BIBLIOGRAFÍA

Standard materials of the five most common organic constituents that normally found in gunshot residues were utilised. These compounds include DPA, EC, MC, 2- NDPA, and 4-NDPA. 4-Nitrotoluene was used as an internal standard.

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Table ‎4.4. The response factors of six substances were used in this study

Analyte Response factor

Diphenylamine 27.66

Methylcentralite 16.50

Ethyl centralite 35.27

2-Nitrodiphenylamine 31.14

4-Nitrodiphenylamine 2.91

A baseline experiment was performed at day zero to provide information concerning the ability to quantitatively extract the organic components of gunshot residue from a piece of cotton fabric. The results from the study are shown in Figure 4.9. The amount of each component dosed on the cotton fabric was 4x10-7 mg. Therefore, if there is no loss of materials within the extraction process, there should be 4x10-7 mg extracted. As can be seen from the data in Figure 4.9, the retrieval of MC, EC and 2-NDPA fall with a 95% confidence interval of the true value (4x10-7 mg) when subjected to a t-Test. However, the retrieval amount of DPA and 4-NDPA are well outside this range.

Figure ‎4.10. Detected compounds at day 0, 1, 5 and 10 at ambient temperature

The effect of storage for a period of 5 and 10 days at two temperatures, ambient and 4

o

C, was determined. The data from these lists is shown in Figures 4.10 and 4.11.

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4.2.4 Discussion

A series of experiments were performed to investigate the effect of storage time and temperature on the preservation of the organic components of GSR. A quantitative study was performed to show how critical the storage time and temperature are upon the concentration of organic residues retrieved when the samples are stored in nylon bags. This result will potentially be important in influencing storage protocols for the storage of this type of evidence.

In order to perform quantitative analysis with sufficient accuracy it was necessary to adopt a method that employed an internal standard. 4-Nitrotoluene was chosen because it has similarity in structure and physical properties to the target substances.

The data from day zero of the study produced some interesting results. The initial amount of each organic loaded onto the cloth was 4x10-7 mg. Extraction at time zero shows that all the materials cannot be extracted without loss. However, the recovery of MC, EC and 2-NDPA is relatively good with the amount recovered being 3.61x10-7, 3.81x10-7 and 3.78x10-7 mg respectively. Surprisingly, the recovery of DPA and 4- NDPA is not as good, with only 1.46x10-7 and 1.47x10-7 mg of these materials being recovered (respectively). While 4-NDPA and DPA do have the lowest boiling points of the materials utilised in this study (Table 4.5), they still have relatively high boiling points and it is not thought that the low recovery of these two components is due to loss through evaporation due to their increased volatility.

Table ‎4.5. Physical properties for key organic compounds in GSR

Compound Flash point Boiling Point Melting/freezing Point

Diphenylamine 153 °C 302 °C 53 °C

Ethyl centralite 325-330 °C 325-330 °C 73 °C

Methylcentralite 142.6 °C 350 °C 122 °C

2-Nitrodiphenylamine 346 °C 346 °C 74 °C

4-Nitrodiphenylamine 190 °C 211 °C 132 °C

The reduced recovery of DPA could be a result of a reaction with the acetone used for the extraction. DPA can react with acetone to produce 9,9-dimethyl-10H-acridine (Figure 4.12).

In theory both 2-NDPA and 4-NDPA can also react with acetone. However, because 2- NDPA (Figure 4.14) is a much weaker base than 4-NDPA (Figure 4.13), due to intra- molecular hydrogen bonding between the secondary amine and nitro groups, the reaction with acetone will be much slower with 2-NDPA compared to 4-NDPA. This is therefore postulated as a potential explanation as to why the recovery of 2-NDPA is significantly greater than 4-NDPA.

Figure ‎4.12. The structure of 9,9-dimethyl-10H-acridine compound

Figure ‎4.13. The structure of 4-Nitrodiphenylamine

Figure ‎4.14. The structure of 2-Nitrodiphenylamine

The effect of increased time and storage temperature is to reduce the concentration of all components. The MC, EC and 2-NDPA are always retrieved in larger concentration than the DPA and 4-NDPA. Several studies reported the influence of the time being a factor in the detection of GSR [14, 54, 124, 217, 218]. Time is the major factor that the forensic scientist must take into account while performing any examination of gunshot residues. This factor may directly affect whether a positive or negative result for gunshot residues analysis is produced [14]. After a certain period of time, detection of

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GSR becomes extremely difficult in terms of detecting the compounds within the limit of detection.

The issues associated with the time delay between discharge of a weapon and swabbing to collect GSR are well documented [14, 124]. The amount of GSR on the hand of the suspect has been shown to decrease rapidly with time as a consequence of environmental exposure [219, 220]. The environmental exposure can contribute to degradation of the organic materials in gunshot residues [139] In addition to the time alone, the consequences of environmental exposure need to the taken into account [139, 219]. Physiochemical processes such as diffusion through air and absorption metallic surfaces play a role.

This study differs from those previously described in the literature. In this study the sample was stored in sealed nylon bag throughout the experiment. The natural assumption is that under these conditions the sample will be preserved. However, these results clearly show that the time delay between collection of the sample and analysis is also an equally important factor in the analytical protocol, and not just the time between discharge of the weapon and collection/storage of the sample.

The results presented here clearly show that the storage temperature is a key factor. Storage of the samples at 4oC results in significantly greater recovery of all of the organic components. While it is known that the long-term storage of ammunition results in it degradation and associated changes to its chemical composition due to oxidation processes [29]. Storage of the sample at 4oC is likely to result in a lower rate of degradation through oxidation reactions and would provide a plausible explanation of the data presented within this study.

Different container types have been used in to store GSR evidence [139]. The nylon bags used in this study are widely accepted as being the method of choice due to their low permeability to volatile materials [221].

Studies have been conducted to determine the best container for the storage of volatile materials, including GSR. Paint can, mason jar, and nylon bag containers were utilised. Nylon bags have been found to have better performance compared to the other containers due to their low permeability to volatile materials [221]. The findings from this study clearly show that there is a strong relationship between storage, time and temperature when analysing the organic residues from GSR. The data clearly shows that

minimising storage time and refrigeration samples during storage are highly recommended in order to minimise the loss of the organic GSR during this phase of the process.