EN ISO 14111:1999

Natural gas - Guidelines to traceability in analysis (ISO 14111:1997)

Migrated from Progress Sheet (TC Comment) (2000-07-10): See mail from P. Pieters dated on 98-03-31 (BL)

Erdgas - Leitlinien für die Rückführbarkeit in der Analytik (ISO 14111:1997)

Diese Internationale Norm enthält allgemeine Leitlinien für die Umsetzung und Anwendung von Konzepten zur Rückführbarkeit in der Erdgasanalytik. Ihr Zweck besteht darin, die Grundlagen für die Entwicklung spezifischer Verfahrensanweisungen zur Rückführung in anderen Internationalen Normen der Erdgasanalytik festzuschreiben. Anmerkung: Neben der Analyse von Erdgas könnte diese Internationale Norm auch für andere Bereiche der Gasanalyse und verwandte Gebiete, so z.B. Luftgütemessung, Kraftfahrzeug-Abgasanalyse und Referenzgasherstellung von Nutzen sein.

Gaz naturel - Lignes directrices pour la traçabilité en analyse (ISO 14111:1997)

Zemeljski plin - Smernice za sledljivost pri analizah (ISO 14111:1997)

General Information

• Status: Published
• Publication Date: 23-Mar-1999
• ICS: 75.060 - Natural gas
• Technical Committee: CEN/TC 238 - Test gases, test pressures and categories of appliances
• Drafting Committee: CEN/SS N21 - Gaseous fuels and combustible gas
• Current Stage: 6060 - Definitive text made available (DAV) - Publishing
• Start Date: 24-Mar-1999
• Completion Date: 24-Mar-1999
• Ref Project: SIST EN ISO 14111:2000 - Natural gas - Guidelines to traceability in analysis (ISO 14111:1997)

Overview

EN ISO 14111:1999 (ISO 14111:1997) - "Natural gas - Guidelines to traceability in analysis" - provides general guidance on implementing metrological traceability in natural‑gas analysis. Rather than prescribing rigid protocols, this International Standard clarifies fundamental concepts of chemical traceability, identifies practical problems, and recommends feasible approaches based on reference gas mixtures and reference materials. It is intended to support the development of specific traceability protocols in other natural‑gas analysis standards.

Key topics and technical requirements

• Traceability concept: Definition of traceability as an unbroken chain of comparisons to national or international standards, with stated uncertainties.
• Traceability chains: Structure and hierarchy of calibration/traceability chains linking field methods to reference and definitive methods via certified reference materials.
• Reference materials and gas mixtures: Role and preparation of reference gas mixtures for calibration and verification; guidance for using reference materials for chemical calibration.
• Uncertainty and accuracy: Relationship between traceability, measurement uncertainty, trueness, precision and bias; role of traceability in uncertainty assessment.
• Chemical composition and SI units: Considerations for expressing natural‑gas composition consistently with SI units and established metrological practice.
• Implementation guidance: Practical advice for designing validated analytical systems and assembling traceability chains for natural‑gas measurements (note: the standard does not prescribe specific protocols).
• Supporting content: Definitions, examples (interlaboratory test data), and bibliography are provided in annexes to assist users.

Practical applications

EN ISO 14111 is a guidance document used to:

• Establish and document calibration chains for laboratory and on‑line natural gas analyzers.
• Support uncertainty budgets for gas composition, calorific value and related derived quantities (e.g., Wobbe index).
• Design quality assurance programs and validation strategies for measurement systems.
• Inform preparation and verification of reference gas mixtures used for instrument calibration.
• Serve as a reference when developing or updating measurement procedures in related fields such as air quality, vehicle emissions, and general gas analysis.

Who should use this standard

• Analytical chemists and laboratory managers performing natural‑gas composition analysis.
• Gas producers, suppliers and transmission operators responsible for custody transfer and contractual measurements.
• Instrument manufacturers and calibration laboratories producing or using reference gas mixtures.
• Quality assurance, metrology and regulatory bodies shaping measurement requirements and conformity assessment.
• Standards developers referencing traceability principles in other natural‑gas analytical standards.

Related standards (selected)

EN ISO 14111 references and complements other documents including:

• ISO 5725 series (accuracy, trueness and precision)
• ISO 6142 / ISO 6143 (preparation and comparison of calibration gas mixtures)
• ISO 6974 / ISO 6976 (natural gas analysis and calorific value calculations)
• ISO Guide 30, Guide 33, Guide 35 (reference materials and certified reference materials)

EN ISO 14111 is valuable as a foundation for implementing traceability and improving measurement confidence in natural‑gas analysis.

