Two paths to the same temperature - with different consequences
In cryopreservation, a distinction is made between two basic storage methods: liquid-phase storage, where samples are directly immersed in liquid nitrogen (LN2), and gas-phase storage, where samples are stored above the LN2 level in cold nitrogen vapor. Both methods have their specific advantages and disadvantages, and the choice between them has far-reaching consequences for sample safety, regulatory compliance and the total cost of ownership of your cryobank.
In this article, we analyze both methods in detail and help you to make an informed decision that suits your requirements.
Liquid phase: Direct immersion in LN2
Advantages
- Lowest possible temperature: At -196°C, all biological activity is completely suspended. This temperature is maximally stable - as long as LN2 is in the tank, the temperature remains constant at -196°C.
- Maximum homogeneity: All samples - regardless of their position in the tank - are exactly at -196°C. There are no temperature gradients.
- Optimal cell protection: The liquid phase offers the best possible protection for extremely sensitive cell types that are damaged by even the slightest temperature fluctuations.
Disadvantages
- Cross-contamination risk: If a vial is leaking, nitrogen can penetrate. When thawed, the infiltrated nitrogen expands suddenly and can cause the vial to burst - a safety risk for staff and a contamination risk for the sample. It has also been documented that pathogens can be transferred between samples via the liquid nitrogen.
- GMP concerns: Due to the risk of cross-contamination, liquid phase storage is not permitted in many GMP-regulated applications or requires additional protective measures (e.g. over-wrap bags).
Gas phase: Storage in cold vapor
Advantages
- No risk of cross-contamination: As the samples are not in contact with liquid nitrogen, the possibility of cross-contamination via the nitrogen medium is eliminated. This is the main reason why the gas phase is preferred in many regulated environments.
- GMP compliant: Gas phase storage is recognized by most regulatory authorities as a safer method and is generally easier to qualify.
- Easier handling: Taking and storing samples is easier as no handling of the LN2 liquid is required.
Disadvantages
- Temperature gradient: A temperature gradient exists in the gas phase: the positions near the LN2 surface are colder (approx. -190°C), while the upper levels, especially near the tank opening, can be warmer (approx. -150°C to -170°C).
- Sensitivity to tank access: Each opening of the tank leads to a greater temperature increase in the gas phase than in the liquid phase. The eccentric opening of the Consarctic® BSD+ and BSF+ tanks significantly minimizes this effect.
The Consarctic® solution: Optimized vapour phase storage
Consarctic® has designed its stainless steel tanks specifically for optimized gas phase storage. Several design features help to minimize the temperature-related disadvantages of the gas phase:
Eccentric opening
The offset tank opening on the BSD+ models reduces gas exchange when the lid is opened. Most of the tank contents remain shielded from the room air, which minimizes the temperature rise.
Cover system with insulation
The lid is fitted with additional insulation, which reduces the heat input from above and thus reduces the temperature gradient in the gas phase.
Temperature mapping as proof
As part of the IQ/OQ qualification, Consarctic® technicians carry out a complete temperature mapping of the tank. This documents the temperature at each rack position and proves that all positions are within the specified limits.
When which method?
The decision between gas and liquid phase depends on several factors:
- Regulatory requirements: For GMP-regulated applications (stem cell products, CAR-T, ATMP), the gas phase is usually the preferred - and often mandatory - method.
- Sample type: For particularly sensitive samples where the temperature gradient of the gas phase is not acceptable, the liquid phase with over-wrap protection may be the better choice.
- Access frequency: Tanks with a high access frequency benefit in particular from the eccentric opening design in the gas phase storage.
Your decision, our expertise
There is no universally correct answer to the question of gas phase versus liquid phase - but there is a right answer for your specific situation. The experts at Consarctic® will analyze your requirements and recommend the optimal storage configuration for your cryobank.
Would you like to discuss the advantages and disadvantages of both methods for your specific application? Contact us for an individual consultation.
