Ion mobility spectrometry identifies compounds based on the time required for ionized molecules to drift through an electric field. In operation, sample molecules in the gas phase are ionized using either a corona discharge source or a radiation source such as 63Ni or 241Am. The ions then enter a field-free drift region with an electric gradient, where they drift toward a collector. The time required for an ion to reach the collector depends on several factors, including shape, mass, and charge. In theory, unknowns may be identified via comparison of their drift profiles to an on-board library. IMS is widely used in field portable chemical detectors due to its small size, portability, low power requirement, sensitivity, and quick analysis time (less than a few seconds in most cases). Disadvantages of IMS include relatively high cost, high false positive rate (i.e., low target specificity), and limited number of target chemicals. Most of the systems were designed specifically to detect CWAs, but several IMS manufacturers have broadened their product line to include detection capability for TIC/TIMs, narcotics, and explosives.