantes analytiques
Trda mineralna goriva – Smernice za potrjevanje alternativnih analitičnih metod
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INTERNATIONAL ISO
STANDARD 11726
First edition
2004-08-15
Solid mineral fuels — Guidelines for the
validation of alternative methods of
analysis
Combustibles minéraux solides — Lignes directrices pour la validation
de variantes analytiques
Reference number
ISO 11726:2004(E)
©
ISO 2004
---------------------- Page: 1 ----------------------
ISO 11726:2004(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
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Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2004
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2004 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 11726:2004(E)
Contents Page
Foreword. iv
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 General. 2
5 Preliminary work on the alternative method .3
5.1 General. 3
5.2 Selectivity and specificity . 3
5.3 Range and linearity. 3
5.4 Sensitivity . 3
5.5 Limit of detection . 4
5.6 Limit of quantitation. 4
5.7 Ruggedness. 4
5.8 Accuracy . 4
5.9 Precision . 4
6 Defining the alternative method to be validated. 4
7 Procedure. 5
7.1 Measurement of precision and trueness. 5
7.2 Comparison with reference materials. 6
7.3 Comparison with International Standard method within a laboratory — Single fuel . 10
7.4 Comparison using the International Standard method within a laboratory — Range of
fuels. 12
7.5 Comparison using the International Standard Method, between laboratories. 13
8 Validation report. 13
Annex A (informative) Explanation of rationale of method A . 14
Annex B (informative) Example calculations. 16
Bibliography . 24
© ISO 2004 – All rights reserved iii
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ISO 11726:2004(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 committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 11726 was prepared by Technical Committee ISO/TC 27, Solid mineral fuels, Subcommittee SC 5,
Methods of analysis.
iv © ISO 2004 – All rights reserved
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INTERNATIONAL STANDARD ISO 11726:2004(E)
Solid mineral fuels — Guidelines for the validation of alternative
methods of analysis
1 Scope
This International Standard describes procedures for validating alternative methods of analysis for coal and
coke either directly by comparison with the relevant International Standard method or indirectly by comparison
with reference materials that have been exhaustively analysed using the relevant International Standard
method.
The statistical analysis methods used are parametric, i.e. their use is possible only when the characteristic is
expressed as a simple number on an approximately linear scale. The results from some methods, for example
the Gray-King coke type, are not so expressed and the methods given here should be used only if the data
are converted to a parametric scale.
2 Normative references
The following referenced documents are indispensable for the application 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 5725-6, Accuracy (trueness and precision) of measurement methods and results — Part 6: Use in
practice of accuracy values
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
accuracy
closeness of agreement between a test result and the acceptable reference value
NOTE The term accuracy, when applied to a set of results, describes a combination of random components and a
common systematic error or bias component.
3.2
bias
difference between the expectation of the test results and an accepted reference value
NOTE Bias is a systematic error as contrasted to random error. There may be one or more systematic error
components contributing to the bias. A larger systematic difference from the accepted reference value is reflected by a
larger bias value.
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ISO 11726:2004(E)
3.3
precision
closeness of agreement between independent test results obtained under prescribed conditions
NOTE 1 Precision depends only on distribution of random errors and does not relate to the accepted reference value.
NOTE 2 The measure of precision is usually expressed in terms of imprecision and computed as a standard deviation
of the test results. Higher imprecision is reflected by a larger standard deviation.
NOTE 3 “Independent test results” means results obtained in a manner not influenced by any previous result on the
same or similar material.
3.4
trueness
closeness of agreement between the average value obtained from a large series of test results and an
accepted reference value
NOTE The measure of trueness is usually expressed in terms of bias.
4 General
An International Standard method is a measurement method that has been subjected to a standardization
process to satisfy various requirements. Among these requirements, taken from ISO 5725-6, Clause 8, are the
following.
The method must be applicable to a wide range of levels of characteristics to cover most materials that
are internationally traded.
EXAMPLE A method for the determination of sulfur content in coal must be applicable to as many internationally
traded coals as possible.
Equipment, reagents and personnel must be available on an international basis.
The costs of performing the tests must be acceptable.
The precision and trueness of the measurement method should be acceptable for the users of the results.
Many analytical methods for coal and coke are based on traditional combustion or wet-chemical analysis and
the results are highly dependent on the test conditions. They are frequently time-consuming, labour- and skill-
intensive and unsuited to automation. However, they meet the requirements of International Standard
measurement methods, both in being internationally available and in providing acceptable levels of trueness
and precision in international coal trade.
