• Table of Content
• Foreword
• Scope
• Uncertainty
• Analytical Measurement
• The Process
• Step 1: Specification
• Step 2: Identification
• Step 3: Quantification
• Step 4: Calculation
• Reporting Uncertainty
• Appendix A: Examples
  • Example 1: Calibration Standard
  • Example 2: Standardising NaOH
  • Example 3: An Acid/Base Titration
  • Example 4: In-house Validation Studies
  • Example 5: Determ. Cadmium Release
  • Example 6: Crude Fibre in Animal Feed
  • Example 7: Lead in Water - double IDMS
• Appendix B: Definitions
• Appendix C: Analytical Proc.
• Appendix D: Analyse U Sources
• Appendix E: Statistics
• Appendix F: MU at the Limit
• Appendix G: Common Sources
• Appendix H: Bibliography

Foreword to the Second Edition

Many important decisions are based on the results of chemical quantitative analysis; the results are used, for example, to estimate yields, to check materials against specifications or statutory limits, or to estimate monetary value. Whenever decisions are based on analytical results, it is important to have some indication of the quality of the results, that is, the extent to which they can be relied on for the purpose in hand. Users of the results of chemical analysis, particularly in those areas concerned with international trade, are coming under increasing pressure to eliminate the replication of effort frequently expended in obtaining them. Confidence in data obtained outside the user's own organisation is a prerequisite to meeting this objective. In some sectors of analytical chemistry it is now a formal (frequently legislative) requirement for laboratories to introduce quality assurance measures to ensure that they are capable of and are providing data of the required quality. Such measures include: the use of validated methods of analysis; the use of defined internal quality control procedures; participation in proficiency testing schemes; accredition based on ISO 17025 [h1], and establishing traceability of the results of the measurements.

In analytical chemistry, there has been great emphasis on precision of results obtained using a specified method, rather than on their traceability to a defined standard or SI unit. This has led the use of "official methods" to fulfil legislative and trading requirements. However as there is now a formal requirement to establish the confidence of results it is essential that a measurement result is traceable to defined standard such as a SI unit, reference material or, where applicable, a defined or empirical (sec. 5.2.) method. Internal quality control procedures, proficiency testing and accreditation can be an aid in establishing evidence of traceability to a given standard.

As a consequence of these requirements, chemists are, for their part, coming under increasing pressure to demonstrate the quality of their their results, and in particular to demonstrate their fitness for purpose by giving a measure of the confidence that can be placed on the result. This is expected to include the degree to which a result would be expected to agree with other results, normally irrespective of the methods used. One useful measure of this is measurement uncertainty.

Although the concept of measurement uncertainty has been recognised by chemists for many years, it was the publication in 1993 of the "Guide to the Expression of Uncertainty in Measurement" [h.2] by ISO in collaboration with BIPM, IEC, IFCC, IUPAC, IUPAP and OIML, which formally established general rules for evaluating and expressing uncertainty in measurement across a broad spectrum of measurements. This EURACHEM document shows how the concepts in the ISO Guide may be applied in chemical measurement. It first introduces the concept of uncertainty and the distinction between uncertainty and error. This is followed by a description of the steps involved in the evaluation of uncertainty with the processes illustrated by worked examples in appendix a.

The evaluation of uncertainty requires the analyst to look closely at all the possible sources of uncertainty. However, although a detailed study of this kind may require a considerable effort, it is essential that the effort expended should not be disproportionate. In practice a preliminary study will quickly identify the most significant sources of uncertainty, and as the examples show, the value obtained for the combined uncertainty is almost entirely controlled by the major contributions. A good estimate of uncertainty can be made by concentrating effort on the largest contributions. Further, once evaluated for a given method applied in a particular laboratory, the uncertainty estimate obtained may be reliably applied to subsequent results obtained by the method in the same laboratory, provided that this is justified by the relevant quality control data. No further effort should be necessary unless the method itself or the equipment used is changed, in which case the uncertainty estimate would be reviewed as part of the normal re-validation.

The first edition of the EURACHEM Guide for "Quantifying Uncertainty in Analytical Measurement" [h.3] was published in 1995 based on the ISO Guide.

This second edition of the EURACHEM Guide has been prepared in the light of practical experience of uncertainty estimation in chemistry laboratories and the even greater awareness of the need to introduce formal quality assurance procedures by laboratories. The second edition stresses that the procedures introduced by a laboratory to estimate its measurement uncertainty should be integrated with existing quality assurance measures, since these measures frequently provide much of the information required to evaluate the measurement uncertainty. The guide therefore provides explicitly for the use of validation and related data in the construction of uncertainty estimates in full compliance with formal ISO Guide principles. The approach is also consistent with the requirements of ISO 17025:1999 [h.1]

NOTE:
Worked examples are given in appendix a. A numbered list of definitions is given at appendix b. The convention is adopted of hyperlinking defined terms upon their first occurrence in the main body of the text, with a reference to Appendix B in square bracket. The definitions are, in the main, taken from the International vocabulary of basic and general standard terms in Metrology (VIM) [h.4], the Guide [h.2] and ISO 3534 (Statistics - Vocabulary and symbols) [h.5]. appendix c shows, in general terms, the overall structure of a chemical analysis leading to a measurement result. appendix d describes a general procedure which can be used to identify uncertainty components and plan further experiments as required; appendix e describes some statistical operations used in uncertainty estimation in analytical chemistry. appendix f discusses measurement uncertainty near detection limits. appendix g lists many common uncertainty sources and methods of estimating the value of the uncertainties. A bibliography is provided at appendix h.