Re: Do cells need to be stored in the vapor part of liquid Nitrogen?

Cells can be  stored in liquid nitrogen either in the liquid phase or in 
the vapor phase. In most cases cells are stored in small dewars with few 
vials actually immersed in the liquid nitrogen phase and the remaining on 
top in the vapor phase.  There are advantages and disadvantages of both.

Storage of cells in the liquid phase are better stored due to the constant 
temperature without fluctuation. However there is a risk of liquid 
nitrogen entering the cryovials leading to the explosion of the vials.  
More importantly, since the liquid nitrogen is in close contact with the 
vials, there is a chance of contamination from one vial to the other, 
especially with mycoplasma.  It is therefore important to make sure that 
cells that are actually stored in the liquid phase are  pathogen-free.  
Further, to avoid explosion, it is always a good idea to loosen the cap of 
the vial during thawing to allow for any contaminating liquid nitrogen to 
escape.  

The advantage of storing cells in the vapor phase is that there is neither 
the threat of contamination from other vials or danger of explosion of the 
cryovials.  It is for this reason that most companies that make cryogenic 
storage material recommend this method. However, the level of liquid 
nitrogen needs to be closely monitored to ensure enough vapor phase to 
maintain uniform temperature.