10 Apr


Reference samples
In order to identify samples recovered from the scene of crime, reference samples are needed for comparison. Reference samples are provided by a suspect and, in some cases, a victim. Traditionally, blood samples have been taken and these provide an abundant supply of DNA; however, they are invasive and blood samples are a potential health hazard. Buccal swabs that are rubbed on the inner surface of the cheek to collect cellularmaterialhavereplacedbloodsamplesinmanyscenarios.Insomecircumstances plucked hairs may be used but this source of material is not commonly used. FTA® cards can be used to store both buccal and blood samples (Figure 3.4). The FTA® card is a cellulose based paper which is impregnated with chemicals that cause cellular material to break open – the DNA is released and binds to the card. The chem- icals on the card also inhibit any bacterial or fungal growth and DNA can be sta- bly stored on FTA® card for years at room temperature as long as the card remains dry.

Storage of biological material
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

Figure 3.4  FTA cards can be used to store both blood and buccal cells. The cellular material lyses on contact with the card. The DNA binds to the card and is stable for years at room temperature

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