An estimation of the measurement uncertainty for an optical emission spectrometric method

 A validation procedure based on the ISO/IEC 17025 standard was used to demonstrate the long-term stability of a calibration process and to assess the measurement uncertainty of a standard test method for optical emission vacuum spectrometric analysis of carbon and low-alloy steel (ASTM E 415–99a). The validation was used to provide documented evidence that the selected method fulfils the requirements and that the method is ”fit for purpose”. A test for drift was applied to determine statistically whether the analytical results vary systematically with time. The accuracy and traceability of the optimised method were tested by an analysis of closely matched matrix certified reference materials (CRMs). The measurement uncertainty estimations took account of the precision study, the bias and its uncertainty, and the qualification of uncertainties not considered in the overall performance studies. Received: 2 November 2002 Accepted: 2 January 2003 Acknowledgement The author expresses gratitude to Dr. Aleš Fajgelj for helpful discussions during the 3rd Central European Conference on Reference Materials and Measurements. Presented at CERMM-3, Central European Reference Materials and Measurements Conference: The function of reference materials in the measurement process, May 30–June 1, 2002, Rogaška Slatina, Slovenia Correspondence to T. Drglin     

A new terminology for the approaches to the quantification of the measurement uncertainty

A new terminology for the approaches to the quantification of the measurement uncertainty is presented, with a view to a better understanding of the available methodologies for the estimation of the measurement quality and differences among them. The knowledge of the merits, disadvantages and differences in the estimation process, of the available approaches, is essential for the production of metrologically correct and fit-to-purpose uncertainty estimations. The presented terminology is based on the level of the analytical information used to estimate the measurement uncertainty (e.g., supralaboratory or intralaboratory information), instead of the direction of information flow (“bottom-up” or “top-down”) towards the level of information where the test is performed, avoiding the use of the same designation for significantly different approaches. The proposed terminology is applied to the approaches considered on 19 examples of the quantification of the measurement uncertainty presented at the Eurachem/CITAC CG4 Guide, Eurolab Technical Report 1/2002 and Nordtest Technical Report 537. Additionally, differences of magnitude in the measurement uncertainty estimated by various approaches are discussed.     

A simplified approach to the estimation of analytical measurement uncertainty

The present study summarizes the measurement uncertainty estimations carried out in Nestlé Research Center since 2002. These estimations cover a wide range of analyses of commercial and regulatory interests. In a first part, this study shows that method validation data (repeatability, trueness and intermediate reproducibility) can be used to provide a good estimation of measurement uncertainty.In a second part, measurement uncertainty is compared to collaborative trials data. These data can be used for measurement uncertainty estimation as far as the in-house validation performances are comparable to the method validation performances obtained in the collaborative trial.Based on these two main observations, the aim of this study is to easily estimate the measurement uncertainty using validation data.     

Appropriate rather than representative sampling, based on acceptable levels of uncertainty

Appropriate sampling, that includes the estimation of measurement uncertainty, is proposed in preference to representative sampling without estimation of overall measurement quality. To fulfil this purpose the uncertainty estimate must include contribution from all sources, including the primary sampling, sample preparation and chemical analysis. It must also include contributions from systematic errors, such as sampling bias, rather than from random errors alone. Case studies are used to illustrate the feasibility of this approach and to show its advantages for improved reliability of interpretation of the measurements. Measurements with a high level of uncertainty (e.g. 50%) can be shown to be fit for some specified purposes using this approach. Once reliable estimates of the uncertainty are available, then a probabilistic interpretation of results can be made. This allows financial aspects to be considered in deciding upon what constitutes an acceptable level of uncertainty. In many practical situations ”representative” sampling is never fully achieved. This approach recognises this and instead, provides reliable estimates of the uncertainty around the concentration values that imperfect appropriate sampling causes. Received: 28 December 2001 Accepted: 25 April 2002     

Compliance decision criteria—problems encountered and actions taken in the EU and Codex

