# Airborne and Dermal Collection Methods of Gunshot Residue for Toxicity Studies
## 3 Results/Discussion  
### GSR Particle Identification (Filter and DST)  
FE-SEM/EDS analysis was used to identify airborne GSR particles on two sample substrates. Characteristic GSR elements and particles were detected on the single and triple shot DST samples (Figure 2A) but not on the filters (Figure 2B). The DST served as a particle collection surface that allowed for the identification of GSR after a single shot (Figures 2A and 3). Traditionally, GSR morphology is identified as a spherical particle containing a mix of Pb, Ba, and Sb [3]. The particles collected on the DST were slightly misshapen from an ideal spherical shape (Figure 3). These findings align with a previous study that collected GSR directly from the ammunition primer onto carbon adhesive tape and found irregular morphology of some particles [50]. A potential rationale for the non-spherical particles collected is the collection method, with particles needing to directly adhere to the DST and in the process of impaction slight alterations to the shape may have occurred. Additional elements (Al, K, O, Si, Sr, Ti) aside from characteristic GSR (Ba and Pb) were used as further validation of the source of the collected particles [51, 52]. The elemental composition of all samples will be discussed in detail below.  
DST is routinely used forensically for collecting GSR from suspects' hands; however, in this study, it was used to collect airborne particles. In contrast, the complex woven nature of the filters (Figure 2B) was a difficult matrix for particle identification with no GSR particles detected. However, additional chemical analysis, described below, provides data that GSR was present on the filter samples. It is likely that GSR particles were present on  
Figure 3: FE-SEM/EDS data for DST single shot sample. A representative GSR particle on DST from the single shot sampling shows the morphology and elemental composition. The elemental overlay of Ba (yellow), K (green), Ti (purple), O (red), and C (blue) along with a combined image (in the top left) are displayed at 1000\(\times\) magnification; the scale bars indicate 25 μm.  
Figure 2: Representative FE-SEM/EDS images of DST and Filters. A representative image of a particle for the DST collection (**A**) is displayed with elemental overlay for Pb (yellow) and C (blue) after a single shot at 6500\(\times\); the scale bar indicates 5 μm. No particles were identified on the filter samples (**B**) which provided a complex matrix for imaging at 1000\(\times\); the white scale bar indicates 10 μm.  
the filter samples but that the smaller particles collected on the filters (under 2.5 \(\upmu\)m) were deposited deep into the layers of the filter during the active sample collection. A study investigating size-selective traffic emissions was able to obtain elemental and morphological characterization of PM\({}_{2.5}\) particles on PTFE membrane filters [53; 54]. Sampling times and exact filter materials in the study differed from those in the current work. Differences in sampling times, 12 h vs. 1 min in this study, likely resulted in different particle loading amounts on the filters. Previous studies using borosilicate glass PTFE-coated filters, like the ones in this study, have documented the complex woven background [55]. The lower particle loading and filter material used in this study likely prevented visualization and characterization. FE-SEM/EDS is the standard analysis for use on carbon adhesive tape in forensic sciences, but chemical characterization may not be adequate for use on filter samples unless the filters have a higher loading of sample than in this study.