Forensic experts will be looking for traces of nerve agent on the clothes of Sergei Skripal and his daughter, Yulia, and in the area where the two were attacked, in an effort to track down where the deadly substance was made, researchers say.

If small amounts of pure agent can be recovered, scientists at Britain’s military research lab at Porton Down will analyse them for telltale impurities that can potentially reveal which process was used to make the agent, and hint at where it was produced.

Nerve agents are made in several steps, and each one creates byproducts that are carried over to the next stage of the reaction, meaning that the final agent usually carries a chemical signature of how it was made, and sometimes the equipment used in manufacture.

“Porton Down will certainly be trying to look at environmental samples which will provide the best sort of clue,” said Alastair Hay, an environmental toxicologist at Leeds University who investigated the use of chemical weapons against Iraqi Kurds in Halabja in 1988.

“They’ll be looking for impurities and inadvertent reaction products and those would give some indication as to the possible method of manufacture.”

Quick Guide How hard is it to make a nerve agent? Show Nerve agents are not hard to make in principle, but in practice it takes specialised facilities and training to mix the substances safely. The raw materials themselves are inexpensive and generally not hard to obtain, but the lethality of the agents means they tend to be manufactured in dedicated labs. The main five nerve agents are tabun, which is the easiest to make, sarin, soman, GF and VX. The latter was used to kill Kim Jong-nam, the half-brother of the North Korean leader, Kim Jong-un, at Kuala Lumpur airport last year. VX is particularly stable and can remain on clothing, furniture and the ground for a long time without proper decontamination. All pure nerve agents are colourless organophosphorus liquids which, after they were discovered to be highly poisonous in the 1930s, became the dominant chemical weapons of the second world war. Once made, the substances are easy to disperse, highly toxic, and have rapid effects. Most are absorbed swiftly through the skin or inhaled, but they can also be added to food and drink. The agents take their toll on the body by disrupting electrical signals throughout the nervous system and the effects are fast and dramatic. Victims find it increasingly hard to breathe. Their lungs produce more mucus which can make them cough and foam at the mouth. They sweat, their pupils constrict, and their eyes run. The effects on the digestive system trigger vomiting. Meanwhile the muscles convulse. Many of those affected will wet themselves and lose control of their bowels. At high doses, failure of the nerves and muscles of the respiratory system can kill before other symptoms have time to develop. There are antidotes for nerve agents, such as oxime and atropine, which are particularly effective against VX and sarin, but they should be given soon after exposure to be effective.

But the procedure, known as impurity profiling, is far from straightforward. For the analysis to work, scientists need samples of pure nerve agent collected from clothes or the crime scene. Any contamination, for example from soil, saliva, food or drink, may well frustrate any attempt to read the impurity signature. While blood and urine samples can help identify the nerve agent itself, the forensic signature of the impurities is lost once the agent passes through the body.

“To do impurity profiling you need the pure agent and not something that’s been contaminated,” a chemical weapons expert told the Guardian. “You might take it from the clothing, but dirt or even exposure to the atmosphere can mean a lot of the signature is lost.”

If scientists can collect enough of the pure nerve agent to analyse for impurities, they may be able to work out which chemical route was used to synthesise the agent. And that could be vital. “I’m sure that the scientists at Porton Down will have a reasonable idea of the different routes to manufacture, and who might be using which process and this will obviously help,” said Hay. If they have built up a library of nerve agents made by labs around the world, they can search it for an impurity signature that matches the material used in Salisbury.

Another avenue for tracing the nerve agent to its source draws on a forensic method used to track the origins of illegal drugs. To work out whether heroin comes from Afghanistan or South America, scientists have analysed the drug for slightly different isotopes, or atomic forms, of hydrogen. The same approach can, in principle, be used to trace the origins of nerve agents, because hydrogen in different parts of the world have subtly different ratios of isotopes.

But chemical forensics may only reveal so much. “They will probably try to do it, every little piece helps, but I would bet on more traditional methods of tracing the source,” the chemical weapons expert said. “I’d say it’s difficult, if not downright impossible to do.”