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Scientific proof of a chemical weapons attack in Syria could take at least three weeks.

Such was the pressure to get a conclusion the 20-member team of inspectors, including experts from the Organisation for the Prohibition of Chemical Weapons, left Syria three hours early, defying a warning that travelling at night was dangerous.

They flew to the Netherlands and the samples have been sent to two laboratories in Europe, thought to be in Sweden and Finland.

The team collected samples from the rebel-held areas in the Ghouta suburb of Damascus three times, taking blood, tissue and hair samples from victims.

They also took samples of soil, clothing and rocket fragments.

The experts will be testing for Sarin, mustard gas and other toxic agents.

The Organisation for the Prohibition of Chemical Weapons said: “The evidence collected by the team will now undergo laboratory analysis and technical evaluation according to the established and recognised procedures and standards.

"These procedures may take up to three weeks.”

Any results will be sent at first for ‘eyes only’ to UN General Secretary Ban Ki-moon.

The team was led by Swedish Professor Ake Sellstrom was the chief inspector of special UN unit set up after the Gulf War in the early 1990s to discover if chemical weapons were used during that conflict.

To ensure fairness and to avoid contamination each sample is split into eight parts. At least one part of each sample would have been handed to the Syrian government.

Discovering if Sarin gas was used will take biochemists to the cutting edge of science.

It will be difficult – but not impossible – to detect the chemical breakdown products of any nerve agents that may have been used in the Damascus attack.

Sarin is a small molecule that quickly breaks down within the human body and in the environment after it is released.

However a tiny amount of breakdown product can persist in the victims’ blood for between 16 and 26 days.

Traces in soil may last even longer, up to six months.

It can be traced using gas or liquid mass spectrometers that can detect the smallest trace of chemicals and checks samples for up to 2,000 marker chemicals.

One of the key markers for Sarin is the smaller breakdown chemical isopropyl methylphosphonic acid (IMPA).

But Japanese scientists who analysed blood samples of sarin victims in the Matsumoto and Tokyo underground attacks of 1994 and 1995, failed to find breakdown products in some of the individuals who were known to be exposed to the lethal agent.