Proficiency testing Measurement Evaluation

Over the past decade, a major trend has been the development of the use of proficiency testing (PT) or evaluation materials (Fox 2000). PT materials are a type of reference material, which aid in assessment of analytical laboratory measurement quality. There will be an increased use of such materials as part of laboratory accreditation programs and other new quality assurance efforts, including internal audits. At the same time, a number of providers have used PT schemes to produce a form of RM intended to meet the ever-growing need for RMs required for routine QC use (Jenks 1995,1997). 

This chapter deals with handling the data generated by analytical methods. The first section describes the key statistical parameters used to summarize and describe data sets. These parameters are important, as they are essential for many of the quality assurance activities described in this book. It is impossible to carry out effective method validation, evaluate measurement uncertainty, construct and interpret control charts or evaluate the data from proficiency testing schemes without some knowledge of basic statistics. This chapter also describes the use of control charts in monitoring the performance of measurements over a period of time. Finally, the concept of measurement uncertainty is introduced. The importance of evaluating uncertainty is explained and a systematic approach to evaluating uncertainty is described. 

More recently, proficiency evaluation programs have produced similar traceability linkages, but with a novel twist. The proficiency test material (transfer standard) is sent without attachment of results. The laboratory under test receives both an independent confirmation of measurement ability and, indirectly, a traceability linkage when the test is concluded and the assigned value of the measurand is disclosed. It is unlikely that proficiency evaluation will supplant traditional forms of providing traceability links. However, the concept does provide metrologists with an additional tool. 

This paper has described new approaches to providing quality assurance within the operation of proficiency evaluation programs. The approaches are used in a new NIST program to monitor providers of proficiency testing in an environmental field. A part of that monitoring is conducted by means of indirect proficiency evaluation of the providers. The new program demonstrates that both direct and indirect proficiency evaluation can provide traceability links, in chemical metrology, to measurements carried out at NIST. 

The selection of an analytical technique and the development of procedures and operating conditions are only one part of the process for achieving reliable measurements. Among the first steps of the validation figure the assessment of the method precision this parameter must be assessed between analysts and days. Appropriate precision, although necessary for reliable work, is not sufficient to guarantee accurate results Evaluation of sample matrix effect, development, and analysis of spiked samples, analysis of Certified Reference Materials (CRM) of similar composition for which the Pb concentration has been established by other laboratories and comparison of results between laboratories (proficiency testing) are equally important [13]. 

The issues of method validation and assessment of measurement uncertainty in the determination of potentially toxic trace elements in rice are of permanent interest for the scientific community. In this context, the sources of uncertainty associated with the determination of Cd, Cu, Pb, and Zn have been recently estimated in rice through an interlaboratory comparison [30]. Four Brazilian laboratories participated in the proficiency test. The analytical technique used were FAAS, ET-AAS, and ICP-AES. The rice samples were supplied by the Institute for Reference Materials and Measurements (IRMM), Joint Research Center of the European Commission, within the scope of the interlaboratory comparison International Measurement Evaluation Programme (IMEP) 19 Trace Elements in Rice (see also Chapter 7 in this book). Three out of the four laboratories reported values close to the reference values. It was emphasized that, in order to establish a reliable uncertainty budget, all significant sources of uncertainty should be identified. 

Ion-selective electrodes (ISEs) represent the current primary methodology in the quantification of S-Li [11-13], Moreover, ISE modules are parts of large and fully automated clinical chemistry analysers. In practice, the validation parameters are most often chosen in terms of judging the acceptability of the new measurement system for daily use. For this reason, the first approach was to study whether the detected imprecision fulfilled the desired analytical quality specifications. Secondly, proficiency testing (PT) results from past samples were of great value in predicting future bias. The identity of the three ISE methods was evaluated using patient samples. The analytical performance was checked after 6 months routine use. Without any exception, method validations always mean an extra economical burden. Therefore, the validation parameters chosen and processed have to be considered carefully. 

The variation of sensitivity between different sensors was also checked. Calibration curves with five different sensors were performed. A Relative Standard Deviation of 13, 13 and 42% of calibration slopes (sensitivity) were obtained for Cu, Pb and Cd respectively. These variations should have limited consequence on bias and precision when the standard addition method is used. However, for Cd, variations in the limit of quantification between two electrodes could be expected. Finally, the accuracy of the method was evaluated by the measurement of a SWIFT reference material used during the 2nd SWIFT-WFD Proficiency Testing exercise (Table 4.2.2). The reference value was chosen as the consensus value of the selected data population obtained after excluding the outliers. The performances of the device were estimated according to the Z-score (Z) calculation. Based on this score, results obtained with the SPEs/PalmSens method were consistent with those obtained by all methods for Pb and Cu ( Z < 2) while the result was less satisfactory for Cd (2 < Z < 3). 

Quality issues related to analytical measurements have been described in full detail in several books [1-3] and highlighted several principles, e.g. validation of methods, quality assurance protocols incorporating the use of CRMs, independent assessment of method performance by participation in proficiency testing schemes, and accreditation. Method performance studies also represent a very important aspect to evaluate the state-of-the-art of a particular type of analysis at the development stage e.g. for testing the applicability of a standard method) or to improve the quality of measurements, e.g. prior to the certification of reference materials. 

To deal with the issues presented by the GAO, Congress amended 305 of TSCA. The amendment required the EPA to conduct research to develop, test, and evaluate radon and radon progeny measurement methods and protocols, and to study the feasibility of establishing a mandatory proficiency program, which would include the recommendations of the GAO report. Congress also required EPA to establish additional user fees to fund the research of measurement methods and protocols for the fiscal years 1991, 1992,1993, 1994, and 1995.56... 

Evaluation of precision performance of quantitative measurement methods Evaluation of the linearity of quantitative measurement methods Method comparison and bias estimation using patient samples Assessment of laboratory tests when proficiency testing is not available... 

The performance of the GFPC and LSC methods for the measurement of gross alpha and beta activity in water was evaluated by participating in Proficiency Tests (PT) organized by Instituto de Radioprotefao e Dosimetria IRD/CNEN, which is available on a routine basis three times per year. The results obtained for the measurement of gross alpha... 

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