How gunshot residue testing works

The Missouri State Highway Patrol crime lab, which performed the analysis on the evidence collected from Lisa Barlow's hands, typically analyzes about 20 to 30 gunshot residue tests each month.

The lab analyzes these cases using a scanning electron microscope with an energy dispersive X-ray, capable of detecting particles 1/10 the width of a human hair, and what's commonly referred to as a GSR kit.

The kit consists of sticky adhesives stubs about half an inch in diameter designed to catch residue off bare skin and preserve it for analysis in the lab.

The sooner police collect samples for a GSR test, the greater chance of accuracy, and the lesser risk of residue having been rubbed off or lost.

Because the residue is "easily transferrable like baking flour," the window of opportunity scientists recommend is usually about four to six hours after the subject would have handled the gun, said trace evidence criminalist Jenny Smith.

Slipping plastic bags over the hands to protect the evidence is also good practice, though Smith said she wasn't certain how widespread it was used among law enforcement agencies.

-- Bridget DiCosmo