Every lab has unique procedures when caring for their biochemical materials, but most teams operate on similar fundamentals. These practical methods are shared among colleagues to minimize waste and maximize chemical potential, but we usually don't discuss why our guidelines work. This article will explore one such process of preservation - desiccation.
The major questions covered:
- What is desiccation?
- Do chemicals expire, and why?
- What is necessary for desiccation?
- How is desiccation performed in a laboratory?
- What chemicals need to be desiccated?
If you’ve ever spent too much time outside on a very hot or cold day with low humidity, you’ve probably experienced a form of desiccation.
Desiccation can be defined in a broad sense as the state of being very, very dry. This can be assessed in the physical properties of biology and chemistry.
Biologically, desiccation occurs when an organism loses a certain quantity of its retained water, as when plants are not properly watered or have too much light exposure. This change is usually detrimental. Different organisms have higher susceptibility to desiccation damage: snails, frogs and salamanders are just a few vertebrates which cannot survive extended periods of desiccation. One commonly referenced example is a snail doused in salt (NaCl). The Na+ and Cl- ions disrupt the balance of cell membranes, and the snail exudes high quantities of mucus to displace the salt. It does this with such vigor that the body dehydrates - essentially death by desiccation.
In chemical contexts, desiccation is commonly regarded as a method of preservation. DNA is often found in ancient, dry remains due to the deoxygenated tissue being preserved from putrefaction. Whereas DNA in decomposing cells might remain intact for only months in a degrading environment, DNA in tissue kept ideally deoxygenated or dry can remain intact for decades (and for mitochondrial DNA, centuries or millenia).
Desiccation is also a preserving method for laboratory-based compounds. Some substances are naturally dry; others are saturated. In both cases, desiccation methods can be used to deter chemical contamination and promote longevity.
Water in a substance can represent a problem on account of contamination or hydrate bonds that change chemical composition. A sample’s water content and thus composition will change based on humidity and temperature.
Shelf Life and Expiration
Desiccation is necessary to preserve the freshness and durability of certain chemical substances. Think of placing food in cold, dry places and even in the near vicinity of baking soda. This environment extends shelf life by reducing the amount of ambient moisture and bacteria. For chemicals, dehydrating their surroundings prevents them from “expiring,” or going bad, too quickly.
Standard shelf life can be defined by the limit of time during which the substance (if properly stored) experiences no chemical or physical changes. Alternatively, a chemical’s expiration date – affected by the same characteristics as shelf life – entails the period of time a standard is viable after its first use. This is usually shorter than shelf life, one year being the common maximum. For substances that require it, desiccation will extend shelf-life and provide stable conditions before the standard expires.
It is difficult to isolate one reason for the “expiration” of chemical substances and laboratory products. The relative stability of certain chemicals will make their period of usefulness shorter after they've been used. Transpiration losses from a container’s outlets and water affinity are responsible for how much moisture might be lost or gained in an exposed environment. Stability and transpiration losses help determine the shelf life of a standard chemical. The quantity of the substance kept together will also influence its longevity; smaller portions usually have a shorter shelf life.
Chemical substances do expire even if they are kept in the best conditions. Organic objects exposed to the elements lose their freshness, and compounds also break down. Their properties lose viability; reactions become less effective; experimental results are thus no longer accurate. In this sense, a chemical product will expire without being used, especially if left unprotected from destructive forces like humidity. To keep often expensive substances functional for longer, desiccation is a useful procedure.
Equipment: Desiccators versus Desiccants
Knowing the theory of desiccation is the first stage. The second is knowing how to perform it. Desiccators and desiccants are two important components to keeping chemicals pristinely dry.
A desiccator is a container, usually made of glass or plastic, which maintains the dryness of chemical materials. They might be equipped with indicators to confirm humidity levels or suspended above a desiccant to improve performance. A vacuum desiccator has an additional vacuum to promote drying. Reduced pressure is ensured by sealing the desiccator. The use of a desiccator is generally favored to the addition of other chemical desiccants directly into a sample.
Desiccants are substances which attract and prevent the movement of water in the environment. These hygroscopic substances are often used to keep containers, fridges or testing equipment consistently desiccated. Some examples of useful desiccants include silica (used in the packets found in new clothes and dried fruits), activated charcoal, calcium chloride and calcium sulfate. In the case of the unfortunate snail, NaCl is the desiccant.
Three functional groups of drying agents are employed to prevent humidity-induced change. The agent can combine with water at varying rates depending on conditional temperature, like anhydrous sodium sulfate; others, including alkali metals, react irreversibly with water; the final option is a molecular sieve. A multitude of compounds can function as laboratory drying agents.
Desiccants can remove the moisture in solvents if a reaction is intolerant to water. They are also responsible for drying solids stored above or near them. If this method doesn’t work, vacuum-drying desiccators or molecular sieves are employed.
Meter indicators are used to determine whether desiccants have lost their functionality. Certain indicators will change colors, often from blue or white to pink, once the desiccant begins losing its effect. The desiccant must then be exchanged for a new one.
