Insuch a case, the progressive work strategy is againrecommended in order to make a more reliable decisionfor this sample.Final remarksThe sample similarity issue deserves some discussion.Differential pulse ASV is a technique that exhibits highselectivity [10]; in fact, the same manufacturer’s protocol(Metrohm Application Bulletin 241/1e) is proposed fordifferent types of water matrices. Moreover, the fact thatstandard addition calibration is used on every samplecould eliminate any matrix effects. Therefore, the use of amixture CRM solution (as a sample) during the validationsteps could be reasonable, even considering that harborsmay contain various materials (e.g., hydrocarbons) thatcould impact on to the similarity between the CRM andthe sample. Unfortunately, protocols or criteria forestimating the uncertainty associated with this issue arenot available. Alternatively, the testing laboratory coulduse a “negative” real sample under the scope of theaccreditation (e.g., an analyte concentration below theLQ)fortified with the mixture CRM solution at the desiredconcentration level (i.e., the sample is considered a blank,requiring a recovery experiment). This could be anrequirement of the particular ISO 17025 auditors duringthe accreditation process.Currently, ISO 17025 auditors accept validation (and IQCresults) that are obtained in the laboratory to be accredited.In terms of method validation, the current work can be con-sidered a“partial method validation”(i.e., a fit-for-purposevalidation of those features subject to the available requi-sites; this concept requires further harmonization fromnormalization/accreditation entities). This means that if acomplete validation is required (say by a particular ISO17025 auditor); extra work must be performed by a testinglaboratory to adapt (and then validate) the present method.By contrast, it can be considered fit-for-purpose, accordingto the AOAC criteria [20], and consistent with the US EPAcritical levels for the analytes/matrix [17]. Conventionally,it is necessary to validate the entire concentration range (i.e., to cover the full scope of the method). This would implythat the proposed protocol must be repeated, at least at alow level (say, close to the experimentally determined LQ).In our opinion, validating the method at a very low level,far from the critical level of the analyte, althoughinformative, has no practical relevance (e.g., in terms ofassessing the risk associated with an analyte); and shouldbe reconsidered in some instances (another consequence ofthe unharmonized partial validation concept).Also, ISO 17025 auditors have tended to demand (at thevery least) method verifications the two extreme concen-tration levels in the concentration range of the method,which could be alternated between working routinesessions (unharmonized task). As before, there must be
less interest in verifying the method at the level without
risk. Similarly, with the current experimental design, there
is no information on the uncertainty of samples below the
target level. However, the real practical interest when
assessing the uncertainty level is mainly in the samples
close to or above the critical level (e.g., sample 14 in
Table4). Again, this point deserves more attention and
revision (harmonization).
Conclusion
Fit-for-purpose “partial” validation could provide to the
testing laboratories almost ready-to-use (or ready-to-validate)
methods that can be implemented more easily, with prior
knowledge of the main features of the method (mainly
accuracy), IQC behavior, and also potentially critical aspects
and possible solutions included as part of the SOP. By
following a systematic protocol, harmonization between
research laboratories performing method development tasks
and testing laboratories can be improved. Finally, the use of
harmonized validation–IQC–uncertainty statistics provides
consistency during decision tasks.
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