TCS Daily


Smoke, No Fire

By Noah Shachtman - April 9, 2003 12:00 AM

It's a scene being replayed all over Iraq. American soldiers stumble upon a mysterious liquid or powder. The material is tested - and it's shown to be a nerve agent, or mustard gas. Embedded reporters and military flacks rush to tell the world that, at last, they've found the "smoking gun" that proves Saddam had banned weapons all along.

And then, a few hours later, further analysis shows that the whole thing was just a false alarm. The sample has to be sent to a lab, where a third and final determination can be made about whether or not the material is toxic.

What's going on here? Why do these "false positives" - as they're called in weapons inspectorese - keep popping up? Why are these tests so consistently inconsistent?

To begin with, the assessments are designed to be oversensitive. If they were people, they'd be hypochondriacs. Diesel fumes, burning grass - even breath mints have been known to set off alarms.

"We'd rather err on the side of caution. 'False positives' are a lot better than 'false negatives,'" Miguel Morales, a spokesperson for the U.S. Army Soldier and Biological Chemical Command.

Still, Peter LeJeune, an adjunct fellow at the Potomac Institute for Policy Studies, replied, "There's no excuse for thinking you have sarin one moment and Chanel No. 5 the next."

He's exaggerating for effect, of course. Nerve agents like sarin are chemically quite similar to pesticides. Thiodiglycol, an organic sulphur compound, is found in mustard gas - and in plastics, dyes, and lubricants, too.

So it's not surprising that one of the Army's first tests for blister and nerve toxins - M8 Chemical Detection Paper - will give "false positives" if it touches brake fluid or insect repellent.

The papers come in tan booklets of 25 strips, coated with three dyes that are soluble in chemical agents. They're used to detect liquid poisons. Put a few droplets of unknown goo on a strip, and in about 30 seconds, the bead will turn red if there's something vaguely like mustard gas or another blister agent; dark green if there's a nerve toxin like VX, and yellow for a poison like sarin.

The papers are used by themselves, and also as part of the M256A1 Chemical Agent Detector Kit, a black tacklebox that's a grunt's answer to his geeky brother's home chemistry set.

Each kit has a dozen miniature experiments, each wrapped in its own little foil envelope. Inside is a collection of liquid reagents that react to a type of poisons, and to a dye. Again, a different color is produced for each kind of agent. The tests are used for telling whether or not there are toxins in the air.

These exams are more sensitive than the strips - they can sniff out just .0005 milligrams of nerve gas in a cubic meter of air. But they're tricked by the same kinds of "false positives" that fool the papers, according to Dr. Griffin Davis, with Johns Hopkins Hospital.

Specialized Army units will also use a CAM ("Chemical Agent Monitor") to monitor the air. Sucking vapor samples in through a nozzle, the CAM analyzes using ion mobility spectroscopy. The vapor gets zapped with a radioactive material, like americium-241, then passes through an electrical field, and then to an ion detector. The substance is identified by the amount of time it takes to run this little gauntlet.

CAMs are also very delicate - maybe too much so. Breath mints, burning grass, and ammonia all interfere with the CAM, according to Defense Department documents. So does exhaust from a diesel truck, said Andrew Wolf, a chemist with the U.S. Army Soldier and Biological Chemical Command.

To keep samples away from all these contaminants, results from the CAM, M8, and M256 tests are being re-checked at the Fox mobile laboratories. These three-man, twenty-ton vehicles have more than a million dollars worth of chemical gear inside.

The most precious item in this haul is a gas chromatic mass spectrometer. It's a big way of a relatively straightforward process: the suspect vapor is broken down into its component parts. Each part is then identified by its unique spectral signature.

But this Fox can be less than sly, as it's masters admit.

"It's not a highly sophisticated lab," said Peter Keating, a spokesperson for General Dynamics, the defense contractor putting the Foxes together for the Army. "It's a reconnaissance vehicle, to identify areas to be avoided. It (purposely) will give a positive for sarin or for a similar pesticide."

Besides, you can't expect chemical tests to be that accurate, he added, "when you're bouncing around a battlefield."
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