Proficiency Analytical Testing

Figure 4 is a spectrum of a filter from the Proficiency Analytical Testing Program (PAT) conducted by the National Institute of Occupational Safety and Health (NIOSH) which contained 103 micrograms as determined by this lab. PAT filters are prepared with 2 mg of sodium silicate as a contaminant. A baseline approximation as described fo Figure 1 is not possible due to the interfering peak at 845 cm. When a spectrum of this type is encountered, a line is drawn from the two minimums on either side of the quartz doublet peak as illustrated by the dotted line in Figure 4. 

There are other routinely practiced procedures in terms of intralaboratory and interlaboratory quality control, such as the control chart, round-robin interlaboratory testing, and various proficiency tests offered by different agencies, such as the PATs (Proficiency Analytical Testing) by AHA in the US and WASPs (Workplace Analysis Scheme for Proficiency) by the Health and Safety Laboratory in Great Britain. In normal circumstances, a combination of these quality control measures should be used in performing workplace air analysis. 

National Institute of Occupational Safety and Health (NIOSH) Proficiency Analytical Testing 45... 

At this time, there is no known method to determine the absolute accuracy of the asbestos analysis. Results of samples prepared through the Proficiency Analytical Testing (PAT) Program and analyzed by the OSHA-SLTC showed no significant bias when compared to PAT reference values. The PAT samples were analyzed from 1987 to 1989 (N=36) and the concentration range was from 120 to 1,300 fibers/mm. ... 

Each laboratory engaged in asbestos counting shall participate in the Proficiency Analytical Testing Program, the / bestos Analyst Registry or equivalent. 

Analytical Methods Committee, Handling false negatives, false positives and reporting limits in analytical proficiency tests. Analyst, 122, 495, 1997. 

It must be remarked that terminology is not consistent and there are many widely used synon)ms. Quality control in this Chapter refers to practices best described as internal quality control. Quahty assessment is often referred to as external quality control, proficiency testing, interlaboratory comparisons, round robins or other terms. Internal Quality Control and External Quality Assessment are preferred because they best describe the objectives for which the RMs are being used, i.e. the immediate and active control of the results being reported from an analytical run or event, and an objective, retrospective assessment of the quality of those results. 

Thompson M, Wood R (1993) International harmonized protocol for proficiency testing of (chemical) analytical laboratories. J of AOAC Intern 76 926-937. 

Assuming an analytical error of 2 % which is based on the analysis of the results of a number of proficiency tests and collaborative trials (Daas and Miller 1998), a maximum uncertainty of 0.24% would imply a 0.1% probability of rejecting a good result. Given that the uncertainty of a content to be assigned is below a predetermined value, the results of the collaborative trial are acceptable otherwise it is recommended to repeat the trial in whole or in part. 

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). 

Magnitude of the response caused by a certain amount of analyte Alternatively, certified reference materials, samples analyzed with reference methods or proficiency test materials may be apphed... 

The principle of proficiency testing schemes consists in analyzing one or more samples sent to the laboratories by an external body. The analytical results returned to the organizer are evaluated in comparison to the assigned value(s) of the sample(s). 

ISO/REMCO N 280 (1993) Proficiency testing of chemical analytical laboratories. Geneva... 

IUPAC (1993) Analytical Chemistry Division, Commission on General Aspects of Analytical Chemistry The International Harmonized Protocol for the Proficience Testing of (Chemical) Analytical Laboratories. Pure Appl Chem 65 2123... 

Lawn RE, Thompson M, Walker RF (1993) Proficiency testing in analytical chemistry. Royal Society of Chemistry, London... 

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. 

The previous chapters of this book have discussed the many activities which laboratories undertake to help ensure the quality of the analytical results that are produced. There are many aspects of quality assurance and quality control that analysts carry out on a day-to-day basis to help them produce reliable results. Control charts are used to monitor method performance and identify when problems have arisen, and Certified Reference Materials are used to evaluate any bias in the results produced. These activities are sometimes referred to as internal quality control (IQC). In addition to all of these activities, it is extremely useful for laboratories to obtain an independent check of their performance and to be able to compare their performance with that of other laboratories carrying out similar types of analyses. This is achieved by taking part in interlaboratory studies. There are two main types of interlaboratory studies, namely proficiency testing (PT) schemes and collaborative studies (also known as collaborative trials). 

There are two main types of proficiency testing scheme. First, there are those set up to assess the competence of a group of laboratories to undertake a very specific analysis, e.g. lead in blood or the number of asbestos fibres in air collected on membrane filters. Secondly, there are those schemes used to evaluate the performance of laboratories across a certain sector for a particular type of analysis. Because of the wide range of possible analyte/matrix combinations it is not practicable to assess the performance of laboratories when analysing all the possible sample types. Instead, a representative cross-section of analyses is chosen (e.g. determination of different pesticide residues in a range of foodstuffs or the determination of trace levels of metals in water samples). 

Provided the sample matrix and analyte concentration are appropriate, matrix Certified Reference Materials (CRMs) can make ideal proficiency testing samples. The assigned value is the certified value given on the certificate accompanying the CRM. The certificate will also give an uncertainty estimate for the certified value, and the use of CRMs allows the traceability of analytical data to be established. However, matrix CRM availability is limited and the materials are often expensive. Hence, Certified Reference Materials are seldom used as PT samples. 

If the analytical method used by participants in the proficiency testing round has been validated by means of a formal collaborative trial, then the repeatability and reproducibility data from the trial can be used. The repeatability standard deviation gives an estimate of the expected variation in replicate results obtained in a single laboratory over a short period of time (with each result produced by the same analyst). The reproducibility standard deviation gives an estimate of the expected variation in replicate results obtained in different laboratories (see Chapter 4, Section 4.3.3 for further explanation of these terms). 

If data from a collaborative study are not available to estimate the reproducibility of a method, a general model such as the Horwitz function can be used. The Horwitz function is described in Chapter 4, Sections 4.4 and 4.6.2. It can be used to predict the value of or based on the concentration of the analyte in the proficiency test material. The disadvantage of this approach is that the chosen model may not accurately represent the true reproducibility of the method. 

This chapter has considered two of the types of interlaboratory comparison exercise in which your laboratory may participate. It is important to remember that proficiency testing schemes and collaborative studies have different aims. The former is a test of the performance of the laboratory, whereas the latter is used to evaluate the performance of a particular analytical method. Laboratories should participate in proficiency testing schemes (where an appropriate scheme is available) as this provides an independent check of the laboratory s performance. This chapter has described the key features of proficiency testing schemes and explained how the results from participation in a scheme should be interpreted. 

Lawn, R., Thompson, M. and Walker, R. F., Proficiency Testing in Analytical Chemistry, ISBN 0-85404-432-9, The Royal Society of Chemistry, Cambridge, UK, 1997. 

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