Standards Content (Sample)

SLOVENSKI STANDARD
01-december-2000
Zemeljski plin - Smernice za sledljivost pri analizah (ISO 14111:1997)
Natural gas - Guidelines to traceability in analysis (ISO 14111:1997)
Erdgas - Leitlinien für die Rückführbarkeit in der Analytik (ISO 14111:1997)
Gaz naturel - Lignes directrices pour la traçabilité en analyse (ISO 14111:1997)
Ta slovenski standard je istoveten z: EN ISO 14111:1999
ICS:
75.060 Zemeljski plin Natural gas
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

INTERNATIONAL IS0
STANDARD 14111
First edition
1997-03-15
Natural gas - Guidelines to traceability
in analysis
Gaz na turel - Lignes directrices pour la traCabilit6 en analyse
Reference number
IS0 14111 :I 997(E)
IS0 14111:1997(E)
Page
Contents
............................................................................................ 1
1 Scope
2 Normative references .
3 Definitions .
4 Fundamental principles of metrological traceability . 4
5 Elaboration of the traceability concept . 6
6 Chemical composition and the SI system . 7
7 Traceability in natural-gas analysis . 10
Hierarchy of reference gas mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
9 Role of traceability in uncertainty assessment
10 Implementation of traceability in International Standards for
...................................................................... 17
natural-gas analysis
11 Examples .
12 Summary .
Annexes
A Supplementary terms and definitions . 23
............................. 26
B Example of typical interlaboratory test results
.................................................................................. 28
C Bibliography
0 IS0 1997
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 the publisher.
International Organization for Standardization
Case postale 56 l CH-1211 Geneve 20 l Switzerland
Internet: [e-mail]
x.400: c=ch; a=400net; p=iso; o=isocs; s=central
Printed in Switzerland
ii
IS0 14111:1997(E)
@ IS0
Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work of
preparing International Standards is normally carried out through IS0
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. IS0
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
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.
International Standard IS0 14111 was prepared by Technical Committee
lSO/TC 193, Natural gas, Subcommittee SC 1, Analysis of natural gas.
Annexes A to C of this International Standard are for information only.

@ IS0
IS0 14111:1997(E)
Introduction
At a time when assurances of measurement accuracy in natural-gas
analyses are increasingly being sought, every analytical chemist
responsible for the design and operation of systems used in such analyses
needs to be aware of, and adopt, suitable means by which he or she will
be able to provide these assurances. This implies that the analyst must
employ validated methods in which each result is securely linked, through
a series of reference materials (reference gas mixtures), to accepted
metrological standards. The formal structure which the analyst creates in
doing this constitutes what is called a traceability chain. Only by this
means will the analyst be able to secure and support a proper estimate of
measurement accuracy (uncertainty).
This seemingly simple concept is elaborated in considerable detail in this
International Standard. The practical considerations involved in the
establishment of a satisfactory traceability chain give rise to challenging
problems, particularly in natural-gas analysis, but relevant and useful
advice is provided.
At present, traceability of measurement is universally defined through the
existence of unbroken calibration chains ending at the level of international
or national measurement standards realizing appropriate SI units. This
concept originates from the field of physical metrology, where it has been
implemented with apparent success. Transfer of the metrological scheme
to chemical analysis and other domains in the field of testing is, however,
a highly difficult task, for which standard methods are not yet available.
Therefore it is not possible, at present, to standardize the implementation
of measurement traceability in natural-gas analysis, or in other areas of
chemical analysis.
For the reasons indicated above, this International Standard does not give
any specific traceability protocols. Instead, its purpose is to
- clarify fundamental concepts involved in chemical traceability;
identify basic problems in the application of metrology in chemistry;
indicate feasible solutions on a reference material basis;
implementations using reference gas
- assist in the design of practical
mixtures;
- serve as a reference document for the application of the traceability
concept in other International Standards for natural-gas analysis.
iv
IS0 14111:1997(E)
INTERNATIONAL STANDARD @ IS0
- Guidelines to traceability in analysis
Natural gas
1 Scope
This International Standard provides general guidelines on the implementation and application of traceability
concepts in the analysis of natural gas. Its purpose is to lay down the foundations for the development of specific
traceability protocols in other International Standards for natural-gas analysis.
NOTE - Besides the field of natural-gas analysis, this International Standard could also be useful as a guidance document in
other areas of gas analysis and in related fields such as air quality measurement, vehicle emission monitoring and reference-
gas mixture preparation.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this
International Standard. At the time of publication, the editions indicated were valid. All standards are subject to
revision, and parties to agreements based on this International Standard are encouraged to investigate the
possibility of applying the most recent editions of the standards indicated below. Members of IEC and IS0 maintain
registers of currently valid International Standards.
- Part 1: Probability and general statistical terms.
IS0 3534-l : 1993, Statistics - Vocabulary and symbols
IS0 5168:- 1) , Measurement of fluid flow - Evaluation of uncertainties.
IS0 5725-l :I 994, Accuracy (trueness and precision) of measurement methods and results - Part I: General
principles and definitions.
IS0 5725-2: 1994, Accuracy (trueness and precision) of measurement methods and results - Part 2: Basic method
for the determination of repeatability and reproducibility of a standard measurement method.
IS0 5725-3:1994, Accuracy (trueness and precision) of measurement methods and results - Part 3: Intermediate
measures of the precision of a measurement method.
IS0 5725-4:1994, Accuracy (trueness and precision) of measurement methods and results - Part 4: Basic
methods for the determination of the trueness of a standard measurement method.
IS0 5725-6:1994, Accuracy (trueness and precision) of measurement methods and results - Part 6: Use in
practice of accuracy values.
Preparation of calibration gas mixtures - Weighing methods (including addendum 1).
IS0 6142:1981, Gas analysis -
1) To be published. (Revision of IS0 5168:1978)