Other, non-standard methods of analysis are in use when
a) most of the material tested comes from the same source and the variation of its characteristics is
relatively small. In such cases a simpler, less expensive method may be adequate;
b) an instrumental or automated version of the standard method provides much cheaper analysis of large
numbers of samples. Such equipment may be much more expensive than the standard equipment or of a
highly proprietary nature;
c) an instrumental method based on an analytical principle different from that of the standard method is
available. Such methods have similar characteristics to 4 b) above.
In some cases, if it is possible to write a generic description of the method and the equipment is widely
available, methods of types 4 b) and 4 c) above can be developed as International Standards. If an
International Standard method already exists, then where both methods continue, however, the new method
2 © ISO 2004 – All rights reserved
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ISO 11726:2004(E)
should be tested against the International Standard method to ensure that it gives results comparable to the
old method for trueness and accuracy. This is part of the process of issuing the alternative method as an
International Standard.
Even if the equipment is widely available, it might not be possible to convert the method into an International
Standard because of the proprietary nature of the equipment, speed of development and rapid obsolescence
of such equipment.
The commercial pressure for cheaper, more rapid analysis has, however, meant that many analyses are
carried out on equipment of this type. Some users develop their own in-house methods or use such methods
for contractual purposes if agreed between both parties, provided that they can be assured that the alternative
method produces results that are comparable for accuracy and trueness with the International Standard
method.
The purpose of this International Standard is to give guidelines for such a validation, as applied to methods for
testing of coal and coke. It is not intended to infer that the use of such alternative methods complies with the
relevant International Standards nor is it for use in writing alternative International Standards. If the intention is
to develop a new method into an International Standard, the procedures given in ISO 5725-6 should be used.
To summarize, alternative methods requiring validation range from simplified versions of the International
Standard method to proprietary automated instrumental methods using principles entirely different from those
of the International Standard method.
5 Preliminary work on the alternative method
5.1 General
Before any detailed comparison with the International Standard method is undertaken, it is necessary to
investigate the performance characteristics of the alternative method. When buying specific commercial
equipment, information on these aspects should be sought from the manufacturer. Many of the characteristics
given below are applicable only to methods where the sample is in liquid form for the determination. For direct
determination on solids (e.g. ash), little preliminary work is possible. Some of the main performance
characteristics are given below, drawn from Reference [2].
5.2 Selectivity and specificity
Selectivity of a method refers to the extent to which it can determine particular analyte(s) in a complex mixture
without interference from the other components in the mixture. A method that is perfectly selective for an
analyte or group of analytes is said to be specific. The applicability of the method should be studied using
various samples, ranging from pure standards to mixtures with complex matrices. Standard addition of pure
analyte to coal/coke solutions should be used. In each case, the recovery of the analyte(s) of interest should
be determined and the influences of suspected interferences stated. Any restrictions in the applicability of the
technique should be documented in the method.
5.3 Range and linearity
The working range for a method is determined by examining samples with different analyte concentrations
and determining the concentration range for which acceptable accuracy and precision can be achieved. Whilst
the working range of the analyte in solution may be determined using pure analyte or synthetic matrices
containing analyte, the true range and linearity cannot be determined until a detailed comparison with the
International Standard method is made on fuel samples.
5.4 Sensitivity
This is the difference in analyte concentration corresponding to the smallest difference in the response of the
method that can be detected. It is represented by the slope of a calibration curve and can be determined by a
least-squares procedure, or experimentally, using fuel samples containing various concentrations of the
analyte.
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ISO 11726:2004(E)
5.5 Limit of detection
The limit of detection of an analyte is determined by repeat analysis of a blank test portion and is the analyte
concentration whose response is the equivalent to the mean blank response plus three standard deviations.
Its value is likely to be different for different types of sample.
5.6 Limit of quantitation
This is the lowest concentration of analyte that can be determined with an acceptable level of accuracy and
precision, i.e. it is usually the lowest point on the calibration curve (excluding the blank). It should be
established using an appropriate standard or sample; it should not be determined by extrapolation.
NOTE Within this International Standard, “accuracy” is known as “trueness”.
5.7 Ruggedness
This is sometimes called robustness. Each time a method is used, small variations are inevitably introduced in
the procedure, which may or may not have a significant influence on the performance of the method. The
ruggedness of a method is tested by deliberately introducing small changes to the method, for example mass
of sample and temperature of combustion, and examining the consequences. A large number of factors may
need to be considered, but because most of these will have a negligible effect, it is normally possible to vary
several at once.
5.8 Accuracy
The accuracy of a method is the closeness of the obtained analyte value to the true value. The overall
accuracy can only be established by analysing suitable reference materials or comparison with the
International Standard method (see Clause 7). For intermediate stages (i.e. solution finishes), an estimation of
accuracy can be obtained by spiking test portions with chemical standards. The value of spiking is limited; it
can only be used to determine the accuracy of those stages of the method following the spiking.
