How should duplicate samples be collected for quality assurance?

The correct approach for collecting duplicate samples for quality assurance is to pour from a larger bottle into two smaller bottles. This method ensures that both samples are taken from the same source and are likely to have similar properties, thus allowing for a valid comparison of results. When using a larger bottle, you can control the environment and conditions under which the samples are taken, which helps in minimizing variability between the duplicate samples.

This method provides a practical way to ensure consistency in the sampling process, as both smaller bottles receive water from the same initial volume, reducing the potential for contamination or differing sample conditions. This is crucial when testing for contaminants such as Legionella, where consistency is key for accurate measurement and comparison of results.

The other methods discussed would not provide the same level of assurance. For instance, using one bottle to collect samples does not allow for the retrieval of two distinct duplicates, making it impossible to compare results effectively. Collecting two samples simultaneously might not truly reflect the same water conditions due to potential fluctuations in water quality during the collection process. Using pre-filled sample bottles could work, but it does not guarantee that both samples come from the same batch of water in the way that pouring from a larger bottle does.