@ IS0
IS0 14111:1997(E)
Determination of composition of calibration gas mixtures - Comparison methods.
IS0 6143:1981, Gas analysis -
Checking of calibration gas mixtures by a comparison method.
IS0 6711 :I 981, Gas analysis -
Determination of composition with defined uncertainty by gas chromatography -
IS0 6974-l :- *), Natural gas -
Part I: Guidelines for tailored analysis.
Determination of composition with defined uncertainty by gas chromatography -
I SO 6974-2:- *), Natural gas -
Part 2: Measuring-s ys tern characteristics and statistics for data processing.
IS0 6976: 1995, Natural gas - Calculation of calorific values, density, relative density and Wobbe index from
composition.
Model for quality assurance in design, development, production, installation and
IS0 9001 :I 994, Quality systems -
servicing.
Part 1: Metrological confirmation
IS0 10012-I :I 992, Quality assurance requirements for measuring equipment -
s ys tern for measuring equipment.
IS0 10723:1995, Natural gas - Performance requirements for on-line analytical s ys terns.
IS0 Guide 30:1992, Terms and definitions used in connection with reference materials.
IS0 Guide 33:1989, Uses of certified reference materials.
General and statistical principles.
IS0 Guide 35:1989, Certification of reference materials -
BIPM/IEC/IS0/0IML/IFCC/IUPAC. International vocabulary of basic and general terms in metrology (VIM), second
edition, 1993.
3 Definitions
For the purposes of this International Standard, the following definitions apply.
3.1 traceability: A property of the result of a measurement or the value of a standard whereby it can be related
to stated references, usually national or international standards, through an unbroken chain of comparisons all
having stated uncertainties.
NOTES
1 The concept is often expressed by the adjective “traceable ”.
2 The unbroken chain of comparisons is called a “traceability chain ”. [VIM]
3.2 (measurement) standard, etalon: A material measure, measuring instrument, reference material or
measuring system intended to define, realize, conserve or reproduce a unit or one or more values of a quantity to
serve as a reference.
EXAMPLES
a) 1 kg mass standard;
b) 100 &I standard resistor;
standard ammeter;
c)
d) caesium frequency standard;
e) standard hydrogen electrode;
f) reference solution of cortisol in human serum having a certified concentration. [VIM]
2) To be published. (Revision, in parts, of IS0 6974:1984)
@ IS0
IS0 14111:1997(E)
3.3 reference material: A material or substance one or more of whose property values are sufficiently
homogeneous and well established to be used for the calibration of an apparatus, the assessment of a
measurement method or for assigning values to materials.
A reference material may be in the form of a pure or mixed gas, liquid or solid. Examples are water for the
NOTE -
calibration of viscometers, sapphire as a heat-capacity calibrant in calorimetry, and solutions used for calibration in chemical
analysis. [ISO Guide 301
34 . Terms related to accuracy and uncertainty
NOTE - Since traceability essentially serves the purpose of assessment and control of accuracy, viz the uncertainty of
measurement, the following terms relating to accuracy and uncertainty are also key terms of this document. The definitions,
taken from IS0 3534-1, have been adapted to usage in the field of measurement instead of testing, by substitution of
corresponding terms ( “measurement result” instead of “test result ”, and “true value” instead of “accepted reference value ”).
In some cases, the notes to the definitions have also been modified.
3.4.1 accuracy: The closeness of agreement between a measurement result and the true value of the
measurand.
NOTE - The term accuracy, whe n applied to a set of measurement results, describes a combination of random components
bias corn pone nt. [Adapted from IS0 3534-I]
and a corn mon systematic error or
3.4.2 trueness: The clo seness of ag reement b etween the average value obtained from a large series of
the true value of the m ea surand.
meas urement results and
NOTES
The measure of trueness is usually expressed in terms of bias.
2 Until recently, “accuracy” was used with the meaning of “trueness ”. This usage no longer conforms with international
standardization. [Adapted from IS0 3534-l I
3.4.3 precision: The closeness of agreement between independent measurement results obtained under
presc ribed conditions.
NOTES
1 Precision depends only on the distribution of random errors and does not relate to the true value.
2 Precision is a qualitative term relating to the dispersion between the results of measurements of the same measurand,
carried out under specified conditions of measurement. Quantitative measures of precision such as variance or standard
deviation critically depend on the variation implied by the specified measurement conditions. Repeatability and reproducibility
are two particular concepts of precision, relating to the endpoints on the scale of variability in measurement conditions.
[Adapted from IS0 3534-l 1
.
measurement result which characterizes the range of values within
3.4.4 uncertainty . An estimate a ttach ed to a
.
.
which the true valu asserted to Ire
NOTES
1 Uncertainty of measurements comprises, in general, many components. Some of these components may be estimated on
the basis of the statistical distribution of the results of series of measurements and can be characterized by experimental
standard deviations. Estimates of other components can only be based on experience or other information.
2 Uncertainty should be distinguished from an estimate attached to a measurement result which characterizes the range of
values within which the expectation is asserted to lie. This latter estimate is a measure of precision rather than of accuracy and
should be used only when the true value is not defined. When the expectation is used instead of the true value, the expression
“random component of uncertainty” must be used. [Adapted from IS0 3534-I]
For suggested further reading see annex C, reference [IT].
3.4.5 Further comment on main terms
of
Since the termi nology relating to accuracy/uncertainty measurement has recently undergone substantial
changes, a short comment on th e meaning of the main ter ms will be given.
@ IS0
