Assessment of Persistence of Gunshot Residues Produced by Firearms from Criminal Cases in the Republic of Kosovo

Abstract

Gunshot residue (GSR) is a material formed during firearm discharge with a specific spheroidal/noncrystalline morphology and chemical composition. The examination of gunshot residue by SEM/EDS is an important tool in forensic studies and presents supporting evidence in criminal investigations. This study is aimed at exploring the number of particles characteristic of GSR identified by SEM/EDS as a function of time, gun caliber, the number of shots, and weather conditions. Firearms typically used in criminal cases in Kosovo were studied, and the experiments were conducted outdoors in the summer and winter seasons. Nine people made different numbers of shots from one to nine and followed a common office routine. An optimized and validated SEM/EDS protocol was applied with a sensitivity of 95%, a bias of −5%, a repeatability of 2% (RSD), a within-lab reproducibility of 2% (RSD), and an expanded uncertainty of the number of GSR particles of 6% at coverage factor k = 2. The results showed that GSR particles could be identified by SEM/EDS five to seven hours after shooting, depending on the weapon used and number of produced shots. The results will benefit forensic scientists by providing a supportive tool for hypothesizing the time interval between firearm discharge and GSR sampling.

1. Introduction

Gunshot residue (GSR) is a material that is formed during firearm discharge; it settles primarily on the shooter’s hands, face, and clothing but can be found in the air and on surrounding surfaces [1,2]. Its chemical composition reflects those of the primer, gunpowder, bullet, and cartridge case. The examination of gunshot residue (GSR) is an important tool in forensic studies as supporting evidence in criminal investigations [3,4].

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.

2. Materials and Methods

2.1. SEM-EDS Measurement

The collected samples were analyzed by a scanning electron microscope (Quanta 650, FEI, USA) coupled with an EDS (energy-dispersive X-ray spectrometer) with an SDD (silicon drift detector) (EDAX, Octane plus detector). The optimal experimental conditions of the SEM/EDS method presented in [12] were applied in this study: accelerating voltage 25 kV, working distance 10 mm, spot size 5, minimal size of particles 0.5 µm, area of scanned frame 8 mm², image resolution (scan speed 3 µs per pixel) 1536 × 1024 pixels. Secondary electron and backscattered electron images were observed. The identification of GSR was performed in an automatic mode using the building software TEAM^TM Basic V4.4.1. The automatic scanning mode was used to study each sample. Each positive GSR sample was confirmed by manually scanning. X-ray signals were collected from 0 to 20 KeV at 20 eV per channel. The live time of spectra acquisition was 5.0 or 10 s., the detector resolution was set at (128 ± 15) eV, and the dead time was ≤30%. The SEM-EDS spectra were acquired in the energy range in which the SEM/EDS system detected chemical elements. A particle was identified as GSR if it possessed a spheroidal/noncrystalline morphology and contained Pb, Sb, and Ba [9].

2.2. Procedures

2.2.1. Number of GSR Particles as a Function of Time

Nine people (experts from the Kosovo Ballistics Laboratory) participated in the test. The volunteers thoroughly washed their hands before firing; immediately after cleaning, the GSR was sampled to check for possible contamination, and the results were negative. Each person made one shot; the group used the same weapon. The sampling was performed from each shooter’s right hand once per person at different time intervals: 0, 1, 2, 3, 4, 5, 6, 7, and 8 h after the firearm discharge. After the shooting, each candidate went about their normal daily activities, without washing their hands. The outdoor experiments were repeated in one year during the winter and summer seasons. The series of experiments during different seasons were performed by randomly chosen people from the group of volunteers. Six series of experiments were carried out using six weapons of different calibers.

The weapons used in the experiments are listed in

Table 1. Types of firearms used in the test.

Group	Type of Firearm	Caliber
	Zastava M70AB2 rifle	7.62 × 39 mm
High caliber	Zastava M48 bolt-action rifle	7.92 × 57 mm
	RPD light machine gun	7.62 × 54 mm
	Walther PPK pistol	7.65 × 17 mm
Low caliber	Glock 19 pistol	9 × 19 mm
	Zastava M70 pistol	7.65 × 17 mm

. The chosen firearms were brought to the Kosovo Forensic Agency as a part of the forensic investigation of real criminal cases. The high- and low-caliber ammunition used in the tests were provided by the Ballistics Laboratory of the Kosovo Forensic Agency and were typically used for testing and research. Based on the data from the Ballistics Laboratory, the studied weapons and ammunitions were those the most frequently used in the criminal cases in the Republic of Kosovo for which the Kosovo Prosecution and Court required expertise. Additionally, weapons presented for legalization and registration in the Integrated Ballistics Identification System (IBIS) database were included in the tests. The low-caliber Glock 19 pistol (9 × 19 mm) used by the Kosovo Police was also studied.

