## 3 Results  
### 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 Figures 1 and 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 \(\upmu\)m 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. 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]. Figure 1: A typical SEM picture and EDS spectrum of GSR particles generated from the discharge of a high-caliber firearm (model: Zastava M70 AB2 rifle; cal. 7.62 \(\times\) 39 mm). Figure 2: A typical SEM picture and EDS spectrum of GSR particles generated from the discharge of a low-caliber firearm (Glock 19 pistol (9 \(\times\) 19 mm)). 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 Tables 5 and 6 as well as in Figures 3 and 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 M70 AB2 rifle (7.62 \(\times\) 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.