It is now recognised that there is an intimate relationship between the final analytical result and the sampling, the measurement uncertainty and the recovery factor used to obtain that result. Within the food sector in the EU this was identified by the SCOOP Task 9.1 “Preparation Of A Working Document In Support Of The Uniform Interpretation Of Legislative Standards And The Laboratory Quality Standards Prescribed Under Directive 93/99/EEC”. The recommendations from this Report have been accepted in the Food Contaminants and the Undesirable Substances in Feed Sectors. These are given in the SANCO document “Report To The Standing Committee On The Food Chain And Animal Health On The Relationship Between Analytical Results, The Measurement Uncertainty, Recovery Factors And The Provisions In EU Food And Feed Legislation With Particular Focus On The Community Legislation Concerning: (A): Contaminants In Food (Council Regulation (EEC) No 315/93 of 8 February 1993 Laying Down Community Procedures For Contaminants In Food), and (B): Undesirable Substances In Feed (Directive 2002/32/EC Of The European Parliament And Of The Council Of 7 May 2002 On Undesirable Substances In Animal Feed).” Similar considerations were identified in the Codex Alimentations Commission where the guidelines “The Use Of Analytical Results: Sampling, Relationship Between The Analytical Results, The Measurement Uncertainty, Recovery Factors And The Provisions In Codex Standards” are being progressed through the Codex system. Both of these Reports stress that before deciding whether a sample is in compliance with a legislative limit the uncertainty should be deducted and the results corrected for recovery. Thus that there is a difference between the legal specification and enforcement limit will be stressed and that this should be appreciated when specifications are being set. The rationale behind the Reports is described. Presented at AOAC Europe/Eurachem Symposium March 2005, Brussels, Belgium

---

     

A need for clearer terminology and guidance in the role of reference materials in method development and validation

 The fact that various definitions and terminology applied to measurements in analytical chemistry are not always consistent and straightforward, by not only answering the question ”what”, but also ”how”, leads to their various interpretations. This results in non-uniform implementation of very basic and essential metrological principles in chemistry. Such a diverse situation is not conducive to the endorsement of harmonised measurements all across the world, to serve as a tool for improving the quality of life in its broadest sense for all its citizens. The discussion in this paper is focused on problems associated with terminology and definitions of ’reference material’ and ’validation’. The role of reference materials in measurement processes for purposes other than calibration and validation principles in analytical chemistry are also discussed in this paper. Where possible, potential solutions are proposed, but more often, questions of essential importance are raised in order to initiate international discussion which will hopefully lead to equally understandable answers. Received: 2 November 2002 Accepted: 3 February 2003 Acknowledgements   The author is grateful to Aleš Fajgelj for his comprehensive comments on the topic described in this paper. Sincere thanks also to Philip Taylor, Ewa Bulska, Emilia Vassileva, Miloslav Suchanek and Margreet Lauwaars for their contribution during fruitful discussions on validation. Presented at the CERMM-3, Central European Reference Materials and Measurements Conference: The function of reference materials in the measurement process, May 30–June 1, 2002, Rogaška Slatina, Slovenia Correspondence to N. Majcen     

Development of metrology for pH measurement in Thailand

It has not been long that metrology is well accepted as an important part in analytical chemistry since it helps the chemists to receive the best measurement and accurate results with traceability. The National Institute of Metrology Thailand (NIMT), which is a public agency under the supervision of the Ministry of Science and Technology, not only focuses on physical standards but also provides and maintains standards in chemical field. pH measurement is one of the most widely used in the laboratories including industries and medical area in Thailand. The chemical laboratory starts working on the project with the objective of disseminating an accurate result in routine pH measurement. In 2002, the laboratory provided a service in calibration of pH meter and organized the first local interlaboratory comparison program (NIMT–C-ILC-1: pH buffer) in pH measurement. There were three buffer solution samples in the range of acid, neutral, and base. A total of 44 laboratories participated in this program. The NIMT chemical laboratory also participated in the proficiency testing program that was conducted by PSB Corporation Testing Group in Singapore. In 2003, NIMT started research in preparation of secondary buffers by using highly accurate pH meters with glass electrode systems. The laboratory produced three secondary buffers, which were pH 4.01, 6.86, and 9.18 with uncertainty 0.020 pH at 25°C. The competence of the laboratory was shown by the measurement results of the pilot study (APMP.QM-P06), which was organized by the APMP electrochemical analysis working group (EAWG/TCQM) in 2005. The title of this study was “pH determination of two phosphate buffers by Harned cell method and glass electrode method”. NIMT aims to achieve for establishment of the primary method for pH measurement in the near future. Presented at -- “BERM-10” -- April 2006, Charleston, SC, USA     

