One method used to identify explosives is a CT scan. Most people are now familiar with the term, even if the only place they've heard it was on a medical TV drama. The "CT" stands for computerized tomography, which is a method for generating a three-dimensional image from multiple 2-D X-ray images captured from multiple angles.
Modern CT scanners use multiple emitters to fire X-rays through an object to a sensor on the opposite side. The sensor records the gross structures it "sees" as well as the amount of energy received, giving an indication of the density of the material it passed through. The computer combines all of these images into one virtual object that appears on a computer display. The virtual object can be rotated on any axis and zoomed in or out to inspect details of interest.
Contraband objects with telltale shapes, like guns, are fairly easy to spot. The latest equipment stores data on densities of objects, both common and deadly, and uses X-ray diffraction to identify and display those in a telltale color. An operator at an airport or other high-threat location can pay closer attention to a container colored in red and determine whether it's plastic explosive or cold cream.
There are several limitations to CT as an explosives detector. The software that flags an explosive may give a false positive alert on something innocuous, like a child's Play-Doh. CT scanners are large-typically, the size of a small truck-and expensive. They're a chokepoint for any venue that has to process a lot of people or items in a short time.
The weakest link is the operator. Even when operators are rotated every 20 minutes or more frequently, fatigue causes them to miss objects that a fresher eye might see immediately. Watching a video display of wireframe images of everyday objects gets boring very quickly, and even the most conscientious employee's mind wanders. When the one-in-a-thousand Bad Thing goes by, it's easy to not see it.
Not all explosives detectors rely on the sense of sight. Some called "sniffers" don't exactly rely on the sense of smell. Labeling an Ion Mobility Spectrometry (IMS) instrument as a "sniffer" might be blasphemy to an analytical chemist, but it's a lot easier to say.
Ions are atoms or molecules containing an unequal number of protons and electrons, so they have a positive or negative charge. In an IMS instrument typically used for explosives detection, a sample is heated to produce a vapor, which is then ionized by a radiation source such as 63Ni or 241Am (there's something similar in the smoke detectors in your house).
The ions move through a drift chamber and an electric field that causes them to separate by size, mass, and geometry. As the ions arrive at a detector at the far end of the drift chamber, those size, mass, and shape characteristics form a signature unique to the compound from whence they came. If the signature matches one the device is programmed to detect, like PETN, it sounds an alarm.
Smiths Detection is one of the largest manufacturers of IMS instruments for explosives detection, and you've probably seen their machines at airport security checkpoints. Typically, an operator uses a glorified coffee filter to swab luggage or some other suspicious item, and places it in a recess for analysis. More often than not, a green light comes on, and you're on your way. The magic described above happens that fast. IMS detectors don't have the operator fatigue problems of CT scanners, but they're still expensive and have limited throughput.
SpectraFluidics has developed a new detection technology that is a combination of Free-Surface microFluidics (FSF) and Surface-Enhanced Raman Spectroscopy (SERS). A key component is a consumable "microfluidic chip" exposed to ambient air around the sample. The fluid, the channels that contain it, and nanoparticles suspended in it are specific for the substances to be detected.
A Raman laser, which works by measuring activity on the surface exposed or excited by the laser, produces a profile that is compared to that of the target substance. The effect is a detector that can be either fixed or handheld and costs a fraction of an IMS instrument. The microfluidic chips are consumed in the analysis process. The company plans to ship its first operational units this year.