* Tokyo sarin attack provides hints for Syria investigation
* Samples needed to prove use can fade within weeks
* Western powers say evidence of use by Syria in mounting
By Anthony Deutsch
AMSTERDAM, May 15 (Reuters) - It took Tokyo police just three hours to confirm the use of sarin gas when members of a doomsday cult attacked five crowded subway cars in 1995, killing 13 people and sickening thousands of passengers.
The same lethal nerve gas is now at the centre of alleged chemical weapons use in Syria's two-year-old civil war, but the UN-appointed inspectors tasked with the investigation have a much harder challenge than faced Japan's hospital laboratories.
In Tokyo, high concentrations of sarin were immediately found at the scene, where assailants released the colourless, odourless chemical from plastic bags by puncturing them with sharpened umbrella tips.
In Syria, about two months have passed since the first allegations of sarin use arose on March 19, when rebels and government forces blamed each other for an attack in Aleppo province in which dozens are said to have been killed.
Syria has not allowed testing at the site and has refused the UN investigators access to determine the use of sarin, which interferes with the mechanism by which neurotransmitters act on muscles, preventing them from relaxing. Death usually occurs because the muscles involved in breathing cannot function.
The UN team of more than 15 experts from the Organisation for the Prohibition of Chemical Weapons and the World Health Organization are waiting in Cyprus for permission to enter.
"What is needed is the collection and analysis of an authentic sample," said Ralf Trapp, an independent consultant on chemical and biological weapons control.
"The longer the UN investigation team has to wait on the outside, the lower is the chance of actually finding a meaningful sample," he said.
Western powers including the United States and Britain have said there is a growing body of evidence of chemical weapons use in Syria and they are considering taking firmer action to stop the fighting, which has claimed about 70,000 lives.
Syria is one of just eight countries not to have joined the Chemical Weapons Convention, which took effect in 1997, and it is therefore not subject to an inspection regimes. Intelligence agencies believe it possesses one of the largest stockpiles of chemical weapons, including sarin.
In the 1995 attack, Tokyo's fire department first suspected acetonitrile, a colourless liquid that is a manufacturing by-product and is only mildly toxic, said Japanese doctors who helped treat more than 1,400 patients at St. Luke's Hospital.
One clue that put them on the right track to identify the correct agent was reduced enzyme activity in the nervous systems of more than 500 patients flooding emergency rooms.
Another factor that helped in identification was the experience of doctors who had treated victims of a smaller sarin attack less than a year earlier in Matsumoto that killed seven.
One way to test for sarin is to examine the blood or urine of people or animals believed to have been exposed.
Turkey's foreign minister said last week Syrian casualties crossing the border showed signs of exposure to chemical weapons, the use of which President Barack Obama said in August marked a "red line" that could prompt intervention.
With the 2003 Iraq invasion in mind, Washington has been cautious of basing policy on false intelligence and said it was seeking greater levels of proof in Syria.
Scientific evidence would raise the chance of intervention against Syrian President Bashar al-Assad to end the conflict.
Speed is crucial because the quality of a sample, whether from human tissue or soil, deteriorates rapidly.
Within just a few weeks, the level of sarin may no longer be measurable on metal fragments, the impact crater of a rocket, or in a victim's body.
The biological symptoms fade even more quickly, said professor Hirotaro Iwase, a forensic pathologist at Chiba University, who performed autopsies after the Tokyo attacks.
One such symptom is a constriction of the pupils or frothing of the mouth, which were mentioned by the Israeli intelligence agency last month as proof of sarin use in Syria.
"If you come across multiple patients with these symptoms in an area where agricultural chemicals are not around, you suspect the use of sarin," Hirotaro said. (Additional reporting by Kiyoshi Takenaka in Tokyo; Editing by Will Waterman)