temperature of less than -135°C is maintained. Such ultra-low temperatures can only be attained by specialised electric freezers or more usually by immersion in liquid or vapour phase nitrogen. The advantages and disadvantages can be summarised as follows: Storage in liquid phase nitrogen allows the lowest possible storage temperature to be maintained with absolute consistency, but requires the use of large volumes (depth) of liquid nitrogen which is a potential hazard.There have also been documented cases of cross contamination by virus pathogens via the liquid nitrogen medium. For these reasons ultra-low temperature storage is most commonly in vapour phase nitrogen. For vapour phase nitrogen storage, the ampoules are positioned above a shallow reservoir of liquid nitrogen, the depth of which must be carefully maintained. A vertical temperature gradient will exist through the vapour phase, the extremes of which will depend on the liquid levels maintained, the design of the vessel, and the frequency with which it is opened. Temperature variations in the upper regions of a vapour phase storage vessel can be extreme if regular maintenance is not carried out. Modern designs of liquid nitrogen storage vessels are increasingly offering improved vapour phase storage technology. Loss of entire cell stocks through inadequate storage maintenance is distressingly common. All liquid nitrogen storage vessels should minimally include alarms that warn of low liquid nitrogen levels and should also be constantly temperature monitored and alarmed.. This is particularly true of vapour phase storage systems. The bulk liquid nitrogen storage vessel should not be allowed to become less than half full before it is re-supplied. This will ensure that at least one liquid nitrogen delivery can be missed without catastrophic consequences. It is highly recommended that valuable cell stocks should be backed up by storage at a second site. ECACC offers a Safe Deposit Service for this purpose. Method Advantages Disadvantages Electric (-135 o C) Freezer • Ease of Maintenance • Steady temperature • Low running costs • Requires liquid nitrogen back-up • Mechanically complex • Storage temperatures high relative to liquid nitrogen Liquid Phase Nitrogen • Steady ultra-low (-196 o C) temperature • Simplicity and mechanical reliability • Requires regular supply of liquid nitrogen • High running costs • Risk of cross-contamination via the liquid nitrogen Vapour Phase Nitrogen • No risk of cross- contamination from liquid nitrogen • Low temperatures achieved • Simplicity and reliability • Requires regular supply of liquid nitrogen • High running costs • Temperature fluctuations between -135 o C and -190 o C Table 3. Comparison of ultra-low temperature storage methods for cell lines.
23 sigma-aldrich.com 7.3 Inventory Control All ultra-low temperature storage vessels should include a racking / inventory system designed to organise the contents for ease of location and retrieval.
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