IS0 14111:1997(E)
“Accuracy ”, “trueness” and “precision” are qualitative terms used to express the smallness of expected
measurement errors. Hereby accuracy as the more general term refers to the total measurement error, trueness to
the systematic component(s) of the measurement error and precision to the random component(s) of the
measurement error.
and “random uncertainty (dispersion)” are qualitative terms used to
“Uncertainty ”, “systematic uncertainty”
express the extent of expected measurement errors, as the counterparts of accuracy, trueness and precision,
respectively. Accuracy and uncertainty are reciprocal terms: high accuracy is equivalent to small uncertainty, and
the same is true for both the other pairs of reciprocal terms - trueness/systematic uncertainty and
precision/random uncertainty (dispersion).
For quantitative expressions of accuracy or uncertainty, the common measures, derived from the results of
repeated measurements, are:
“bias” for systematic uncertainty
and
“standard deviation” for random uncertainty (dispersion).
NOTES
1 This clause gives those terms and definitions which are essential to understand before proceeding further in the text. Other
terms and definitions used in the text, for which it is not necessary to have an exact understanding at this stage, are given in
annex A.
2 This document mainly employs terms which have been defined previously by committees within ISO, OIML (International
Organization of Legal Metrology), BIPM (Bureau International des Poids et Mesures) and IEC (International Electrotechnical
Commission), as well as terms and definitions which are being proposed with revisions of other International Standards or
Guides.
3 In producing this document, it has been acknowledged that there are serious problems in applying some terms, which
originate from physical metrology, to the field of chemical metrology. Furthermore, no international vocabulary of basic and
general terms for chemical metrology is yet available. Therefore additional notes and remarks are appended to the definitions
given both here and in annex A wherever this has been felt necessary for clarification.
4 Fundamental principles of metrological traceability
4.1 Traceability and accuracy
In recent years, the term “traceability” has come into considerable vogue, but in doing so it has (in common with
many other technical terms) tended to lose its proper scientific pedigree. Thus it has been turned into a general-
purpose catchword, (mis)used in a variety of generous interpretations, extending down as far as nothing much
more than a tenuous synonym for reliability. In this document, however, it is used exclusively in the original and
authentic scientific sense of metrological traceability.
In this sense, traceability is essentially a means of providing an assurance that the accuracy of the results from one
measurement system or technique can be related in a known way (transferred) to the results from another. For
example, the result of an “everyday” (field) method should be demonstrably traceable to the result of a reference
method, and the result of a reference method should be demonstrably traceable to the result of a definitive
method. Traceability is usually mediated by some kind of (certified) reference object or material having known
metrological qualities.
4.2 Structure of traceability chains
Self-evidently, the literal meaning of traceability is the ability to trace. In metrology (the science of measurement)
this implies the existence of an unbroken, identifiable and demonstrable pathway between the measurement
process in question and some quantity or set of quantities regarded as “fundamental” or “indisputable ”. Such a
pathway is called a traceability chain; the most complete chains have clear links all the way back to SI units.
The purp
ose of all claims for t raceability is to esta b lish, or guarantee the accuracy of measuremen t. Measurement
I
consists almost al ways of the comparison an un
of known, the value of which is desired, with a sta n dard, the value
@ IS0 IS0 14111:1997(E)
of which is taken as known. In physical measurements, the known is often an object calibrated (using a defined
method) against a higher standard within a hierarchical structure. By moving upwards through the various levels in
such a hierarchy, traceability to primary standards can be obtained.
The major conceptual elements which are usually present in a typical traceability hierarchy are indicated in figure 1;
what is needed in order to address any real metrological question is the existence, at each level, of reference
objects or materials that can be used in realizing the standard represented by that level.
Level 0
Level 1
Level 2
Level 3
Test sample
Level 4
Figure 1 - Conceptual traceability hierarchy
The transfer of information between the various levels in the hierarchy is accomplished by methodology
interconnections which create the traceability chain. Such a chain typically has many links between each level in
the hierarchy. Each link is formed by either the whole or, more commonly, some sub-system or part of the defined
method, and will probably involve auxiliary standard objects or materials (e.g. thermometers or mass pieces) which
realize values or scales of subsidiary or subordinate physical properties.
In apt analogy with a mechanical chain, it is clear that a traceability chain is no stronger than its weakest link. The
best chains have few links, each of which is very secure. When the pathway is fully defined and documented, an
assignment of uncertainty can be made at any point in the chain and at each level in the hierarchy. If the pathway is
broken (that is, if linking information is missing), uncertainties of measurement cannot be assigned at that point,
and complete traceability cannot be obtained. Consequently, the measured value is then not traceable to SI units,
perhaps not even to primary or secondary standards, but just as far as to where the break occurs. A statement
about traceability without reference to the end-point of the chain is of no value.
4.3 Traceability in chemical analysis
In essence, then, traceability is an information retrieval process. In chemical analysis, the information needed to