Laboratory Desiccation: Desiccators, Desiccants, and Freeze-Drying
Most desiccation is conducted by researchers and managers of lab material. Storage requires an intimate knowledge of each chemicals’ needs to promote accurate quality control and conserve resources stored in large quantities.
For laboratory work, desiccator- or desiccant-mediated preservation is employed for temperature-sensitive or decomposition-prone substances otherwise heated on a hot plate or in a microwave system. Proteins, enzymes, microorganisms and plasma are all heat-sensitive and need to be dried in an alternative way.
When the container is large enough to fit a sample currently in use, a desiccator is reliable for preserving dryness. Vacuum desiccators remove any previous moisture from a chemical. In other cases, the addition of desiccants keep substances properly dry.
Chemicals are stocked in boxes and bags of different sizes, so the number of desiccants used depends on the container's size and contents.
- Standards that will be used in the short-term only need one desiccant.
- Multiple desiccants are used if the product will be preserved, weighed and used for over a year.
Desiccants must be replaced. The longer the desiccants last, the better. One bag might provide moisture relief for three to six months depending on how it's sealed within the material, so expired desiccants should be removed from the shelf more often than expired chemicals.
- For frequently used chemicals like TRIS and ampicillin, the containers of which are opened more often for use, two 8-oz desiccants will keep the chemical desiccated if stored in bulk.
- A less frequented substance like penicillin might have three or four desiccants stored with it.
- Other materials can get up to five desiccants for extended periods of shelf stasis.
Not all laboratories operate on the same formula of product-desiccant ratio, but the general rule of mutual linear increase will apply.
In addition, freeze-drying – also known as cryodessication or lyophilization – may be necessary for preservation. Freeze-dry techniques use dilute solutions to remove ice sublimes and volatile liquids from a biological or pharmaceutical substance. A freeze-drying unit may be implemented to perform this technique, freeing ice and moisture before removing it in vapor form.
Freeze-drying is a common practice for pharmaceutical and biochemical industries. Biopharmaceutical products include vaccines and Streptokinase. Probiotic powders are also produced by freeze-drying bacteria. Alternatively, a biochemical substance with low molecular weight may be freeze-dried to remove solvents when filtration membranes are inefficient.
Chemicals Susceptible to the Dangers of Moisture
What laboratory chemicals are more susceptible to the dangers of moisture? Desiccation is not essential for all substances, especially if they are already in liquid form. Reason would suggest that solids and crystalline structures are more vulnerable to the characteristic dampening of humidity. There are some cases in which liquids must be desiccated, and this can be achieved through distillation. Gases, too, can be desiccated with absorptive material.
What of the functional groups of laboratory substances? Antibiotics seem to be frequently registered. Penicillins and aminoglycosides are particularly common: ampicillin, carbenicillin, geneticin, hygromycin B and kanamycin monosulfate are all recommended to be stored desiccated at -20°C. So too is the glycopeptide antibiotic vanomycin hydrochloride. NTC, a streptothricin, is desiccated and stored at 4°C.
Likewise, some enzymes like D luciferin are better off desiccated. The reducing agent TCEP-HCl and some buffers – TRIS and MOPS – are, too. Another product is IPTG, which must also be protected from light. Even some dyes must have their moisture regulated.
It’s apparent that a large number of chemical substances must be kept desiccated. Because desiccated materials represent a breadth of laboratory chemicals, it’s essential for those involved in the storage and use of these products to be vigilant of manufacturer recommendations.
Preservation instructions should be listed on the label or supplier’s description of the product. If storage directions are overlooked, there is a potential for both the product and the research it’s used for to be squandered.
Desiccation. (2017, June 01). Retrieved June 08, 2017, from https://en.wikipedia.org/
Harris, T. (2002, August 22). How Freeze-Drying Works. Retrieved June 12, 2017, from http://science.howstuffworks.com/
Howard-Reed, C., Liu, Z., Cox, S., Samarov, D., Lebber, D., & Little, J. (2011). Assessing the shelf-life of a prototype reference material for product emissions testing. National Institute of Standards and Technology. Retrieved June 9, 2017, from https://www.nist.gov/publications/.
Multi-Agency Radiological Laboratory Analytical Protocols Manual (MARLAP): Laboratory Sample Preparation (Vol. 2). (2004). Retrieved June 12, 2017, from https://www.epa.gov
Perrin, D. D., Perrin, D. R., & Armarego, W. L. (1980). Purification of Laboratory Chemicals (2nd ed.). Oxford: Pergamon Press. Retrieved June 9, 2017, from https://erowid.org/.
GoldBio Staff Writer
Megan Hardie is an undergraduate student at The Ohio State
University’s Honors Arts and Sciences program. Her eclectic
interests have led to a rather unwieldly degree title: BS in
Anthropological Sciences and BA English Creative Writing,
Forensics Minor. She aspires to a PhD in Forensic Anthropology
and MA in English. In her career, she endeavors to apply the
qualities of literature to the scientific mode and vice versa,
integrating analysis with artistic expression.
Category Code: 79104, 79107, 79108, 79109