Come si rivelano gli agenti nervini ed altre notizie sulla guerra chimica

In questi giorni si discute se si siano usate armi chimiche in Siria, in particolare agenti nervini. Sono al lavoro gli ispettori dell’ONU che usano ovviamente le armi della chimica. Già ma come si fa a rivelare l’uso oltre ogni ragionevole dubbio di tali sostanze?

La questione è di enorme interesse e ad essa sono dedicati anche libri ed articoli recenti. In anteprima per il nostro blog una piccola parte del più recente libro firmato come coautore da Vincenzo Balzani

frontcover_9783527334797

In uno dei capitoli di questo nuovo libro, il cui indice completo trovate qui, si parla dellle tecniche più innovative per rispondere alla domanda.

(per la cortesia di V. Balzani, P. Ceroni e A. Juris e della Wiley una parte del cap. 14)

Capitolo 14. Technological applications of photochemistry and photophysics

sezione 14.3 Luminescence sensors

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14.2.6 Detecting warfare chemical agents

Ratification of the Chemical Weapons Convention by more than 165 States has reduced the risk of warfare chemical agent use; however, there is increasing concern about terrorist attacks involving these compounds. Practically all the analytical techniques are used for detection of warfare chemical agents. The most employed ones are gas and liquid chromatography, but fluorescence also plays an important role [29].

Figure 14.8 shows the chemical formulae of Sarin, Soman and Tabun, three organophosphorus compounds used as extremely potent nerve agents. The formulae of model compounds, used for obvious reason in this research field rather than the agents themselves, are also shown.

 

fig1balzaFigure 14.8   Chemical formulae of nerve agents Sarin, Soman and Tabun and their mimics.

The general mechanism for the fluorescent detection of such compounds involves nucleophilic attack of the probe molecule on the electrophilic agent to form a phosphate ester, which leads to the generation of a signal through suppression of photoinduced electron transfer (PET) from the nitrogen atom to the fluorophore (see e.g. Figure 14.9)[30].

fig2balzaFigure 14.9 Fluorescent detection of phosphoryl nerve agents:  phosphorylation of an amine participating in PET quenching turns on the fluorescence.

The chemical formula of Sulfur Mustard (SM), also called mustard gas/HD, is shown in Figure 14.10 with two model analogues. Sulfur Mustard has frequently been used on a large scale against military and civilian targets since the beginning of the 20th century, causing millions of casualties. Because of its ease of preparation compared with other chemical warfare agents, the use of this chemical compound by terrorist groups or rogue nations represents a serious threat to society and national security.

fig3balzaFigure 14.10 Chemical formulae of Sulfur Mustard and two of its mimics.

Designing a fluorescent detection method for Sulfur Mustard is challenging because of the absence of highly electrophilic sites such those of nerve agents as well as of any traditional molecular recognition sites. Nevertheless, two rapid, highly selective, and sensitive fluorescent detection methods have been devised.

One such method is based on the reaction of a dithiol with the Sulfur Mustard model compound CEES to form a receptor, which has high affinity for Cd2+ (Figure 14.11). Such a receptor can displace Cd2+ from a Cd2+ complex with 4-methylenesculetin (ME), a strongly fluorescent molecule that does not emit when complexed with Cd2+ [31]. This method has been used successfully for the detection of the model compound present on surfaces and in soil samples.

fig4balzaFigure 14.11 Strategy for fluorescence detection of the Sulfur Mustard mimic CEES.

The second method, which is again based on the displacement sensing assay concept, relies on the fact that when a dansyl-based fluorophore is attached to a gold nanoparticle through an imidazole group, its fluorescence is quenched through an energy transfer process. Exposure of the functionalized gold particle to the Sulfur Mustard simulant CEME, whose sulfur atoms have strong affinity for gold, causes the detachment of the appended fluorophore switching on its fluorescence (Figure 14.12)[32].

fig5balzaFigure 14.12 Method for detection of the Sulfur Mustard mimic CEMS using gold nanoparticles.

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Ma se queste sono le idee all’avanguardia, nella pratica attuale ci sono varie procedure che si rifanno alla chimica analitica e che servono egregiamente allo scopo, anche se occorre dire che non tutti i documenti a riguardo sono “open”, disponibili; una gran parte sono anche classificati e in questo la chimica non ci fa una gran bella figura. Ci sono sia procedure che rivelano l’uso che procedure che rivelano la produzione. Per esempio per rivelare la produzione di VX si ricorre all’individuazione di un intermedio, l’EMPTA o acido O-Etill metilfosfonotioico:

O-Ethyl_methylphosphonothioic_acid_Structural_Formula_V.1.svg

(EMPTA) è un composto organofosforico. Si tratta di un composto a doppio uso: da una parte si usa come intermedio nella sintesi di pesticidi e prodotti farmaceutici, e dalll’altra è un precursore nella sintesi degli agenti nervini come l’agente VM e l’agente VX. L’individuazione dell’EMPTA è indicato come la causa della decisione americana di distruggere nel 1988 l’impianto farmaceutico di  Al-Shifa in Sudan.[1]

  1. ^ Claudine McCarthy (2005). “EMPTA (O-Ethyl methylphosphonothioic acid)” (Google Books excerpt). In Eric Croddy, James J. Wirtz. Weapons of mass destruction: an encyclopedia of worldwide policy, technology, and history. pp. 123–124. ISBN 1-85109-490-3.

Un ampio documento che è una vera e propria review sulla questione è il seguente:

Kim, K., Tsay, O. G., Atwood, D. A. and Churchill, D. G. (2011) Destruction and Detection of Chemical Warfare Agents. Chem. Rev., 111, 5345–5403.

Qualcosa si trova anche senza passare per le forche caudine delle riviste a pagamento e quindi eccovi qui un breve elenco per le tecniche più importanti, ma senza pretesa di essere esasutivo:

http://www.google.it/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CGcQFjAG&url=http%3A%2F%2Fwww.zenobi.ethz.ch%2FAnalytik5_2011%2Fchemical_warfare.pptx&ei=1Y8dUsqCEef07AavhIGQAg&usg=AFQjCNGAQKAx4Xzek_jJVsH_mjmhtzHIPQ&sig2=GgBTsa1upxRG8lrFpOYlrw&cad=rja

http://www.fas.org/irp/threat/cbw/drdc2006.pdf

http://www.biomedicale.univ-paris5.fr/enseignement/toxico/M2THERV_2011_2012/ANNALES%202010:2011/C3/Publi%20sujet%20module%20tox%20analytique.pdf

http://www.ecbc.army.mil/pr/news/DART.pdf

http://www.dtic.mil/dtic/tr/fulltext/u2/a418514.pdf

Ernest H. Braue Jr.,Michael G. Pannella Microchimica Acta 1988, Volume 94, Issue 1-6, pp 11-16  FT-IR analysis of chemical warfare agents

(c.dellavolpe)

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