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When storing biological samples at cryogenic temperatures with liquid nitrogen, two primary options merit consideration. The initial choice involves vapour cryogenic phase storage, while the alternative entails directly placing the samples within the liquid phase of nitrogen.
Liquid nitrogen was initially utilized for cryogenic storage in the 1950s, enabling the preservation of samples at temperatures as low as -196°C. Since then, LN2 has been widely acknowledged as the benchmark for cryogenic storage. When it comes to the prolonged storage of samples below the glass transition point of water (-135°C), a critical threshold at which nearly all biological functions cease, a cryogen such as liquid nitrogen stands out as the sole viable solution.
Originally, the sole method involved immersing sample containers in the liquid phase. However, technological advancements soon introduced dry or vapour phase storage, where samples are stored above the LN2 rather than within it.
Both designs, involving the submersion of samples in the liquid phase or storing them in the vapour phase, continue to be utilized today, each presenting its own set of advantages and disadvantages. Ultimately, the optimal choice depends on the specific needs and requirements of the user.
Storage
This is where the advantages and disadvantages of each solution become most apparent. Both designs have their merits and drawbacks stemming from the distinct interior configurations of the freezers.
Both types of freezers incorporate liquid phase nitrogen. However, in vapour phase freezers, samples are stored in the vapour created above the liquid nitrogen, rather than being directly immersed in it. Some critics previously highlighted limitations in space for vapour phase storage freezers, citing the lower third of the freezer as unusable due to the liquid nitrogen base. It is essential to note that this criticism is outdated, primarily referring to modified liquid phase freezers for nitrogen vapour storage. Modern units specifically designed for this purpose do not typically encounter such storage limitations.
A crucial aspect universally acknowledged in the design of vapour phase cryogenic freezers is the minimization of contamination risk—a significant concern with liquid phase LN2 freezers. Vial explosion, a potential issue in liquid phase storage, is absent in vapour phase storage. This phenomenon can occur when even a single drop of LN2 enters a sample vial, leading to violent explosions upon warming, posing both potential danger and sample loss.
Improper storage in liquid nitrogen raises the risk of sample contamination, especially when valuable samples share storage with potentially infectious ones. Certain contaminants, such as Hepatitis B and Vesicular Stomatitis Virus, maintain high infectivity even when suspended in liquid nitrogen. Reports indicate that Hepatitis B retained infectivity after two years in liquid nitrogen.
Vapour phase LN2 freezers address contamination concerns by storing samples above the LN2, minimizing the risk of transmission through the liquid itself.
Additionally, the potential contamination of the outer layer of containers poses risks during retrieval. Unlike with liquid phase freezers, vapour phase freezers present a lower risk of user contamination or injury from splash-back, as they do not require direct contact with the liquid nitrogen. Ignoring the potential for contamination, LN2 drag out alone can pose a significant threat not only to operators but also to the surrounding area.
Access
Liquid phase LN2 freezers are often preferred for extended sample storage durations, as direct submersion in the liquid is deemed more efficient and reliable over an extended period.
Conversely, freezers designed for vapour storage excel in sample accessibility. Retrieving samples from vapour suspension is notably easier. For instance, the HEco vapour phase freezer facilitates effortless sample access through its lid, offering high storage density without compromising hold time or sample security. The HEco also incorporates an internal carousel, bringing samples to the operator, thereby eliminating the need to struggle with reaching samples at the rear of the freezer—a potential hazard with liquid phase freezers.
Some argue that liquid storage consumes less LN2. However, highly efficient and purpose-built vapour storage freezers, such as the HEco, consume nearly 50 percent less liquid nitrogen than an equivalent capacity liquid storage freezer.
Temperature Control
While researchers were historically constrained in temperature control options, the introduction of vapour phase freezers has significantly enhanced temperature adjustment effectiveness.
Operators have harbored skepticism concerning temperature uniformity in vapour phase freezers, particularly in earlier versions that exhibited significant temperature variations between the top and bottom of the freezer. However, contemporary vapour phase freezers have addressed these initial challenges, with current models proficiently maintaining a temperature of -190°C in the top compartment. This achievement is noteworthy, especially when considering that modern vapour phase freezers, such as the HEco, can sustain cryogenic temperatures for several weeks even in the absence of the LN2 source.
In contrast, liquid phase freezers operate at a fixed temperature of approximately -196°C. Nevertheless, advancements in vapour storage, exemplified by the Variō freezer from Chart, now provide opportunities for temperature control. The Variō stores liquid nitrogen in a separate chamber rather than within the freezer itself. This liquid nitrogen is then propelled through a heat exchanger to generate vapour. Consequently, this unique freezer can operate across a broad temperature range, from -20°C to -150°C.
Conclusion
The decision-making process between liquid and vapour phase freezers may not be straightforward, as it involves considerations such as accessibility, temperature control capabilities, and storage needs tailored to the specific requirements of a cryo-room or facility.
Cardboard freezer boxes, a kind of vapour cryogenic phase storage method, are containers specifically designed for the storage of samples in freezers, particularly in laboratory and research settings. These cryo freezer boxes lab are constructed from sturdy and insulated cardboard materials that can withstand low temperatures, providing a cost effective and practical solution for organizing and protecting various samples.
Researchers commonly use cryo storage boxes to store and organize vials, tubes, or other sample containers in a systematic and efficient manner within a freezer. These boxes play a crucial role in maintaining sample integrity and facilitating easy access and identification during experiments or storage.
Yongkang Medical is a company that specializes in the manufacturing and distribution of high quality cryogenic boxes for medical applications or labs. Contact us to order cardboard cryo freezer box.