Estimation of the uncertainty of mass measurements from in-house calibrated analytical balances

The uncertainty evaluation of mass measurements when using “in-house” calibrated analytical balances is revisited according to the Guide to the expression of Uncertainty Measurement (GUM). The calibration of analytical balances is discussed according to the guidelines of several bodies such as ASTM, UKAS and DKD/PTB. The remainder components of uncertainty can be estimated from the balance data sheet specifications.     

Estimation of the efficiencies and uncertainties of the extraction and cleanup steps of pesticide residue determination in cucumber using <Superscript>14</Superscript>C-carbaryl

The estimation of the uncertainty of measurement has, nowadays, become an integral part of analytical results. The uncertainty and efficiency of extraction and cleanup are very important components of a pesticide residue analytical method. In this work, for a quick review of extraction and cleanup efficiency and to evaluate the individual analysis steps during method adaptation, ¹⁴C-carbaryl was applied at all fortification levels. Then, further analyses, such as ethylacetate extraction, filtration, evaporation and cleanup, were performed. The calibration of gel chromatograph column, performed with both ¹⁴C-carbaryl and fortification mixture (dichlorvos, malathion and chlorpyrifos), showed that pesticide fractions came through the column between the 8- and 23-mL fractions. The overall recovery of ¹⁴C-carbaryl after the extraction and cleanup step was 0.91, with the relative uncertainty of 0.084. Using the “bottom-up” approach, the uncertainty of extraction u _cEX and cleanup u _cGPC were 0.033 and 0.107, respectively. The combined standard uncertainty u _c associated with the described analytical method was 0.112. Similar values were obtained using the alternative “top-down” approach: uncertainty of extraction u _ct1 was 0.039, uncertainty of cleanup u _ct2 was 0.108 and the combined standard uncertainty u _cAV was 0.081. Both approaches showed that the uncertainty of cleanup was the main source of combined standard uncertainty.     

Estimation of uncertainty in routine pH measurement

A procedure for estimation of measurement uncertainty of routine pH measurement (pH meter with two-point calibration, with or without automatic temperature compensation, combination glass electrode) based on the ISO method is presented. It is based on a mathematical model of pH measurement that involves nine input parameters. Altogether 14 components of uncertainty are identified and quantified. No single uncertainty estimate can be ascribed to a pH measurement procedure: the uncertainty of pH strongly depends on changes in experimental details and on the pH value itself. The uncertainty is the lowest near the isopotential point and in the center of the calibration line and can increase by a factor of 2 (depending on the details of the measurement procedure) when moving from around pH 7 to around pH 2 or 11. Therefore it is necessary to estimate the uncertainty separately for each measurement. For routine pH measurement the uncertainty cannot be significantly reduced by using more accurate standard solutions than ±0.02 pH units – the uncertainty improvement is small. A major problem in estimating the uncertainty of pH is the residual junction potential, which is almost impossible to take rigorously into account in the framework of a routine pH measurement.¹ Received: 11 August 2001 Accepted: 22 February 2002     

Uncertainty sources in UV-Vis spectrophotometric measurement

An overview is given of the most important uncertainty sources that affect analytical UV-Vis spectrophotometric measurements. Altogether, eight uncertainty sources are discussed that are expected to have influence in chemical analysis. It is demonstrated that the well-known intrinsic (or “physical”) sources of uncertainty that originate from the instrument itself (repeatability of spectrophotometer reading, spectrophotometer drift, stray light, etc.) often have significantly lower contributions to the combined uncertainty of the result than the “chemical” sources of uncertainty that originate from the object under study (interference from the constituents of the matrix, decomposition of the photometric complex, etc.). Although selectivity of a photometric procedure is often considered more a validation topic than an uncertainty topic, it is very often important to include it also in the uncertainty budget.Usually the most difficult part of uncertainty estimation of a chemical measurement result is to evaluate the magnitude of the actual uncertainty components, especially the chemical ones. For most of the uncertainty sources discussed in this paper, approaches for their evaluation are given. A generic uncertainty budget for absorbance is presented. Electronic Supplementary Material Supplementary material is available for this article at     

