# Chemical Analysis of Gunpowder and Gunshot Residues
## 3 Analysis of Gunpowder and Gunshot Residue
### Chemical Analysis
#### 3.2.1 Sample Preparation  
Solvent extraction uses solvents with different physicochemical properties to extract selected compounds from the sample due to their different partition coefficients [94]. Several solvents have been employed to extract organic compounds from unburnt SG [26; 28; 95] and spent cartridges [53], including both aqueous solutions [53; 96; 97] and organic solvents [23; 26; 28; 37; 95; 97; 98; 99; 100; 101]. For example, [PERSON] et al. [26] used methanol to extract selected compounds for unburnt SG. However, the authors noticed that most analysed samples did not dissolve completely, leading to additional centrifugation and filtration steps before GC-MS analysis [26]. Dichloromethane [97; 98], acetonitrile [37; 95], ethanol [99], methyl-ethyl-ketone [21; 23], and methylene chloride [28; 95; 100] were also used in the extraction of organic compounds of explosives, SG and GSR samples. [PERSON] et al. [101] used acetone to extract compounds from collection devices, choosing it over dichloromethane due to its lower toxicity and ability to dissolve most explosive compounds. Due to its inefficiency, solvent extraction has seen little use in the chemical analysis of SG or GSR, and it is now being replaced by more modern and efficient extraction methods, such as SPME or HSSE. Solid-phase microextraction (SPME) is a solvent-free extraction technique that allows the extraction and enrichment of volatile and semivolatile organic compounds from the vapour phase of solid, liquid and gas samples at trace or ultra-trace levels [26; 27; 31; 92; 27]. The compounds in the vapour phase become adsorbed to the polymeric phase that coats the fused silica fibre [92; 92; 15; 27], which is usually composed of polydimethylsiloxane (PDMS), carboxene (CAR), polyacrylate (PA), divinylbenzene (DVB), or of a mixture of these sorbent phases [73; 82; 102; 103; 104]. This extraction process results from an equilibrium established between the sample and extraction phases that is dependent on the characteristics of the sorbent coating and target compounds, the concentration of the analytes, as well as sampling temperature and time [92; 26; 15]. After the extraction process, the analytes are desorbed from the fibre and transferred to a GC apparatus by thermal desorption in the injection port [92]. SPME is an environmentally friendly extraction technique because of its solvent-free nature (when thermal desorption is employed). It is also relatively cheap, sensitive, and straightforward [26; 92]. Furthermore, it eliminates other analytical steps since the compounds can be directly transferred from the sample to the fibre and from the fibre into the injector of the GC [92; 103]. Despite all its advantages, SPME also has some limitations, including the inability to extract non-volatile compounds when using headspace extraction and the fact that it is mainly a laboratory-based technique, which limits its in situ use. Moreover, it presents low reproducibility and is more time-consuming when compared to solvent extraction [102; 104]. Nevertheless, this extraction technique is recurrent in several forensic areas, including explosives [105] and arson [106] investigations, as well as in forensic ballistics and gunpowder analysis [26; 27; 102; 103; 104; 107; 82]. [PERSON] et al. [82; 103] were the first to employ SPME in forensic ballistics to estimate the time since discharge of several elements, namely rifles [103], pistols, revolvers [82], spent cartridges, and shotguns [103]. The authors focused on several OGSR compounds using SPME combined with GC-TEA (thermal energy analysis) and GC-FID (flame ionisation detector) [82; 103]. Concurrently, [PERSON] et al. [72] studied three standard-loading approaches for the SPME procedure, concluding that in-vial loading of the standard is the most suitable for quantifying trace analysis of selected compounds in SG and GSR. The procedure achieved forensic differentiation among various 9 mm calibre ammunition types by determining the volatile compounds and their relative abundance.