By Fred Pleitgen and Andrew Carey, CNN
The Hague, Netherlands (CNN) - When U.N. weapons inspectors left their hotel to investigate claims of chemical weapons use in the suburbs of Damascus in late August, most of the experts travelling in the convoy of armoured SUVs were not United Nations staff at all.
In fact, nine of the 12 were inspectors from the Organization for the Prohibition of Chemical Weapons (OPCW).
The OPCW is the implementing body of the Chemical Weapons Convention (CWC), an international treaty which bans the possession of chemical weapons, and has been signed and ratified by 189 countries around the world.
Syria has just applied to join, presenting the OPCW with perhaps the most challenging assignment in its near 20-year history, as the country is still locked in a deadly civil war.
The OPCW's experts have monitored the cataloguing and destruction of chemical weapons in countries ranging from the United States and Russia to Libya. They have also worked in Iraq, which was the first time its inspectors were sent into a live battlefield.
"We try to get as much information as we can about what we are doing," Franz Ontal, OPCW's head of inspector training, recently told CNN, during an exclusive visit to the organization's lab and staging facility in the Netherlands.
"We want to know what the target site looks like; we want to know what we are after. The information we get is what's going to inform the inspectors about the kind of protective equipment they are going to wear."
Ontal walked us through the OPCW's warehouse and explained the inspectors' rules of operations. Everyone we met was at pains to stress there was no deviation from those rules during the recent deployment in Syria to investigate reports of an August 21 gas attack on the rebel-controlled outskirts of Damascus.
The U.S. and its allies have accused the Syrian government of using sarin gas – a potent nerve agent. Syria has denied the allegations.
Once they have located the site of a possible chemical weapons attack, the inspectors use special electronic detectors to give them an initial readout of the type of chemicals they might be facing, and in what concentration. Two different machines, using different technologies, are used to increase confidence in the result.
In addition to chemical experts, the inspection team also includes munitions experts. That's because they may be dealing with unexploded ordnance. In addition, shell remnants often contain traces of the chemical residue inspectors are looking for.
Ontal told us inspectors rarely find chemical agents in their pure form, but, crucially, they can detect residues even if only fine traces are left.
"The holy grail for environmental sampling is the pure agent, the agent itself. [But] that might not be practical; we do not expect to find agent by the time we arrive. So we need to look for secondary evidence. That could be munitions fragments, or the delivery device itself, or whatever they used to deliver the agent. Munitions fragments can inform us of many things; they can still hold agent, if there is some liquid left."
Identifying the munitions and the delivery device can also assist the investigative process, by providing clues about who might have been behind the attack.
During the process of sample-gathering, protecting the chain of custody is of utmost importance. Inspectors do not take samples from third parties, so they said they did not get samples from opposition doctors working in field hospitals right after the alleged chemical attack in August.
"The chain of custody is absolutely critical," Dr. Hugh Gregg, the head of OPCW's lab told us. "It is our inspectors in the field that take the samples. We do not take samples from a third party. Our inspectors ensure that they know absolutely where the samples came from. They take pictures, they log it, they time-track it, and all of that information follows the sample."
The OPCW says this is the only way to guarantee the investigation's independence, and make sure it stands up to intense international scrutiny and criticism.
Samples collected in the field are transported in a stainless steel container, each strong enough to withstand a plane crash. When they reach the lab, scientists unpack them carefully, monitoring them with a chemical weapons detector.
In most cases, the samples will be split and sent to several of the OPCW's partner laboratories for analysis, to ensure objectivity in the findings. Partner labs are found across the world, from the United States and Russia to Germany and Iran. Samples taken from Syria followed the same process.
During our tour of the facility in the Netherlands, the inspectors explained the effect of sarin, and how it incapacitates or kills those who come into contact with it.
A highly-volatile liquid, sarin works by blocking an enzyme needed to control bodily functions. The inspectors said victims start moving and shaking uncontrollably, suffering significant involuntary release of bodily fluids. Most fatalities are caused by suffocation or drowning, as the victim’s saliva floods their lungs, they said. Children and the elderly are especially vulnerable.
Even though sarin dissipates quickly, Gregg says his scientists can find traces of it months after it was deployed, by using a process called gas chromatograph-mass spectrometry (GC-MS).
"The routine GC-MS analysis that we would do for environmental samples can see things down below one part per million. It can see samples that have been there for weeks or months," he told us, standing in front of a GC-MS device at the OPCW's lab.
Now that the regime of Bashar al-Assad has said it plans to join the Chemical Weapons Convention, putting its chemical weapons stockpile under international control, the OPCW knows it has a major new assignment on its hands.
Speaking from previous experience, Gregg knows it is not an easy task. "For inspectors to go and catalogue that they would actually have to go and to witness how many artillery shells there are, and how much they contain on an average fill.
"They would have to look at storage containers, they would have to figure all that out. But cataloguing something would depend on how many sites there are; how many different munitions there are. It could take months."
And, of course, there is still a bloody civil war going on in Syria, which would make that mission even more daunting. Their work may be focused on safeguarding international security, but the OPCW also has the safety of its own staff to consider.