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The chemical classes covered under the Chemical Weapons Convention (CWC) and listed on Schedules 1-3: nerve agents (sarin, soman, VX, the novichok / A-series exposed by the 2018 Salisbury and 2020 Navalny incidents), vesicants (sulphur and nitrogen mustards, lewisite), blood agents (cyanides) and choking agents (phosgene, chlorine); the GC-MS / LC-MS/MS / NMR analytical workflow under OPCW Designated Laboratory accreditation; and the chemistry of the 2017 Khan Shaykhun, 2018 Douma and 2013 Ghouta investigations.
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On 21 August 2013, rockets carrying sarin-filled warheads impacted residential areas in Ghouta, a suburb of Damascus, Syria, in the early hours of the morning. Conservative estimates by Medecins Sans Frontieres placed the death toll at approximately 1,400. The United Nations Mission to Investigate Allegations of the Use of Chemical Weapons in the Syrian Arab Republic, led by chief inspector Ake Sellstrom, collected samples from the sites within weeks and transmitted them under chain of custody to four OPCW-designated laboratories in Europe. The laboratories worked independently, without knowledge of each other's findings, and reported their results to the OPCW Technical Secretariat. All four confirmed the presence of sarin and its characteristic degradation products: isopropyl methylphosphonic acid (IMPA), methylphosphonic acid (MPA), and the nerve-agent-specific biomarker isopropyl methylphosphonofluoridate in environmental samples.
The Ghouta investigation illustrates everything that is distinctive about chemical warfare agent (CWA) analysis in a forensic context: the chemistry is unambiguous if the right analytical methods are applied, the confirmation requires multiple independent designated laboratories working from a shared chain of custody, and the results carry legal and geopolitical weight that very few other forensic chemistry findings ever achieve. The stakes in CWA casework are unlike those in any other sub-discipline of forensic chemistry: an OPCW finding of CWA use can trigger treaty obligations, international sanctions, military responses, and International Criminal Court referrals.
This topic covers the chemistry of the four major CWA classes, the Schedule structure of the Chemical Weapons Convention, the analytical workflow under OPCW Designated Laboratory accreditation, and the key twentieth and twenty-first century cases that turned chemistry results into history.
The three schedules of the CWC are not a simple list of banned chemicals. They are a carefully calibrated control regime that distinguishes between agents, precursors, and dual-use industrial chemicals, with different verification obligations attached to each.
The Chemical Weapons Convention (CWC), adopted at the Conference on Disarmament in Geneva in 1992 and entering into force on 29 April 1997, is the most comprehensive arms control treaty in the history of chemistry. As of 2024, 193 states are parties to the convention, the largest membership of any arms control agreement. The treaty is implemented and verified by the Organisation for the Prohibition of Chemical Weapons (OPCW), headquartered in The Hague, Netherlands.
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Practice Forensic Chemistry questionsThe CWC's control regime is built around three schedules of chemicals, each with different production, transfer, and reporting obligations.
Schedule 1 lists chemicals with little or no use outside chemical weapons. This includes the nerve agents proper (sarin, GB, isopropyl methylphosphonofluoridate; soman, GD, pinacolyl methylphosphonofluoridate; cyclosarin, GF; tabun, GA, ethyl N,N-dimethylphosphoramidocyanidate; VX, O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate; and the Soviet-era R-VX or Russian VX), the vesicants (sulphur mustard, HD, bis(2-chloroethyl) sulphide; nitrogen mustards HN-1, HN-2, HN-3; lewisite, L), and biological toxins with CWA potential (saxitoxin, ricin). Schedule 1 chemicals may only be produced in quantities up to one metric tonne per year per state party, only at single small-scale facilities, and only for research, medical countermeasure development, or OPCW-approved purposes.
Schedule 2 lists chemicals that are significant precursors to Schedule 1 agents or have limited legitimate commercial uses. These include thiodiglycol (a direct precursor for sulphur mustard synthesis), methyl phosphonic dichloride and methyl phosphonic difluoride (precursors for nerve agent P-F bond formation), PFIB (a high-toxicity industrial chemical), and amiton. States parties must declare production and trade of Schedule 2 chemicals above defined threshold quantities.