Confidence in testing services – new expectations, new rules, new challenges. 5th Eurolab Symposium offered important information and stimulating discussions

The 5th Eurolab Symposium on 4th and 5th October in Edinburgh was the highlight event of Eurolab in 2000, Eurolab’s 10th anniversary year. Under the title: ”Confidence in Testing Services – new expectations, new rules, new challenges” it reflected recent changes and developments in the laboratory scene and focussed on new tasks and challenges. The most essential changes for laboratories are connected with the new standard ISO/IEC 17025 ”General requirements for the competence of testing and calibration laboratories”. Keywords in this respect are uncertainty of measurement and traceability to SI units, requirements on personnel, proficiency testing, customer orientation, and management systems.     

Metrological traceability is not always a straight line

The two most important concepts in metrology are certainly “traceability to standards” and “measurement uncertainty evaluation”. So far the questions related to these concepts have been reasonably solved in the metrology of “classical quantities”, but for the introduction of metrological concepts in new fields, such as chemistry and biology, a lot of problems remain and must be solved in order to support international arrangements. In this presentation, the authors want to develop the strategy implemented at Laboratoire national de métrologie et d’essais (LNE) in metrology in chemistry and biology. The strategy is based on: (1) pure solutions for calibration of analytical instruments, (2) use of certified reference materials (matrix reference materials), and (3) participation to proficiency testing schemes. Examples will be presented in organic and inorganic chemistry. For laboratory medicine, proficiency testing providers play an important role in the organization of External Quality Assessment Schemes. For the time being, the reference value or the assigned value of the comparison is calculated with the results obtained by the participants. This assigned value is not often traceable to SI units. One of the methods suggested by LNE is to ensure the metrological traceability to SI units of the assigned value for the more critical quantities carried on analytes by implementing the Joint committee for traceability in laboratory medicine reference methods.     

The traceability of analytical measurements

 Traceability to the System International (SI) is an important prerequisite for international comparability and uniformity of chemical measurements to ensure mutual recognition of the results. In theory, all measurements can be traced back to the seven base units of the SI. Although the traceability system works well for most physical measurements, in many analytical and in some spectrophotometric measurements this system is not satisfactory. This paper describes the particular and practical problems and the contribution of the Romanian National Institute of Metrology in this field. The paper discusses the following concepts: clearly defined targets in the form of requirement specification, knowledge of trueness and/or measurement uncertainty, and traceability through an unbroken chain of calibration to primary standards. Traceability and uncertainty being two concepts inherently coupled, two examples of assessment of the uncertainty of measurement results are given for two spectrophotometric methods currently used in chemical laboratories. Received: 17 July 1996 Accepted: 2 September 1996     

Measurement uncertainty – a reliable concept in food analysis and for the use of recovery data?

 Steps which are taken to implement the concept of measurement uncertainty in analytical chemical laboratories should take full account of existing internationally agreed protocols for analytical quality assurance and reflect the needs of particular analytical sectors. For the food sector this may mean that for official purposes the use of the term measurement uncertainty is replaced by the term measurement reliability and that a quantitative estimation of this is made based on existing collaborative trial data. In many analytical sectors, the differing strategies currently followed for the determination and use of recovery information are an important cause of the non-comparability of analytical results. Guidelines which are being prepared for the estimation and use of recovery information in analytical measurement may provide a more unified approach which includes measurement uncertainty as a key concept in the use of recovery data. Received: 4 November 1997 · Accepted: 3 February 1998     

Uncertainty due to the quantification step in analytical methods

The quantification step is an important source of uncertainty in analytical methods, but it is frequently misunderstood and disregarded. In this paper, it is shown how this uncertainty is closely related to the linear response range of a method, and to the Pearson correlation coefficient of the calibration line. So, if there is a need for a pre-fixed quantification uncertainty, the linear response range will be affected. Some practical cases are given showing the quantification uncertainty significance. The theoretical equation giving the value of the quantification uncertainty is deduced from which new conclusions can be taken out. Because of that, the quantification uncertainty can easily be calculated and the parameters that really affect its value are shown along the paper. Some final considerations about detection limits and two-point calibration lines are also given. The paper can also be considered a reflection on uncertainty owed to calibration and on their consequences on the analytical methodology.     

