Bomb Detector

When detonated in strategic, population-dense, or confined spaces, bombs are especially destructive. For example, a bomb planted by political terrorists in a suitcase was responsible for the explosion of Pan Am Flight 103 over Lockerbie, Scotland, on December 21, 1988, that claimed 270 lives. Given the devastation that bombs can cause, and the risk they pose to national security, the detection of bombs is a important priority in airports and elsewhere.

Despite the fact that x-ray examination may not detect some bombs, the technique is still a mainstay in bomb detection. For example, x-rays are the best way to reveal the presence in luggage of suspicious shapes. Plastic explosives can be molded to resemble common objects. Also, explosives are not metallic, and so will escape metal detection. A well-trained operator is a key part of this bomb detection strategy. A newer version of the x-ray examination places a reflector on the opposite side of an object from the x-ray beam. As the rays are scattered back, they are analyzed by a sophisticated computer program, which can reveal differences in the outgoing and incoming beams that were caused by passage of the beams through suspicious material.

Another version of the x-ray dual energy technology sends two x-ray beams through the object at the same time. One of the beams distinguishes organic material (i.e., food, leather objects, paper) and displays them as red. The other beam distinguishes inorganic objects (i.e., metal clips, umbrella, metal pens) as green or blue. The color difference helps the operator quickly scan packages and baggage for object that are suspicious by their shape or chemistry. A similar method, which uses radio waves instead of x-rays, is called quadrupole resonance technology.

Another optical device is computer tomography, a technique that has been adapted from the CAT scan xray technology used in the medical operating room. In

A dust sample is taken from a laptop computer and the particles analyzed for explosives residue by a Barringer explosives detection device.

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tomography, an object is scanned and then a computer analyzes the x-ray image. If areas of the package have not been adequately revealed, the x-ray source can be rotated so as to produce a detailed view of the specific area. In this way packages and baggage can be examined in great detail.

Some bomb components can leave a scent. Until a few decades ago, specially trained dogs were a mainstay of bomb detection squads. Specially trained dogs are still used today to check out packages or locations that are difficult to examine using a machine. A dog's nose is actually a bit more sensitive than the sensitivity of detection machinery that is currently available. However, a dog and handler costs approximately $50,000 a year, whereas a piece of detection equipment represents a one-time cost of $20,000 to $40,000. Thus, machines are becoming more prevalent.

One such technology utilizes gas chromatography and a property called chemiluminescence. In gas chromatography, chemicals of different composition can be separated from each other based on their differing speeds in a stream of gas (selection of the gas can determine the rate of movement of different compounds). A compound in the gas, which will then glow, will recognize an isolated compound that has a certain chemical group in its structure. The glowing (chemiluminescence) registers on an optical detector, revealing the presence of the explosive chemical.

Devices known as sniffers detect vapor given off by certain explosives. Chemicals such as nitroglycerin are readily detected. But, a sniffer can miss explosives such as plastic explosives that do not readily vaporize. Thus, a sniffer should be used only as part of a bomb detection regimen that involves other detection techniques.

Another device detects chemicals present in bombs by concentrating the air collected from a target location. The air is drawn through a filter, where explosive chemicals collect, due to their tendency to be heavier than the air molecules around them. The filter is analyzed using ion mobility spectrometry

The spectrometric technique is very sensitive. Less than a nanogram (10₉ of a gram) of explosives residue can be detected. To put this into perspective, a fingerprint on a luggage handle left by someone had been handling explosives will typically contain 100,000 times more of the residue.

An ion mobility spectrometer (IMS) is a spectrometer capable of detecting and identifying very low concentrations of chemicals based upon the differential migration of gas phase ions through a homogeneous electric field. IMS devices come in a wide range of sizes (often tailored for a specific application) and are capable of operating under a broad range of conditions. Systems operated at higher pressure (i.e. atmospheric conditions, 1 atm or 760 Torr) are also accompanied by elevated temperature (above 100°C), while lower pressure systems (1-20 Torr) do not require heating. Elevated temperature assists in removing ion clusters that may distort experimental measurements.

n its simplest form an IMS system measures how fast a given ion moves in a uniform electric field through a given atmosphere. The molecules of the sample need to be ionized, usually by corona discharge, atmospheric pressure photoionization (APPI), electrospray ionization (ESI), or a radioactive source, eg. a small piece of ⁶³Ni or ²⁴¹Am, similar to the one used in ionization smoke detectors.