5.9 Precision
The precision of a method is a statement of the closeness of agreement between mutually independent test
results and is usually stated in terms of standard deviation. It is generally dependent on analyte concentration,
and this dependence should be determined and documented. It may be stated in different ways depending on
the conditions for which it is calculated. Repeatability is a type of precision relating to measurements made
under repeatability conditions, i.e. same method, same material, same operator, same laboratory, different
time but within a narrow time period. Preliminary estimations of precision of the alternative method may be
made, for example, by comparing the results of duplicate samples for the ruggedness tests.
6 Defining the alternative method to be validated
Once the preliminary work on the alternative method (see Clause 5) has shown that it is likely to be suitable
for the intended purpose, the test conditions for the method should be chosen and clearly and unambiguously
defined in a manner similar to the way in which International Standard methods are defined. Critical test
parameters vary with the type of test and cannot be exhaustively listed in this document. Examples of some
parameters commonly found in coal and coke analysis are as follows:
a) mass of sample and solid reagents, plus critical range;
b) condition of sample, moisture content, particle size, particle size range;
c) accuracy of measuring equipment for temperature, mass, volume;
d) purity of reagents, accuracy of solution concentration;
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ISO 11726:2004(E)
e) furnace temperature, with critical dimensions of the hot zone where relevant;
f) length of time of combustion/heating;
g) atmosphere in the furnace/oven;
h) in spectroscopic determinations, cell path length, wavelength;
i) calibration procedures.
When automated instruments are operated under pre-set conditions, these conditions should be defined as
closely as possible; all variable settings of the instrument should be defined. Particular attention should be
paid to those instruments whose settings can be altered by modification of a computer program where any
change might not be immediately apparent to the operator. Some method of checking that the program has
not been altered from the standard conditions should be devised.
The test procedure should be written, again in a manner similar to that of an International Standard, so that
subsequent operators are able to follow the method identical to that used during validation.
7 Procedure
7.1 Measurement of precision and trueness
7.1.1 Precision
Measure the precision of results in terms of the standard deviation of a set of analyses carried out under
repeatability conditions. The precision of the alternative method is measured directly by making replicate
analyses of samples.
If the alternative method is to be validated using reference samples, then calculate the standard deviation of
the International Standard method from the repeatability limit given in the International Standard method.
If a direct comparison is to be made between the two methods, then determine the precision of the
International Standard method directly by analysing samples in replicate. This will be a more accurate
measure than that calculated from the International Standard because the precision of the two methods on the
analysis of the same fuels is compared, rather than a precision determined at the time of development of the
International Standard on fuels whose identity is unlikely to be known.
7.1.2 Trueness
Estimate the trueness either by comparing the results obtained by analysing a reference material using the
alternative method with the reference value (see 7.2) or by comparing results on the same fuels using both the
alternative and International Standard methods (see 7.3).
Measurement of trueness can only be an estimate, the errors of which are measured by considering the
variability of the differences between the results. The greater the variability, the greater the estimation error,
the more results that are compared, the lower the estimation error.
Two different statistical analysis methods (A and B) are given in this International Standard.
Method A (see 7.2.2.1 and 7.3.2.1 for details) is recommended as the most rigorous. Decide before starting
on the maximum tolerable bias, MTB, and design the test to be sufficiently sensitive to detect that bias, should
it exist. Carry out a sufficient number of analyses to make the statistical test powerful enough to conclude
either that
a) the bias is significantly greater than zero and not significantly less than MTB; or
b) the bias is significantly less than MTB and not significantly greater than zero.
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ISO 11726:2004(E)
In a simpler test (method B; see 7.2.2.2 and 7.3.2.2 for details), compare a fixed number of results and, unless
the mean difference fails a null hypothesis test, it can be concluded that no bias exists between the two
methods. If the estimation error is too great, however, it is also possible that a bias at an unacceptable level
could exist. To obviate such an ambiguous conclusion, use method A.
The difficulty with method A is in deciding what value to assign for MTB. However, with method B, it is
necessary to a make a judgement, after the result, as to what bias levels are tolerable. For either method,
therefore, it is necessary to consider the practical implications of any possible bias and to make a rational
judgement on what level is unacceptable.
7.2 Comparison with reference materials
7.2.1 Sources of reference materials
Coal and coke reference materials samples are widely available commercially. Before use, investigate the
traceability, authority and methods of analysis. Obtain details of homogeneity trials, stability trials, the methods
used for certification and the uncertainty and variations in the stated analyte values from the producer and use
them to judge the pedigree. In order to compare the alternative method with the International Standard
method, use only reference materials that have been analysed using the International Standard method.
