Freezer liquid nitrogen

Heat added to an ice-water mixture melts some of the ice, but the mixture remains at 0 °C. Similarly, when an ice-water mixture in a freezer loses heat to the surroundings, the energy comes from some liquid water freezing, but the mixture remains at 0 °C until all the water has frozen. This behavior can be used to hold a chemical system at a fixed temperature. A temperature of 100 °C can be maintained by a boiling water bath, and an ice bath holds a system at 0 °C. Lower temperatures can be achieved with other substances. Dry ice maintains a temperature of -78 °C a bath of liquid nitrogen has a constant temperature of-196 °C (77 K) and liquid helium, which boils at 4.2 K, is used for research requiring ultracold temperatures. 

The vials are then transferred into freezer boxes and stored in the Liquid Nitrogen storage tank the next day. 

Root powdering and demineralization. Sixty-five frozen and pulp-free tooth roots were ground in a Waring blender and powdered under liquid nitrogen in a freezer mill (Spex, Edison NJ, USA). The sieved powder (< 450 pm) was demineralized in dialysis bags in 0.5 M EDTA, pH 7.4, at 4°C with regular replacement of the solution. Demineralization was continued until no further calcium release could be detected by atomic absorption spectrometry. 

Store the tissue on the chuck if sections are to be cut in the immediate future if not, wrap the tissue in aluminum foil, and place it in a small sealable plastic bag. Store at -70°C or in the vapor phase of a liquid nitrogen freezer (see Note 6). 

Evidence of the stability and recovery of the seeds and banks should be documented. Storage containers should be hermetically sealed, clearly labelled and kept at an appropriate temperature. An inventory should be meticulously kept. Storage temperature should be recorded continuously for freezers and properly monitored for liquid nitrogen. Any deviation from set limits and any corrective action taken should be recorded. 

Evan inflates two balloons to the same size and heats one over some boiling water while the other cools down in the freezer. A quick recomparison of the two balloons shows that while the heated balloon expanded, the cooled balloon shrunk. Ask your instructor what happens when a balloon is dipped into — 196°C liquid nitrogen and then pulled back out. 

Fractions were trapped at the detector outlet in glass U-tubes chilled in liquid nitrogen. The tubes were sealed and stored in a freezer until further investigations could be made. 

A variation of the Bead Beater is the freezer mill, which uses small magnetic bars rather than beads to pulverize the sample. Cooling is provided by immersing the sample chambers in a liquid nitrogen bath. The freezer mill is intended for larger samples (>500 mg) but may be customized for smaller samples. Both the freezer mill and the Bead Beater require that the sample be placed in secondary containers, which may or may not be disposable. Disposable secondary containers offer the very attractive advantage of reduced time and effort for cleaning. 

In order to use a programmable freezer it is essential to have a supply of piped liquid nitrogen from a pressurised cylinder and this is also very convenient for topping up liquid nitrogen freezers. 

When the samples are frozen (at — 70°C or — 120°C) they should be transferred to a liquid nitrogen freezer. These are of two types. In the older version the vials are actually immersed in liquid nitrogen but in the newer models the samples are simply held in the vapour above a sea of liquid nitrogen. 

For the liquid phase freezer, ampoules are transferred to canes (Fig. 7.5) or cryoboxes and immersed CAREFULLY into the liquid nitrogen vat. Gloves and goggles or visors must be worn to protect yourself from splashes of liquid nitrogen. Canes have the problem that vials can be lost from them or are difficult to remove when required, and the locator system of cryoboxes (Thermolyne Appendix 3) has advantages. 

The liquid phase freezers are more difficult to use but because of their narrow neck they are less likely to lose their liquid nitrogen than are the wide necked vapour phase models. In either case it is imperative to top up with liquid nitrogen to the required level twice a week. It is a good idea to keep valuable cells in two different freezers. Pathogenic samples must be stored in vapour phase to avoid spread of contamination on explosion. 

Suitable liquid nitrogen freezers (and canes) are available from... 

A number of organisations store frozen cells for sale or distribution and will also store cells for individuals as a back-up for departmental liquid nitrogen freezers. These include the European Collection of Animal Cell Cultures at Porton Down, the American Type Culture Collection Cell Repository at Rockville, MD, and the Human Genetic Mutant Cell Repositary at Camden, N.J. (Appendix 3). 

Place the freezing ampoules inside a polystyrene box, seal the box, place in a -70°C freezer overnight. Then transfer the ampoules to liquid nitrogen storage after 24 h. 

An important aspect of isozyme work is that provision must be made for the careful handling of tissues to keep them solidly frozen at all times. Proteins are particularly sensitive to thawing events. This means that dry ice or liquid nitrogen procedures must be used during the collection and transportation of the tissues in addition, at least a —70° freezer is needed for the storage of tissues. 

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