Biological material collected for DNA analysis should be stored in conditions that will slow the rate of DNA degradation, in particular low temperatures and low humidity. A cool and dry environment limits the action of bacteria and fungi that find biological material a rich source of food and can rapidly degrade biological material.
The exact conditions depend on the nature of the samples and the environment in which the samples are to be stored. Buccal swabs and swabs used to collect material at a crime scene can be stored under refrigeration for short periods and are either frozen directly or dried and then stored at -20 °C for longer term storage. Blood samples will normally be stored at between -20 and -70 °C. Buccal and blood samples collected using FTA®® cards can be stored for years at room temperature. Some items of evidence, like clothing, can be stored in a cool dry room; in temperate regions of the world DNA
has been recovered from material stored at room temperature for several years . When samples are not frozen, for example clothing, they are stored in acid-free paper rather than plastic bags, to minimize the build up of any moisture. Once the DNA has been extracted from a sample, the DNA can be stored short term at 4 °C but should be stored at -20 to -70 °C for long term storage.
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