Some Clarity on Aldehydes Used as Biological Fixatives
Posted on Wed, Jul 15, 2009 @ 01:31 PM
By Courtney Drayer, Technical Services Representative
Aldehydes are carbonyl groups (C = O) that are bonded to either 2 hydrogen atoms (Formaldehyde) or a hydrogen and an alkyl or phenol group (higher aldehydes). Aldehydes are known for their pungent smells but also include some culinary favorites such as vanilla, cinnamon, and spearmint oil. In the biological sciences, certain aldehyde solutions are used to fix tissue samples for analysis. Chemical fixatives preserve biological specimens by disabling enzymatic degradation, preventing microbial colonization, and increasing cellular strength. Common aldehydes used in chemical fixation include formaldehyde, paraformaldehyde, and gluteraldehyde.
There are many misconceptions surrounding the definitions of these chemicals. Formaldehyde (HCOH) is a naturally occurring and synthesized gas (HCOH) that hydrates when dissolved in water forming methylene hydrate (HOCH2OH). The maximum solubility of formaldehyde in water is 37% by weight and 40% by volume. Solutions that contain the maximum amount of Formaldehyde dissolved in water are defined as 100% formalin. A 10% formalin solution is 10% of a 37-40% formaldehyde solution or 3.7-4% formaldehyde solution.
As formalin solutions age, the methylene hydrate can polymerize. Precipitation of these polymers from solution can form a flammable white powder called paraformaldehyde. Likewise, the dissolution of 4g of paraformaldehyde into 100ml of water will form a 10% formalin solution. Many scientists add some amount of methanol (usually 10%) to their formalin solutions in order to stabilize the solution and prevent polymerization. Additionally, monobasic sodium phosphate monohydrate and dibasic sodium phosphate anhydrate can be added to formalin solutions in order to buffer the low pH caused mainly by the oxidation of formaldehyde to formic acid.
In addition to formaldehyde and paraformaldehyde, gluteraldehyde (OHCCH2CH2CH2CHO) is a fixative used for biological preservation in preparation for Electron Microscopy. Gluteraldehyde solutions are able to fix samples quicker than formalin solutions due to the dialdehyde structure of the molecule but the larger molecular structure causes penetration of the fixative to slow. An additional benefit of gluteraldehyde use is that OSHA currently has no regulations regarding exposure limits to gluteraldehyde whereas the permissible exposure limit for formaldehyde is 0.75 ppm.
Additional concerns exist concerning the best practice disposal of formalin solutions. While solutions without significant alcohol are not regulated by the EPA, the same reasons that make formalin a useful fixative makes it questionable for drain disposal in sewer systems as formalin can deactivate the useful and necessary bacteria used in sewage treatment plants. Other reasons for the collection of formalin are due to the toxic and carcinogenic effects of Formaldehyde. Fortunately there are many formaldehyde-free alternatives available on the market today. Information on some alternatives can be found at http://www.sustainablehospitals.org/cgi-bin/DB_Report.cgi?px=W&rpt=Subcat&id=18!18 . However, this list is not all inclusive and many scientists have found some success with other alternatives.
If your institution needs any help developing a best management strategy for formalin (or any other aldehyde solution) please contact an account manager and we will help you figure out a plan that will work for you and your team!