Schedule 3 lists chemicals with some legitimate commercial use but significant CW potential. This includes phosgene (carbonyl chloride, used in polyurethane and polycarbonate manufacture, also a World War I-era choking agent), hydrogen cyanide (used in nylon and acrylonitrile production, also a blood agent), triethanolamine (potential precursor), and chloropicrin (used as a soil fumigant and a chlorinated nerve irritant in WWI). Schedule 3 chemicals face declaration and export-notification requirements.
Every nerve agent shares one mechanism: it phosphorylates the serine residue in the active site of acetylcholinesterase, locking the enzyme in an inactive state, and the resulting accumulation of acetylcholine at nerve junctions kills by convulsions and respiratory failure.
Nerve agents are organophosphorus (OP) compounds that inhibit acetylcholinesterase (AChE), the enzyme responsible for terminating nerve impulses at cholinergic synapses by hydrolysing acetylcholine to choline and acetic acid. All G-series and V-series agents, and all A-series (novichok) agents, share this mechanism, though they differ in volatility, skin-penetration, lethality, and chemical stability in environmental matrices.
Sarin (GB, isopropyl methylphosphonofluoridate, MW 140) is highly volatile (vapour pressure 2.1 mmHg at 20°C), moderately water-soluble, and relatively short-lived in the environment due to hydrolysis. It was synthesised by Gerhard Schrader at IG Farben in 1938 while researching insecticides. Its inhalation LCt50 (median lethal concentration-time product) is approximately 35 mg.min/m3 for humans. Sarin's degradation in aqueous environments produces isopropyl methylphosphonic acid (IMPA, a key biomarker), and further to methylphosphonic acid (MPA). Both IMPA and MPA are specific OPCW verification analytes. The 1995 Tokyo subway attack by Aum Shinrikyo (13 deaths, approximately 5,500 injured, including some with permanent ocular damage) and the 2013 Ghouta attack (~1,400 deaths) are the two largest-casualty sarin incidents.
VX (O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate, MW 267) is a viscous liquid with very low volatility (vapour pressure 0.0007 mmHg at 20°C) and very high skin-penetration toxicity. It persists in the environment for weeks to months. Its primary route of entry is skin absorption, not inhalation. VX was developed by Ranaji Ghosh and VR Webb at ICI (UK) in the early 1950s and later independently by US and Soviet programmes. The 2017 assassination of Kim Jong-nam at Kuala Lumpur International Airport involved the application of VX by two female operatives, each carrying one precursor (A: 4-amino-2,6-dimethylpyrimidine derivative in acetonitrile; B: sarin/VX precursor mixture), mixed on the victim's face. Malaysian forensic laboratories and the OPCW confirmed VX in blood, eye, and face-swab samples.
The A-series (novichok) agents are a class of OP compounds developed in the Soviet Union's Foliant programme (1970s-1990s) and first publicly disclosed by chemist Vil Mirzayanov in his 1992 article in Moscow News and later in his 2008 book "State Secrets." The Foliant agents include A-230, A-232, and A-234, with some reported to be 5-10 times more toxic than VX. They differ structurally from classic G- and V-agents in using phosphoramidates or phosphonates with non-standard leaving groups. The 2018 Salisbury poisoning (Sergei and Yulia Skripal, DS Nick Bailey injured; Dawn Sturgess killed in a second exposure) was attributed by the OPCW and UK government to A-234 (Novichok). The OPCW Designated Laboratory at the DSTL Porton Down confirmed A-234 in blood and environmental samples. The 2020 Berlin poisoning of Alexei Navalny was likewise confirmed as A-234 by DSTL Porton Down, Charite Hospital Berlin, and two other OPCW laboratories.
Sulphur mustard was the chemical weapon of the First World War, and it was still being used against Kurdish civilians in Halabja in 1988. Understanding its chemistry is not historical. It is practical.
