Being an old timer, I have been around since before the advent of vials of glutaraldehyde. In the 1970s, the Merck index described a method for purifying glutaraldehyde using activated charcoal.

Basically, glutaraldehyde begins to polymerize when stored at room temperature. Since best fixation is achieved with the monomeric form, this purification protocol was designed to remove the more complex forms. Storage under nitrogen in a sealed vial slows the polymerization, and lower concentrations degrade more slowly. Storing at 4°C is advised.