Assessment of performance and uncertainty in qualitative chemical analysis

Introduction

Many decisions with socio-economic or individual impact depend on qualitative analysis, including decisions related to food safety, clinical diagnosis, and forensic evidence, are based primarily on qualitative, rather than quantitative, chemical analysis. Qualitative analysis is analysis that returns a classification rather than a numerical value, such as the identity of a chemical substance, the type of plastic of a microparticle, the potential source of an oil spill, the presence of a banned sports doping substance, or the presence of accelerant in fire debris. Sometimes, such classifications rely solely on qualitative tests; others may use measurement results (such as line frequencies in a spectrum) to reach a conclusion.

Qualitative analysis, like quantitative analysis, needs to be demonstrably reliable. Part of the purpose of the guide is accordingly to show how the performance of a qualitative analysis procedure can be quantified to ensure its fitness for purpose. Practical difficulties and limitations in reliable quantification of low false result rates are discussed, and recommendations are made for checking the validity of these analyses. Brief recommendations are also made for ensuring that any measurements undertaken in the course of a qualitative analysis are reliable.

Although laboratories accredited following the ISO/IEC 17025 standard are not currently required to express qualitative analysis results with uncertainty, the guide provides some metrics that can be used to convey the level of confidence in conclusions from qualitative analysis. In addition, the guide provides suggestions for reporting results with an indication of confidence if desired. 

This guide does not make recommendations on whether or not laboratories should routinely report information on the confidence, or uncertainty, associated with qualitative analysis results. Nevertheless, an understanding of performance, or of the uncertainty associated with a qualitative analysis result, can help laboratories to better advise customers; it can also help to identify quality improvements or determine (for example) whether additional confirmatory tests might be needed.

This Eurachem/CITAC guide accordingly aims to provide some tools to help improve the quality of qualitative analysis and of their interpretation, to make sure that interests dependent on these analyses are well protected.

Content of the guide

This guide includes information on 

• The nature and relevance of qualitative analysis.
• Different types of qualitative analysis - for example, those that depend on qualitative tests alone, and others that use quantitative information to come to a conclusion.
• Experimental and other strategies for estimating false response rates, which are fundamental to establishing reliability.
• Expressions of confidence in qualitative analysis.
• Reporting qualitative analysis results.
• Examples showing how false response rates and other metrics can be determined.
• The statistical basis for some important measures of confidence in a qualitative result.

Availability

The guide may be downloaded from this website at no cost.

• Download the Guide in Czech (PDF, 1.4 Mb) (Published 2022-12-20)
• Download the Guide in English (PDF, 1.8 Mb) (Published* 2021-11-17)
• Download the Guide in Farsi (PDF, 2.3 Mb) (Published 2024-04-04)
• Download the Guide in German (PDF, 1.4 Mb) (Published 2024-04-04)
• Download the Guide in Russian (PDF, 2.6 Mb) (Published 2024-06-18)

* First published 2021-11-11. Publication dates above are dates of file publication on this website.

Translation

Translation into other languages is permitted for members of Eurachem. Other offers of translation should be directed to the Eurachem Secretariat for permission. The Eurachem policy on maintenance and development of Eurachem guidance, available on the Policies page, gives further information on translation.

Citation

This guidance should be cited* as
"R Bettencourt da Silva and S L R Ellison (eds.) Eurachem/CITAC Guide: Assessment of performance and uncertainty in qualitative chemical analysis. First Edition, Eurachem (2021). ISBN 978-0-948926-39-6.
Available from www.eurachem.org."

*Subject to journal requirements.

Previous editions

There are no earlier editions of this Guide.

Use of uncertainty information in compliance assessment. (2nd ed. 2021)

Content

In order to decide whether a result indicates compliance or non-compliance with a specification, it is necessary to take into account the measurement uncertainty associated with the result. This guide provides guidance on how uncertainty may be taken into account in deciding compliance with a limit.

The guide is applicable to decisions on compliance with regulatory or manufacturing limits where a decision is made on the basis of a decision rule, together with a measurement value and the associated measurement uncertainty.

The guide includes a discussion and general recommendations, including the use of "guard bands" to improve the probability of correct acceptance or correct rejection. This is followed by more detailed guidance on establishing rules for interpretation and by several examples.

This second edition has been amended to take into account the developments in other international guides and Standards, including ILAC G8, "Guidelines on Decision Rules and Statements of Conformity" and JCGM 106, "Evaluation of measurement data – The role of measurement uncertainty in conformity assessment".

Major changes in this second edition are:

• the idea of an acceptance limit is introduced;
• decision rules that provide for conditional or inconclusive results are introduced (sometimes called “non-binary” decision rules);
• use of the lognormal distribution is introduced for some asymmetric cases;
• an Annex has been added to introduce the ideas of "global" and "specific" risks.

Availability

The guide may be downloaded from this website at no cost.

• Download the Guide in English (PDF, 1.2 Mb) (Published* 2021-04-29)
• Download the Guide in Farsi (PDF, 1.7 Mb) (Published 2021-12-09)
• Download the Guide in Russian (PDF, 837 kB) (Published 2023-02-13)
• Download the Guide in Ukrainian (PDF, 986 kB) (Published 2022-05-30)

* Publication dates are dates of publication on this website.

Translation

Translation into other languages is permitted for members of Eurachem. Other offers of translation should be directed to the Eurachem Secretariat for permission. The Eurachem policy on maintenance and development of Eurachem guidance, available on the Policies page, gives further information on translation.

Note: The First edition of this guide is available in English, Farsi and Portuguese.

Citation

This guidance should be cited* as
"A. Williams and B. Magnusson (eds.) Eurachem/CITAC Guide: Use of uncertainty information in compliance assessment (2nd ed. 2021). ISBN 978-0-948926-38-9.  Available from www.eurachem.org."

*Subject to journal requirements.

Previous editions

Earlier editions of this guide are available in the publication archive on this website.

Blanks in method validation

A Supplement to the Eurachem Guide "The Fitness for Purpose of
Analytical Methods"

Contents

Blanks are an important tool and are used in the determination of most performance characteristics during a validation process. They are also often included in each analytical run during routine use of the measurement procedure. There are many different types of blanks and the analyst must consider which blanks to include during preparation of the validation plan.

The aim of this supplement is to describe the different types of blanks which may be used during method validation and to provide guidance for situations where it may be difficult to obtain a suitable blank matrix. It is intended to be used in conjunction with "The Fitness for Purpose of Analytical Methods - A Laboratory Guide to Method Validation and Related Topics (3rd ed.)"

Availability

This supplementary guidance is available in the following languages:

• Download the guide in English (published 2025) (pdf, 245 kb).

Citation

This publication should be cited* as:
"H. Cantwell (ed.) Blanks in Method Validation - Supplement to Eurachem Guide The Fitness for Purpose of Analytical Methods, (2nd ed. 2025). Available from http://www.eurachem.org"

*Subject to journal requirements

Translations

Translation into other languages is permitted for members of Eurachem. Other offers of translation should be directed to the Eurachem Secretariat for permission. The Eurachem policy on maintenance and development of Eurachem guidance, available on the Policies page, gives further information on translation.

The previous edition of this supplement is available in a number of translations. Please see previous editions here.

Previous Editions

The first edition of this supplement, and its translations, is available here.