Proficiency-testing difficulty

The test material is chosen to fulfill the aims of the study. In a proficiency testing scheme or a method validation study, the test material is usually as near as possible to typical field samples. There is no advantage in competently analyzing an artificial sample if the same laboratory has difficulty with real samples. The organizing laboratory must know the composition of the test material, and must be sure that the analyte for which a quantity is to be measured is present in about the desired amount. For pure materials this is not a problem, but for natural test materials or complex matrix materials, the organizing laboratory may have to do some analyses before the samples can be sent out to the participants. If the value of the measurand is to be established by an independent laboratory before the study, then the identity requirement is also fulfilled when the measurand is stated. 

Designated laboratories or laboratories seeking designation must participate in one of the usually two proficiency tests conducted annually. The proficiency tests conducted within the same year should therefore have the same degree of difficulty in order to accommodate a fair and consistent proficiency-testing scheme among the participants. 

An attempt to judge the degree of difficulty of the first 10 conducted proficiency tests is made in Figures 11 and 12. The degree of difficulty for each proficiency test is measured in the percentage... 

Figure 11. The degree of difficulty of proficiency tests 1 to 10, measured for the designated laboratories...

The sum of these three bars represents the total number of spiking chemicals a participant has been tested for during the first 10 proficiency tests, with a maximum of 72. These numbers depend on the frequency of participation as regular participants, and therefore differ from the other participants. The fact that not all participants were tested on exactly the same spiking chemicals plus taking into account the spread in the degree of difficulty of past proficiency tests (see Section 4.3) limits the extent to which the participants performances can be compared. The absolute numbers of... 

Another element is the use of a cocktail of different Scheduled chemicals in a single proficiency test. Applying a cocktail of chemicals may not be very realistic, however, it is considered unavoidable in order to train participants in the analysis of as many different Scheduled chemicals as possible in a small number of tests. The chemicals, used for spiking the samples, have been reviewed, which revealed that certain Scheduled chemicals have not been covered in the first 10 proficiency tests. Therefore, it would be recommendable to include such chemicals in future scenarios. The same recommendation applies to several identified categories of chemicals, which have been applied in previous proficiency tests, but created difficulties (repeatedly) for some of the designated laboratories, as well as for most of the nondesignated laboratories that regularly participate in the scheme. 

There were differences identified in the degree of difficulty between the first 10 proficiency tests. These differences are a result of the changing sample composition, that is, different matrices, spiking chemicals, and background materials. It is mainly a laboratory s experience in the analysis of CWC-related chemicals that seems to allow the participants to cope with these varying challenges. The Technical Secretariat tries to maintain a similar degree... 

There is one element within the proficiency test process that causes difficulties for many of the participants. The proficiency-testing scheme trains participants to not report chemicals that are considered irrelevant within the scenario of the test and which, in real off-site analysis, could reveal confidential information that is considered not relevant in the context of the CWC. A review on how the test scenarios were applied revealed that participants are not really in a position to judge the relevancy of chemicals. It rather should be the Technical Secretariat making a final decision on the relevancy of chemicals instead, and, for that reason, accept the reporting of any chemical given that it is a degradation product of a Scheduled chemical or directly related to a Scheduled chemical. 

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