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Testing for sarin

Dear Reader,

One of the key problems for civilized society and decent people is how should we react to an alleged use of sarin or another chemical weapon. I hold the view that their use in civilians is a clear war crime, the question of “is it a war crime to attack civilians / non combatants with a chemical weapon ?” is a nobrainer the answer should be perfectly clear to any normal person.

Even if we ignore for a moment the international law which bans the use of chemical weapons in war (for example the Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or other Gases, and of Bacteriological Methods of Warfare) I think that it is very difficult to imagine a scenario where the use of poison gas on a battlefield would be legal. While it is possible to make a choice regarding where a bullet is fired, a cloud of gas or droplets will drift in the wind towards civilians, prisoners of war and other people who should be protected by international law. To my mind the use of such an indiscriminate weapon is wrong because of the reckless endangerment of innocent bystanders. It is a very cruel and inhumane way of waging war. So I hope we can all agree the use of poison gas is deeply wrong.

Now we have agreed that it is wrong, we must take some steps to discourage these disgusting practices, to detect their use and to punish the vile criminals responsible for waging poison gas warfare. One of the great problems with modern chemical warfare agents is that many of them are shortlived, after release into the environment they tend to degrade.

While this instability may make it harder to detect them, it does not make it impossible to prove that they have been used. The way to deal with the problem is to look for the breakdown products. For example if bis-(2-hydroxyethyl sulfide) (AKA thiodiglycol) is found in the urine of a person then it is a sign that the person has been exposed to the classic mustard gas (HD).

In the case of the nerve gases many of them degrade into methylphosphonic acid via methylphosphonic acid esters such as the isopropyl hydrogen methylphosphonate which is formed by the hydroylsis of sarin. In a similar way somon will form 3,3-dimethylbutan-2-yl hydrogen methylphosphonate, cyclosarin will form cyclohexyl hydrogen methylphosphonate while VX can form ethyl hydrogen methylphosphonate. One good paper about this was published by M. Kanamori-Kataoka and S. Yasuo, Journal of Health Science, 2008, 54(5), 513-523.

Degradation products of somon

Degradation products of somon

One good method for looking for these compounds is to form the tert-butyldimethyl silyl derivatives of the residue compounds before running gas chromatography, the reason for forming the derivatives is that they are able to vapourise and travel through the GC column while the alkyl hydrogen methylphosphonates can not travel through the column. I have been known to use derivatives to allow me to run GC on my compounds (typically carbohydrates, paint chemicals and fatty acids) and I can tell you that forming a derivative makes GC experiments work far better, but back to the nerve agent chemistry for a while. One of the things which M. Kanamori-Kataoka and S. Yasuo report is that calcium and other metal ions in biological and environmental samples tend to bind to the nerve agent degradation products and make it harder to detect them. They found that sample clean up with ion exchange resin made it much more easy to detect the compounds.

I would be inclined to use a polystyrene resin with sulfonic acid groups on it to remove all traces of calcium and magnesium from the sample, I would rather not say what ion exchange resin I would use in case I am accused of plugging one company’s product instead of another company.

After the clean up of the sample it has been reported by T. Nakajima, K. Sasaki, H. Ozawa, Y. Sekijima, H. Morita, Y. Fukushima and N. Yanagisawa (Archives of Toxicology, 1998, 72(9), 601-603 that N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide can be used to form the silyl derivatives needed for GC. Here is a picture of the reagent.

N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide

N-methyl-N-(tert-butyldimethylsilyl)-trifluoroacetamide

Back to the calcium chemistry. A. Mermer and P. Starynowicz, Acta Crystallogr.,Sect.B:Struct.Sci., 2011, 67, 399 report the crystal strucutre of the polymer of bis(hydrogen methylphosphonato)-calcium while Guang Cao, V.M.Lynch, J.S.Swinnea and T.E.Mallouk reported in inorganic chemistry, 1990, 29, 2112  a polymer of a hydrated calcium salt of methylphosphonic acid. It is interesting that G.Cao, H.Lee, V.M.Lynch and T.E.Mallouk in the same journal reported the magnesium salt in 1988, volume 27 on page 2781.

The solid of A. Mermer and P. Starynowicz is a layered solid which has infinite 2D layers of calcium ions which are separated by layers of methyl groups. Here is one view of the solid which shows the alternation of the two different types of layers.

Layered solid formed from MePO3H2 and calcium

Layered solid formed from MePO3H2 and calcium

The layers are more clear when the calcium and oxygen atoms are removed.

Without calcium and oxygen atoms

Without calcium and oxygen atoms

The fact that the solid contains infinite layers of calcium ions linking the organophosphorus molecules together will make it very insoluble, the calcium salt reported in inorganic chemistry is also a similar layered solid.

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