Changes may arise through such ways as contamination, breakage, evaporation, accidental scratching or bending, or loss through improper or careless packaging. Each different item or similar items collected at different locations must be placed in separate containers. Packaging evidence separately prevents damage through contact and prevents cross-contamination. The chain of custody rule provides that the party seeking to introduce into evidence the results of an expert analysis has the burden of proving that the specimen or object analyzed was, in fact, derived or taken from the particular person or place alleged. This proof is customarily adduced by testimony which traces the location and custody of the specimen from the time it was secured by crime scene technician.
Chain of custody is an essential quantum of proof in any case involving physical evidence.
In most cases the chain of custody can be sufficiently proven by the testimony of the investigator who secured the specimen or object and the analyst who examined it. The examination of evidence, whether it is soil, blood, hair, glass, fibers, etc., often requires comparison with a known standard or control. The quality and quantity of control specimens often determine the evidential value of crime-scene evidence, and these control specimens must be treated with equal care. In police work, there is perhaps no experience more exasperating or demoralizing than to watch valuable evidence excluded from use against the accused because of legal considerations. Since the 1960s, the Supreme Court has been particularly concerned with defining the circumstances under which the police can search for evidence in the absence of a court-approved search warrant.