Where an International Standard method contains a major element of choice, quote the subclauses from the
International Standard giving details of that variation.
Reference materials should be clearly labelled and stored under the specified conditions and should be
safeguarded against contamination or loss of determinand.
Select reference materials to be of the same type of fuel and to have the same analyte concentration ranges
as those which are to be analysed routinely by the alternative method. Some factors which should be
considered are coal rank, coke type or manufactured fuel type.
Where the range of fuels to be tested is very wide or reference materials of the appropriate type cannot be
obtained, test the method against the International Standard method (see 7.3 or 7.4).
Two methods are given for the estimation of trueness. The recommended method, method A, requires the
specification, before any analysis is done, of the greatest bias (the maximum tolerable bias) that the user is
prepared to risk; testing continues until an unambiguous conclusion is reached. In a simpler method,
method B, a fixed number of analyses is performed and a conclusion drawn from the result. In the latter
method, there is a possibility that an alternative method is considered to be unbiased even though there is a
risk that it is biased to an unacceptable extent.
7.2.2 Estimation of trueness
7.2.2.1 Method A
Decide on a value for the maximum tolerable bias, B (see 7.1.2).
Calculate the standard deviation of the International Standard s method using Equation (1):
ISO
r
s = (1)
ISO
22
where
s is the standard deviation of the International Standard method under repeatability conditions;
ISO
r is the repeatability limit for the International Standard method.
6 © ISO 2004 – All rights reserved
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ISO 11726:2004(E)
Calculate the value of g using Equation (2):
B
g = (2)
s
ISO
Using Table 1, calculate the number of replicate tests, n, necessary to identify the MTB.
Table 1 — Values for factor g for calculating the number of analyses required
0 1 2 3 4 5 6 7 8 9
0 — — — 4,170 2,728 2,195 1,872 1,659 1,506 1,389
10 1,295 1,218 1,154 1,099 1,051 1,009 0,972 0,938 0,907 0,880
20 0,855 0,832 0,810 0,790 0,772 0,755 0,739 0,724 0,710 0,696
30 0,684 0,672 0,660 0,649 0,639 0,629 0,620 0,611 0,602 0,594
40 0,586 0,579 0,571 0,564 0,558 0,551 0,545 0,539 0,533 0,527
50 0,521 0,516 0,511 0,506 0,501 0,496 0,491 0,487 0,483 0,478
60 0,474 0,470 0,466 0,463 0,459 0,455 0,451 0,448 0,445 0,441
70 0,438 0,435 0,432 0,429 0,426 0,423 0,420 0,417 0,414 0,411
80 0,409 0,406 0,404 0,401 0,399 0,396 0,394 0,392 0,389 0,387
90 0,385 0,383 0,380 0,378 0,376 0,374 0,372 0,370 0,368 0,366
NOTE The number of sets required corresponding to a given g factor is the sum of the column and row headings.
Analyse the reference material using the alternative method n times.
Calculate s , using Equation (3):
ALT
2
x
()
∑
2
x −
∑
n
s = (3)
ALT
n −1
()
where
x is the sum of all the results;
∑
2
x is the sum of the squares of the results;
∑
n is the number of the results.
Recalculate g and hence n. If the new value for n is greater than the number of analyses already done, then
carry out additional analyses to reach n. Continue with this process until enough analyses have been carried
out.
Calculate the value of the mean difference of the analytical values from the reference value, d , using
Equation (4):
x
∑ i
dR=− (4)
n
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ISO 11726:2004(E)
where
x is the analytical value of the ith determination;
i
R is the reference value.
Calculate the statistic t from Equation (5):
c
dn
t = (5)
c
s
ALT
Compare with the value of t from Table 3 at (n-1) degrees of freedom.
t
If t > t then it can be concluded that the alternative method is biased in comparison with the International
c t
Standard method. Otherwise, it can be concluded that any bias is significantly less than the maximum
tolerable bias.
Repeat this process for reference fuels taken from across the range of fuels to be tested.
7.2.2.2 Method B
Using the method as defined (see Clause 6), analyse the reference material using the alternative method
n times (at least 5) under repeatability conditions.
Calculate the mean, x , of the results using Equation (6), and hence its difference from the reference value,
d , and the standard deviation of the results, s , using Equation (3).
ALT
x
∑
(6)
x =
n
Calculate the statistic, t , using Equation (5).
c
Compare with the value of t from Table 3 at (n-1) degrees of freedom.
t
If t > t then it can be concluded that the alternative method is biased in comparison with the International
c t
Standard method.
Equation (5) implies that the sensitivity of the test to detect any bias is dependent on the number of analyses,
n, and the standard deviation s .
