5.1 ETDs are used by first responders, security screeners, the military, and law enforcement to detect and identify explosive threats quickly. ETDs typically operate by detecting chemical agents in residues and particles sampled from surfaces and can have detection limits for some compounds extending below 1 ng. An ETD is set to alarm when its response to any target analyte exceeds a programmed threshold level for that analyte. Factory settings of such levels typically balance sensitivity and selectivity assuming standard operating and deployment conditions.
5.2 A LOD is commonly accepted as the smallest amount of a particular substance that can be reliably detected in a given type of medium by a specific measurement process (2, 3). The analytical signal from this amount shall be high enough above ambient background variation to give statistical confidence that the signal is real. Methods for determining nominal LOD values are well known (for example, Hubaux and Vos (7) and Practice D6091), but pitfalls exist in specific applications. Vendors of ETDs often report detection limits for only a single compound without defining the meaning of terms or reference to the method of determination.
5.3 When deployed, individual ETD performance (for example, realistic LODs) is influenced by: (1) ETD manufacturing differences, history, and maintenance; (2) ETD operating configurations (for example, thermal desorption temperature, analyzer temperature, and type of swab); and (3) environmental conditions (for example, ambient humidity and temperature and chemical background). As a result, realistic LOD values for an ETD may be poorly estimated by the factory specifications. These fundamental measures of ETD performance are critically important for assessing the ability of an ETD to detect trace levels of particular compounds in a particular setting, so a reliable and accessible method is needed to determine realistic LOD values, especially in the field.
5.4 Technical Challenges and Pitfalls to the Determination of LOD Values in ETDs and the Setting of Optimal Alarm Thresholds:
5.4.1 Scope—There are over 230 explosive materials currently listed by the Bureau of Alcohol, Tobacco, Firearms, and Explosives.4 There are many technologies used for detection, and ETD manufacturers design their systems and balance operating conditions to provide detection capabilities across as many analytes as possible. However, a very limited subset of analytes is normally used to test and verify ETD performance. Therefore, default ETD operating conditions and alarm thresholds may not be optimally set to detect reliably certain compounds deemed important in particular scenarios.