Estimation of measurement uncertainty in organic analysis: two practical approaches

Estimation of measurement uncertainty has become a more regularly performed part of the whole analytical process. However, there is still much on-going discussion in the scientific community about ways of building up the uncertainty budget. This study describes two approaches for estimation of measurement uncertainty in organic analysis: one which can be used for single sets of measurements and the other based on validation studies. In both cases the main contributions to the uncertainty are presented and discussed for the analysis of PCBs in mussel tissue, but the approaches can be extended to other organic pollutants in environmental/food samples. The main contributions to the uncertainty budget arise from calibration, sample preparation, and GC–MS measurements. A comparison of the relevant sources and their contributions to the expanded uncertainty is presented.     

Uncertainty in analytical results from solid materials with electrothermal atomic absorption spectrometry: a comparison of methods

The combined uncertainty in the analytical results of solid materials for two methods (ET-AAS, analysis after prior sample digestion and direct solid sampling) are derived by applying the Guide to the Expression of Uncertainty in Measurement from the International Standards Organization. For the analysis of solid materials, generally, three uncertainty components must be considered: (i) those in the calibration, (ii) those in the unknown sample measurement and (iii) those in the analytical quality control (AQC) process. The expanded uncertainty limits for the content of cadmium and lead from analytical data of biological samples are calculated with the derived statistical estimates. For both methods the expanded uncertainty intervals are generally of similar width, if all sources of uncertainty are included. The relative uncertainty limits for the determination of cadmium range from 6% to 10%, and for the determination of lead they range from 8% to 16%. However, the different uncertainty components contribute to different degrees. Though with the calibration based on reference solutions (digestion method) the respective contribution may be negligible (precision < 3%), the uncertainty from a calibration based directly on a certified reference material (CRM) (solid sampling) may contribute significantly (precision about 10%). In contrast to that, the required AQC measurement (if the calibration is based on reference solutions) contributes an additional uncertainty component, though for the CRM calibration the AQC is “built-in”. For both methods, the uncertainty in the certified content of the CRM, which is used for AQC, must be considered. The estimation of the uncertainty components is shown to be a suitable tool for the experimental design in order to obtain a small uncertainty in the analytical result.     

Measurement uncertainty of selenium determination in the reference material Seronorm Trace Elements Serum by hydride generation atomic fluorescence spectrometry

 An analytical result is not considered to be complete without a statement of the measurement uncertainty associated with it. Evaluation of measurement uncertainty is therefore an essential part of every determination. In the present study the measurement uncertainty of the result of determination of selenium in the reference material Seronorm Trace Elements Serum was studied. The combination of a digestion procedure using sulfuric acid, V₂O₅ in H₂SO₄ and hydrogen peroxide and the continuous flow approach of hydride generation atomic fluorescence spectrometry (HG-AFS) detection was used for selenium determination. The total uncertainty budget was calculated with the help of the GUM Workbench program, in which computations follow the rules of the ’ISO guide to the expression of uncertainty in measurement’. The major sources of uncertainty were found to be due to the recovery of the procedure, measurement of peak heights and the purity of Na₂SeO₃. A selenium mass fraction of 77.1±4.8 ng/g (with a coverage factor of 2.1, 95% probability) was determined, which is comparable with the recommended value of 80 ng/g. Received: 13 September 2002 Accepted: 23 December 2002 Acknowledgements We would like to thank Tinkara Buˇcar, B.Sc. for useful discussions and the IAEA Vienna, for financial support of the project 11553/RO. Presented at CERMM-3, Central European Reference Materials and Measurements Conference: The function of reference materials in the measurement process, May 30–June 1, 2002, Rogaška Slatina, Slovenia Correspondence to V. Stibilj