ALT
If the estimated bias is d , then the true bias will lie within the 95 % confidence limits given by Equation (7):
ts⋅
tALT
d
...
SLOVENSKI STANDARD
SIST ISO 11726:2005
01-november-2005
7UGDPLQHUDOQDJRULYD±6PHUQLFH]DSRWUMHYDQMHDOWHUQDWLYQLKDQDOLWLþQLKPHWRG
Solid mineral fuels -- Guidelines for the validation of alternative methods of analysis
Combustibles minéraux solides -- Lignes directrices pour la validation de variantes
analytiques
Ta slovenski standard je istoveten z: ISO 11726:2004
ICS:
75.160.10 Trda goriva Solid fuels
SIST ISO 11726:2005 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
---------------------- Page: 1 ----------------------
SIST ISO 11726:2005
---------------------- Page: 2 ----------------------
SIST ISO 11726:2005
INTERNATIONAL ISO
STANDARD 11726
First edition
2004-08-15
Solid mineral fuels — Guidelines for the
validation of alternative methods of
analysis
Combustibles minéraux solides — Lignes directrices pour la validation
de variantes analytiques
Reference number
ISO 11726:2004(E)
©
ISO 2004
---------------------- Page: 3 ----------------------
SIST ISO 11726:2005
ISO 11726:2004(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2004
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2004 – All rights reserved
---------------------- Page: 4 ----------------------
SIST ISO 11726:2005
ISO 11726:2004(E)
Contents Page
Foreword. iv
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 General. 2
5 Preliminary work on the alternative method .3
5.1 General. 3
5.2 Selectivity and specificity . 3
5.3 Range and linearity. 3
5.4 Sensitivity . 3
5.5 Limit of detection . 4
5.6 Limit of quantitation. 4
5.7 Ruggedness. 4
5.8 Accuracy . 4
5.9 Precision . 4
6 Defining the alternative method to be validated. 4
7 Procedure. 5
7.1 Measurement of precision and trueness. 5
7.2 Comparison with reference materials. 6
7.3 Comparison with International Standard method within a laboratory — Single fuel . 10
7.4 Comparison using the International Standard method within a laboratory — Range of
fuels. 12
7.5 Comparison using the International Standard Method, between laboratories. 13
8 Validation report. 13
Annex A (informative) Explanation of rationale of method A . 14
Annex B (informative) Example calculations. 16
Bibliography . 24
© ISO 2004 – All rights reserved iii
---------------------- Page: 5 ----------------------
SIST ISO 11726:2005
ISO 11726:2004(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 committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 11726 was prepared by Technical Committee ISO/TC 27, Solid mineral fuels, Subcommittee SC 5,
Methods of analysis.
iv © ISO 2004 – All rights reserved
---------------------- Page: 6 ----------------------
SIST ISO 11726:2005
INTERNATIONAL STANDARD ISO 11726:2004(E)
Solid mineral fuels — Guidelines for the validation of alternative
methods of analysis
1 Scope
This International Standard describes procedures for validating alternative methods of analysis for coal and
coke either directly by comparison with the relevant International Standard method or indirectly by comparison
with reference materials that have been exhaustively analysed using the relevant International Standard
method.
The statistical analysis methods used are parametric, i.e. their use is possible only when the characteristic is
expressed as a simple number on an approximately linear scale. The results from some methods, for example
the Gray-King coke type, are not so expressed and the methods given here should be used only if the data
are converted to a parametric scale.
2 Normative references
The following referenced documents are indispensable for the application 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 5725-6, Accuracy (trueness and precision) of measurement methods and results — Part 6: Use in
practice of accuracy values
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
accuracy
closeness of agreement between a test result and the acceptable reference value
NOTE The term accuracy, when applied to a set of results, describes a combination of random components and a
common systematic error or bias component.
3.2
bias
difference between the expectation of the test results and an accepted reference value
NOTE Bias is a systematic error as contrasted to random error. There may be one or more systematic error
components contributing to the bias. A larger systematic difference from the accepted reference value is reflected by a
larger bias value.
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3.3
precision
closeness of agreement between independent test results obtained under prescribed conditions
NOTE 1 Precision depends only on distribution of random errors and does not relate to the accepted reference value.
NOTE 2 The measure of precision is usually expressed in terms of imprecision and computed as a standard deviation
of the test results. Higher imprecision is reflected by a larger standard deviation.
NOTE 3 “Independent test results” means results obtained in a manner not influenced by any previous result on the
same or similar material.
3.4
trueness
closeness of agreement between the average value obtained from a large series of test results and an
accepted reference value
NOTE The measure of trueness is usually expressed in terms of bias.
