Evidence packaging
If there is one rule about packaging of fire scene evidence that is to be tested for accelerant residues, it is that airtight containers must be used. Accelerants are generally volatile substances, that is, they evaporate easily. If airtight packaging is not used, then some or all of the accelerant may evaporate before the analysis is completed. Some crime laboratories will store all of their fire scene evidence in one cabinet or locker, usually an explosion-proof one. If some of the packages are leaking, then much of the other evidence may be contaminated with these vapors. If this takes place over a long enough period of time, false positive conclusions may be the result. In addition, the leaking evidence container may lose so much accelerant to evaporation, that, when it is finally examined, little or no accelerant may be found. Over the years, a number of containers have been used for storage of fire debris suspected to contain flammable residues. The most popular have been metal containers. Many fire scene investigators employ unused paint cans. These make excellent containers for fire residues because they are made to be airtight when sealed properly. They can be heated without danger of breaking and are generally rugged and easy to transport. Holes are easily punched in the top for access to the evidence without removing the top. They usually have a protective coating on the inside to retard rust. If this gets scratched, then the inside may rust quickly, especially when wet materials are stored there. Glass jars have also been used as containers for fire residues. Generally these are smaller than paint cans, which limit them to small samples. Either new Mason jars or used peanut butter or vegetable jars work. They are fragile and must be heated carefully to avoid breaking. Their metal tops are suitable for punching holes in to get access to the evidence. Bags have also been used for packaging fire residue evidence. Paper bags are totally unsuitable because they can disintegrate if exposed to water and, more importantly, they “breathe,” that is they will allow hydrocarbon or other flammable vapors to escape. Some plastic bags have also been used, albeit with mixed success. Poly ethylene bags are very common, but not suitable. They can be sealed easily but may be reactive to some hydrocarbon vapors and some solvents. They are not impervious to hydrocarbons and thus, evidence will be lost. Contamination of the fire residues inside the bag may occur if the outside of the bag is exposed to hydrocarbons. On the other hand, polyvinylidine bags are quite suitable for storage, being impervious to solvents and flammable materials such as those found in fire residues. They come in various sizes and can accommodate many different types of evidence, however, they do not lend themselves well to hole punching for access to the evidence. Of course, as with all evidence, once the fire residues have been packaged, they must be sealed and properly labeled. They should be transported to the laboratory as soon as possible so that they can be analyzed with a minimum of sample loss due to evaporation of volatile materials. Fire residues contained in soil present special packaging problems. It is well known among forensic chemists who analyze fire residues that microbes in soil can and do eat hydrocarbon residues. If this is not mitigated, soil microbes can consume so much accelerant over time that the result may be that the remainder cannot be identified. Recent research indicates that soil microbes can be quite picky about what types of hydrocarbons they eat. The laboratories that are concerned about this issue simply put such evidence in a refrigerator to slow down the action of microbes. This can be very expensive and of limited benefit. Recent research has shown that certain chemicals can retard microbial action without compromising the evidence. Chief among these is household bleach.