support the result comprises details of the analytical methods and reference materials used, together with all the
associated uncertainties.
As discussed in greater detail in IS0 Guide 35, traceability is much more difficult to realize in quantitative chemical
analysis (chemical metrology) than in physical metrology, mainly due to the complexities of the overall analytical
process Nevertheless, the concept of traceability is similar, at least in principle.
Analyses must be made by comparison of the relevant attributes of the sample against the known attributes of
reference materials. This may be done either directly, or indirectly by means of scales or instruments that have
been calibrated using (one or more) accepted reference values.
@ IS0
IS0 14111:1997(E)
The additional complexities arise because a proper correspondence between the sample and reference material
may be difficult to achieve for a variety of reasons.
Firstly, several reference materials realizing various levels of composition for each target component may be
required for a multi-component sample. Secondly, the matrix which contains the analyte could have a significantly
disturbing effect on the analysis. Similarly, any other chemical compound present in the sample may have an
interferent effect on the determination of the target compound. Finally, the sampling procedure itself is a significant
error source, e.g. due to lack of homogeneity of the bulk material from which the sample is taken, and to
contamination as well as degradation of the sample.
The exact requirements and procedures (protocols) necessary to ensure traceability must, therefore, depend upon
the specific problem being addressed. In chemical metrology, the proper transfer of accuracy can only be achieved
with very detailed protocols. Any such protocol should be considered as a fundamental part of the particular
analytical method, and can therefore become an integral part of an international standard method.
5 Elaboration of the traceability concept
5.1 Distinction from related concepts
Despite what may appear above as a clear identification of what is meant by traceability, there remain differing
interpretations of just what the concept can involve. These differences seem to arise because usage of the term is
fed not from a single discipline, but from such diverse sources as legal regulation of operational practices,
monitoring the performance standards of instruments or machines, and quality assurance in manufacturing
processes, as well as from pure metrological science.
Thus, the current main interpretations of traceability, discernible to the present authors, are
a) Traceability = Ability to provide complete information about every step involved in or relevant to arriving at a
measurement result, by documented records.
b) Traceability = Ability to provide evidence that measurement results are equivalent to results obtained by an
authoritative laboratory.
Traceability = Ability to demonstrate that a measuring system regularly produces accurate results on selected
d
measurands.
= Ability to prove the validity of individual measurement results by complete reduction to, for
d) Traceability
example, property values realized by measurement standards or reference materials, or to accepted values of
physical constants.
These concepts are termed, respectively, “administrative ”, “authoritarian ”, “demonstrative” and “definitive ”. They
are increasingly purposeful in the order given.
The administrative concept (a) is of little concern in science because, while extensive documentation may be
necessary, it is not sufficient to achieve the intent of traceability, namely the assurance of adequate accuracy of
measurement. With regard to this goal, the authoritarian concept (b) is also rather unsatisfactory, since it merely
refers to apparently correct results instead of demanding procedural correctness (i.e. the presence of proper
metrological links).
The definition of traceability given in 3.1, adopted from the International Vocabulary of Basic and General Terms in
Metrology (VIM), permits interpretation in the senses intended by both concept (c) and concept (d). As the main
difference, traceability according to the definitive concept (d) implies assurances of validity for individual
measurement results, and therefore demands considerably more than the demonstrative concept (c) where the
aim is verification of overall measurement system performance.
5.2 Requirements for secure traceability chains
The formal requirements for secure metrological traceability are clearly embedded in definition 3.1, interpreted and
illuminated in accordance with the definitive concept (d) defined in 5.1. The main features can be enumerated in
more detail as follows.
0 IS0 IS0 14111:1997(E)
ity chain between the test object or sample and the standard reference
a) There shall exist an unbroken traceabi
traceability is to be claimed. The latter should normally be a national or
object or reference material to which
international standard, which may be 2 realization of the appropriate SI unit.
b) The traceability chain normally has to include intermediate standard objects or materials in a hierarchical
structure. These intermediate standards shall be of established metrological provenance.
c) The various levels in the hierarchical structure shall be linked by specified and validated test methods which, by
comparisons between objects or materials, allow the transfer of information pertaining to accuracy from one
level to the next lower level. The protocols by which comparisons are made shall be sufficiently well defined
that a result is adequately reproducible.
ard objects or materials used
d) For each test method, any auxiliary stand shal I be traceable to relevant definitive
standards through an auxiliary traceability chain.
stimate of uncertainty to each measurement to
It shall be possible to assign an e in the traceability chain, and
e)
transfer or combine al I of these tn such a way that the desired result carries a p roven assurance 0 f accuracy.
5.3 Applications to chemical analysis
In (quantitative) chemical analysis, direct traceability of individual results to (realizations of) fundamental units is
normally prohibitive, in particular for field analyses, due to the reasons explained in 6.4. As an executable