4 General
An International Standard method is a measurement method that has been subjected to a standardization
process to satisfy various requirements. Among these requirements, taken from ISO 5725-6, Clause 8, are the
following.
The method must be applicable to a wide range of levels of characteristics to cover most materials that
are internationally traded.
EXAMPLE A method for the determination of sulfur content in coal must be applicable to as many internationally
traded coals as possible.
Equipment, reagents and personnel must be available on an international basis.
The costs of performing the tests must be acceptable.
The precision and trueness of the measurement method should be acceptable for the users of the results.
Many analytical methods for coal and coke are based on traditional combustion or wet-chemical analysis and
the results are highly dependent on the test conditions. They are frequently time-consuming, labour- and skill-
intensive and unsuited to automation. However, they meet the requirements of International Standard
measurement methods, both in being internationally available and in providing acceptable levels of trueness
and precision in international coal trade.
Other, non-standard methods of analysis are in use when
a) most of the material tested comes from the same source and the variation of its characteristics is
relatively small. In such cases a simpler, less expensive method may be adequate;
b) an instrumental or automated version of the standard method provides much cheaper analysis of large
numbers of samples. Such equipment may be much more expensive than the standard equipment or of a
highly proprietary nature;
c) an instrumental method based on an analytical principle different from that of the standard method is
available. Such methods have similar characteristics to 4 b) above.
In some cases, if it is possible to write a generic description of the method and the equipment is widely
available, methods of types 4 b) and 4 c) above can be developed as International Standards. If an
International Standard method already exists, then where both methods continue, however, the new method
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should be tested against the International Standard method to ensure that it gives results comparable to the
old method for trueness and accuracy. This is part of the process of issuing the alternative method as an
International Standard.
Even if the equipment is widely available, it might not be possible to convert the method into an International
Standard because of the proprietary nature of the equipment, speed of development and rapid obsolescence
of such equipment.
The commercial pressure for cheaper, more rapid analysis has, however, meant that many analyses are
carried out on equipment of this type. Some users develop their own in-house methods or use such methods
for contractual purposes if agreed between both parties, provided that they can be assured that the alternative
method produces results that are comparable for accuracy and trueness with the International Standard
method.
The purpose of this International Standard is to give guidelines for such a validation, as applied to methods for
testing of coal and coke. It is not intended to infer that the use of such alternative methods complies with the
relevant International Standards nor is it for use in writing alternative International Standards. If the intention is
to develop a new method into an International Standard, the procedures given in ISO 5725-6 should be used.
To summarize, alternative methods requiring validation range from simplified versions of the International
Standard method to proprietary automated instrumental methods using principles entirely different from those
of the International Standard method.
5 Preliminary work on the alternative method
5.1 General
Before any detailed comparison with the International Standard method is undertaken, it is necessary to
investigate the performance characteristics of the alternative method. When buying specific commercial
equipment, information on these aspects should be sought from the manufacturer. Many of the characteristics
given below are applicable only to methods where the sample is in liquid form for the determination. For direct
determination on solids (e.g. ash), little preliminary work is possible. Some of the main performance
characteristics are given below, drawn from Reference [2].
5.2 Selectivity and specificity
Selectivity of a method refers to the extent to which it can determine particular analyte(s) in a complex mixture
without interference from the other components in the mixture. A method that is perfectly selective for an
analyte or group of analytes is said to be specific. The applicability of the method should be studied using
various samples, ranging from pure standards to mixtures with complex matrices. Standard addition of pure
analyte to coal/coke solutions should be used. In each case, the recovery of the analyte(s) of interest should
be determined and the influences of suspected interferences stated. Any restrictions in the applicability of the
technique should be documented in the method.
5.3 Range and linearity
The working range for a method is determined by examining samples with different analyte concentrations
and determining the concentration range for which acceptable accuracy and precision can be achieved. Whilst
the working range of the analyte in solution may be determined using pure analyte or synthetic matrices
containing analyte, the true range and linearity cannot be determined until a detailed comparison with the
International Standard method is made on fuel samples.
5.4 Sensitivity
This is the difference in analyte concentration corresponding to the smallest difference in the response of the
method that can be detected. It is represented by the slope of a calibration curve and can be determined by a
least-squares procedure, or experimentally, using fuel samples containing various concentrations of the
analyte.
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5.5 Limit of detection
The limit of detection of an analyte is determined by repeat analysis of a blank test portion and is the analyte
concentration whose response is the equivalent to the mean blank response plus three standard deviations.
Its value is likely to be different for different types of sample.
5.6 Limit of quantitation
This is the lowest concentration of analyte that can be determined with an acceptable level of accuracy and
precision, i.e. it is usually the lowest point on the calibration curve (excluding the blank). It should be
established using an appropriate standard or sample; it should not be determined by extrapolation.
