The Forgotten Evidence
McGinn, Terrence, Law & Order
The analysis of gunshot residue (GSR) utilizing Scanning Electron Microscopy interfaced with energy dispersive x-- ray (SEM/EDX) has undergone extensive research and development since its inception into the forensic world. At present it appears to be the most reliable technique for detection and identification of GSR. This is due to its high degree of sensitivity and specificity.
The American Society of Testing Materials defines GSR as that residue formed by the condensation of the vaporized bullet and primer materials that segregate into metallic and compound particles. In order to qualify as GSR, the elemental content of these particles must be: lead, antimony and barium; or antimony and barium. The morphology of each particle must also be considered. These metallic particles must be spheroidal in appearance and range in diameter from 0.5 microns to 50 microns-a micron is a millionth of a meter.
When a weapon is discharged, the components of a primer undergo an intense exothermic reaction: there is a rapid increase in temperature followed by a sharp decrease. This super-- cooling phenomenon leads to a unique spheroidal formation that is generally not observed in the natural environment. In other words, these metals could be detected on the hands of an individual who did not recently discharge a firearm. However, these particles would not be spheroidal and the concentration of these particles would be extremely small compared to that found in GSR.
The Scanning Electron Microscope (SEM) allows observation of each particle for these morphological characteristics. These conditions must be met to satisfy the elements of the definition and to conclusively report the presence of GSR.
The SEM is a complex but straightforward analytical instrument. The instrument utilizes a focused beam of electrons upon the specimen. Simultaneously, several electron probe-- induced signals are produced from the specimen. These signals generate images of the specimen surface. Material composition is also determined by an x-ray. There are many parameters within the instrument's controls that can be regulated. They are varied based upon the sample specimen and the data being sought in the analysis.
The major primer components in a center-fire cartridge are antimony sulfide, barium nitrate and lead styphnate. These compounds, as well as their relative amounts, will vary based upon brand and caliber. Thus, the analyses of each will yield different results, both qualitatively and quantitatively. In addition, numerous supporting metallic particles- zinc, copper or nickel- can and will most likely be observed. The interpretation of all the data is significant when rendering the results. With regard to law enforcement, the qualitative analysis (whether GSR is present) is of greatest importance.
There are several field or color tests available to law enforcement for the detection of possible GSR. Color tests are presumptive tests that are indicative of a certain component or components of the residue produced from the discharge of a firearm. The first is the paraffin test for nitrates, a common byproduct of the discharge of a firearm. However, there are numerous everyday chemical compounds that contain nitrates. Some of these nitrate-containing compounds are fertilizer, urine, certain pharmaceuticals, matches and explosives.
A second test is the modified Griess test, which reacts with nitrites, another byproduct of a firearm discharge. Nitrites are used extensively in the manufacture of dyes, photographic chemistry and as preservatives in meats. Another procedure is the sodium rhodizonate test, which is presumptive for lead and barium, both of which can be found frequently in the environment. Lead is found in numerous construction materials such as piping, solder, aerosols, batteries and pigments. Barium is found in road flares, paints and ceramics.
The problem with these color tests is that they cannot fulfill the criteria to meet the GSR definition. …