Validation of a Standard Operating Procedure (SOP) for Forensic Soils Investigation in Brazil

Abstract

Soil traces are useful as forensic evidence due to their potential to transfer and adhere to different types of surfaces on a range of objects or persons. Several works have been developed in forensic soils, under different analytical approaches. However, in Brazil, only the researches from the Group of the Federal University of Paraná have developed works with soils under a forensic approach. Focus has been given on the sequential chemical analyses and mineralogical techniques once they presented a large potential to discriminate soil samples, even that originated from the same bedrock. In this way, this work aimed to test a Standard Operating Procedure (SOP) for forensic soil sampling and an analytical protocol in Brazil between the academy and the scientific police through a blind simulated crime scene scenario. Samples were collected at four sites located in the Curitiba Metropolitan Region. All soils were classified as Inceptisol (Cambissolo), and the parent material in Curitiba is claystone and in Colombo is limestone. Around 3 g of sample composed by silt + clay fraction were isolated and analyzed by sequential chemical procedure: i) extraction of poorly ordered iron and aluminum oxides with ammonium oxalate (AO); ii) extraction of crystalline iron oxides with sodium dithionite-citrate-bicarbonate (DCB); and iii) extraction of poorly ordered aluminosilicates and gibbsite with NaOH 0.5 mol L -1 . All data were transformed by square root and formed a data matrix subsequently analyzed in a principal component analysis (PCA). Most of the samples were properly grouped according to their provenance at all four sites tested, showing the potential of the Standard Operating Procedure (SOP) in a real crime scene. The sampling procedures presented in the SOP were detailed enough to allow the appropriate police work in forensic cases in any part of Brazil. As future considerations, modifications to the sample treatment and analytical protocol could be made depending on the context of the forensic work. Intense anthropogenic activities, such as domestic waste disposal in urban areas, would potentially reduce the discrimination power of such a proposed chemical analytical protocol.

Sand fraction is not suitable for forensic investigations in subtropical soils

“…The level of grouping of the replicate samples ( Figures 6 and 7) was lower when the finer fractions of the soils were used. Testoni et al (2019) applied sequential chemical analyses to assess a limited quantity of clay and silt fraction and obtained the grouping of the same samples (1 to 5, 6 to 10, 11 to 15, and 16 to 20) with similarity level higher than 95 %. Data from analysis of the finer fraction of the soil also separated into clear groups of soils formed from the same parent material (two sites under claystone and two sites under marble) but collected from different sites (Testoni et al, 2019).…”

“…To investigate the usefulness of the sand fraction for forensic discrimination in this study, twenty soil samples were selected by sampling: 1) at two distinct geologies (claystone and limestone) and 2) at four locations under a simulated crime scene in southern Brazil. Finer sized particles (silt + clay) analyses using the same soils have already been tested and allowed a clear grouping of the samples according to their location of origin at all four sites tested, ascertaining the ability to use chemical characterization to test a link between a questioned sample and a known crime scene (Testoni et al, 2019).…”

“…Design of the simulated crime scene for surface soil sampling. Source: modified fromTestoni et al (2019).Rev Bras Cienc Solo 2020;44:e0190174…”

The Use of a Sequential Extraction Technique to Characterize Soil Trace Evidence Recovered from a Spade in a Murder Case in Brazil*

“…Some work in Brazil has concentrated on sequential chemical extraction of short-range order materials (SOR) and crystalline minerals from the clay fraction in forensic cases (16)(17)(18)(19)(20)(21)(22). The sequence of analyses adopted mixed methodologies for extraction of short-range order phase (SOR) and crystalline Fe oxides in nonforensic studies (23)(24)(25)) (with modifications): (i) 0.2 mol/ L pH 3.0 ammonium oxalate (AO); (ii) dithionite-citrate-bicarbonate (DCB); and (iii) 0.5 mol/L NaOH.…”

“…To avoid these transitory variations, several authors (16,(18)(19)(20)(21) used more aggressive extraction methods to analyze different and less transient mineral phases. The extraction of SOR by AO and NaOH allows access to the elements with intermediate mobility and solubility in relation to soluble and exchangeable and crystalline forms (20,25,26,(29)(30)(31)(32). This perspective is essential in the discrimination of soils which are highly similar pedologically (i.e., the same soil class and parent material) but separated by short distances (i.e., 10 to hundreds of meters).…”