alternative, traceability of performance (see 7.2.3), in particular of calibration, but e.g. also of separation or
specificity, can be established using either reference analytical methods of known performance or reference
materials of known accuracy. Concerning the latter alternative, as the more typical one in chemical analysis,
traceability of performance is essentially reduced to traceability of the reference materials used in calibration.
These, in turn, must be traced back further along a chain consisting of higher-level reference materials and
measurement systems or methods, until reaching a reference standard of definitive accuracy. Then, in
consequence, accuracy can be assessed on every lower level down to the field measuring system.
As explained in more detail in 6.3, chemical composition can, in principle, be traced back to (primary realizations of)
an SI unit of a physical quantity of composition, and the chemical species concerned. In fortunate cases such as
major parts of gas analysis, traceability of reference material to fundamental units can be established, cf. 7.2 and
8.3.
However, in many other fields of chemical analysis, the step relating complex material composition to fundamental
units, through a fundamental method, is too wide to be implemented with full command of accuracy. Then
traceability chains necessarily terminate at the level of primary reference materials, of complex composition. As a
consequence, alternative methods are needed for the assessment of accuracy of these primary reference
materials.
6 Chemical composition and the SI system
6.1 Quantities for portions of substances
In chemical metrology, the relationships between quantities associated with samples of substances are elaborated.
Since matter is usually defined as “anything that has a mass and occupies space ”, the two most commonly
recognized physical quantities designating the amount of a sample of matter are mass m (unit: kg) and volume v
(unit: m3).
The number of entities N (no unit, dimensionless) in a sample of substance is another such quantity. These entities
may be atoms, molecules, ions, etc., or any combinations of these.
A fourth such quantity is the amount of substance ~1 (unit: mole). The mole is directly based on a specific number of
entities, the number of atoms in 12 g of carbon-l 2. When the mole is applied, the elementary entities have to be
specified. For the mole, it is not possible yet to realize an unambiguous standard. Therefore standards for molar
quantities are made using the standard of mass and accepted reference values of atomic/molar masses as
proportionality constants.
@ IS0
IS0 14111:1997(E)
6.2 Quantities and units of chemical composition
The basic task of chemical analysis is to determine the composition of substances. As an extreme case, complete
analysis of an entirely unknown substance amounts to the qualitative identification of all its constituents and the
quantitative determination of their proportions. In general, the task will be to determine accurately the content of
one or several specified constituents of a substance with approximately known composition. Here the meaning of
the term “constituent” largely depends on the context. In the case of a pure substance, that is, of a chemical
compound, composition usually refers to the constituent chemical elements, while in the case of a mixture it refers
to the constituent pure substances.
From the side of physical metrology, it is often argued that chemical analyses are essentially measurements of a
single physical quantity, the amount of substance n, and therefore, in principle, should be traceable to the mole as
the SI unit of the amount of substance. This assertion is based upon a fundamental misconception. In mixture
analysis, the measurand never happens to be the amount of substance as such but always in conjunction with a
specified chemical species, the content of which is to be determined in a given mixture. Obviously the
misconception mentioned above is due to erroneously considering chemical species as measuring objects. In
mixture analysis, however, the measuring objects are the mixtures to be analysed, while the individual chemical
species define the various measurands, that is, the quantities to be measured. The determination of the contents
of two different species in a mixture, e.g. the determination of the water content and the determination of the
sulfur dioxide content in air, are two fundamentally different measuring tasks - such as the determination of the
mass and the determination of the volume of a material body.
The claim that chemical analysis essentially deals with the measurement of a single quantity of composition is
mistaken. Instead, the scope of chemical analysis consists of measurement of as many different quantities of
composition as there are different analytically relevant chemical species.
For the expression of mixture composition, a number of different quantities are used, which are quotients of two
(not necessarily like) quantities, expressing the amount of a specified mixture component and the amount of the
mixture. The common quantities of composition are mass concentration, volume concentration and amount-of-
substance concentration (molar concentration), and mass fraction, volume fraction and amount-of-substance
fraction (molar fraction). Among these, the mass fraction and the molar fraction have the benefit of being
independent of the state (temperature and pressure) of the mixture. In gas analysis, however, the volume fraction
is still in use.
From the previous argumentation it follows that mixture composition cannot be adequately expressed in the SI
system, unless it is complemented by the chemical species of the mixture constituents. In fact, specification of the
composition of a mixture requires
specification of every mixture constituent;
a)
the numerical value of the proportion or concentration of every mixture constituent.
b)
6.3 Traceability of mixture composition to fundamental units
As explained in the previous subclause, specification of the composition of a mixture with N components involves
(N + 1) fundamental metrological units or entities: N qualitative ones, defining the mixture components, and a single