NOTE Within this International Standard, “accuracy” is known as “trueness”.
5.7 Ruggedness
This is sometimes called robustness. Each time a method is used, small variations are inevitably introduced in
the procedure, which may or may not have a significant influence on the performance of the method. The
ruggedness of a method is tested by deliberately introducing small changes to the method, for example mass
of sample and temperature of combustion, and examining the consequences. A large number of factors may
need to be considered, but because most of these will have a negligible effect, it is normally possible to vary
several at once.
5.8 Accuracy
The accuracy of a method is the closeness of the obtained analyte value to the true value. The overall
accuracy can only be established by analysing suitable reference materials or comparison with the
International Standard method (see Clause 7). For intermediate stages (i.e. solution finishes), an estimation of
accuracy can be obtained by spiking test portions with chemical standards. The value of spiking is limited; it
can only be used to determine the accuracy of those stages of the method following the spiking.
5.9 Precision
The precision of a method is a statement of the closeness of agreement between mutually independent test
results and is usually stated in terms of standard deviation. It is generally dependent on analyte concentration,
and this dependence should be determined and documented. It may be stated in different ways depending on
the conditions for which it is calculated. Repeatability is a type of precision relating to measurements made
under repeatability conditions, i.e. same method, same material, same operator, same laboratory, different
time but within a narrow time period. Preliminary estimations of precision of the alternative method may be
made, for example, by comparing the results of duplicate samples for the ruggedness tests.
6 Defining the alternative method to be validated
Once the preliminary work on the alternative method (see Clause 5) has shown that it is likely to be suitable
for the intended purpose, the test conditions for the method should be chosen and clearly and unambiguously
defined in a manner similar to the way in which International Standard methods are defined. Critical test
parameters vary with the type of test and cannot be exhaustively listed in this document. Examples of some
parameters commonly found in coal and coke analysis are as follows:
a) mass of sample and solid reagents, plus critical range;
b) condition of sample, moisture content, particle size, particle size range;
c) accuracy of measuring equipment for temperature, mass, volume;
d) purity of reagents, accuracy of solution concentration;
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e) furnace temperature, with critical dimensions of the hot zone where relevant;
f) length of time of combustion/heating;
g) atmosphere in the furnace/oven;
h) in spectroscopic determinations, cell path length, wavelength;
i) calibration procedures.
When automated instruments are operated under pre-set conditions, these conditions should be defined as
closely as possible; all variable settings of the instrument should be defined. Particular attention should be
paid to those instruments whose settings can be altered by modification of a computer program where any
change might not be immediately apparent to the operator. Some method of checking that the program has
not been altered from the standard conditions should be devised.
The test procedure should be written, again in a manner similar to that of an International Standard, so that
subsequent operators are able to follow the method identical to that used during validation.
7 Procedure
7.1 Measurement of precision and trueness
7.1.1 Precision
Measure the precision of results in terms of the standard deviation of a set of analyses carried out under
repeatability conditions. The precision of the alternative method is measured directly by making replicate
analyses of samples.
If the alternative method is to be validated using reference samples, then calculate the standard deviation of
the International Standard method from the repeatability limit given in the International Standard method.
If a direct comparison is to be made between the two methods, then determine the precision of the
International Standard method directly by analysing samples in replicate. This will be a more accurate
measure than that calculated from the International Standard because the precision of the two methods on the
analysis of the same fuels is compared, rather than a precision determined at the time of development of the
International Standard on fuels whose identity is unlikely to be known.
7.1.2 Trueness
Estimate the trueness either by comparing the results obtained by analysing a reference material using the
alternative method with the reference value (see 7.2) or by comparing results on the same fuels using both the
alternative and International Standard methods (see 7.3).
Measurement of trueness can only be an estimate, the errors of which are measured by considering the
variability of the differences between the results. The greater the variability, the greater the estimation error,
the more results that are compared, the lower the estimation error.
Two different statistical analysis methods (A and B) are given in this International Standard.
Method A (see 7.2.2.1 and 7.3.2.1 for details) is recommended as the most rigorous. Decide before starting
on the maximum tolerable bias, MTB, and design the test to be sufficiently sensitive to detect that bias, should
it exist. Carry out a sufficient number of analyses to make the statistical test powerful enough to conclude
either that
a) the bias is significantly greater than zero and not significantly less than MTB; or
b) the bias is significantly less than MTB and not significantly greater than zero.
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In a simpler test (method B; see 7.2.2.2 and 7.3.2.2 for details), compare a fixed number of results and, unless
the mean difference fails a null hypothesis test, it can be concluded that no bias exists between the two
methods. If the estimation error is too great, however, it is also possible that a bias at an unacceptable level
could exist. To obviate such an ambiguous conclusion, use method A.
