Written by Christopher Burgess in Valid Analytical Methods and Procedures.Royal Society of Chemistry. 2000.pp. 15-191 Introduction
The importance of sampling in method validation and, in particular, intercomparison of methods cannot be over emphasised. If the test portion is not representative of the original material, it will not be possible to relate the analytical result measured to that in the original material, no matter how good the analytical method is nor how carefully the analysis is performed. It is essential that the laboratory sample is taken from a homogeneous bulk sample as a collaborator who reports an outlying value may claim receipt of a defective laboratory sample.
It is important to understand that sampling is always an error generating process and that although the reported result may be dependent upon the analytical method, it will always be dependent upon the sampling process. The essential question in the inter-comparison of analytical methods is, �If the same sample (or a set of identical aliquots of a sample) is analysed by the same method in different laboratories, are the results obtained the same within the limits of experimental error?�. It is apparent, therefore, that the selection of an appropriate sample or samples is critical to this question and that the sampling stage should be carried out by a skilled sampler with an understanding of the overall context of the analysis and trial.
Any evaluation procedure must cover the range of sample types for which the method under investigation is suitable, and details of its applicability in terms of sample matrix and concentration range must be made clear. Similarly, any restrictions in the applicability of the technique should be documented in the method. In particular, Crosby and Patel�s General Princbles of Good Sampling Practice and Prichard provide readily digestible guidance to current best practices in this area.
2. What is a sample?
The Commission on Analytical Nomenclature of the Analytical Chemistry Division of the International Union of Pure and Applied Chemistry has pointed out that confusion and ambiguity can arise around the use of the term �sample� and recommends that its use is confined to its statistical concept. When being used to describe the material under analysis, the term should be qualified by the use of �laboratory sample� or �test sample�, for example.
One of the best treatments of sampling terminology is given in recommendations published by IUPAC� which describes the terms used in the sampling of bulk or packaged goods. In this example, the sampling procedure reduces the original consignment through lots or batches, increments, primary or gross samples, composite or
aggregate samples, subsamples or secondary samples to a laboratory sample. The laboratory sample, if heterogeneous, may be further sample is deemed to be the end of the sampling procedure.
Once received into the laboratory the laboratory samples or test samples will be recorded and then be subjected to analytical operations, beginning with the measuring out of a test portion and proceeding through various operations to the final measurement and reporting of results/findings.
The problems associated with sampling in many areas of chemical testing have been addressed and methods have been validated and published. Where specific methods are not available, the analytical chemist should rely upon experience or adapt methods from similar applications.When in doubt, the material of interest and any samples taken from it should always be treated as heterogeneous.
It is important when documenting a sampling procedure to ensure that all of the terms are clearly defined, so that the procedure will be clear to other users. The use of sampling plans may be appropriate and guidance is available for procedures based upon attributes or variables.
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