Who the hell knows, since the British aren't making any chemical samples available to the international organization of chemical weapons experts, as per the Convention on Chemical Weapons to which both Britain and Russia are signatories.
Meanwhile Britain has advised anyone in the area of the Skripals that day to 'wash their clothes' as a precaution. Does that sound like the advice you'd give to someone if the chemical contamination remains present for many years?
No one is going to bother wasting time with impotent international organizations, who’re just going to be ignored anyway. Britain is going to show their intelligence to the USA and France, but advocating anything else is advocating doing nothing.
I tend to buy the British account in this issue and I am glad the Trump administration supports the British in this issue.
I think that washing the clothes would work about as well as getting under your desk in a nuke attack!