Angelique Botha

Final report on international key comparison CCQM-K53: Oxygen in nitrogen

Gravimetry is used as the primary method for the preparation of primary standard gas mixtures in most national metrology institutes, and it requires the combined abilities of purity assessment, weighing technique and analytical skills. At the CCQM GAWG meeting in October 2005, it was agreed that KRISS should coordinate a key comparison, CCQM-K53, on the gravimetric preparation of gas, at a level of 100 ?mol/mol of oxygen in nitrogen. KRISS compared the gravimetric value of each cylinder with an analytical instrument. A preparation for oxygen gas standard mixture requires particular care to be accurate, because oxygen is a major component of the atmosphere. Key issues for this comparison are related to (1) the gravimetric technique which needs at least two steps for dilution, (2) oxygen impurity in nitrogen, and (3) argon impurity in nitrogen. The key comparison reference value is obtained from the linear regression line (with origin) of a selected set of participants. The KCRV subset, except one, agree with each other. The standard deviation of the x-residuals of this group (which consists of NMIJ, VSL, NIST, NPL, BAM, KRISS and CENAM) is 0.056 ?mol/mol and consistent with the uncertainties given to their standard mixtures. The standard deviation of the residuals of all participating laboratory is 0.182 ?mol/mol. With respect to impurity analysis, overall argon amounts of the cylinders are in the region of about 3 ?mol/mol; however; four cylinders showed an argon amount fraction over 10 ?mol/mol. Two of these are inconsistent with the KCRV subset. The explicit separation between two peaks of oxygen and argon in the GC chromatogram is essential to maintain analytical capability. Additionally oxygen impurity analysis in nitrogen is indispensable to ensure the preparative capability. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

A case study for the provision of measurement traceability and measurement uncertainty for the South African reference materials

The South African Reference Materials (SARMs) have traditionally been certified through interlaboratory studies involving reputable national and international laboratories. The ISO definition of a Certified Reference Material (CRM) has changed recently. In view of these changes re-certification of the SARMs with the use of primary methods to provide measurement traceability and complete evaluation of the uncertainties associated with the methods are urgently needed. The CSIR-National Metrology Laboratory of South Africa (CSIR-NML) has embarked on a project to re-certify some of the rock type South African Reference Materials (SARM 3 and 4) for selected trace elements using the primary method of isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS). Analysis data published in the literature for these SARMs since their production and certification in 1978 was also collected and critically evaluated (data due to be published). Based on this work, new recommended values with associated expanded uncertainties are proposed for some trace elements of SARM 3 and SARM 4.

The new ISO Guide 80: Guidance for the in-house preparation of quality control materials (QCMs)

Abstract Reference materials (RMs) are widely used in measurement laboratories for a variety of purposes, and it is important to recognise that the material most appropriate for a particular application should be used. Certified reference materials (CRMs) are used for method validation, the calibration of a measurement system and all other aspects of the evaluation of the measurement system where the trueness of the measurement result is required. For other aspects, such as quality control, precision studies, the checking of the variability between operators, where the results are compared relatively, any suitable reference material can be used. ISO/REMCO, the ISO Committee on Reference Materials, has prepared ISO Guide 80, a guidance document for the in-house preparation of quality control materials (QCMs). QCMs are mostly used to monitor the performance of laboratory methods that have already been validated over time to be able to detect change or when a method goes out of statistical control. QCMs are RMs and as such have to be sufficiently homogeneous and stable for the intended use. QCMs are usually prepared in-house by laboratory staff for in-house use only, and therefore, the requirements for “in-house” QCMs are less demanding than those for a CRM. For example, transport issues are not of concern. The quality assessment of QCMs should involve homogeneity and stability assessments, and a limited characterisation of the material to provide an indication of its relevant property values and their variation, prior to use.

