Evaluating polar and non-polar solvents extraction efficiency of gunpowder components by FTIR

Abstract

Introduction: Crimes involving firearms have been increasing and is a current transnational concern. To face such issues the development of analytical procedures is paramount to increase the efficacy of criminal investigation on homicides and other gun related crimes. The identification of an ammunition type or brand from a simple residue would represent a major contribution for gunshot residues analysis. Such a breakthrough will benefit homicides and war crimes investigation. The composition of gunpowder varies and each ammunition type contains a given formula and often specific components [1–3] suggesting that Fourier-transform infrared spectroscopy (FTIR) might be a useful technique for the rapid analysis of gunpowders [2]. Indeed, FTIR has an outstanding discriminatory ability when coupled with chemometrics analysis [3]. Here we present the optimization of the extraction organic solvent for analysis of ammunition contents by FTIR which is an essential step for the discriminatory study of ammunition brands.

Materials and methods: An ammunition of caliber .22RL was purchased at Soldiers®. The sample was dissolved in eight different solvents (Acetone, Acetonitrile, Dichloromethane (DCM), Ethanol, Isooctane, Methanol, Methyl Ethyl ketone and n-Hexane), subjected to ultrasound for 30 min and followed by centrifugation. The supernatant was collected and analyzed on a Perkin Elmer 65 FTIR spectrometer coupled to an ATR accessory. A spectral window of 4000-525 cm⁻¹ was used and 32 scans were accumulated at 1cm⁻¹ resolution.

Results: Figure 1 shows the FTIR spectra of methanol and n-hexane for the sample used. From the spectra it is clear that methanol is a better extractor than n-hexane. Of the eight solvents used, methyl ethyl ketone, acetone and methanol had the better extraction efficiency of gunpowder (GP) while n-hexane and dichloromethane were not able to extract GP.

Figure 1. FTIR spectra of sample E dissolved in methanol (blue) and n-Hexane (red).

Discussion and conclusions: This study showed that the best solvents for extraction of GP are the more polar solvents containing carbonyl (C = O) and hydroxyl (OH) groups. The best extraction solvents were Methyl ethyl ketone and acetone followed by methanol.

Acknowledgments

The authors acknowledge funding by Egas Moniz – Cooperativa de Ensino Superior