In liquid phase storage, samples are submerged directly in liquid nitrogen at -196 degrees Celsius. In vapour phase storage, samples are held above the liquid surface in nitrogen vapour at temperatures typically between -150 and -190 degrees Celsius depending on their position in the vessel. The temperature in vapour phase varies with height, which is why there is no single vapour phase temperature for a given vessel.
The critical practical difference is cross-contamination risk. Liquid nitrogen is not sterile. In liquid phase storage, any pathogen in the tank environment has direct physical contact with every stored sample. The HFEA Code of Practice and associated UK regulatory guidance increasingly reflect the expectation that vapour phase storage and closed carrier systems are the appropriate standard for licensed human reproductive material. A pilot study published in PeerJ examining automated liquid-phase embryo storage found stable temperatures at -195 degrees Celsius in storage areas and -177 degrees Celsius in retrieval zones, illustrating the temperature gradient that characterises liquid phase access events. Any new vessel purchased for primary UK IVF storage should support vapour phase as a baseline specification.
