The cold chain is the unbroken process that keeps a temperature-sensitive material within a defined temperature range from the moment it is produced to the moment it is used. The chain metaphor is deliberate: a break in any one of the storage, packaging, transport and receipt links can undo the care taken in all the others. Venom samples, containing proteins and peptides, are materials of this kind.
Drying is the first step in increasing the stability of venom; why and how it is done was described in the article on venom lyophilization. Even a dry sample changes over time, and the way to slow that change is to keep the temperature low and steady. This article looks at storage equipment, the arrangement of storage and the cold chain in shipping.
Temperature levels
| Level | Typical equipment | Common use in the laboratory |
|---|---|---|
| 2-8 °C | Laboratory refrigerator | Short-term holding, samples that have been reconstituted and will be used soon |
| About −20 °C | Standard laboratory freezer | Medium-term storage of dry samples and aliquots |
| About −80 °C | Ultra-low temperature freezer | Long-term archive and reference samples |
| About −78 °C | Dry ice (solid carbon dioxide) | Transporting frozen samples |
| About −196 °C | Liquid nitrogen and vapour-phase tanks | Special materials such as cells and tissues |
Which level is chosen depends on the material, its form and the storage period. A lower temperature does not always mean better; the reliability of the equipment, how often the sample will be accessed and the possibility of backup in the event of a failure are taken into account as well. The storage condition specific to a product is read from that product's documents rather than from general habit; the section of the safety data sheet devoted to storage and the product record are the sources of this information.
Arrangement in the deep freezer
The inside of a freezer is not at the same temperature at every point. Shelves near the door and the upper compartments warm up more each time the lid is opened, while the rear and lower areas are more stable. Valuable samples and those to be kept for a long time are therefore placed away from the door. Mapping the temperature at different points of the interior when a new unit is commissioned makes these differences visible in numbers.
Freezers with an automatic defrost function are not considered suitable for sample storage. During the defrost cycle the internal temperature rises at regular intervals and the sample undergoes repeated warming and cooling unnoticed. Laboratory-grade units have no such cycle; the ice that builds up is removed manually during planned maintenance.
The length of time the door stays open is the largest source of temperature fluctuation. The way to shorten it is to find the sample without searching for it:
- Every box and shelf has a location code, and the sample record includes that code.
- Labels are chosen from material resistant to low temperatures and condensation, and written with an indelible pen or printed.
- Tubes are of the leak-proof, screw-cap type; glass containers are chosen with care because of the risk of cracking at very low temperatures.
- Samples are divided into single-use aliquots, so that the main stock does not come out unnecessarily.
- The inventory list is kept outside the freezer and updated with every entry and removal.
Monitoring, alarms and the contingency plan
The proof of a cold chain is the record. Freezers are monitored with a data logger that measures continuously and are connected to an alarm system that gives an audible and remote warning when the temperature exceeds the set limit. Because a record can show afterwards when a problem began and how long it lasted, it is far more valuable than the reading on the display at any given moment.
Failure is treated not as a possibility but as a matter of time. Backup power to keep the unit running during a power cut, spare capacity able to take the contents of a failed freezer immediately, and a short instruction setting out who does what when an alarm sounds are the basic parts of the plan. With the lid kept closed, a well-insulated unit can hold its internal temperature for a while; because that time varies with the unit and how full it is, it is established by measurement rather than by guesswork.
The cold chain in shipping
The moment the sample leaves the facility is the most fragile link in the chain. An insulated shipping box, a suitable coolant and a temperature logger travelling with the package are the basic tools at this stage. Dry ice is mostly used for frozen samples; for the 2-8 °C range, pre-conditioned gel packs are preferred.
Dry ice passes directly from solid to gas and releases carbon dioxide. A box containing dry ice is therefore closed in a way that allows the gas to escape; in a completely sealed container the accumulated gas builds up pressure. Carbon dioxide accumulating in closed, poorly ventilated spaces is a separate risk to staff. Dry ice is also a subject of the transport rules and may require a declaration; the general framework of dangerous goods transport was described in the article on ADR, IMDG and IATA. The amount of coolant is determined by taking into account the length of the journey and any possible delays.
Checks on receipt
- Check the external condition of the box and its labels.
- See whether the dry ice has run out, or what condition the gel packs are in.
- Read the data from the temperature logger and note any deviation.
- Transfer the sample to the intended storage conditions without delay.
- Enter the delivery date and time and your observations in the sample record.
A shipment showing a deviation is neither used nor discarded straight away; the situation is shared with the sender and how the sample is to be assessed is decided together.
How the chain continues on the buyer's side
The cold chain does not end with delivery. How the sample is stored in the receiving laboratory, how many times it is thawed and how long it stays open directly affect the consistency of later measurements. Thawing is part of the chain too: frozen liquid samples are mostly thawed in a controlled way on ice or in a refrigerator, droplets left on the tube wall are collected with a brief spin and the sample is used without delay. At this last link, leaving the sample in a warm environment unnecessarily can undo all the earlier care. Maintaining the cold chain is one of the buyer obligations defined for our products; the forms and the related headings are given on the products page. The change that emerges over time is itself covered in the article on venom stability.
