Drafting of Standards

Besides formal aspects of standardization, analytical standard methods have to follow a general structure with several obligatory technical instructions in the normative body of the standard. The ISO Directives, Part 2 (ISO/IEC, 2001) give advice for the structure of ISO standards. Standard developers can gain supplementary information from a model manuscript (ISO, 1998a). See Table 1.2 for an example of the structure of a EN ISO standard method.

This concept ensures a strict definition of the application range of the standard to be applied. In addition, informative annexes may be presented in order to give further examples and information to the user. The normative part of an analytical standard method in CEN and ISO includes at least the clauses listed below (ISO/IEC, 2001; ISO, 1998). Additional clauses, such as specific definitions or a list of minimum requirements needed, may be added to the standard, if relevant.

Table 1.2 Structure of EN/ISO 15061 (2001). EN/ISO 15061 Water quality - Determination of dissolved bromate: Method by liquid chromatography of ions

Foreword Introduction

1 Scope

2 Normative references

3 Interferences

4 Principle

5 Essential minimum requirements

6 Reagents

7 Apparatus

8 Quality requirements for the separator column

9 Sampling and sample pretreatment

10 Procedure

11 Calculation

12 Expression of results

13 Test report

Annex A (informative) Eluents

Annex B (informative) Regeneration solutions

Annex C (informative) Example of column switching technique

Annex D (informative) Interlaboratory trial

Annex E (informative) Checked interferences

Bibliography

Title

The title shall be concise and represent the parameter (sometimes also the analytical technique and matrix, if relevant) treated in the standard (ISO, 1998a). The user of the standard shall be aware that the method is strictly limited to the mentioned sample matrices and any analytical result based on an extension or alteration of the standard method in the user's laboratory cannot refer to the standard method.

Foreword

A foreword shall appear in each standard. It gives the designation and name of the technical committee and subcommittee that prepared the standard (ISO, 1998a).

Introduction

The introduction is an optional element containing commentary about the technical content of the standard or background information (ISO/IEC, 2001).

Scope

The scope specifies briefly the applicability (e.g., parameter to be determined, working range to be applied, appropriate sample matrix types) and the limitation of the method (ISO, 1998a). Limitation means exclusion of any expansion or changes of the standard method (e.g., addition of parameters or sample matrix types not listed).

Normative references

This clause lists a number of other standard methods essential for the application of the standard. Draft International Standards may also be cited in the list. All other documents, for instance any used for the development of the standard, may be listed in an informative bibliography (ISO, 1998a).

Interferences

This section gives information on the technical limitations of the standard method caused by, for example, sample matrix effects (coloured samples can interfere with the photometric detection or element specific spectral interferences (AAS, ICP-OES) or chemical interferences (precipitation reactions, formation of reaction byproducts). These details are validated experimentally in laboratories participating actively in the standardization work. The documentation of the interferences may help potential users of a specific standard to decide whether the standard method could be applicable for the requirements of their analytical businesses.

Principle

This clause gives a brief overview of the procedural basis of the analytical method.

Reagents

This clause contains a list of reagents and/or solutions used in the method (ISO/IEC, 2001) including information on the required purity grade as well as concentrations of solutions.

Apparatus

This section defines the analytical system to be used for the determination of the parameters listed in the title of the standard (ISO/IEC, 2001). The suitability of the specified apparatus has also been checked experimentally. Standards developers generally check different technical systems and appropriate alternative equipment, if this exists. Finally, the standards developers (working group members) decide which one of the possible alternative systems shall be part of the normative body of the standard. If applicable, alternative systems can be presented in an informative annex.

CEN and ISO standard developers should not refer to a sole supplier (monopoly situation). Equipment offered by a single manufacturer should not be specified. Where such equipment is not commercially available, detailed specifications for the equipment shall be given in order to enable all users to test comparable apparatus and systems (ISO/IEC, 2001).

Sampling and sample pre-treatment

Generally, this clause refers to an international standard, if such exists. If necessary, specific preconditions and methods of sampling or pre-treatment steps for the preservation of the samples (e.g., filtration, acidification, bottle material, storage conditions) are given.

Procedure

This clause gives advice about all of the procedural elements used for determination of the parameters. This includes the preparation of the test sample, the set-up procedure of the analytical system, the calibration strategy, and the measurement of the sample.

Calculation

This element gives instructions on how to convert a measured value obtained from the parameter of interest into a mass concentration, including the method of calculation (e.g., use of the inverse calibration function, consideration of blank values).

Expression of results

This clause defines the report format of the calculated results (e.g., dimension, number of significant figures).

Test report

The test report contains a minimum of information on the sample (e.g., result, identification data of the sample, applied standard method).

Interlaboratory trial

The presentation of statistical results of data from interlaboratory trials can be handled differently in CEN and ISO. CEN presents these data generally in the normative body of the standard, whereas ISO puts them in the informative annex of the standard. Besides the different philosophies in ISO and CEN on the layout of a standard, the organization and evaluation of an interlaboratory trial is obligatory for the standard developers in CEN and ISO, and the quality of the statistical data from the interlaboratory trial is the final categorical factor in the decision to publish the standard method, to postpone or to withdraw the project from the working list. The criteria for interlaboratory trial data to be met are given in ISO 5725, Part 2 (ISO, 2002).

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