These detectors involve electron capture, mass spectrometry, or ion mobility spectroscopy techniques. The electron capture detector is highly sensitive and selective to polar compounds. An inert carrier gas enters the detector and is ionized by a radioactive foil. When a high voltage is applied, a steady standing current is produced. The entrance of a compound with an attached electronegative group causes a decrease in the standing current since these compounds tend to absorb electrons and turn into negative ions. Mass spectrometry detectors offer high sensitivity and selectivity; however, their complexity and high costs have prevented commercialization. Ion-mobility spectroscopy detectors are commercially available. Since they do not require gas supplies, they are simple and convenient to operate, but they lack sensitivity and produce a high rate of false alarms. Explosives detectors are evaluated on a number of parameters. Their major requirement is sensitivity, since explosives produce extremely low concentrations of vapors. They must also be selective and respond to only those vapors associated with explosives. In addition, the detectors must be capable of detecting a wide variety of explosives: a difficult task since the types of vapors vary. They must respond rapidly if they are to examine large numbers of packages or letters. Finally, they must be simple to operate. Data tables provide information on the capabilities of a number of detectors and on various techniques. Six references are included. For related articles, see NCJ 74479.