Day Three: Three elements must be fused together to be gunshot residue

Lead, barium, and antimony are the three things looked for when testing for gunshot residue, Foster says.

She says the residue is left near the hand of someone firing a gun. She is then shown a cartridge of ammunition, and asked if she can point out what a "primer cap" is.

When shown on an overhead projector, which all the court can see, she is shown a piece of ammunition, and points out the "primer cap" in the center of the bullet. It is the materials IN this primer cap which cause the residue when fired.

Elana Foster tells the prosecution that the materials are found in ammunition, lead-based paint, fireworks, and sometimes in a mechanic-type environment. She explains that gunshot residue particles they look for the three materials together. All three need to be fused together in a single particle for it to be able to say it came from a discharged firearm.

Scanning electron microscope (SEM), Foster says, is the tool used to find such residue, and she compares it to plugging your video camera into a TV so that you can see what you point the camera at. When they do the analysis they are looking at a monitor at the materials.

Foster says the lowest amount of residue, the sensitivty level of the SEM can see particles not even the width of a hair.