A retention sample originating from the same consignment as the analytical sample must always be kept in the laboratory - when possible in the original container - for use if the results of the analysis are disputed. This is usually prepared by the lead unit from the sample as received. The sample should therefore be large enough to provide an adequate reserve even when a number of replicate tests are required.

Sometimes, however, the retention sample is prepared by the sampling inspector when the analytical sample is taken. In this case the two samples should be separately packaged and transferred together to the laboratory. The retention sample is then labelled as such and given a registration number before it is forwarded with the analytical sample for storage in the lead unit.

Once all the required tests have been performed, any remaining portions of the sample should be resealed in their original containers. They should then be labelled with the date on which they may be discarded and placed in a locked cabinet in central store, if necessary at low temperature. Samples found to comply with specification should be kept for at least 6 months. Those that do not should be kept for at least one year, or for any longer period specified in current regulations.