The search for sarin's fingerprint comes down to 3 numbers, lots of testing and retesting

WASHINGTON - Three simple numbers will prove whether sarin was used to gas Syrians last month: 99-125-81.

Chemists this week around Europe are feeding samples of bodily tissue and dirt collected after chemical attacks in Syria into sophisticated machines, waiting for those three numbers to read out in a bar graph on a computer screen. The numbers are sarin's fingerprint, said Carlos Fraga, a chemist who specializes in nerve agent forensics at the Pacific Northwest National Laboratory in Richland, Wash. "You're always going to see that."

In a process that takes about two weeks, chemists have to turn that solid dirt and tissue first into liquid and then into gas.

Chemists dissolve the samples by putting them into a solvent, such as methanol, and shaking them, Fraga said. Then that's injected into a gas chromatograph, which looks like a big oven. It heats the liquid, turning it into a gas, then acts as a giant sorting machine. The suspected sarin is separated, but at this point scientists still can't figure out what it is. It's just not mixed up with everything else anymore.

The separated chemicals are injected into a mass spectrometer, which hits the molecules with an electron beam that knocks out an electron to give the molecule a positive charge. The machine can't analyze sarin when it is in its normal neutral level, Fraga said, but when ionized, it breaks apart into a telltale pattern. It's that pattern, shown as a bar graph on a computer screen,that reads the atomic masses of the chemical fragments — the molecular fingerprint.

Each chemical has a special distribution based on the fragments the molecules split into. For sarin, that's 99-125-81.

Sarin should take exactly the same time to run through each test — the time varies a bit from lab to lab based on individual equipment — so the clock provides another method of confirmation, Fraga said.

Once scientists get that 99-125-81 reading, he said, "You have that 'CSI' moment and you're, 'Oh, man, there it is.'"

If it were a television crime drama, the credits would soon be rolling. But the reality of international diplomacy and chemistry doesn't work so fast.

One test in one lab won't be nearly enough. There will be gas chromatography-mass spectrometry tests, considered the gold standard. Then there will be liquid chromatography-mass spectrometry tests, high resolution spectroscopy, infrared spectroscopy and others. Chemists are looking for other chemical signatures of the nerve agent and using different techniques to prove the same thing. There's cleaning, prepping, checking more samples, checking against known sarin quantities, checking against blanks.

And that's just one lab. Other labs will be doing the same to make sure everyone's getting the same answer.

There are probably 20 different tests for each initial 99-125-81 hit.

"That's the boring part, but you have to check everything out," Fraga said.

Further, the chemists are working with agents that can kill by blocking nerve cells from sending messages to each other. People exposed to sarin or other nerve agents can suffer convulsions, excessive saliva, pinpoint pupils, blurred vision and respiratory distress as well as vomiting, diarrhea, confusion, headache, changes in heart rate, loss of consciousness and paralysis. So throw in glove boxes, airlocks and high-tech filter systems, said Ralf Trapp, a France-based international chemical weapons disarmament consultant and scientist.

And in the end, nothing chemists find can precisely prove just who did the gassing. This is the chemical equivalent of a medical examiner saying that gaping hole in the body is a gunshot from this calibre weapon.

Chemists might discover that what was used was not sarin, but soman or VX or another banned chemical agent. Yet except for the name of the weapon and chemical signature, the results are the same. Sarin is the chief suspect because of the Syrian government's history, but the other nerve agents cause similar deadly results as those seen on videos taken at the scene of the Syrian attack.

A lot is riding on this, so there's one key dictum to chemists who are considered tops in the business: Get it right.

It's all about eliminating the likelihood of false positives, even if they are a one-in-a-billion chance, Trapp said. False positives are unlikely "given the high standard of the labs involved," but that's why more than one lab is used. Just to make sure.