The use of chemical weapons on 4th April in Syria, which prompted the American bombing of an airfield, was blamed on Syrian government forces by the US and UK.
UK ambassador to the UN, Matthew Rycroft, has stated scientists at Porton Down ‘have analysed samples obtained from Khan Shaykhun’, the site of the incident, and ‘these have tested positive for the nerve agent Sarin, or a Sarin like substance.’
This prompts a simple question. Given that the Russians, amongst others, are disputing the UK/US assessment, why not present them with the evidence as revealed in analysis?
The Russians may not take our government’s word for it, but I think they would trust Porton Down scientists. Scientists in Russia and UK (and USA as well) tend to trust each other, I believe, and confer too.
It is worth keeping this background in mind. Although not mentioned much in our press, tests showed that the 2013 chemical attacks could not credibly be blamed on Assad’s forces. President Obama had been all set to bomb Syria when, at the 11th hour, General Dempsey, Chair of the Joint Chiefs of Staff, who knew of the true state of intelligence, had a word with him. Obama called off the bombing, apparently mindful that congress would be apprised of the intelligence.
The Americans and Russians and Syrians went on to defuse the situation by agreeing on the destruction of Syria’s stockpile of chemical weapons. So even if those stocks had ever been used in the past – which Syria denies – they could not be used in the future. Still, just to be sure, the exact molecular profile of the chemicals was ascertained and recorded at the time.
So, about the present controversy: if there is evidence that Assad was behind the recent Khan Shaykhun incident, then Porton Down scientists have it.
At least, they will, if they have indeed analysed soil samples gathered by rescue workers from the scene of the 4th April attack, as claimed. For the scientists can obtain the complete chemical profile of a chemical agent, including impurities that are present at very low concentrations, when the sample comes from the soil. They can determine how the sarin was synthesized and whether the sample matches the ‘kitchen sarin’ used by opposition fighters in 2013 or the military-grade sarin from Syria’s former stockpile.
The strange thing, however, is that the UK’s ambassador Rycroft has spoken of the sample containing a ‘sarin-like substance’. Analysis of a sample molecule identifies it by its ‘signature’ or else just does not identify it. If it’s not identified, you can’t say anything about what kind of molecule it is.
Ambassador Rycroft’s statement is therefore confusing: its uncertain reference to ‘sarin or sarin-like substance’ is incompatible with a claim that the chemical was Syria’s military grade sarin. Either Porton Down scientists showed the sample to be military grade Sarin, in which case Assad would be strongly implicated, or it failed to, in which case he would be in the clear.
If UK/US authorities have definite evidence implicating Assad, they have no reason not to share it (and, one might have thought, quite a strong incentive to do so). As long as they fail to, the reasons for doubt will remain.
A special word of thanks goes to Paul McKeigue, Professor of Genetic Epidemiology and Statistical Genetics at the University of Edinburgh, for his advice on the science and its application to the question at hand.
 In the words of Ambassador Rycroft, Assad’s responsibility is ‘highly likely’. (This is the same form of words used by the Joint Intelligence Committee in its 2013 briefing to Prime Minister Cameron before the emergency debate on support for Obama’s proposed bombing of Syria. Then, as now, hearers of the words were asked to take on trust that there was more compelling – and yet still not definitive – evidence than they were being shown. Having the opportunity to debate the matter at length, the MPs voted against.)
 The most readable, now classic, source on the chemical evidence of 2013 is Seymour Hersh’s ‘The Red Line and the Rat Line’, London Review of Books, 17 April 2014 https://www.lrb.co.uk/v36/n08/seymour-m-hersh/the-red-line-and-the-rat-line. Seekers of further or more comprehensive information and analysis are referred to the extensive wiki resource of A Closer Look On Syria http://acloserlookonsyria.shoutwiki.com/wiki/Main_Page
 The bombing had looked so certain to go ahead that the UK parliament was recalled from recess to debate it. http://www.telegraph.co.uk/news/worldnews/middleeast/syria/10277522/Syria-vote-crisis-timeline.html
 Obama’s surprise reversal of decision followed consultation with his ‘most senior military advisor’ https://www.theguardian.com/world/2013/aug/31/syrian-air-strikes-obama-congress
 For how a lab in the Netherlands showed in 2012 that this type of analysis was feasible see Report of the Temporary Working Group on Sampling and Analysis to the Nineteenth Session of the OPCW Scientific Advisory Board. Annex 2, Agenda Item 8 [Internet]. 2012 Sep. Available from: https://www.opcw.org/fileadmin/OPCW/SAB/en/sab-19-01_e_.pdf
 I am informed that this is a combination of retention time in the gas chromatography step and mass/charge ratio in the mass spectrometry step.
 Unless it turns out that there was some ‘misunderstanding’ about how the sample was gathered or the testing done. I mention this possibility because while soil samples can yield definite analysis of the chemical used, samples from blood or tissue (physiological samples) cannot. In 2013, the early use of soil samples (as furnished General Dempsey with the understanding referred to) was in subsequent cases ‘inexplicably’ supplanted by the less useful method of analyzing physiological samples by the US. (Perhaps some institutional learning about this was lost in the transition between presidential teams and perhaps Rycroft was not briefed on the significance of the point. But that, of course, is pure speculation.)