The Skripals were found slumped on a bench in the English city of Salisbury on March 4. Britain has blamed Russia for the attack -- a charge that Moscow has strongly denied.
The OPCW also made no assessment of who was to blame for the March 4 attack on former Russian spy Sergei Skripal, which also affected his daughter Yulia and a local policeman.
Russia won't take on faith any conclusion relating to the Skripal affair until such a time as Russian experts are allowed access to the samples mentioned.
OPCW confirmed that the chemical used was "of high purity", without mentioning the exact name of the substance which will be reserved for the complete classified report it has prepared.
While her whereabouts are unknown, Skripal said she is being supported by police officers who are keeping her informed of their high-profile investigation.
The poisoning has chilled relations between Russia and the West, producing a wave of diplomatic expulsions unseen even at the height of the Cold War.
Yulia Skripal said in a statement released Thursday by British police that her strength is growing daily and expressed gratitude to those who came to her aid.
The diplomatic fallout between Russia and Britain over the poisoning of Sergei Skripal took another bizarre turn as Moscow complained of a lack of information about the former double agent's pets.
head of Russia's SVR foreign intelligence agency said Wednesday the poisoning of Russian former spy Sergei Skripal in Britain was a "grotesque provocation" by the UK and US.
Putin expressed hope a meeting of the world's chemical weapons watchdog would put a 'full stop' to the issue of an ex-spy whose poisoning in Britain has been blamed on Moscow.
British military facility analysing the Novichok nerve agent used to poison Russian spy Sergei Skripal and his daughter said it has 'not identified the precise source' of the substance.
Russian Foreign Minister Sergei Lavrov says ex spy poisoning could be 'in interests' of UK government
Lavrov also suggested that the poisoning of Skripal could also be in the interests of the British special forces who are known for their abilities to act with a licence to kill".
In Washington, the State Department said there was no justification for the Russian move and that the United States "reserves the right to respond."
Russia's mass expulsion of US diplomats Thursday is the latest tit-for-tat move triggered by the poisoning of a Russian former double agent in England.
Investigators had earlier sealed off the bench where the Skripals were found, a pub and restaurant the pair visited, and the grave of the former spy's wife.