Vesicants (blister agents) cause blistering of mucous membranes, skin, and respiratory and gastrointestinal tracts. They are not immediately lethal at typical battlefield concentrations but cause severe, slow-healing wounds and long-term respiratory damage. Sulphur mustard (HD, bis(2-chloroethyl) sulphide, MW 159) is the prototype. It was first used in large quantities in July 1917 at Ypres, Belgium, where German forces deployed mustard in artillery shells against British and Canadian troops, causing more casualties than any other CWA in the war. In the Iran-Iraq War (1980-88), Iraq's military used sulphur mustard extensively against Iranian troops and against the Kurdish population of Halabja in March 1988, killing an estimated 5,000 civilians and injuring 10,000 more in a single attack. Sulphur mustard's mechanism is alkylation of nucleophilic sites (DNA guanine N7, protein cysteine and lysine) via an ethylene sulfonium ion intermediate, causing DNA cross-linking, protein denaturation, and cell death in the epidermis and mucous membranes.
Nitrogen mustards (HN-1, HN-2, HN-3: bis(2-chloroethyl) amine derivatives) are chemically similar but were developed for medical use (nitrogen mustard HN-2 is the basis of mechlorethamine, the first cancer chemotherapy drug). They are less persistent in the environment than sulphur mustard.
Lewisite (L, 2-chlorovinyldichloroarsine, MW 207) is an arsenic-containing vesicant with a rapid blistering action (unlike sulphur mustard, which has a latent period of hours). It was synthesised by W. Lee Lewis at Catholic University in 1918. British Lewisite antidote (dimercaprol, British anti-Lewisite, BAL) was developed for this specific agent and remains in use for arsenic and mercury poisoning. FTIR and GC-MS identify lewisite by its arsenic-chlorovinyl signature and its hydrolysis product 2-chlorovinylarsonous acid (CVAA).
Blood agents inhibit cellular respiration by binding to the iron in cytochrome c oxidase. Hydrogen cyanide (AC, HCN) forms a stable complex with ferric iron in the cytochrome, blocking the terminal electron transfer step of oxidative phosphorylation. It was used in WWI gas attacks (though its volatility made it impractical in field concentrations) and in World War II concentration camp gas chambers (as Zyklon B, adsorbed HCN). Cyanogen chloride (CK, ClCN) is both a blood agent and a lung irritant. Treatment is hydroxocobalamin (Cyanokit) or sodium thiosulphate plus amyl nitrite.
Choking agents act on the respiratory system, causing pulmonary oedema. Phosgene (CG, carbonyl chloride, COCl2) was responsible for approximately 80-85% of chemical-weapon deaths in the First World War, primarily because its delayed pulmonary oedema (onset 4-24 hours post-exposure) caused soldiers to walk away from exposure and collapse hours later. Chlorine (Cl2) was the first large-scale CWA, released as a cloud from 5,700 cylinders at Ypres in April 1915. In the Syrian Civil War, chlorine has been used in improvised barrel-bomb attacks; the 2018 Douma attack, which killed at least 43 people, involved chlorine delivered in cylinders. The OPCW Fact-Finding Mission (FFM) and subsequent Investigation and Identification Team (IIT) investigation confirmed chlorine use at Douma and attributed it to the Syrian Arab Air Force.
An OPCW Designated Laboratory finding is the gold standard of CWA analytical confirmation, not because the chemistry is unique but because the independence, proficiency testing, and chain-of-custody architecture surrounding it are unique.
The OPCW Technical Secretariat maintains a list of Designated Laboratories: national forensic science institutions and defence research establishments that have been certified to receive, analyse, and report on OPCW-collected samples under the conditions specified in the Verification Annex of the CWC. Designation requires demonstrating capability across the standard OPCW analytical suite (GC-MS, LC-MS/MS, NMR) and passing biennial blind proficiency tests (PT rounds) with blinded matrices.
As of 2024, OPCW Designated Laboratories include:
The OPCW analytical workflow for a collected sample is standardised. Environmental samples (soil, water, wipes) and biomedical samples (blood, urine, tissue) arrive at the Designated Laboratory in hermetically sealed, serially numbered containers with an unbroken OPCW chain-of-custody seal. The laboratory documents the seal integrity, opens the outer container in a certified chemical agent laboratory, and registers the samples against the OPCW mission reference.
