For organic residues, the following instruments are used:
1. Gas chromatography-mass spectrometry: GC-MS is most widely used for gunshot residue as it doesn’t need any sample preparation, pre-concentration and analysis are done in continuation. Various detectors like flame ionization, electron capture, thermal energy analysis and mass spectroscopy were combined with GC to detect explosives like TNT, RDX, Pentaerythritol tetranitrate (PETN) and organic residues like nitroglycerine and nitrocellulose.
2. High-performance liquid chromatography: this technique is very useful in the analysis of components in smokeless powder. Both burnt and unburnt smokeless powder and organic components were analysed even if the firearm is of a different Caliber, brand, age etc. It can detect organic residues like diphenylamine, ethyl centralite, dimethylphthalate and 2-nitrodiphenylamine. Polytetrafluoroethylene (PTFE) tapes were used by some scientists for swabbing organic gunshot residue and then for detection of these additives in gunshot residue LC-Qtof-MS (liquid chromatography quadrupole time of flight mass spectrometry) was applied. This technique helps in detecting Ba, Sb, and Pb with great accuracy.
3. Raman spectroscopy: it is used for identifying and analysing various propellant components. There was a recent study in which near-infrared (NIR) Raman spectroscopy combined with advanced statistics was used to differentiate gunshot residue particles originating from different calibre ammunition. It can detect residues like nitrates, sulphates and carbonates.
4. Capillary electrophoresis: there is an advanced technique of capillary electrophoresis which is Micellar electrokinetic capillary electrophoresis (MECE) which helps in the full identification of organic compounds of GSR. This method separates neutral molecules like explosives, stabilizers etc and is cheaper than SEM-EDX . Also, MECE has the potential to examine organic GSR which cannot persist for more than an hour. Residues like diphenylamine, nitroglycerin and enthylcentralite can be detected by MECE.
There are several instruments which are used for detecting inorganic residue which are as follows:
1. Neutron Activation analysis is a non-destructive technique with a sensitivity of about 1 ng. only helping to determine trace quantities of a broader range of elements. By exposing the sample to neutron radiation, some nuclei in the sample absorb neutrons and turn radioactive. Certain elements give off distinctive gamma rays when these radioactive isotopes decay. Scientists can identify and measure the components included in the sample by determining the energy and intensity of these gamma rays. It is used for detecting Sb and Ba.
2. Atomic absorbance spectroscopy: The technique measures the amount of light that free, ground-state atoms in a vaporized sample absorb, usually in the ultraviolet or visible range. Upon atomizing a sample—typically using a flame or graphite furnace—the atoms absorb light at wavelengths that are unique to that element. It is possible to quantify the concentration of the element in the sample by measuring the amount of light absorbed at certain wavelengths. The specificity of AAS is one of its key advantages; even in complex mixes, it can discriminate between various elements.
3. Scanning electron microscope X-ray emission: is an effective analytical method that produces high-resolution images of materials using a concentrated electron beam. When the electron beam strikes a material, it produces a variety of signals that are detected and examined, such as backscattered electrons, secondary electrons, and distinctive X-rays.This combination method preserves the sample for future study or legal review in a non-destructive manner. Because of its great sensitivity and specificity, it is a very useful instrument in forensic science, especially when it comes to identifying and analyzing gunshot residue (GSR) [Shrivastava et al., 2021]. In criminal investigations, the capacity to precisely identify the elemental composition of the minute particles of GSR, such as lead, barium, and antimony, in addition to being able to see them, is vital evidence.
4. Inductively coupled plasma mass spectrometry analysis: this technique is mostly used for the analysis of elements present in the primer like barium, lead and antimony. It is a sophisticated analytical method that is often utilized to identify and measure trace elements and isotopes in a range of materials [Li et al., 2023]. The sample initially goes into a high-temperature plasma and is ionized with an argon plasma torch to start the procedure. The resultant ions are subsequently put into a mass spectrometer, where their mass-to-charge ratio is used to separate them. ICP-MS offers accurate and sensitive measurements of elemental concentrations, generally at parts-per-billion (ppb) or even parts-per-trillion (ppt) levels, by identifying these ions. ICP-MS's broad dynamic range, which enables the quantification of both major and trace elements in a single run, is one of its main advantages. Furthermore, with the right sample preparation, the technique is adaptable and can handle a wide range of sample types, including liquids, solids, and gases
