This research studied parameters affecting the extraction efficiency of NO3-and NO2- from gunshot residue samples (GSRs). The spectrophotometric determination of NO3-and NO2- was carried out using the principle of a Griess reaction. For this work, both sampling and extraction parameters were investigated before selecting the optimal condition for a study of shooting distance. The results showed that a mixed solution of 1% Sulfanilamide and 1% N-(1-Naphthyl) ethylene diamine (NED) in 0.5 molL-1 HCl (Griess reagent) and 30 min reaction time provided good sensitivity for the determination of NO2- at 540 nm. In the case of NO3-, 1% VCl3 in Griess reagent was used to reduce NO3- to NO2- before analysis with the same principle. The optimal condition for a sampling procedure used 150 µL of room temperature deionized water per cotton brush swab for two replicated GSR collections. Samples were stored for 1 week before analysis could be carried out at 4 OC in a refrigerator or at room temperature without significantly different results. On the other hand, 4 mL of room temperature deionized water with and without ultrasonic extraction times of 5 or 30 min respectively were the selected parameters for NO2-and NO3- extraction from GSRs.
The proposed method could be applied completely for spectrophotometric determination of NO3-and NO2- in GSRs. The limits of detection (LOD) of NO2- and NO2- + NO3- were 0.27×10-6 and 3.58×10-6molL-1 respectively with a relative standard deviation of the given slope. A wide linearity range between 2.17×10-6to 217×10-6molL-1 was obtained and a percentage of recovery in the range of 96.24 – 102.32 and 98.04 – 105.96 for NO3- and NO2- was achieved, respectively. This method can be performed for determination of NO3- and NO2- content at a maximum 304.8 centimeter shooting distance. From these findings, the proposed sampling, extracting and determining methods for NO3-and NO2- in GSRs are highly sensitive, precise, robust, simple and inexpensive, with a short analysis time and provide a useful alternative procedure for analytical chemists or forensic scientists in their work.
Keywords: Extraction, NO3- and NO2-, Gunshot residues
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