Performance Criteria for Methods Used

In Section 4.3, some factors which need to be considered in choosing a method of analysis were discussed in general terms. The next step is to consider the properties of a method that will enable a choice to be made. This is done for a specific case, e.g. the determination of residues of chemicals used in veterinary practice to treat animal diseases and to prevent the development and spread of disease where large numbers of animals are kept in close proximity to each other. Such chemicals may be administered by injection, or orally as a constituent of the feed. Some chemicals are metabolized and excreted while others may be partially retained in edible products such as milk, eggs, meat and offal (liver or kidney). The detection and determination of such residues is a very difficult analytical problem. 

Suggest reasons why the determination of veterinary residues in animal products presents difficulties for the analyst. 

Your answer should have included the following  

Congratulations if you thought about the last one and if you realized that the analysis is made more difficult by the presence of large numbers of co-extracted compounds. Hence, a method involving extensive clean-up or purification of the initial extract will be required. In addition, the detection system selected will not only need to be able to measure very low concentrations but also be highly selective to ensure that positive signals are not obtained from co-extracted analytes. 

Since the analytical problem is so difficult, there will not be many methods or techniques available which are satisfactory for the purpose required, i.e. to determine whether residues are present at or above the legal limits. There will be instances where no method is available which matches the criteria initially specified for precision and accuracy. In these cases, the customer must be fully informed of the situation, since developing a new method takes time and money. 

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