2.2.2. Number of GSR Particles as a Function of Time and Number of Shots

Nine people participated in the test. The same precautions described above were applied to avoid contamination. Each person made a different number of shots, from 1 to 9, using the same weapon. Two series of experiments were carried out using two weapons of different calibers (

Table 2. Experimental plan for studying the number of GSR particles as a function of the number of shots and the postdischarge time.

Weapon		High Caliber: Zastava M70AB2 Rifle (7.62 × 39 mm)	Low Caliber: Glock 19 Pistol (9 × 19 mm)
Shooter	Number of Shots per Person	Sampling after Shooting, h	Sampling after Shooting, h
P1	1	0	0
P2	2	1	1
P3	3	2	2
P4	4	3	3
P5	5	4	4
P6	6	5	5
P7	7	6	6
P8	8	7	7
P9	9	8	8
atmospheric conditions		temperature 9 °C, humidity 84%, wind 1 km/h, precipitation 0%	temperature 6 °C, humidity 77%, wind 3 km/h, precipitation 2%

and

Table 3. Types of firearms used in the test.

High caliber	Zastava M70AB2 rifle (7.62 × 39 mm)
Low caliber	Glock 19 pistol (9 × 19 mm)

). The sampling was carried out at different time intervals after the shooting. The time interval increased with an increasing number of shots. The sampling from the hand of the person who produced one shot was performed right after the shooting. The sampling from the hand of a person who fired nine shots was performed after 8 h.

The weapons and ammunitions used in the test are presented in Table 3. The weapons were used in criminal cases and were presented for examination at the Kosovo Forensic Agency during cases under investigation. The studied ammunitions were used for tests and research by the Ballistic laboratory of the Kosovo Forensic Agency.

2.2.3. Sampling of GSR

Samples were collected from right hand using ready-made carbon stubs for GSR sampling (TED PELLA, Inc.; Redding, CA, USA). The stubs were ready to use and did not require carbon coating prior to SEM/EDS measurement. Fifteen dubbings by the stub to the hand surface were made for GSR sampling. The same procedure was applied for the contamination controls taken just after the volunteers’ hand cleaning and before shooting. All controls appeared to be GSR-negative by SEM/EDS.

2.3. Evaluation of Analytical Characteristics of the SEM/EDS Method

The performance characteristics of the SEM/EDS method were studied by measurements of the ENSFI GSR PT test sample coded SPS-5P-2A (GSR 2005 PT edition). The precision and trueness were evaluated.

2.3.1. Precision

Precision was evaluated in conditions of repeatability and intermediate reproducibility [21].

The repeatability conditions included the same measurement procedure, two operators, the same measuring system, the same operating conditions, the same location, and four replicate manual scans of the same test sample made in one day. The repeatability was assessed by the relative standard deviation (RSD, %):

$$\mathrm{RSD}=\frac{\mathrm{standard}\mathrm{deviation}\mathrm{of}\mathrm{the}\mathrm{number}\mathrm{of}\mathrm{GSR}\mathrm{from}4\mathrm{runs}}{\mathrm{mean}\mathrm{number}\mathrm{of}\mathrm{detected}\mathrm{GSR}}\times 100$$

The intermediate (within-lab) reproducibility conditions included the conditions of measurement, out of a set of the conditions mentioned above, over an extended period of time. The ENSFI GSR PT test sample was examined once per week as a control sample for one month. The same instrument was used, but various operators carried out the measurements. The RSD was calculated from the standard deviation of four runs and the average number of detected GSR particles for each group of particles as well as for total number of GSR particles.

2.3.2. Trueness

Trueness was estimated by the sensitivity and bias of the method.

The sensitivity presented the method’s ability to identify positive results. A positive result was a correctly detected and classified PbSbBa particle. The sensitivity parameter was calculated by the equation:

$$\mathrm{sensitivity}=\frac{\mathrm{TP}}{\left(\mathrm{TP}+\mathrm{FN}\right)}\times 100$$

where TP was the number of true positive results and FN was the number of false negative results.