The difficulty with method A is in deciding what value to assign for MTB. However, with method B, it is
necessary to a make a judgement, after the result, as to what bias levels are tolerable. For either method,
therefore, it is necessary to consider the practical implications of any possible bias and to make a rational
judgement on what level is unacceptable.
7.2 Comparison with reference materials
7.2.1 Sources of reference materials
Coal and coke reference materials samples are widely available commercially. Before use, investigate the
traceability, authority and methods of analysis. Obtain details of homogeneity trials, stability trials, the methods
used for certification and the uncertainty and variations in the stated analyte values from the producer and use
them to judge the pedigree. In order to compare the alternative method with the International Standard
method, use only reference materials that have been analysed using the International Standard method.
Where an International Standard method contains a major element of choice, quote the subclauses from the
International Standard giving details of that variation.
Reference materials should be clearly labelled and stored under the specified conditions and should be
safeguarded against contamination or loss of determinand.
Select reference materials to be of the same type of fuel and to have the same analyte concentration ranges
as those which are to be analysed routinely by the alternative method. Some factors which should be
considered are coal rank, coke type or manufactured fuel type.
Where the range of fuels to be tested is very wide or reference materials of the appropriate type cannot be
obtained, test the method against the International Standard method (see 7.3 or 7.4).
Two methods are given for the estimation of trueness. The recommended method, method A, requires the
specification, before any analysis is done, of the greatest bias (the maximum tolerable bias) that the user is
prepared to risk; testing continues until an unambiguous conclusion is reached. In a simpler method,
method B, a fixed number of analyses is performed and a conclusion drawn from the result. In the latter
method, there is a possibility that an alternative method is considered to be unbiased even though there is a
risk that it is biased to an unacceptable extent.
7.2.2 Estimation of trueness
7.2.2.1 Method A
Decide on a value for the maximum tolerable bias, B (see 7.1.2).
Calculate the standard deviation of the International Standard s method using Equation (1):
ISO
r
s = (1)
ISO
22
where
s is the standard deviation of the International Standard method under repeatability conditions;
ISO
r is the repeatability limit for the International Standard method.
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Calculate the value of g using Equation (2):
B
g = (2)
s
ISO
Using Table 1, calculate the number of replicate tests, n, necessary to identify the MTB.
Table 1 — Values for factor g for calculating the number of analyses required
0 1 2 3 4 5 6 7 8 9
0 — — — 4,170 2,728 2,195 1,872 1,659 1,506 1,389
10 1,295 1,218 1,154 1,099 1,051 1,009 0,972 0,938 0,907 0,880
20 0,855 0,832 0,810 0,790 0,772 0,755 0,739 0,724 0,710 0,696
30 0,684 0,672 0,660 0,649 0,639 0,629 0,620 0,611 0,602 0,594
40 0,586 0,579 0,571 0,564 0,558 0,551 0,545 0,539 0,533 0,527
50 0,521 0,516 0,511 0,506 0,501 0,496 0,491 0,487 0,483 0,478
60 0,474 0,470 0,466 0,463 0,459 0,455 0,451 0,448 0,445 0,441
70 0,438 0,435 0,432 0,429 0,426 0,423 0,420 0,417 0,414 0,411
80 0,409 0,406 0,404 0,401 0,399 0,396 0,394 0,392 0,389 0,387
90 0,385 0,383 0,380 0,378 0,376 0,374 0,372 0,370 0,368 0,366
NOTE The number of sets required corresponding to a given g factor is the sum of the column and row headings.
Analyse the reference material using the alternative method n times.
Calculate s , using Equation (3):
ALT
2
x
()
∑
2
x −
∑
n
s = (3)
ALT
n −1
()
where
x is the sum of all the results;
∑
2
x is the sum of the squares of the results;
∑
n is the number of the results.
Recalculate g and hence n. If the new value for n is greater than the number of analyses already done, then
carry out additional analyses to reach n. Continue with this process until enough analyses have been carried
out.
Calculate the value of the mean difference of the analytical values from the reference value, d , using
Equation (4):
x
∑ i
dR=− (4)
n
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where
x is the analytical value of the ith determination;
i
R is the reference value.
Calculate the statistic t from Equation (5):
c
dn
t = (5)
c
s
ALT
Compare with the value of t from Table 3 at (n-1) degrees of freedom.
t
If t > t then it can be concluded that the alternative method is biased in comparison with the International
c t
Standard method. Otherwise, it can be concluded that any bias is significantly less than the maximum
tolerable bias.
Repeat this process for reference fuels taken from across
...
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