Why do neither the UK government nor the OPCW release what agent was used for the Salisbury/Amesbury attacks?

Question

For the purposes of this question please assume that the UK / OPCW story is largely true. Any answers along the lines of it being a false flag operation are off topic for this question.

In 2018 there were two incidents of poisoning in Southern England. In March Sergei Skripal, his daughter Yulia, and ex-police officer Nick Bailey were poisoned in Salisbury, and in July Dawn Sturgess died and her boyfriend, Charlie Rowley, survived after they were poisoned in Amesbury.

The explanation for this accepted by the UK government, the Organisation for the Prohibition of Chemical Weapons (OPCW), the G7 and most of the west is that this was an attack by the Russian state targeting Sergei Skripal. The two affected later in Amesbury happened across a discarded perfume bottle that is assumed to have been the method of application in the Skripal attack. A primary independent confirmation of this story is the finding of the OPCW that the same agent was present in both incidents:

The results of the analysis conducted by OPCW Designated Laboratories of environmental and biomedical samples collected by the OPCW team confirm the findings of the United Kingdom relating to the identity of the toxic chemical that intoxicated Mr Charles Rowley and Ms Dawn Sturgess. The toxic chemical compound, which displays the toxic properties of a nerve agent, is the same toxic chemical that was found in the biomedical and environmental samples relating to the poisoning of Sergei and Yulia Skripal and Mr Nicholas Bailey on 4 March 2018 in Salisbury (S/1612/2018, dated 12 April 2018).

S/1612/2018 is has a similar statement:

The results of analysis by the OPCW designated laboratories of environmental and biomedical samples collected by the OPCW team confirm the findings of the United Kingdom relating to the identity of the toxic chemical that was used in Salisbury and severely injured three people.

I have looked quite hard and I can find nowhere that states which specific member of the Novichok class of agents this is referring to, or what evidence the statement that they are the same chemical is based on. As the evidence for the western explanation hinges on this, it would seem to weaken their evidence to not provide this information.

To be able to state the two chemicals are the same there must be some evidence to support this. Even if they are not able to state exactly which chemical it is, they must have some evidence to support the statement. Why is this information not available?

Answer 1

"Sources and methods."

It is quite possible that some of the information the UK government and/or the OPCW have on the Novichok agents comes from espionage. Revealing such information could betray the source to the Russian government. Even if that is not the case, behaving as if it is gives the Russian government something extra to worry about.

There are also no plausible gains to be made from revealing the details. The Russian government are not going to admit to the attack, apologise and pay compensation. The people who believe Russian claims of innocence are not going to be convinced by chemical structures. There's no point in risking the confidentiality of the sources.

Answer 2

FWTW, after the Skripals incident, even though the exact chem used was officially kept confidential and only released to the State Parties...

The name and structure of the identified toxic chemical are contained in the full classified report of the Secretariat, available to States Parties.

several Novichoks were then explicitly added to the Schedule 1 of the convention. So, some academics have inferred that it was one of those, e.g. one paper says:

The Novichok agent A 234 (structural formula on Fig. 1) was alleged by the British government to have been used to poison the Skripals, and its identity was confirmed by the Organization for the Prohibition of Chemical Weapons (OPCW).

Whether those two academics had access to the classified reports, I'm not sure. (They don't cite anything for their assertion of the exact chemical.) But if you're just looking for 'peer reviewed scientific literature', as you say in a comment, that paper fits the bill. And only a bit more googling finds several other such academic papers saying the same thing. Whether this is the academic version of the rumour mill, given the lack of official statement on the exact compound, IDK. One of those papers claiming A-234 was the precise compound used on the Skripals is authored by employees of the U.S. Army Combat Capabilities and Development Command Chemical Biological Center, Aberdeen Proving Ground, MD. So perhaps they had access to some classified info too.

BTW, that (US army) paper finds that A234 was the best in several categories among the compounds they tested: it required the least concentration to take (enzymatic) effect and also had the highest stability in the environment. So, ideal for a covert mission abroad (where you don't want to transport much) and for being capable of 'delayed' use via contaminating objects that someone touches later.

When the Skripals incident happened (2018), the Novichoks were not explicitly on any list as such (although implicitly they were as chemical weapon, broadly construed). Likewise, after the Navalny poisoning (2020), info has emerged that the agent used there was not the (recently adopted!) list either, being a slightly modified version of a chemicals added to the list in 2019.

But the OPCW statement noted that although “the biomarkers of the cholinesterase inhibitor found in Mr. Navalny’s blood and urine samples have similar structural characteristics to toxic chemicals belonging to [Schedule 1],” the specific Novichok agent used to poison Navalny in August 2020 was not among those included in the amendment to the convention’s annex on chemicals.

Gregory Koblentz, who directs biodefense graduate programs at George Mason University, said in an Oct. 6 tweet that the OPCW language suggests a similar Novichok agent, called A-262, was used to poison Navalny. He explained that A-262 agents combine features of two of the chemicals already included in Schedule 1, which were cited by the OPCW as having similar structural characteristics to the agent used on Navalny.

Koblentz detailed in a Sept. 30, 2019, article in The Nonproliferation Review that the presence of an additional nitrogen atom in A-262 and in similar Novichok compounds precluded their inclusion in the updated annex on chemicals, meaning that they were technically not subject to declaration and destruction following the CWC amendment.

The OCPW keeps some data from the general public and only releases it to State Parties, which in this case seems indeed a bit pointless, as they seemingly told Russia what they needed to know to formally evade the convention (stocks wise) by modifying the agent used, slightly.

See also, related, older Q: Which laboratories confirmed Navalny was poisoned by Novichok? on the other 'open secrets' of OPCW. Why they stick to this formal policy I'm not entirely sure, but revising it might take all signatory countries to agree (incl. Russia), IDK.

You might think that the confidentiality commission of OPCW is dominated by Western countries, but in fact, it is not. Here's the (current) membership:

Africa

Mr. Amine Sid (Algeria)
Mr. John Billy-Eko (Cameroon)
Mr. Amadou Ousmane Ba (Senegal)
Ms. Boipelo Motsi (South Africa)

Asia

Mr. Zuo Qi (China)
Mr. Febrizki B. Mukti (Indonesia)
Mr. Reza Najafi (Islamic Republic of Iran)
Ms. Wan Maisarah (Malaysia)

Eastern Europe

Mr. Eduard Kloboucek (Czechia)
Mr. Jerzy Gierasimiuk (Poland)
Ms. Anna Kogteva (Russian Federation)
Mr. Serhii Trotskyi (Ukraine)

Latin America and the Caribbean

Mr. Gustavo Zlauvinen (Argentina)
Mr. Jorge Caravajal (Chile)
Mr. Carlos Alvarez (Cuba)
Mr. Isaac Morales (Mexico)

Western European and Other States

Mr. Christoph Vedder (Germany)
Mr. Ioannis Seimenis (Greece)
Mr. Paul van Rhijn (The Netherlands)
Mr. Jack Marsh (United Kingdom of Great Britain and Northern Ireland)

And as that page opens:

A stringent regime governs the handling of confidential information at the OPCW. The OPCW Policy on Confidentiality is essential to the work of the Organisation because of the intrusive verification measures that are aimed at promoting confidence in compliance with the Convention while respecting States Parties’ legitimate concerns about sensitive information.