Rapid, selective and quantitative determination of nitrite and nitrate ions with capillary electrophoresis: A new screening tool for gunshot residue detection

In the recent years, capillary electrophoresis (CE) has established itself as a versatile analytical tool in many areas of Forensic Science, including illicit drug analysis, DNA fingerprinting, ink analysis, explosive and ion determination etc. CE uses the differential mobility of ions to perform their separation in fused silica capillaries (internal diameter 20-100 micron, length 20-100 cm) and different detection methods including UV absorption, fluorescence, electric conductivity, mass spectrometry etc. High efficiency, high mass sensitivity, ruggedness, easy and low cost operation and high versatility are the typical features of CE methods, which may be of the highest interest for the forensic analysts. Quite recently, a simple CE method has been published for the rapid determination of nitrite and nitrate, which are major inorganic components of gunshot residues (1). The aim of the present work was to study the application of this method to the analysis of nitrite and nitrate in residues of firearm discharge. The used method proved simpler, cheaper and faster than the traditional approaches to gunshot residue (GSR) analysis based on flameless atomic absorption spectrometry (AAS), atomic emission spectrometry (AES), inductively coupled plasma-mass spectrometry (ICP-MS), or scanning electron microscopy (SEM). The analyses were carried out in a bare fused-silica capillary (75 mm inner diameter) with a 100 mM borate buffer (pH 9.24). The detection was by direct UV absorption at 214 nm wavelength. Separation took place under the voltage of - 15 kV. Bromide was used as the internal standard. Sensitivity was about 1 mM for both nitrite and nitrate. Reproducibility (intraday and day-to-day) was good with relative standard deviations (RSDs) < 1.0% for relative migration times and < 4.5% for peak areas in both standard solutions and real matrices. The method has been applied successfully to a number of samples obtained in firing tests. The distribution of nitrite and nitrate on targets used in tests was studied. Samples from hands of shooters and of non shooters were analyzed and their results were used to establish a suitable interpretative framework. Shooting tests were performed with different weapons and different cartridges. Hair and skin samples from a victim shot in the head were also successfully analyzed for the presence of nitrite and nitrate. This preliminary study indicates that CE offers a sound potential for forensic GSR investigation as a screening technique prior to more expensive and time consuming analytical techniques. The importance of CE in a procedure able to detect GSR produced by leadfree ammunition, where the evidential value of analytical results obtained with SEM can be poor (2), is finally shown.