9.10.2 Methods

It is important that the results obtained in analyses are valid. If analysis of water supplies is to be useful, methods must yield consistent results; however, different methods of analysis can in some cases give different results on the same water sample. To ensure consistency, at least one standard method of analysis is suggested in the fact sheet for each characteristic listed in the Guidelines. Wherever possible, the recommended method should be used, as it is well documented, has undergone extensive evaluation (preferably through comprehensive inter-laboratory comparison programs), is readily available and is widely used by larger water suppliers. If other methods are to be used, it is important that they meet these criteria, and give results that are consistent with the standard methods. Even minor variations to documented methods can lead to inaccurate measurements.

Whatever analytical technique is used, it must give a result that can be compared to the listed health and aesthetic guideline values. This is especially true in relation to the limit of detection for the method. Wherever possible, the method used should have a limit of detection that is less than the guideline value. The limit of detection for each suggested method has therefore been provided in the relevant fact sheet.

The whole analytical process, from sampling through to presentation of results, needs to be managed in accordance with sound quality assurance principles and should include quality control checks as part of the quality assurance process. Quality assurance principles are set out in documents such as ISO 9001, and supported by programs such as the National Association of Testing Authorities (NATA) schemes. Wherever possible, analyses should be undertaken at NATA-accredited laboratories.

As part of the testing procedure, certified reference materials should be used so that results can be benchmarked against a specified reference. This will help ensure that measurement results from different methods and laboratories are comparable.