A scent transfer unit is a vacuum device used to collect scent evidence from a crime scene or item of evidence. The unit was invented by Bill Tolhurst (a former president of the National Police Bloodhound Association) while working for the Niagara County Sheriff's Department. Several law enforcement agencies have bought scent transfer units.
The device works by sucking an odour from a desired object onto a sterile gauze pad placed over the air intake. The resulting scent pads can either be given to a tracking dog, used in a 'scent lineup', or preserved in a freezer. Tolhurst has claimed that evidence pads can be stored successfully in a frozen state for more than 11 years.
Although the reliability of scent evidence in general has been called into question, certain experiments have demonstrated successful use of scent transfer units.