What do the chemical signatures of deadly nerve agents tell us about their origins?

From Guardian Science Weekly podcast: Ian Sample talks to two fellow Guardian reporters and a professor of environmental toxicology about the Salisbury spy poisoning. Last week, the city of Salisbury was thrust into the spotlight when two people were found in critical condition in a local park. Details began to emerge that the man, in his late sixties, was a former Russian spy. The woman found in a comatose state beside him, his daughter. Speculation mounted that they were poisoned, but by what? And by whom?