Contributed by Jenny Smith
Tape products often become items of evidence for the trace evidence chemist. Criminals find tape; particularly duct tape, useful for ligatures, gags, and restraints and to wrap up improvised explosive devices and drugs. A questioned tape end can be physically matched to a known roll of tape to give individualizing evidence or can be associated by class characteristics. The tape itself can bare fingerprints or saliva (DNA) when cut with the teeth and often trace evidence such as hair and fibers may be caught in the adhesive. As with many commercial products that trace evidence chemists are asked to compare, it is important first to establish the variability of the product in order to assess its significance.
Duct tape is made up of as many as 50 chemical components; plastics, rubbers, fillers, stabilizers, tackifiers, colorants, etc. Each of these are subject to market fluctations and are frequently changed. Herein lies the variability. In fact, duct tape has been found to be variable between manufacturers, within the same manufacturers and even within the same batch of tape. The comparison of duct tape by a trace analyst will involve the use of polarized light microscopy, infra-red spectroscopy and elemental analysis encompassing most of the skills of the analyst.
Because of this changing market, tape products can provide useful investigative leads and should always be collected from the scene of a crime.