The use of Trace Detectors by Customs and Security staff is quite common, especially in the screening of items which cannot undergo a thorough X-ray inspection. The presence of particles on the surface of inspected items may be an indication of the presence of a larger concentration of suspect substances inside. Trace Detectors are available for mobile (QS-H150) and stationary mode (QS-B220). The samples - even at a nanogram level - are collected from contaminated surfaces by wiping the surfaces with sample traps. The traps containing explosives are then subjected to a thermal desorption during which substances particulates are volatilized and resulting vapors are introduced into the detectors where their analysis takes place. Specific substances (threats) will trigger an alarm.