The bias was defined as the difference between the number of GSR particles obtained in the laboratory and the assigned value of PbSbBa particles on the standard test stub. The bias parameter was calculated by:

$$\mathrm{bias}=\frac{\mathrm{Number}\mathrm{of}\mathrm{detected}\mathrm{GSR}-\mathrm{Assigned}\mathrm{number}\mathrm{of}\mathrm{GSR}}{\mathrm{Assigned}\mathrm{number}\mathrm{of}\mathrm{GSR}}\times 100$$

The number of detected GSR particles was calculated as an average number from four measurements of the ENSFI GSR PT test sample.

3. Results

3.1. Analytical Characteristics of the Optimized SEM-EDS Method

The SEM/EDS measurement protocol was optimized by varying the acceleration voltage, working distance, and spot size to achieve the optimal amounts of the EDS detector counts, contrast, and brightness in the SEM image [12]. The outputs of the testing of GSR produced by discharging the studied weapons by the optimized SEM/EDS method are presented on Figure 1 and Figure 2. As can be seen from the presented spectra, the spheroidal particles identified by SEM contained Pb, Sb, and Ba at the same time. Additionally, potassium, tin, and aluminum (Figure 1) as well as potassium, iron, and sulfur (Figure 2) were identified. The elemental profiles of the studied particles corresponded to the classification of the particles characteristic of GSR presented in ASTM 1588 [9].

The number of PbBaSb particles with a diameter of 0.5 µm on the surface of the microscopic stub was used as a measurand in the validation study. The method characteristics in the studied range of 0–100 PbSbBa particles are presented in

Table 4. SEM/EDS method performance among the groups of PbSbBa GSR particles in the ENSFI GSR PT test sample, coded SPS-5P-2A (GSR 2005 PT edition).

Diameter of GSR Particles, µm	Number of GSR Particles		Repeatability, RSD% (n = 4)	Within-Lab Reproducibility, RSD% (n = 4)	Sensitivity, %	Bias, %
	Assigned	Detected
10	3	3	0	0	100	0
2.4	24	24	0	0	100	0
1.2	32	30	3	3	92	−6
0.8	30	29	3	3	96	−4
0.5	14	12	8	8	88	−6
total (0.5–2.4 µm)	100	98	2	2	95	−5
demand [11]			<10	<10%	≥85	≤15

. The measurement protocol was demonstrated to satisfy the requirements of the ENSF guidelines for the studied groups of GSR particles [9]: the precision (as RSD) in repeatability and within-laboratory reproducibility conditions was less than 10%, the sensitivity was higher than 85%, and the bias lower than 15%. The overall method performance was found to be “satisfactory”, with a z-score < 2. The expanded uncertainty of the total number of detected particles was 6% at a 95% confidence level [12].

3.2. Evaluation of the Relationship of the Number of Particles and the Sampling Time and the Caliber of the Ammunition

The number of characteristic GSR particles at different time intervals after an outdoor weapon discharge was studied. Normal daily office activity without hand washing was recommended to the people involved in the tests. Each person made one shot, and the group used the same weapon. The experiments were repeated during the winter and summer seasons by randomly chosen people from the group of volunteers to account for the effects of weather conditions and the particularities of human skin. The results are presented in

Table 5. Number of particles characteristic of GSR as a function of time after shooting with a high-caliber ammunition. Each person made one shot, followed by normal daily activity without hand washing until GSR sampling.

	Number of Particles Characteristic of GSR
Person’s number	P1	P2	P3	P4	P5	P6	P7	P8	P9
time after shooting, h	0	1	2	3	4	5	6	7	8
	Zastava M70AB2 rifle (7.62 × 39 mm)
winter 2021	53	35	28	23	15	8	3	0	0
summer 2021	36	18	9	7	4	0	1	0	0
	Zastava M48 bolt-action rifle (7.92 × 57 mm)
winter 2021	62	44	15	8	0	2	0	0	0
summer 2021	44	25	13	6	3	0	0	0	0
	RPD light machine gun (7.62 × 54 mm)
winter 2021	70	34	29	12	4	1	0	0	0
summer 2021	48	27	18	8	3	1	0	0	0
mean	52	31	19	11	5	2	1	0	0
SD	12	9	8	6	5	3	1	0	0

and

Table 6. Number of particles characteristic of GSR as a function of time after shooting with a low-caliber ammunition. Each person made one shot.

