Comment

1 July 2002

Ion mobility spectroscopy

By Nicholas Sheble

The 700 ion mobility spectroscopy (IMS) systems operating in U.S. airports are part of a spectrum of trace detection technologies the government is studying. IMS leads the homeland security charge.

“If you'll kindly step over to that table sir and remove your shoes...”

This airline passenger is transitioning from state-of-the-art X ray or computer tomography to what the Federal Aviation Agency and the Transportation Security Administration (TSA) plan to use to not only spot check profiled passengers but also eventually check every piece of luggage, cargo, mail, and passenger that boards an aircraft: ion mobility spectroscopy (IMS).

Screeners are further scrutinizing this passenger because he inadvertently left a pair of scissors in his carry-on bag. The agent wipes a special cloth over the surface of the man's shoes and then places the swab into the IMS device. The device heats the swab such that any particles gleaned from the shoes vaporize.

These vaporized substances then ionize (given an electric charge) and drift through a tube. The ions move at different speeds through the tube, depending on their molecular size and structure.

The characteristic speed at which an ion moves is a distinct fingerprint that identifies the original substance.

There are 700 IMS systems operating in U.S. airports. They are part of a spectrum of trace detection technologies the government is studying. Others include Raman spectroscopy, unwieldy mass spectrometry (MS), and MS on a chip. IMS leads the homeland security charge at this point.

Accelerate electrostatically

A typical ion mobility spectrometer comprises an ion molecule reaction chamber, an ionization source associated with the ion reaction chamber, an ion drift chamber, an ion/molecule injection shutter (Bradbury-Nielsen-Shutter) placed between the ion reaction chamber and the ion drift chamber, and an ion collector (Faraday plate).

A carrier gas, normally air or nitrogen, transports the subject gases or vapors into the ion mobility spectrometer. There, an ionization source charges the carrier gas and the analyte molecules. The source is usually ß radiation, but lasers, discharge lamps, and partial or corona discharges also suffice.

Because of the great excess of carrier gas, the ions generated are almost exclusively from carrier gas molecules. Because the mean free path length of the ions is many times smaller than the dimensions of the reaction chamber of the spectrometer, multiple collisions between ionized species and the analyte molecules occur. In these collisions, the charge of the ions transfers to the analyte molecules.

The charged molecules are accelerated by an electrostatic field gradient maintained between counter electrode and Faraday plate, causing them to travel toward the injection shutter interface of the ion drift chamber, in which they quickly reach the terminal velocity.

Basic construction of an ion mobility spectrometer.

Periodically, the potential of the ion shutter is adjusted for a very short period of time to a value permitting a swarm or pulse of ions to pass from the ion reaction chamber into the ion drift region. There, under the influence of the electrostatic field, the ions are pulled to the ion collector (Faraday plate).

The particular ion mobility in the nonionized gas filling of the drift chamber determines the time of arrival at the ion collector plate of each ionic species, both the carrier gas and the analyte gas molecules.

Thus, it is possible to identify the different ionic species by monitoring the time between the introduction of the ions into the drift region at the electric shutter and the arrival of the ions at the collector plate. The quantity of ions collected as a function of drift time records as a current. A microprocessor computes the current to an alarm or an all clear.

Utah games prove utility

The TSA deployed IONSCAN, an IMS-based trace explosives detector from Barringer Instruments (www. barringer.com), at Salt Lake City Airport for the 2002 Winter Olympics. It installed 80 systems at the ticket check-in counters to screen checked baggage.

The technology demonstrated its effectiveness in screening baggage for explosive devices while maintaining high passenger throughput rates. Analysis time is 6-8 seconds.

The device detects RDX, PETN (the active ingredients in the plastic explosive Semtex), TNT, and other explosives at picogram levels. It also detects cocaine, heroin, cannabis, LSD, and other illegal chemicals at the subnanogram level. IT

Next month, the Sensors department discusses the most promising bulk chemical identification technology for airport (still developing): nuclear quadrupole resonance spectroscopy.