Homeland Security Insider

Detecting the undetectable

• By Col. Timothy D. Ringgold
• Jan 03, 2007

The Transportation Security Administration is doing a good job protecting air travelers. Likewise, airport security managers are working hard to keep the public safe and to speed us through the facilities. Until recently, getting through airport security was technologically simple -- bags were run through an X-ray scanner while walking through a conventional metal detector. X-ray scanners and metal detectors alerted airport security personnel to the presence of hidden metal weapons like knives or guns. However, X-ray scanners and metal detectors won't pick up organic-chemical hazards like explosives, biological warfare agents, toxic industrial chemicals and illegal drugs. To detect these substances, security agents look for the telltale traces of chemicals that evaporate from the surfaces of these substances by using various kinds of chemical "sniffers." Ion mobility spectrometers (IMS) are among the most commonly used instruments for this type of detection.

X-ray scanners and metal detectors won't pick up organic-chemical hazards like explosives, biological warfare agents, toxic industrial chemicals and illegal drugs. To detect these substances, security agents look for the telltale traces of chemicals that evaporate from the surfaces of these substances by using various kinds of chemical "sniffers."

The process begins when a security officer wipes an absorbent swab over a person's clothing or luggage and then inserts the swab into a small, heated chamber for analysis. Inside the chamber, traces of organic compounds the swab has picked up evaporate and mix with a carrier gas that is swept into the main part of the instrument. The ion mobility spectrometry technique separates and detects electrically charged particles (ions) that have been sorted according to how fast the ions travel through an electrical field in a tube. Inside the instrument, a radioactive substance, commonly Nickel or Americium, similar to that used in ionization smoke detectors, is housed in a small chamber that is shielded from the outside, but accessible to the inside of the instrument.

The radioactive source gives off high-energy electrons that collide with the sample molecules and the carrier gas to form ions. The small ions travel fast and reach the detector first, with successively larger ions following. In a way, different-sized molecules have different-sized parachutes behind them. The molecules are all drawn through the electric field, but larger molecules are held up more by the carrier gas. It is different than a similar technique -- time-of-flight mass spectroscopy -- because it does not rely primarily on mass, but rather the size and shape of a molecule for separation. IMS also is carried out at atmospheric pressure and has lower power requirements than mass spectroscopy.

Handheld, portable IMS units are now in use in many places, including the battlefield in Iraq, where they are being used by U.S. and coalition forces to detect chemical agents. Federal and some state law enforcement agencies carry IMS instruments on raids for on-site identification of illegal drugs. On-site monitoring of pesticides, chemical warfare agents and industrial chemicals is becoming common.

Today, there are a number of commercial IMS instruments of various designs available. Dennis Koehler, vice president of sales and marketing for Golden, Colo.-based Isonics Corp., said his company's IMS technology can detect and identify toxic chemicals down to parts per billion with good sensitivity, fast operating speeds and operating simplicity. Isonics offers both handheld and portable units that specialize in real-time detection of chemical warfare agents, toxic industrial chemicals and homemade explosives.

Each Isonics unit can be custom configured to identify up to16 different substances at a time. This is important because current IMS technology only sorts molecules by size, not by chemical properties or other identifying features. IMS is not designed for making positive identification of unknown compounds. Instruments for use in airports and other security screening locations are programmed with a set of signatures from known hazardous or illegal compounds. When a sample matches any of these signatures, the person or object can be checked again using more detailed methods. The extra check ensures that heart patients who take nitroglycerin, construction workers who handle explosives or other innocent passengers are not mistakenly identified as criminal or terrorist suspects.

With the Isonics instrument, detection and identification occur subsequent to one another. Communication with an external computer -- data exchange, recording spectra -- is done via a conventional USB port. Optionally, the device can be set to trigger an alarm signal (optic, acoustic) or other pre-determined countermeasures once a designated chemical or homemade explosive signature is detected. In a 2005 taxi cab test, the portable Isonics device found traces of mustard gas simulant on articles of clothing from passengers sitting inside the vehicle.

IMS technique detects concentrations in the pictogram (10-12) to nanogram (10-9 ) range. There is little or no sample preparation, and the analytical response usually occurs within 15 seconds. Untrained operators become skillful with the instrument in a matter of a few hours.

Officials are correct to use a layered approach to security that relies on multiple methods, including individual searches; ion mobility spectrometers; metal detectors; X-rays; intelligence; behavior observation techniques; canines; random searches; air marshals; and additional security measures both visible and invisible to the public. No single method of inspection works all the time. A layered approach strengthens the value of security, creating a much stronger, more formidable system. No longer can we rely solely on metal detectors and X-rays to keep us safe. The war on terrorism and the drive to secure the homeland against those who would do us harm is not only a competition between competing ideologies, but also a contest between competing uses of technology. The race to deploy technology will require us to provide incentives for our best minds and companies to roll up their sleeves and get to work. If the activity since 9/11 is any guide to the future, we are off to an excellent start.