	Number of Particles Characteristic of GSR
Person’s number	P1	P2	P3	P4	P5	P6	P7	P8	P9
Time after shooting, h	0	1	2	3	4	5	6	7	8
	Walther PPK pistol (7.65 × 17 mm)
winter 2021	19	13	9	3	1	0	0	0	0
summer 2021	14	12	6	2	0	0	0	0	0
	Glock 19 pistol (9 × 19 mm)
winter 2021	12	10	8	1	2	0	0	0	0
summer 2021	8	4	2	0	1	0	0	0	0
	Zastava M70 pistol (7.65 × 17 mm)
winter 2021	14	8	6	3	2	0	0	0	0
summer 2021	11	4	3	5	1	0	0	0	0
mean	13	9	6	2	1	0	0	0	0
SD	4	4	3	2	1	0	0	0	0

as well as in Figure 3 and Figure 4.

The results showed that the GSR that originated from a high-caliber ammunition persisted on the shooter’s hands for up to 4–5 h. It can be seen that when the Zastava M70AB2 rifle (7.62 × 39 mm) was used the produced GSR particles could be found on the shooter’s hands, even after 6 h of normal daily office activity. Considering the number of GSR particles, we observed that during the summertime experiment around 30% lower numbers of GSR particles were detected in the samples taken right after the shooting event. The observed tendency was the same (30%) regardless of the type of high-caliber weapon; hence, the possible irregularities in the sampling procedure did not provoke significant deviations in the results. The lower amount of GSR during the summer experiment could be mainly due to the windy weather.

Table 5 and Figure 4 show the results from the study of GSR particles obtained by three types of low-caliber ammunition. The number of detected particles characteristic of GSR were 75% below the number of particles generated by high-caliber ammunition. The GSR particles could be found on the hand of a shooter who performed normal daily activities for up to 4 h after shooting, regardless of the season and the type of gun.

In both series of experiments with high- and low-caliber ammunition, the slope of the curves in the first 3 h was high, demonstrating that the main loss of particles occurred during the first 3 h after the shooting. A decay in the particle loss was observed between 3 and 6 h, depending on the calibers and types of firearms used in this experiment. Five hours after the firearm discharge, particles characteristic of GSR were found in one of six series of experiments. Therefore, five hours could be proposed as a threshold for the time interval if only one shot is to be assumed.

The mean number of characteristic GSR particles calculated from six series of experiments as a function of the time interval between shooting and sampling is presented in Figure 5. An average of two calibers of firearms was calculated, with three firearms in each group. All experiments were repeated during two seasons: summer and winter. Based on the mean results, it was found that the polynomial model can predict the GSR fate with R² > 0.99. The equations are presented in the Figure 5 inset.

3.3. Evaluation of the Number of Characteristic GSR Particles as a Function of the Number of Shots and Sampling Time

To verify the previous hypothesis, the time dependence of the number of persistent GSR particles on the shooter’s right hand (the hand used for shooting) with the number of shots was investigated. Nine participants in the test fired from 1 to 9 shots, and the sampling of GSR was carried out at 0 to 8 h after the discharge. The results are presented in

Table 7. Number of particles at different time intervals after shooting various numbers of shots with high- (Zastava M70AB2 rifle) and a low-caliber (Glock 19 pistol (9 × 19 mm)) firearms. The hands of the shooter were thoroughly cleaned before shooting. The sampling was performed at different time intervals.

	Number of Characteristic GSR Particles
People/Number of Shots	P1/1	P2/2	P3/3	P4/4	P5/5	P6/6	P7/7	P8/8	P9/9
Time after shooting, h	0	1	2	3	4	5	6	7	8
Zastava M70AB2 rifle (7.62 × 39 mm) a	39	27	18	15	13	10	4	2	0
Glock 19 pistol (9 × 19 mm) b	9	11	6	5	6	3	1	0	0

^a Atmospheric conditions: temperature 9 °C, humidity 84%, wind 1 km/h, precipitation 0%. ^b Atmospheric conditions: temperature 6 °C, humidity 77%, wind 3 km/h, precipitation 2%.

and Figure 6.

