Table 1:

Chemical Fixatives

Chemical fixative	Fixation time (Time in solution)	Mechanism	Comment
Crosslinking fixatives – Formaldehyde (Often used as 10% neutral buffered formalin)	12–24 hours	Creating covalent chemical bonds between proteins in tissue, tend to preserve the secondary structure of proteins and may protect significant amounts of tertiary structure as well	Most commonly used fixative in histology, good tissue penetration and good for IHC techniques
Crosslinking fixative - Glutaraldehyde	1 hour	Same as above	More extensive cross linking than formaldehyde interfering with with, gives best overall cytoplasmic and nuclear detail for electron microscopy
Precipitating or denaturing fixatives - alcohols	1–6 hours	Reducing the solubility of protein molecules or by disrupting the hydrophobic interactions that give many proteins their tertiary structure	Examples: Ethanol and methanol: most common precipitating fixatives, fixation frozen sections and smears, rarely used alone for fixing blocks unless studying nucleic acids. Methacarn (Methanol-Carnoy)
Mercury (B-5 fixative)	4–8 hours	Unknown mechanism that increases staining brightness and give excellent nuclear detail	Fast but penetrates poorly and produce tissue shrinkage. Favored for hematopoietic and reticuloendothelial tissues. Change to 70% ethanol after the 4–8 hours.
Methacarn (Methanol-Carnoy)	1–4 hours		Quick fixation and can be stored in fixative for several weeks without apparent harm to tissue morphology or antigenicity. Good RNA quality.