The fact that most samples examined are not pure substances, but are often mixed with dirt or debris, presents a major challenge to the forensic chemist. This may also be an advantage, as every substance collected at a crime scene is a unique mixture of chemical compounds that can ultimately be identified. Arsonists, for example, often use accelerants such as gasoline or kerosene to speed combustion and spread flames in the interior of a building. A forensic chemist may collect samples of burned and unburned materials, extract the volatile hydrocarbons, and separate the components for analysis by gas chromatography.
In a crime scene investigation, they can examine a wide range of materials such as blood stains, urine, and blood gases for traces of poisons or drugs. Kerosene and gasoline are two often tested for substances, mostly in crimes involving arsen.
Forensic Chemists can help in crimes such as; say you had a body, found in a car crashed at the bottom of a steep hill or cliff. The car appears to have just rolled off the road and the body inside the car is burned beyond recognition, a forensic chemist can extract substances from the car to determine whether the car exploded from rolling down the hill or if the car was lit on fire after rolling down the hill. Also, they can judge those kinds of things by the burn patterns left from a fire and they're very good a determining whether a fire was accidental or if it was an arsen.