How to investigate chemical weapons allegations
There have been many accounts - and distressing video footage - suggesting chemical weapons have been used in Syria, most likely by government forces. Ralf Trapp, an international disarmament consultant who specialises in chemical and biological weapons, looks at how such evidence is collected and evaluated.
Investigations of the alleged use of chemical weapons involve a variety of techniques to collect and analyse different types of evidence.
This includes interviews with survivors of the alleged attack and other eyewitnesses, such as first-responders and medical staff who treated the victims.
Inspectors would want to undertake medical examinations of the victims, and take biomedical samples such as urine or blood, which can be analysed for characteristic biomarkers and traces of the agent or its degradation products (metabolites) even if the incident has taken place some time ago.
Inspectors would also want to acquire samples from dead people or dead animals.
They would inspect the site where the alleged attack has taken place to understand the context of the alleged incident, and to search for evidence such as remnants of shells or bombs that would still be contaminated.
They would collect environmental samples such as soil, rubble or vegetation that may have been in contact with the chemical warfare agent and may still contain characteristic degradation products or even traces of it.
Some of the initial chemical analysis would be done in a mobile field laboratory during the investigation.
Other samples would be shipped for off-site analysis under strict protocols to protect the samples and ensure an unbroken chain of custody.
For this off-site analysis, the Organisation for the Prohibition of Chemical Weapons (OPCW) - the implementing agency of the Chemical Weapons Convention - has established an international network of highly competent so-called "designated laboratories" that routinely undergo proficiency testing to ensure that their standard of analysis is up to the needs.
Two or three of them would be selected to analyse the samples, together with a set of controls, which would be prepared by the OPCW Central Laboratory, to ensure the quality and dependability of the results.
In situations such as the ongoing conflict in Syria, reliance on broadcast witness statements, videos and photographic images and even the analysis of samples brought out of the country will always be somewhat problematic as there is no independent way to verify the information.
While video footage may be compelling and show symptoms that may be consistent with certain agents allegedly used, such evidence often also leaves many questions unanswered.
Also, the analysis of samples is a powerful tool to identify any agent that may be present, even if in small concentrations, or of characteristic degradation products that would demonstrate its past presence.
But the results have to be evaluated in the context of whether the authenticity of the sample has been established and documented, and whether the sample has been protected from interference through a rigid protocol to ensure an unbroken chain of custody.