The analytical sequence is: GC-MS in electron ionisation (EI) and chemical ionisation (CI) modes to identify intact agents and volatile degradation products; LC-MS/MS to identify polar, non-volatile degradation products (alkyl methylphosphonic acids, thiodiglycol-sulphoxide from sulphur mustard); 31P NMR (phosphorus NMR) to confirm the presence of phosphorus-containing analytes consistent with nerve agents (diagnostic: chemical shift and coupling pattern); and immunoassay for biomedical sample pre-screening. Results are entered into the OPCW Central Analytical Database (OCAD), which contains confirmed spectra of agents and degradation products for all scheduled chemicals.
The biennial OPCW proficiency test is the only quality gate that certifies a laboratory as actually capable of what it claims to be capable of, using real-matrix blind samples rather than performance on paper.
The OPCW Technical Secretariat runs biennial proficiency test (PT) rounds for Designated Laboratories. Each PT round consists of a set of coded samples sent to all participating laboratories. The samples contain one or more agents or degradation products at realistic concentrations in challenging matrices (soil, water, simulated urine, blood plasma) and may include decoy compounds and interferents. Laboratories analyse the samples blind and report all detected chemicals to the Secretariat, which scores the results against the known content.
A Designated Laboratory must achieve satisfactory performance in consecutive PT rounds to maintain designation. Consistent failures, or failures at specific analyte classes, trigger a review of the laboratory's methods and equipment. The OPCW PT is considered the gold standard of external proficiency testing for CWA laboratories, more stringent than commercial PT schemes, because it uses actual agents at realistic concentrations in real-matrix blind samples.
India's DRDO DRDE (Defence Research and Development Establishment) in Gwalior, Madhya Pradesh, is the only OPCW Designated Laboratory in South Asia and one of very few in the Asia-Pacific region. DRDE was founded in 1962 under DRDO and undertakes research on protective equipment, medical countermeasures, and detection technologies for chemical, biological, radiological, and nuclear (CBRN) threats alongside its OPCW analysis function. Its designation reflects India's position as a CWC state party since 29 April 1997 (entry into force date) with declared possession of chemical weapons (India was one of a small number of states parties to declare and subsequently destroy a legacy chemical weapons stockpile under OPCW verification, completing destruction by 2009).
At the national level, CWA analysis capability in other jurisdictions is similarly concentrated in defence-adjacent institutions: DSTL Porton Down (UK) began as the Chemical Defence Experimental Station in 1916 and has been the central UK CWA analysis laboratory throughout the CWC era. Edgewood Chemical Biological Centre (US Army) traces its history to the US Chemical Warfare Service established in 1917. FOI in Sweden has been a neutral-country designated laboratory since CWC entry into force and played a pivotal role in analysing samples from multiple Syrian conflict incidents. The Netherlands Forensic Institute (NFI), by contrast, is a civilian forensic science institution, representing the crossover between CWA Designated Laboratory capabilities and general forensic practice.
A forensic chemist receives a soil sample collected by an OPCW inspection team from a crater site in a conflict zone. LC-MS/MS analysis confirms the presence of isopropyl methylphosphonic acid (IMPA) and methylphosphonic acid (MPA), and GC-MS in EI mode confirms isopropyl methylphosphonofluoridate. What conclusion is supported by these findings?
| A-series (novichok) |
| A-234 (Novichok) |
| Skin or inhalation |
| Estimated 5-10x more toxic than VX |
| AChE inhibition; phosphoramidate mechanism |
| A-234 and phosphoramidate hydrolysis products |
| Vesicant (sulphur mustard) | HD (sulphur mustard) | Skin and inhalation | ~1,500 mg.min/m3 (inhalation, severe effect) | DNA alkylation; ethylene sulfonium intermediate | Thiodiglycol-sulphoxide, bis(2-hydroxyethylthioethyl) sulphide in urine |
| Blood agent | HCN (hydrogen cyanide) | Inhalation | ~100 mg.min/m3 (inhalation) | Cytochrome c oxidase Fe(III) binding; oxidative phosphorylation blockade | Thiocyanate, cyanohaemoglobin in blood |
| Choking agent | Phosgene (CG) | Inhalation | ~3,200 mg.min/m3 (inhalation) | Acylation of lung alveolar proteins; delayed pulmonary oedema | Trichloromethanol degradation products; protein carbamylation adducts |