After sample collection, the other steps do not follow in a specific order. For example, DNA can be prepared from fresh sample, or the sample can first be transported off-site then stored (short term) or archived (long term).
A number of options are available for sample collection, transport, archiving, and DNA purification. These can be categorized into those that are amenable to room temperature archiving and transport, and those that are not. In general, untreated biological samples are not stable at room temperature.
Biological samples collected directly into a tube, multiwell plate, or other vessel are typically not stable at room temperature. To maintain nucleic acid integrity, samples should be processed immediately, stored over the short term (with or without an added chemical stabilizer) at a temperature appropriate for the sample type, or collected onto a matrix suitable for room temperature archiving. For short-term storage, conditions vary by sample type. For example, blood samples can be stored at 4 °C for up to 48 h before isolating DNA, while tissue samples and pelleted cells may be stored at -80 °C for several weeks or months. Cells may be frozen directly, or a stabilizing agent (see below) may be added. Most tissues should be “flash frozen” in liquid nitrogen for best results. Biological samples that require refrigeration or freezing also require wet or dry ice, respectively, for shipping. In addition, human samples must be labeled as potential biohazards. Shipping of these samples to less accessible regions may be limited to certain days of the week, and the shipper must be willing to deal with biohazardous materials.
Chemical stabilizers may be used to stabilize DNA, RNA, and/or protein. These chemicals allow samples to be stored for short periods (several days or weeks) at room temperature or for longer periods (12+ mo) at -20 °C or -70 °C. Chemical stabilizers require an additional kit or method to isolate nucleic acids.
A straightforward way to prepare a sample for room temperature archiving and transport is to collect the sample directly onto a paper matrix. The options for paper matrices include a variety of untreated matrices (e.g., Guthrie or Whatman 903 cards) and chemically treated matrices (i.e., Whatman FTA technology). When nucleic acids will be isolated, we recommend using chemically treated matrices designed to stabilize nucleic acids and denature proteins. Additional details on Whatman FTA technology are provided later in this chapter.
Other products are available for room temperature archiving. However, these products do not stabilize the nucleic acids in the samples themselves; they stabilize previously purified nucleic acids.
See Table 3.1 for a summary of the different options for sample collection, transport, archiving, and DNA purification, including advantages and disadvantages.