Colorimetrics & automated colorimetrics The use of
colorimetric test kits for explosive detection is one of the most simple methods for officers, and widely used method for the detection of explosives. Colorimetric detection of explosives involves applying a chemical reagent to an unknown material or sample and observing a
color reaction. Common color reactions are known and indicate to the user if there is an explosive material present and in many cases the group of explosives from which the material is derived. The major groups of explosives are
nitroaromatic,
nitrate ester, and
nitramine explosives, as well as inorganic nitrate-based explosives. Other groups include
chlorates and
peroxides which are not nitro based explosives. Since explosives usually contain nitrogen, detection often is based around spotting nitrogenous compounds. As a result, traditional colorimetric tests have a disadvantage: some explosive compounds (such as
acetone peroxide) do not contain nitrogen and are therefore harder to detect.
Dogs Specially trained dogs can be used to detect explosives using their noses which are very sensitive to
scents. While very effective, their usefulness becomes degraded as a dog becomes tired or bored. These dogs are trained by specially trained handlers to identify the scents of several common explosive materials and notify their handler when they detect one of these scents. The dogs indicate a 'hit' by taking an action they are trained to provide — generally a passive response, such as sitting down and waiting. The explosive detection canine was originated at the
Metropolitan Police Department in Washington, D.C. in 1970, by then trainer Charles R. Kirchner. The explosive detection canine was first used in Algeria in 1959 under the command of
General Constantine. Recent studies suggest that
mass spectrometric vapor analysis techniques, such as
secondary electrospray ionization (SESI-MS), could support canine training for explosive detection.
Honey bees This approach couples trained
honey bees with
advanced video computer software to monitor the bee for the strategic reaction. Trained bees serve for 2 days, after which they are returned to their hive. This proven system is not yet commercially available. Biotechnology firm
Inscentinel claims that bees are more effective than sniffer dogs.
Mechanical scent detection Several types of machines have been developed to detect trace signatures for various explosive materials. The most common technology for this application, as seen in US airports, is
ion mobility spectrometry (IMS). This method is similar to
mass spectrometry (MS), where molecules are ionized and then moved in an electric field in a vacuum, except that IMS operates at atmospheric pressure. The time that it takes for an ion, in IMS, to move a specified distance in an electric field is indicative of that ion's size-to-charge ratio: ions with a larger cross-section will collide with more gas at atmospheric pressure and will, therefore, be slower.
Gas chromatography (GC) is often coupled to the detection methods discussed above in order to separate molecules before detection. This not only improves the performance of the detector but also adds another dimension of data, as the time it takes for a molecule to pass through the GC may be used as an indicator of its identity. Unfortunately, GC normally requires bottled gas, which presents logistical issues since bottles would have to be replenished. GC columns operated in the field are prone to degradation from atmospheric gases and oxidation, as well as bleeding of the
stationary phase. Columns must be very fast, as well, since many of the applications demand that the complete analysis be completed in less than a minute.
Spectrometry Technologies based on
ion mobility spectrometer (IMS) include
ion trap mobility spectrometry (ITMS), and
differential mobility spectrometry (DMS).
Amplifying fluorescent polymers (AFP) use molecular recognition to "turn off" or quench the
fluorescence of a polymer.
Chemiluminescence was used frequently in the 1990s, but is less common than the ubiquitous IMS. Several attempts are being made to miniaturize, ruggedize and make MS affordable for field applications; such as an
aerosol polymer that fluoresces blue under UV but is colorless when it reacts with nitrogen groups. One technique compares reflected
ultraviolet,
infrared and
visible light measurements on multiple areas of the suspect material. This has an advantage over olfactory methods in that a sample does not need to be prepared. A patent exists for a portable explosive detector using this method. Mass spectrometry is seen as the most relevant new spectrometry technique.
X-ray machines Specially designed
X-ray machines using
computed axial tomography can detect explosives by looking at the density of the items. These systems that are furnished with dedicated software, containing an explosives threat library and
false-color coding to assist operators with their dedicated threat resolution protocols. X-ray detection is also used to detect related components such as
detonators, but this can be foiled if such devices are hidden inside other electronic equipment. Adding
marker substances (X-ray opacifiers) to commercial explosives is also an option.
Neutron activation Specially designed machines bombard the suspect explosives with neutrons and read the resulting
gamma radiation decay signatures to determine the chemical composition of the sample. The earliest developed forms of
Neutron Activation Analysis use low-energy neutrons to determine the ratios of nitrogen, chlorine, and hydrogen in the chemical species in question and are an effective means of identifying most conventional explosives. Unfortunately, the much smaller thermal
Neutron cross sections of carbon and oxygen limit the ability of this technique to identify their abundances in the unknown species, and it is partly for this reason that terror organizations have favored nitrogen absent explosives such as
TATP in the construction of
IEDs. Modifications to the experimental protocol can allow for easier identification of carbon and oxygen-based species, (e.g. the use of
inelastic scattering from
fast neutrons to produce detectable gamma rays, as opposed to simple absorption occurring with the
thermal neutrons), but these modifications require equipment that is prohibitively more complex and expensive, preventing their widespread implementation.
Silicon nanowires for trace detection of explosives Silicon nanowire configured as
field effect transistors have been demonstrated to detect explosives including
TNT,
PETN and
RDX in sensitives superior to those of canines. The detection in this method is performed by passing a liquid or vapor containing the target explosive over the surface of a chip containing tens to hundreds of silicon nanowire sensing elements. Molecules of the explosive material interact with the surface of the nanowires and induce a measurable change in the electrical properties of the nanowire. ==Detection aids==