GSR (gunshot residue) wipe tests are used by law enforcement to determine whether a suspect fired a gun or was close to someone who did. They can also be used to determine if an accused person touched a gun, a weapon or anything else that came into contact with a discharged firearm.
The presence of GSR on a person’s hands indicates that they have either: (i) fired a firearm; (ii) been in very close proximity to the shooting and brushed against a gun at the time of discharge; or (iii) come into direct physical contact with the person who fired the gun. For this reason, investigators swab people’s hands to look for the presence of these distinctive chemicals. If GSR is found on the hands, chemical testing (usually a modified Griess test and/or sodium rhodizonate) and scanning electron microscopy (SEM-EDX) can be conducted to confirm the findings.
Despite the importance of GSR analysis, there are many issues that may affect the validity of a results from this method. One issue is that over time, GSR can be lost from a person’s hand by washing or accidental or deliberate removal. This can have significant impact on a case.
Another problem is that GSR can easily transfer from an accused person’s hand to other surfaces – and then to the fingers of others. This can be a result of cross contamination during the handling and sampling process, or it may simply be due to the fact that GSR particles are so small that they can attach to almost anything.