HOW do homicide detectives know how long a person has been dead? In the first 12 to 24 hours after death, visible changes in the body give an accurate enough estimate. But when corpses are days or weeks old it becomes more difficult because the complex chemistry of decay takes over and no one understands very much about it.

But now chemists, anthropologists and other scientists in Tennessee are working on a project to develop a device they could "wave over a body" to detect how long the person has been dead. Arpad Vass of the Oak Ridge National Laboratory, the lead investigator on the lab's "time since death" programme, says the first step is to create a look-up table of the chemicals made during a body's decomposition. You would then compare entries in the table with the output of an electronic nose—a device that detects and identifies vapours.

A ...