Racks and Dividers to Organize Small Frozen Samples
One of the most critical features of a storage system for samples frozen at -75ºC or less is the ease with which the samples can be retrieved (Cato and Schmidly, 1991). Metal and cardboard organizers can be used inside of liquid nitrogen tanks and ultracold freezers to maximize the efficiency of storage organization.
A highly standardized organization structure allows the assignment of a precise location for each individual sample, improving the likelihood of retrieving a sample rapidly and accurately. The description provided here is limited to an ultracold freezer, but the principles apply to a liquid nitrogen system as well.
Paisley S. Cato
Virginia Mus. of Natural History
1001 Douglas Avenue
Martinsville, VA 24112 USA
Tel (703) 666-8634
Fax (703) 632-6487
Photographs: Paisley S. Cato
The organizational structure for an ultracold freezer can best be visualized as a hierarchical system consisting of stainless steel racks, cardboard boxes with tops, cardboard dividers, and containers that hold individual samples. The racks subdivide the freezer into subunits; the racks are subdivided by the boxes they hold; and each box is subdivided by cardboard dividers. Each cell in the divider holds one sample. Handles make the racks easier to install and remove from the freezer (Fig. 1).
Figure 1. Use of stainless steel racks to subdivide ultracold freezer.
Materials Tools Supplies
- Aluminum foil
- Cardboard dividers
- Freezer vials (polypropylene)
- Moisture repellent fiberboard boxes, 51/4in x 51/4in x 2in or 3in high
- Permanent marker pen
- Plastic bags
- Ultracold freezer
- Subdivide moisture repellent fiberboard boxes into 100, 81, 64, or 49 units or cells, using cardboard dividers that are inserted into the box (Fig. 2).
Figure 2. Two sizes of cell dividers for fiberboard boxes.
- If freezer vials are of different diameters, modify the boxes by removing part or all of the dividers.
- Develop a numbering system to identify the location of each sample in the freezer. For example, the numbering system might include a freezer number, a rack number, a box number, and a cell number. A simplified system could use numbers for only the freezer, box and cell; in the latter scheme, location number 3.202.54 would refer to a freezer vial sample stored in the third freezer, and the 54th cell within box no. 202.
- Number the boxes with permanent ink, and post the numbering system on the freezer.
- Store larger samples in aluminum foil or plastic bags that can be stored in the boxes. For collections with large quantities of samples that are too big to fit into the standard 51/4in box, use larger, standard-sized boxes with a rack support that is custom-designed and built to the freezer and box specifications.
Ultimately, the quantity of samples that can be stored in a single freezer depends not only on the size of the freezer, but on the variation in sizes of the individual samples as well. An ultracold freezer can hold anywhere from nine to 36 racks, depending on the style of freezer (upright or chest), its capacity (in cubic feet), and the size of the boxes used for storage. Each rack is designed to hold seven to 16 boxes, depending on the size of the boxes.
When storing boxes in a freezer in metal racking systems, the entire rack must be removed to retrieve boxes located on the bottom, or in the rear. Care must be taken to ensure that the rack is not tipped to allow the boxes to fall out. One option when choosing a stainless steel rack is to be sure that the shelving in the racks has a lip or other restraint to minimize the potential for boxes falling during retrieval.
Although it is possible to organize a set of large boxes in a freezer without the rack support, it is difficult to maintain the organizational structure if the collection is heavily used; boxes are too easily returned to the freezer out of place.