Isotope dilution ICP-MS analysis of trace elements in a Syenite rock reference material

Isotope dilution inductively coupled plasma magnetic sector field mass spectrometry (ID-ICP-MS) was applied to the trace analysis of selected elements of Syenite rock-type reference material SARM 2 as part of the continuing work programme on the re-evaluation of certain South African Reference Materials. The results for Ba, Cd, Cu, Mo, Ni, Pb, Sr and Zn are compared with the initial certification data for SARM 2, as well as published data for these elements since 1978. Complete uncertainty statements were calculated according to the ISO GUM and EURACHEM/CITAC guidelines for quantification of uncertainty of measurement.

Principles for the assessment of homogeneity and stability in the new ISO Guide 35:2017

The production of reference materials (RMs) is a key activity for the improvement and maintenance of a worldwide coherent measurement system. General requirements for all types of RM production are set out in ISO 17034. These general requirements include the assessment of homogeneity and stability. Technical detail is, however, left entirely to other guides, of which ISO Guide 35 is the most general and comprehensive. This paper provides an overview of the provisions for stability and homogeneity assessment in ISO Guide 35:2017, with particular attention to changes from the 2006 edition. Important changes include new guidance on the need for experimental studies when substantial prior experience is available; a larger range of homogeneity study designs; improved guidance on the use of accelerated stability studies; additional, and more flexible, guidance on within-unit homogeneity assessment; greater attention to post-certification monitoring; and new provisions for end-user storage and repeated sampling from individual RM units.

Supplementary comparison EURAMET.QM-S4: automotive gas mixtures

The capabilities for the preparation of certified reference materials of carbon monoxide, carbon dioxide, and propane in nitrogen have been compared. The participating national metrology institutes each submitted a gravimetrically prepared gas mixture of a specified target composition typical for the automotive industry to the coordinating laboratory. All mixtures were analyzed by the coordinating laboratory using a gas chromatograph equipped with a thermal conductivity detector in three runs under repeatable conditions. Based on the calibration curve, reference values were assigned to the amount fractions of carbon monoxide, carbon dioxide, and propane. The degrees of equivalence were established as the difference between the gas composition as calculated from preparation and the measured one, and its associated uncertainty. All participants obtained satisfactory results. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by EURAMET, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Outline for the revision of ISO Guide 35

The production of reference materials (RMs) is a key activity for the improvement and maintenance of a worldwide coherent measurement system. As detailed in ISO Guide 33, RMs with different characteristics are used in measurements, such as calibration, quality control and method validation, as well as for the assignment of values to other materials. Currently, ISO Guide 35 is in its third edition after it was revised in 2006. The Guide was developed to support best practices in the value assignment to specified properties of Certified Reference Materials (CRMs). This Guide gives general guidance and explains concepts to assist the understanding and development of valid methods to assign values to the properties of a reference material, including the evaluation of their associated measurement uncertainties, and the establishment of their metrological traceability. From the outcome of a systematic review of ISO Guide 35 among the members of ISO/REMCO, the ISO Committee on Reference Materials, it followed that there is a need for revising the current edition of ISO Guide 35. The mandate for the revision is focused on editorial updates to explain the concepts in more detail. It is not envisaged that major technical changes will be introduced. This paper explains the approach and rationale for the revision of ISO Guide 35 and invites comments from the users of the current edition of ISO Guide 35.

General guidance for the value assignment of reference materials: a proposed scope for the revision of ISO Guide 35