In specified intervals, a sample of the ions is let into the drift chamber; the gating mechanism is based on a charged electrode working in a similar way as the control grid in triodes works for electrons. For precise control of the ion pulse width admitted to the drift tube, more complex gating systems such as a Bradbury-Nielsen design are employed. Once in the drift tube, ions are subjected to a homogeneous electric field ranging from a few volts per centimeter up to many hundreds of volts per centimeter. This electric field then drives the ions through the drift tube where they interact with the neutral drift molecules contained within the system. Separation of chemical species is achieved based upon the ion mobility (a parameter that is dependent of ion mass, size, and shape) where they arrive at the detector for measurement. Ions are recorded at the detector in order from the fastest to the slowest, generating a response signal characteristic for the chemical composition of the measured sample. Often the detector is a simple Faraday plate, however, more advanced ion mobility instruments are coupled with mass spectrometers where both size and mass information may be obtained simutaneously.

Perhaps ion mobility spectrometry's greatest strength is the speed at which separations occur--typically on the order of 10's of milliseconds. This feature combined with its ease of use, relatively high sensitivity, and highly compact design have allowed IMS as a commercial product to be used as a routine tool for the field detection of explosives, drugs, and chemical weapons. In the pharmaceutical industry IMS is used in cleaning validations, demonstrating that reaction vessels are sufficiently clean to proceed with the next batch of pharmaceutical product. As a research tool ion mobility has also shown great strides towards the analysis of biological materials, specifically, proteomics and metabolomics.

Star Trek-like technology being developed at The University of Arizona might soon screen airplane passengers for explosives as they walk through a portal similar to a metal detector while hand-held units scan their baggage.

The new device is about 1,000 times more sensitive than the equipment currently used in airports to discern explosives. Rather than analyzing a swab from a person's briefcase, the new technology could detect the traces of explosives left in air that passes over a person who has handled explosives.

"This is a form of tricorder," said M. Bonner Denton, the professor of chemistry at UA in Tucson who's spearheading the new technology. Denton said combining such technology with a walk-through portal would make it simple to screen 100 percent of passengers.

The new device can be pocket-sized. The analyzers currently used in airports are about the size of a table-top microwave oven. Denton, UA scientist Roger Sperline and Christopher Gresham and David Jones of Sandia National Laboratories in Albuquerque, N.M. are working on developing a hand-held analyzer capable of detecting small traces of explosives or illicit drugs.

Such a device could be used at border crossings, Denton said. "This is more sensitive than dogs' noses. It does not suffer from overexposure or a case of sinus. One can store it in the cabinet, then grab the unit, turn it on – and it's running. And it tells you what material has been detected. Dogs just tell you something's been detected."

Denton will talk about this and other portable detection instruments on Monday, March 14, at 2 p.m. Eastern Time (11 a.m Pacific Time) at the 229th American Chemical Society national meeting in San Diego. His talk, "Advanced Instrumental Technologies and Their Impact on Homeland Security and on Forensic Science," will be given in Room 25C of the San Diego Convention Center.

Detecting explosives or drugs means sorting through an environmental mish-mash of chemical signals to pick out the one chemical of interest. That's what a drug-sniffing dog's nose does – picks out the chemical signature of a drug from the chemicals that come from the dirty laundry, candy, food stains, fabrics, toothpaste and everything else inside someone's luggage.

To do the same thing to detect explosives, machines at airport screening stations use a technology called ion mobility spectrometry.

Ions, or charged molecules, move when placed in an electric field. The speed at which an ion moves depends on its size and shape, so each ion has a characteristic speed. The airport analyzers snatch a collection of chemicals gathered from a person's luggage, put those chemicals into an electric field and then search for any ion that has a speed that indicates "explosive."

The machine needs a certain number of molecules to accurately detect and identify a specific chemical. If there are very few molecules of a particular substance, the machine cannot distinguish that molecule from all the others in the mix.

Denton realized that one place to improve detection was the electronics of ion mobility spectrometers. So he adapted circuitry originally developed for use in infrared astronomy.

The new device, called a capacitive transimpedance amplifier, improves the readout circuitry in ion mobility spectrometers.

"This change in readout electronics is key to the vastly improved sensitivity. It boosts the signal while lowering the noise," Denton said. "This is the first radical change in ion detection since the 1930s.