One of the current fields of research is the study of GSR persistence on the shooter and the possibility of hypothesizing the time window after the shooting and sampling [
3]. The study appears very complex due to the influence of many factors on the GSR’s postdischarge fate such as the skin conditions and activities of the shooter, the behavior of the shooter at the crime scene, the type of firearm and ammunition used, the environmental conditions, the transfer processes, etc. Different analytical techniques were applied to investigate the behavior of GSR: SEM/EDS [
3,
4,
5], laser-induced breakdown spectroscopy (LBS) [
6], graphite furnace atomic absorption spectrometry [
7], and ICP-MS coupled with laser ablation [
8]. The detection time interval was influenced by the sensitivity of the instrumental technique and was found to range from 6 h for nasal samples and ICP-MS detection [
8] to 5 days for hand samples and LBS detection [
6].
In criminal investigations, scanning electron microscopy/energy-dispersive X-ray spectrometry (SEM/EDS) presents a “gold standard” for testing a sample for inorganic GSR. SEM/EDS provides two-dimensional data for the identification of a particle as having originated from the discharge of a firearm. According to ASTM E1588, GSR particles possess a spheroidal/noncrystalline morphology, identified by SEM, and an appropriate elemental composition, identified by EDS. Spherical particles containing lead (Pb), barium (Ba), and antimony (Sb) are considered to be characteristic of inorganic GSR [
9]. The applied SEM/EDS protocol and quality control procedures, after method validation, could influence the obtained data [
10,
11]. The working parameters of an SEM/EDS instrument should be studied and optimized for each instrument’s and laboratory’s needs [
12,
13]. Differences in laboratories across countries and different sample types can cause significant variability in results and should be taken into account when interpreting shooting events [
4,
14]. In a recently published review, Blakey et al. discussed the findings on the distribution, deposition, transfer, and persistence of GSR [
3]. Although it has already been studied in depth [
3,
4,
15,
16], the possibilities of establishing or, at least, supposing the elapsed time between firearm discharge by a suspect and GSR sampling requires a specific study for each particular country, criminal case, and firearm. The persistence of GSR on a shooter’s hands, nose, hair, or clothing is strongly influenced by the person’s activity, the substrate being tested, the skin conditions, the firearm and ammunition type, the number of shots, and the outdoor or indoor conditions as well as the collection and analysis techniques [
3,
17]. The total number of detected particles has been reported to vary significantly, even within the ammunition itself [
18]. The GSR collected from the face, hair, and clothing of a shooter showed a longer half-life than the samples taken from the person’s hands [
18]. A decrease in the amount of characteristic GSR collected from a shooter’s hands one hour postdischarge was well-described [
19]. However, secondary transfer or the shooter’s behavior at the crime scene greatly affected the obtained results [
4,
16,
20]. Generally, the characteristic GSR particles may still be detected by SEM/EDS on a shooter’s hands between 4 and 10 h after firearm discharge and normal office activity [
3]. Several forensic protocols recommend GSR sampling, depending on the time following the criminal event. When GSR sampling is performed later than 4 h after a shooting event, samples collected from a suspect’s face, hair, and clothing should be included in the evidential material along with hand samples due to the loss of GSR particles [
18]. The comparison of published data is a complicated task, and enlarging the database of available results may benefit forensic scientists from each country.
In the Republic of Kosovo, the ASTM 1588 classification is applied for characteristic inorganic GSR particles investigated by SEM/EDS, and it is accepted in court as evidence. Our recently published study demonstrated the results from SEM/EDS optimization and method validation by the Kosovo Forensic Agency [
12]. The uncertainty of the number of particles found by the optimized SEM/EDS method is estimated to be 6%, and it should be taken into account in the conclusions based on the number of detected particles. In Kosovo, the GSR testing of a suspect is currently performed regardless of the time after a shooting event. It is important to study the appropriate time interval between firearm discharge and GSR sampling, taking into account the specificity of normal office activity in the Republic of Kosovo as well as the specificity of the firearms used by criminals in this region.
This study is aimed at exploring the number of characteristic GSR particles identified by SEM/EDS as a function of time, gun caliber, number of shots, and weather conditions. The obtained data would allow, on one hand, a broadening of the information extracted from SEM-EDS measurements and, on the other hand, the establishment of the time interval after shooting that is appropriate for GSR sampling. Different firearms typically used in criminal cases in Kosovo were studied, and the experiments were conducted outdoors in the summer and winter seasons to determine the influence of weather conditions. Nine people participated in the experiments in order to determine the influence of specific human skin conditions on the results, and the participating shooters followed a common office routine. The obtained results showed that GSR particles could be identified by SEM/EDS five to seven hours after a shooting event, depending on the weapon used and the number of produced shots. The results may benefit forensic scientists by providing a supporting tool for hypothesizing the time interval between firearm discharge and GSR sampling. In the case of Kosovo forensic practice, the presented results could lead to an update of the sampling protocol regarding the appropriate time interval for GSR sampling for SEM/EDS analysis.