By Anthony Deutsch
AMSTERDAM, Aug 29 (Reuters) - With the world watching and a U.S.-led strike on hold, U.N. inspectors investigating suspected chemical attacks in Syria are under extreme pressure to make sure their findings stand up to the most intense scrutiny.
The inspectors, who have been gathering samples since Monday and may already have some results after testing them through the night at a Damascus hotel, will not point any fingers, but may offer clues by identifying what kinds of munitions were used.
Safeguarding the integrity of samples collected in the outskirts of Damascus, where hundreds of people are believed to have been gassed by a banned toxic substance in an Aug. 21 air assault, is the top priority.
"If they don't do that, they cannot guarantee the chain of custody, meaning that nobody tampered with the sample itself," said Dieter Rothbacher, a chemical weapons expert who trained members of the team.
Abu Akram, a doctor who helped the inspectors at a field hospital, was surprised they refused to take some of the blood samples they had taken from the deceased, who were buried within 24 hours according to Muslim custom, and rocket fragments.
That would have broken the chain of custody guidelines stating samples must be taken by the inspectors themselves.
"So they took some other samples from the site - soil, pieces of furniture and clothing. We also gave them the bodies of dead animals - we gave them a chicken that died from the chemical attacks," Akram said.
The 20-member team, including experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) who served in Iraq, have already been testing for sarin, mustard gas or other toxic agents, people familiar with their methods said.
"If they have, which I am sure they do, onsite analytical capability, they have some analytical results already," said Rothbacher who worked at the OPCW until 2009 and co-owns Hotzone Solutions Group, a training and consultancy company.
"And then, of course, now the question is how the U.N. deals with that result."
Washington said it is convinced Bashar al-Assad's forces were behind the gassing of civilians chemical weapons attacks, but many governments are awaiting the U.N. findings before drawing conclusions about responsibility.
The team, including three World Heath Organisation medical experts, plans to leave Syria on Saturday, U.N. Secretary General Ban Ki-moon said. The decision-making U.N. Security Council will then consider its findings.
Some defence experts said the rebels do not even have the capability to carry out a large scale chemical weapons attack, but Syria's allies, including Russia, said it was the work of opposition forces seeking to sway world opinion against Assad.
The team headed by Swedish scientist Ake Sellstrom crossed the frontline into rebel-held territory in a convoy of six Toyota Land Cruisers marked with U.N. initials. Unidentified snipers opened fire on their first day, but no one was hurt.
Once at the scene, they broke into pairs, some interviewing victims and taking blood and urine samples at hospitals, while others collected dirt and took swabs, or "wipes" from possible chemical munitions.
At the spot where a rocket was said to have struck, the inspectors wore chemical-proof body suits, gas masks and were equipped with air detection sensors and boots with sensitive paper that would change colour if toxins were detected.
"If chemical weapons were used, the team will in my opinion have a pretty good chance of getting (or already having) some sound evidence to show that chemical weapons were used," said Ralf Trapp, a disarmament expert who worked for the Organisation for the Prohibition of Chemical Weapons.
Limited by basic portable instruments, inspectors probably tested only the so-called environmental samples of dirt, wipes from munition fragments, or chemical spills, he said.
Those samples can be cross-referenced against the OPCW's database of more than 1,000 chemical agents and may already have given a positive result for sarin, the nerve agent Syria is believed to have in vast quantities.
"When I find sarin on a shell through analysis, that's a positive. It's always better when you have two or more positive results, meaning a blood sample, bio-markers, or whatever, but it's almost certain if you find sarin on some of these shells you will also find it in the biomedical samples of possible victims," Rothbacher said.
More elaborate bio-metric analysis of blood, hair or urine samples will probably be done in fully-equipped, designated laboratories, of which the OPCW has 22 in 17 countries.
Inspectors will carry the samples in containers, closed with individually-numbered fibre-optic seals.
To maintain the "chain of custody" the inspectors will accompany the samples out of Syria to two or even three of the OPCW partner laboratories, where additional analysis will be done, possibly to confirm preliminary field tests.
Under normal U.N. inspection procedures, the laboratory tests could be performed under the supervision of a Syrian scientist sitting in as an observer.
If the sample goes to an OPCW partner laboratory, the U.N. inspector who took the sample, or the team leader, would witness the analysis to make sure the chain of custody is not broken.
(Additional reporting by Erika Solomon in Beirut; editing by Philippa Fletcher)
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Captions By The Associated Press