The news agencies initially described agent VX as the most poisonous chemical weapon known. They seem to have corrected themselves. The distinction belongs to one or more of the Soviet “Novichok” family, dating to 1987. NTI, the Nuclear Threat Initiative, offers a brief statement in the (pdf) Russia Chemical Stockpile: alleged Undeclared “Novichok Capabilities”. For further confirmation, NCBI states,

The Soviet Union reacted with an extensive program (code name FOLIANT, NOVICHOK) for the development of new, fourth generation chemical weapons, and the result was a technology for binary ammunition with nerve agents exerting enhanced toxicity.

Many of the relics of the Cold War have remained elusive. But a principal of the Novichok program, Vil S. Mirzayanov, wrote a book, State Secrets: An Insider’s Chronicle of the Russian Chemical Weapons Program. In his blog entry, NOVICHOK CHEMICAL FORMULAS ARE NOT TERRORIST WEAPONS , he explains why he thinks it was OK to include the chemical formulas. I think he is correct. In fact, it is probably helpful for the formulas to be public knowledge.

Soviet assays indicated that at least some members of the Novichok family are 5 to 7 times more potent than VX.  But that is the least important distinction. The Soviets wanted a chemical that would be undetectable by technology deployed by NATO. Unlike VX, Novichok has no antidote. Although the effectiveness of antidotes under field conditions has always been questioned, it gives Novichok a psychological edge.

The degree to which a chemical agent can be weaponized depends on many factors, of which toxicity is probably the least important. VX can be used as an “area denial weapon.” Because it is persistent, it can be used to prevent an enemy from occupying, or even transiting an area. In this use, it is actually effective in the military sense without killing anybody. VX is stable in storage, and, as a binary agent, can be mixed in transit to the target from nontoxic precursor chemicals.

(Aljazeera) and (Reuters) North Korea  used a binary form of VX. Two nontoxic chemicals, applied to the face of Kim Jong Nam by two women, combined on his face and in his eyes  to form VX. One of the women, probably the second attacker, suffered symptoms of VX exposure, because the combination also occurred on her hand.

But for assassination, VX is not optimal. The stability of VX   allowed detection post mortem. Prepackaged, “lab-on-a-test-strip” tests, developed primarily for NATO  exist, as well as documented changes in body chemistry. See  NAP (National Academies Press) Chemical and Biological Terrorism: Research and Development to Improve Civilian Medical Response.

An ideal candidate chemical for assassination would have these properties:

• Instability. Decays too rapidly to test for post-use. This is opposite the requirement for a weapon.
• Lack of off-the-shelf tests.
• Binary. Made of innocuous, common, nontoxic precursor chemicals. The precursors of Novichok are utterly commonplace.
• Unavailability of an antidote.
• A novel mode of toxicity, impairing the view of the toxicologist of exactly what happened to the victim. The mode of the Novichok agents is novel.

The past novelty of ricin (Markov) and polonium (Litvinenko) made them useful. They are now too well known.

In May 2015 Vladimir Kara-Murza was poisoned. He received nonspecific treatment at a Moscow hospital, and survived. The poison was not identified.  In February 2017, he was probably poisoned again. He has been allowed to leave Russia for treatment in the west. If the poison is not persistent in the body, examination of samples by western chemical warfare facilities will avail nothing. He walks with a cane, indicative of nerve damage.

The speculation of this article is that one of the hundred-or-so Novichok agents, but not the weaponizable Novichok-5/A-232, was used to poison Kara-Murza. Where avoidance of detection in the west is paramount, an unstable compound  might be chosen. In Russia, the “ultra toxic” solid state derivatives A-242 and A-262 (Mirzayanov,  State Secrets:…, page 145)  could have the advantage of ease of deployment.

Particularly striking is that in neither instance has Kara-Murza any idea as to how the poison was administered. It was administered specifically to him, with no collateral exposure,  possibly from a distance, via a dart made of a volatile or soluble substance.