Proficiency testing calibrants

Table 14 can be regarded as providing a reasonable overall picture, even if the results cannot applied to any particular case. However, if the underlying principle is accepted, it becomes clear that improvements in a single stage, for example the reduction of instrument variation, has a negligible beneficial effect (if this variation was not outside the normal range ). Even if the contribution of repeatability is re-duced to zero, the cumulative uncertainty is reduced by 10% only, i.e. from 2.2 to y(0.0)2 (0.8)2 (1.0)2 + (1.5)2 = 2.0. This statistical view of errors should help to avoid some unnecessary efforts to improve, e.g., calibration. Additionally, this broad view on all sources of error may help to detect the most important ones. Consequently, without participation in proficiency tests, any method validation will remain incomplete. 

En numbers are used when the assigned value has been produced by a reference laboratory, which has provided an estimate of the expanded uncertainty. This scoring method also requires a valid estimate of the expanded uncertainty for each participant s result. A score of En < 1 is considered satisfactory. The acceptability criterion is different from that used for z-, z - or zeta-scores as En numbers are calculated using expanded uncertainties. However, the En number is equal to zeta/2 if a coverage factor of 2 is used to calculate the expanded uncertainties (see Chapter 6, Section 6.3.6). En numbers are not normally used by proficiency testing scheme providers but are often used in calibration studies. 

Traditionally, the education that chemists and chemistry laboratory technicians receive in colleges and universities does not prepare them adequately for some important aspects of the real world of work in their chosen field. Today s industrial laboratory analyst is deeply involved with such job issues as quality control, quality assurance, ISO 9000, standard operating procedures, calibration, standard reference materials, statistical control, control charts, proficiency testing, validation, system suitability, chain of custody, good laboratory practices, protocol, and audits. Yet, most of these terms are foreign to the college graduate and the new employee. 

The NVLAP is comprised of a series of laboratory accreditation programs (LAPs), depending on what is requested and what is needed. Each LAP includes specific calibration and/or test standards and related methods and protocols assembled to satisfy the unique needs for accreditation in a field of testing or calibration. NVLAP accredits public and private laboratories based on evaluation of their technical qualifications and competence to carry out specific calibrations or tests. All are based on the ISO/IEC Guide 25 discussed in Section 2. The process involves an application and the payment of fees. This is followed by an on-site assessment. If deficiencies are found, these must be resolved to the satisfaction of the assessment team. The laboratory then participates in proficiency testing (Section 5.7) and a technical evaluation. 

Radiation monitoring laboratories seeking to achieve optimum proficiency test results with an accreditation standard must use calibration methods that duplicate or at least closely approximate the irradiation protocols described in the accreditation standard. This requirement is particularly important for calibrations using photons with energies below 200 keV where irradiation conditions must recreate the scattered radiation that contributes significantly to the response of the monitoring device. 

Procedures carried out in the laboratory, as opposed to proficiency testing or other interlaboratory collaborations, are known as in-house or internal quality control procedures. When running batches of samples with calibration solutions and unknowns, there are a number of extra samples that can be analyzed that cover different aspects of quality control (QC samples). These QC samples should be documented in the quality manual and be part... 

Extension into physical metrology is less obvious, but has no conceptual barrier. The direct approach could be applied if proficiency tests were conducted by a reference laboratory distributing to device producers, preparing lots of material measures and calibrants such as gage blocks, masses, or thermometers. Distribution of such material measures is already a feature of most measurement assurance programs in physical metrology. Use of the indirect approach would have units randomly selected from production and sent to a reference laboratory. This could prove, in some cases, more economical than direct proficiency testing of the producers of the devices. 

Faison CD, Parris RM, Rasberry SD (1999) NIST Handbook 150 19 Chemical calibration providers of proficiency testing. NIST, Gaithersburg, MD... 

Keywords Traceability Gas standards Calibration Quality assurance Proficiency testing International comparisons... 

In their regular day to day practice, field laboratories use commercial reagents or prepare in-house solutions for the calibration of instruments, and they rely on purity assessment of producers. For method validation and even measurement uncertainty, field labs regularly participate in proficiency testing schemes. In such inter-laboratory comparisons, the reference value is usually obtained as the arithmetic mean of results of participants. 

A common way to assess the matrix bias is to analyse a drinking water (therefore containing a matrix) in the proficiency testing. For this analysis, laboratories calibrate their instruments using standard, commercial or in-house solutions. If a CRM is available, a possible matrix effect can be corrected by adjusting operational instrument parameters to match the certified value. 

Fig. 5 Results of cadmium analyses in the spiked pure water of the proficiency testing using regular calibration solutions and certified solutions...

The certified pure solutions of cadmium and lead have demonstrated the reliability of calibrations performed by laboratories. No bias was observed with the commercial or in-house solutions used by the participants of the proficiency testing. In this case, it demonstrates that there is no particular benefit for laboratories to carry out analyses using metrological-... 

Thus physical measurements would benefit from obtaining direct evidence of comparability by carrying out at least some collaborative studies to check that any difference between the response of their measurement system to the sample of the standard have been correctly accounted for. Proficiency testing schemes could also be more widely utilised. At the present moment undue reliance is placed on the effectiveness of the use of calibrated instruments. 

The term quality control (QC) is applied to procedures used to provide evidence of quality and confidence in the analytical results. It includes use of blanks, replication, analysing reference materials or other well-defined samples and participation in Proficiency Testing schemes. Several other features of analysis forming part of QC are control of reagents and instrumentation, equipment maintenance and calibration, and procedures for checking calculations and data transfer. It should be noted that what is referred to as quality assurance in the UK is known as quality control in Japan. 

The ISO 17025 standard [3] describes monitoring the quality assurance of test and calibration results by, amongst other means, the regular use of CRMs and/or internal quality control using secondary reference materials and by participation in inter-laboratory comparisons or proficiency testing programmes. 

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). 

Around 1600 European laboratories responded to the questionnaire (Table 5.3). From the answers received in the different countries, it was clear that many laboratories are not familiar with the term CRM and that confusion exists between certified and non-certified reference materials, between matrix RMs and calibrants. Besides RMs and CRMs it also became clear that many laboratories were not aware of the existence and even the meaning of proficiency testing. 

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