Primer Residue
Primer residues may also land on the hands of a shooter. The residue is mostly microscopic blobs of the molten metals from the primer cap, the primer compound, the casing, and other metallic components (containing copper (Cu), zinc (Zn), nickel (Ni), aluminum (Al), among others). The major primer elements are lead (Pb), barium (Ba), and antimony (Sb); typically, all three are found in GSR. The minor elements include aluminum (Al), calcium (Ca), chlorine (Cl), potassium (K), silicon (Si), sulfur (S), and tin (Sn). As the blobs ly through the air, they condense into heterogeneous spheres of various sizes (submicron up to 50 or more microns, with most in the 2- to 10-µm range). Several tests have been developed over the years, including the dermal nitrate test (which also tested positive for fertilizers) and atomic absorption (AA). Still used in some laboratories, AA suffers from a number of limitations, including collection problems (swabbing the suspect’s hands with a mild acid solution), a lengthy analysis time, and, most importantly, lack of specificity. The result of an AA analysis yields quantities of the elements tested for but not their distribution. Because GSR particles are aggregates of compounds, no information about the form of the sample is known with AA. Fireworks, matches, and other common objects could yield a positive result by AA. In 1976, Nesbitt, Wessell, and Jones published a method for detecting GSR particles using a scanning electron microscope (SEM) outfitted with an energy dispersive spectrometer (EDS) and imaging system. The particles were collected from the hands of a suspected shooter and placed on a carbon-coated aluminum mount; then they were viewed in the SEM with a detection mode that relates brightness to atomic number. The bright particles on a dark background of car bon were those that were most likely GSR (Figure 1). The EDS detector would then analyze the individual spheres to detect the presence of antimony, barium, and lead—the three main components found to be in GSR but not other high-atomic number particulate matter. The shape and elemental content of the particles denied them as GSR. It is important to note that the primers in 0.22 ammunition differ from other primers and may not be detected by a system screening for PbSbBa particles. This method gained greater acceptance when the technology allowed for the detection and analysis of the method to be automated for unattended operation. Mul tiple samples could be loaded into the SEM, the software calibrated, and then it could be left on its own to run samples overnight. The human operator still needed to verify any positive “hits” because other materials could produce elemental signatures that were overlapping with or confounded the signatures of true GSR particles, such as automotive brake pads.
FIGURE 1 Gunshot residue, more properly called “primer residue,” forms as molten blobs of metals from the discharge of the ammunition. These spheres deposit on surfaces within about 3 ′ of the discharged firearm, depending on the design of the firearms and environmental conditions, such as wind.
The only conclusion that can be drawn from a GSR test is that the subject discharged a firearm, was near a firearm when it discharged, or handled a recently discharged irearm. These factors, plus the potential for contamination during arrest from GSR-rich environments, like police officers’ hands, patrol car seats, and handcuffs, make for a limited application of GSR analysis. Many laboratories continue to offer GSR analyzes, and some perform a high volume of casework (>500 cases) annually.