quantitative one, defining the scale on which component proportions or concentrations are measured.
As a consequence, metrological traceability of mixture composition involves more than just traceability to an SI
unit. In addition, it involves traceability to reference materials as measurement standards, providing primary
In mass spectrometry, traceability even refers
realizations of the chemical species present in the mixture.
exclusively to chemical species. This is due to the fact that in this method molar fractions are measured directly, as
relative particle numbers, and counting does not refer to any scale or unit.
Additional complications arise if measurement-related interferences among mixture components have to be taken
into account. In the absence of any such interactions, a multicomponent mixture, consisting e.g. of several target
components and a single balance component can be rigorously related to a corresponding number of binary
mixtures, each realizing the content of one of the target components. If interferences among mixture components
cannot be safely excluded, such reduction to binary mixtures is not possible. As a consequence, traceability can
then only be established among multicomponent mixtures of closely related composition. In such cases - which,
0 IS0 IS0 14111:1997(E)
unfortunately, are numerous among current analytical problems as e.g. in environmental monitoring - traceability
chains necessarily terminate at complex reference materials of composition similar to that of the analyte. Tracing
further back to (realizations of) fundamental units, that is, to chemical species and SI units of composition, is
discouraged due to the enormous increase in uncertainty that would be the consequence.
6.4 Impracticality of direct traceability to SI units
Due to the complexity of chemical-composition analysis, it is utterly impractible to try to trace back, directly, every
individual analytical result to measurement standards realizing the totality of SI units involved - such as mass,
volume, temperature and pressure, as well as to standards for all the chemical species involved. This would imply
ending up with an uncertainty budget built up from specified uncertainties attributed to every single step or item.
However, in any such costly exercise there will inevitably be missing links in the “traceability network ”.
Moreover, due to cumulative effects, extended error propagation calculations aiming at identification of all error
sources, quantification of their individual contributions and combination into a measure of the overall uncertainty
typically result in notoriously high estimates of uncertainty. This is related to the fact that uncertainty estimation
aims at conservative upper bounds of expected errors which invariably add and never cancel unless correlation
between different error sources is taken into account - which is clearly out of reach for everyday practice.
As an alternative to this “genealogical” approach in uncertainty estimation, the accuracy of an analytical method
can be investigated empirically by direct comparison of measurement results with corresponding reference values
realized by reference materials or obtained by reference methods of measurement. For example, the performance
of a method of composition analysis can be evaluated using suitable reference materials as standards of
composition. For this purpose, the analytical procedures must be fixed as closely as possible, thereby minimizing
the number of variance components that contribute to the dispersion of results. By repeated analysis of reference
materials of known composition (known within definitive uncertainty limits), precision as well as trueness, and with
that the accuracy, of the analytical method can be estimated. Of course, these estimates properly refer to isolated
points on a multidimensional range of composition only, with extension by interpolation and extrapolation requiring
utmost care, e.g. in assessing the additional uncertainty caused by these mathematical procedures. An approach
like this is well suited to provide traceability of performance (in particular of calibration) to reference materials as
standards of composition.
uncertainty on the calorific value of natural gas, dete rmined by composition analysis and
As another example, the
calculation in accordance with IS0 6976, could be investigated by two diff erent methods, as follows:
a) Genealogical approach: Extended error propagation calculation taking into account errors in composition
caused by the analytical method, errors in the calorific values of natural-gas constituents and deviations from
the mathematical model (linear combination of component calorific values). The uncertainty of the analytical
method - typically gas chromatography - is traced back to various sources.
b) Empirical approach: Direct comparison of results obtained by the method under investigation and a suitable
reference method (calorimetry) on a number of gas mixtures covering the composition range considered.
Summarizing, in chemical analysis, direct traceability for every single analytical measurement to (realizations of) all
the relevant SI units, as propagated by metrological standards and guidelines, is often beyond reach, due to the
complexity of the corresponding traceability networks. Reference materials then can provide “intermediate”
standards, realizing appropriate composition levels, with traceability chains sufficiently short to enable realistic
uncertainty assessment. In analytical chemistry, it has become more and more common practice to use reference
materials and especially certified reference materials as intermediate standards.
6.5 Role of fundamental methods
The measurement methods used in present-day chemical analyses are typically indirect methods in which the
target quantity, the concentration or proportion of a specified chemical species, is not measured directly. Instead,
the measured quantity is an appropriate instrumental response variable. Such measuring systems, therefore,
require calibration in the sense that the functional relationship between the response variable and the target
variable is determined empirically, by measuring the response to known values of the target variable, as realized
e.g. by corresponding reference materials as calibrants.