The production of reference materials (RMs) is a key activity for the improvement and maintenance of a worldwide coherent measurement system. As detailed in ISO Guide 33, RMs with different characteristics are used in measurements, such as calibration, quality control and method validation, as well as for the assignment of values to other materials. Certified reference materials (CRMs) are also used to maintain or establish metrological traceability to conventional scales, such as the octane number, hardness scales and pH. Furthermore, selected pure substances are used to maintain the international temperature scale. In order to be comparable across borders and over time, measurements need to be traceable to appropriate and stated references. CRMs play a key role in implementing the concept of traceability of measurement results in chemistry, biology and physics among other sciences dealing with materials and/or samples. Laboratories use these CRMs as readily accessible measurement standards to establish traceability of their measurement results to international standards. The property values carried by a CRM can be made traceable to SI units or other internationally agreed references during production. ISO Guide 35 explains how methods can be developed that will lead to well-established property values, which are made traceable to appropriate stated references. For reference material producers (RMPs), there are three ISO Guides that specifically assist with the set-up of a scheme to produce and certify RMs to ensure that the quality of the RMs meets the requirements of the end users. ISO Guide 34 outlines the requirements to be met by an RMP to demonstrate competence. ISO Guide 35 provides general guidance on property assessment issues and explains the concepts for processes such as the assessment of homogeneity, stability and value assignment for certification of RMs. ISO Guide 31 describes the format and contents of certificates for CRMs and accompanying documents for non-certified RMs, respectively. Currently, ISO Guide 35 is in its third edition after it was revised in 2006. The Guide was developed to support best practice in the value assignment for CRMs. But a user of this type of documentation should always consider that it cannot substitute for ‘critical thinking, intellectual honesty and professional skill’ (section 3.4.8. of the Guide to the Expression of Uncertainty in Measurement (GUM), ISO Guide 98-3: 1993). Obviously, the quality of the ‘product’ RM depends as much on these aspects as on the use of general procedures and methods. Therefore, thorough knowledge of the material and its properties and of the measurement methods used during homogeneity testing, stability testing and value assignment of the material, along with a thorough knowledge of the statistical methods, are needed for correct processing and interpretation of experimental data in a typical certification project. It is the combination of these required skills that make the preparation and certification of RMs so complex. The greatest challenge in these projects is to combine these skills to allow for a smooth implementation of the project plan. Since the release of ISO Guide 35 in 2006, ISO Guide 34 was revised and the preparation of ISO Guide 80 was started. At several occasions, ISO Guide 34 refers (also for A. Botha (&) National Metrology Institute of South Africa, Pretoria, South Africa e-mail: abotha@nmisa.org

Final report on international comparison CCQM-K71: Measurement of stack gas

Industrial stack gas emission measurements are important for process control, control of air pollution, and for implementing legislation regarding carbon dioxide emission rights. Measurements are typically performed using a range of process analysers for carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx), sulphur dioxide (SO2) and miscellaneous hydrocarbons. The calibration of these analysers is often performed using a series of binary mixtures of each component in nitrogen. For reasons of efficiency as well as a better match with true stack gas, the use of multi-component mixtures for this purpose would be preferred. The aim of this key comparison is to evaluate the measurement capabilities of national metrology institutes for carbon monoxide, carbon dioxide, nitrogen monoxide, sulphur dioxide and propane in nitrogen. Ten laboratories participated in the key comparison and one in the associated study. The key comparison reference value is based on the gravimetric preparation data. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Guidance on the contents of accompanying documentation for reference materials (RMs)

The use of reference materials (RMs) is a key activity for the improvement and maintenance of a worldwide coherent measurement system. As detailed in ISO Guide 33, RMs with different characteristics are used in measurement processes, for the purpose of precision control, bias assessment, calibration, assigning values to other materials, and maintaining conventional scales, to name a few. For the establishment of metrological traceability of measurement results to international scales or other measurement standards, proper use of certified reference materials (CRMs) is essential. From the perspective of a reference material producer, the documentation that is provided with an RM is the value-adding component of the material; for the user, the document is critical for the correct implementation and use of the RM in the measurement process. The ISO Committee on Reference Materials (ISO/REMCO) recognised the importance of the documentation that accompanies a reference material as early as 1981 when the first edition of ISO Guide 31 was published. The third edition of the Guide that was published recently considers the appropriate accompanying documentation for all types of reference materials, i.e. CRMs and non-certified RMs.