As can be seen from the presented results, the amount of persistent GSR on a shooter’s hands depended on the number of shots and the caliber of the firearm. The higher caliber showed a higher number of particles. Increased numbers of shots with both studied calibers of ammunition resulted in increased numbers of particles characteristic of GSR. However, both series of results followed the same pattern of loss of particles despite the caliber and number of shots. Eight shots with high-caliber ammunition resulted in a longer persistence of GSR particles up to 7 h. At the same time, after 8 h no GSR particles were detected when low-caliber ammunition was used, even when nine shots were fired. One particle was detected from seven shots with low-caliber ammunition when sampling was performed 6 h after shooting. Exploring the number of particles, it could be seen that the loss of GSR particles followed the same trend. The high-caliber ammunition showed a faster loss of particles in the first two hours after shooting: the slope of the curve in Figure 6 was higher for high-caliber ammunition. At the same time, low-caliber ammunition resulted in a flatter curve with an almost constant lower slope over the studied period.

4. Discussion

The identification of the GSR by SEM/EDS is a standard method used by justice authorities in the Republic of Kosovo. Currently, only the characteristic particles of the GSR determined by SEM/EDS are accepted as evidence of a person involved in or in the vicinity of a shooting incident. According to the ASTM classification, the particles characteristics of GSR are three-component spheroidal particles containing PbBaSb. A single PbBaSb-containing particle detected by SEM-EDS is considered evidence, regardless of the reported questionable significance of one or two GSR particles in a sample [22]. Thus, this study focused on the particles characteristic of GSR identified by SEM/EDS and produced by discharging weapons typically found in the criminal incidents in the Kosovo region. Although an indispensable tool in forensic investigation, GSR research by SEM/EDS is time-consuming and expensive. Thus, we attempted to extract more information from SEM/EDS data by considering the number of detected characteristic GSR particles. A detailed optimization and validation of the SEM/EDS protocol in the laboratory of the Kosovo Forensic Agency was presented in our previous paper [12]. Based on the method’s performance, estimated for each group of characteristic GSR particles in the reference sample SPS-5P-2A (Table 4), i.e., repeatability, within-laboratory reproducibility, sensitivity, and bias, the method was assessed as “fit for purpose” according to the requirements of ASTM 1588 [9] for the intended use of results by courts, prosecutors, and police intelligence in the Republic of Kosovo.

The forensic practice in the Republic of Kosovo revealed that firearms are often used by criminals, reflecting the state in the region after the 1998–1999 war. The data showed that the same firearms were used in many cases, but ammunitions came from different European manufactures. The UN report presented in 2003 revealed that 460,000 civilian small arms could be found in Kosovo region, and 53% of the reported guns were Zastava and Tokarev, while 85% of the assault rifles were Kalashnikov and Zastava [23]. Three high-caliber (Zastava and RPD) and three low-caliber (Walter, Glock, and Zastava) firearms were included in this study (Table 1 and Table 2). Based on data from the Kosovo Forensic Agency’s ballistics laboratory, the studied weapons and ammunition were those most frequently used in criminal proceedings for which the Kosovo Prosecution and Court required expertise. Additionally, weapons presented for legalization and registration in the Integrated Ballistics Identification System (IBIS) database were included in the tests. The low-caliber Glock 19 pistol used today by the Kosovo police was also studied.

The results from the tests with low- and high-caliber firearms (Table 6 and Figure 4) showed that the low-caliber firearms generated, on average, 25% of the total number of detected GSR particles that originated from high-caliber ammunition discharge. The obtained result was logically related to the quantity of the ammunition components. The GSR persisted on a shooter’s hand for approximately 4–5 h, depending of the caliber of the weapon fired if a single shot was made. However, GSR generated from firing a Zastava rifle was found after 6 h normal daily activity (Table 5 and Table 6). The results are in agreement with the findings reported in the literature: characteristic GSRs were identified by SEM/EDS from 4 to 10 h after firearm discharge [3]. Brozek-Mucha [18] estimated that the average half-life of the GSR on the hands of five shooters making nine shots reached up to 30 min, while it reached 170 min on the face and hair. The GSR detection time intervals by SEM/EDS reported in the literature varied from 4 h for hand samples to 48 h for nasal samples [3,24]. The number of GSR particles at the longest interval was between 542 in a nasal mucus sample [24] and less than 5 particles in a hand sample [25]. In this study, one or two GSR particles were identified on the shooter’s hand after 4 and 5 h (the longest time interval) after firing low-caliber and high-caliber firearms, respectively.

