I believe what you are interested in defining is forensic chemistry or the use of chemistry for legal purposes, such as crime scene investigations. Forensic chemistry can include looking at trace evidence such as collecting hairs, glass, soil, paper, and gunshot residue or running toxicology screens to look for drugs or other substances in the body or conducting DNA testing. One of the unique challenges that forensic chemists face is the fact that they may not know the nature of the sample that they are processing and have to use a number of scientific techniques and testing to examine and identify the evidence such as microscopy, spot testing, and mass spectrometry to name a few. A forensic chemist is often called to provide expert testimony and provide evidence to the jury. Forensic chemistry has become a very popular career recently due to television shows such as "Bones," "CSI," and "Dexter."