@ IS0
IS0 14111:1997(E)
termed comparison m ethods or relative methods), fundamental methods are
In contrast to indirect methods (also
those in which the target qua ntity is measured directly. For example, ma ny of the typical methods of “wet
where the target species is separated from the sample, identified
analytical chemistry” are fundamental methods,
and quantified, e.g. by weighing. Such methods evidently do not require calibration in the sense described above.
However, their performance must be validated, that is, their measurement uncertainty (or recovery rate) must be
determined empirically.
called direct methods or absolute methods), an equally important
Besides fundamental methods of analysis (also
branch of fundamental methods are methods for the production of reference materials, e.g. the manufacture of
mixed substances from pure compounds by gravimetry.
Evidently, fundamental methods are indispensable for establishing the final links of traceability chains at the level of
fundamental units. As explained in 6.3, for composition analysis these fundamental units are the SI unit for the
considered physical quantity of composition, and the chemical species of the considered mixture constituents.
Only if this final link can be firmly established, within definitive uncertainty limits, can definitive uncertainty limits
also be attributed to all of the lower-level measurement systems, measurement standards and reference materials.
In the field of gas analysis - with the exclusion of trace analysis and reactive components - gravimetric
preparation of reference gas mixtures is generally considered as a high-accuracy fundamental method. This is true,
provided that all the relevant uncertainty components are assessed carefully, as e.g. the impurities of the parent
gases. Subclause 8.3.1 contains a detailed discussion of the uncertainty components in gravimetric gas mixture
preparation.
7 Traceability in natural-gas analysis
7.1 Analysis of natural gas
Natural gas and natural-gas substitutes are multicomponent gas mixtures, with the major constituents including a
range of hydrocarbons (mostly alkanes) and a number of permanent gases (e.g. helium, nitrogen, carbon oxides and
hydrogen), plus a multitude of trace components such as water vapour and volatile sulfur compounds.
The scope of natural-gas analysis comprises the following:
analysis of composition, in the sense of quantitative determination of each component that is considered
a)
relevant to the calculation of some bulk property of interest, such as calorific value, relative density or Wobbe
index;
b) analysis for important trace components, such as hydrogen sulfide, heavy hydrocarbons or mercaptans.
For the analysis of natural gas, the importance
...