The time between the shooting incident and the GSR sampling is of particular importance, as it could greatly affect conclusions based on whether or not GSR particles are detected in a suspect’s sample. Results based on single-shot tests revealed that GSR particles could be detected on the hand of a shooter who followed normal office activities for up to 4 h after firing, regardless of the season and type of firearm. The results showed that the main loss of GSR particles from a shooter’s hands following routine office activity without hand washing occurred during the first 3 h after shooting in outdoor conditions (Figure 2 and Figure 3). These observations are in agreement with the results presented by Jalanti et al., which showed that the main loss of GSR occurred in the first 2–4 h after the shot; however, it should be noted that their tests were performed in indoor conditions [15]. Depending on the firearm used, GSR could be still identified 6 h after firing the weapon (Table 5). A decay in the particle loss was observed between 3 and 6 h, depending on the calibers and types of firearms used in this experiment. Five hours after the shooting, characteristic GSR particles were found in one of six series of experiments. Therefore, five hours could be proposed as a threshold for the time gap hypothesis, if it is assumed that only one shot was fired in the shooting event. As previously mentioned, one GSR particle has questionable significance [22], although in Kosovo it is still accepted as evidence. To overcome, at least partially, the uncertainty of the forensic conclusions, we can recommend the approach proposed by Brozek-Mucha [18] but with a longer time interval. When more than five hours have passed since the shooting, GSR samples should also be collected from the suspect’s face, hair, and clothing as well as his hands.

In order to extract more information from SEM/EDS measurements, we calculated the mean number of characteristic GSR particles as a function of the postdischarge time (Figure 5). The obtained polynomial functions, presented in the Figure 5 inset, could be used to formulate an alternative hypothesis if it was assumed that only a single shot was produced during the shooting incident. The 6 % uncertainty of the SEM/EDS measurements should be taken into account. Therefore, if 25 ± 2 GSR particles were found on the hand of a suspect, it can be assumed that a large-caliber ammunition was used and that the time interval between the shooting and sampling was approximately 2.5 h. If 10 ± 1 GSR particles were identified, then two hypotheses could be made: (1) if high-caliber ammunition was used, the time interval was greater than 4 h, and (2) low caliber ammunition was used in this shooting event. However, it was observed that the individual results were highly scattered around the mean value due to the different weather conditions, the particularities of shooter’s skin, the sampling irregularities, the type of firearm used, the person’s activity, and the secondary transfer processes. Therefore, the proposed interpretation of the analytical results could only be used as a support tool for forensic specialists. According to Maitre et al., the evaluation of GSR analytical results obtained by SEM/EDS should be carried out in the context of specific circumstances of investigation, taking into account GSR transfer and persistence [26].

The study of the number of characteristic GSR particles as a function of the number of shots and the time of sampling (Table 7 and Figure 6) showed that if more than one shot was fired the time threshold for GSR persistence on a shooter’s hand increased up to 7 h. Firing high-caliber ammunition (Zastava rifle in this test) resulted in a higher amount of detected GSR but showed a faster rate of loss of GSR from the shooter’s hands (Figure 6). However, GSR particles produced by high-caliber weapons could be detected by SEM/EDS an hour later than those originating from low-caliber weapons.

5. Conclusions

Based on the obtained results, it can be summarized that:

• A shooter could appear GSR-negative by SEM/EDS seven hours after firearm discharge;
• Depending on the number of shots and the ammunition in high-caliber and low-caliber weapons, the duration of GSR persistence on a shooter’s hands is extended but does not exceed 7 h;
• The results of this work are to be considered valid only for mobile people who follow normal office activities but not for victims (immovable). Our previous investigation showed that GSR particles could be detected in samples taken from people who had committed suicide, even two days after the incident. This long residual time highlights the limited possibilities of transferring the GSR from one surface to another;
• The results could be influenced by the postdischarge activity of the person involved in the tests, and interference or contamination by secondary transfer during the working day should also be considered.

The obtained results are in good agreement with the SWGGSR recommendations to collect GSR within 4–5 h after a shooting incident [27]. In each of the tests, GSR particles were detected on the shooter’s hands 4 h after firearm discharge. However, later GSR sampling should not be excluded, as on average, from six series of single-shot experiments, GSR could be detected on the shooter’s hand 6 h after outdoor firearm discharge. If the number of shots increased, GSR particles could be detected on the shooter’s hands even later, 7 h after firing a high-caliber firearm. This evidence contrasts some forensic protocols that required the sampling to be carried out no later than 6 h after the shooting event. Hence, the forensic protocol for GSR sampling should take into account, from one side, the wide range of reported time intervals for GSR persistence on the shooter’s hands and, from another side, the fact that a shooter may appear GSR-negative if sampling was carried out at longer postdischarge intervals.