When a technician has a choice in the fixation reagent, zinc formalin and zinc tris are the popular picks for IHC. Zinc formalin provides the fixation time of formalin plus the addition of zinc salts that enhances preservation and antigen retention.
Zinc tris is a non-formalin, zinc fixative. Its fixative action is primarily from the zinc salts which are in a tris buffer with a pH of 7.2 – 7.4. It acts more like a preservative that prevents autolysis from proteolytic enzymes, instead of a fixative which increases the cells rigidity and thus tissue/cell morphology. Tissues must be trimmed as small as possible and remain in the fixative for a longer period of time. If the tissue is not fixed well, antigen preservation is compromised and thus the staining is also. There are some concerns that this fixative may interfere with silver stains and with tissues that are primarily collagen fibers. The fibers appear not to fix very well, making zinc tris a bad fixative of choice for many tissues, unless the fixation is done properly. If it is NOT used properly, the collagen does NOT stain adequately and morphology is compromised. Therefore, stains used for collagen staining, such as Mallory's trichrome and collagen IHC, or silver stains, such as Gomori's, may be compromised if a zinc fixative is used.