Determinação espectrofotométrica de antimônio em amostras de projéteis de fuzis

Abstract

The analysis of the chemical composition of projectiles found in a crime scene is a useful tool to obtain information that aids in the investigation of cases involving rifle shooting, in which the integrity of the fired projectile is not preserved, and ballistic confrontation cannot be used. Antimony is normally present in rifle projectiles and has stood out as one of the key elements for their classification. This work proposes a methodology for the dissolution of rifle bullet samples for the determination of antimony by spectrophotometry, with the aim of using this result in the differentiation of these projectiles by their antimony content, according to the manufacturers. The proposed method was used in the analysis of projectiles from three types of projectiles caliber 7,56x52mm, provided by the Civil Police of the State of Rio de Janeiro: CBC,09; Winchester .308; e CBC .308 . The analysis involved the dissolution of the samples in HNO3 8 mol L-1, at 150 °C. The lead(II) ion was removed from the solution, as it interferes with the determination, through its precipitation as PbSO4 and, later, centrifugation. The spectrophotometric determination of antimony was based on the formation of the tetraiodoantimonate(III) complex (λ = 425 nm) after the addition of a potassium iodide solution, in an acidic medium, and proved to be adequate for the analysis of the projectiles. The analytical curve showed excellent linearity (r = 0,9999) and the limit of detection (1,05 ± 0,02 mg L-1) and quantification (3,49 ± 0.04 mg L-1) obtained were adequate, as the solutions produced by dissolving the samples presented a concentration (20 mg L-1) well above these limits. The study of antimony concentration found in the solutions produced by the proposed dissolution method, as well as the accuracy of the method, determined by the recovery studies (between 99,022 ± 0,003% and 109,719 ± 0,005%) were satisfactory, as they indicate that during the dissolution of the samples and PbSO4 precipitation, there was no appreciable loss of antimony by volatilization or co-precipitation, respectively. The antimony content determined in the projectiles were in a range of 0,3207 a 1,9359 g/100 g. ANOVA and Tukey's test were used to discriminate the projectiles by their types. By analyzing the fragments of the projectiles, it was possible to discriminate only one type (CBC,09) from the others (Winchester .308 and CBC .308) by its Sb content. When comparing projectiles from the same type, a significant difference was observed between the projectiles produced by only one of them (Winchester .308). No significant differences were observed in the Sb contents when analyzing the different sections of the same projectile, indicating that the Sb distribution is relatively homogeneous (considering the sample size and the number of replicates taken for the analysis), an interesting fact, because this method is intended for analyzing rifle bullets that, when found at crime scenes, are fragmented, or deformed. However, when comparing the types choosing specific sections of the projectiles, it was noticed that the sections at the ends of the projectiles presented a greater potential for the differentiation of the projectiles, having differentiated the types Winchester .308 and CBC 7.62x51mm.