An investigation into Forensic Soil Analysis

An investigation into Forensic Soil Analysis

Abstract

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In this review soil examination, preparation and comparison is being discussed. Procedures used by many authors on soil examination are the topic being reviewed here. The review begins with Transport of soil, preparation of soil, Attenuated Total Reflectance – Fourier-transform infrared spectroscopy (ATR-FTIR), Multivariate analysis, Microscopy examination, X-ray Absorption Spectroscopy, pH analysis and Laser Induced Breakdown Spectroscopy (AIBS). Soils may establish evidence that connects a person or object to a particular location.

Introduction

Soils are a very complex material; it can consist of both organic and inorganic components. The soils stem can transfer components onto an object or persons. Due to its complex material through examination of its components it can provide information about its geological origin, dominant vegetation, management and environment. Soil consists of high volume of information that has a great impact on Forensic analysis. Individual analytical techniques have different scales of resolution and relevance depending on the nature of the criminal case and context, each method has its strengths and weaknesses. The soil that transfers from the stem onto an object or person can be recovered and used as evidence in a forensic investigation. Soil identification of source, linking to a crime is dependent of the characteristics of the organic and inorganic samples which can be compared with other samples. For example, the referenced soil sample can be compared with the suspect’s soil sample to aid in placing someone at the scene of the crime. An overview of soil characterisation methods including chemical analysis, mineralogy, infrared spectroscopy and microscopic analysis are presented here. Using soil as evidence has a century of history, dating all the way back to the writings of Sir Arthur Conan Doyle between 1887 and 1893 through his ‘Sherlock Holmes’ book series. Although soil evidence is a historical one, there are a lot of new developments that are barely known by analysts. It has a lot to do with equipment given in the laboratory.

1. Transport of soil to the Laboratory

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When a soil sample is collected for several days before arrival to the laboratory, field-moist soil undergoes significant biological changes at room or elevated temperatures when placed into air-tight containers. Organic matter can release various element ions such as Cadmium Phosphate PO₄^3-, Sulphate SO₄^2-, Borate BO₃^3- and Ammonium NH₄⁺ into the soil. In anaerobic conditions organic matter decomposition can result in loss of Nitrogen (N) from the soil. When transporting soil for a long period of time the sample should be kept in a cool environment – 5 to 10⁰C and the excess water should be removed by drying partially, this keeps the soil moist. A soil sample can be frozen but similar to the effect high temperature has on the soil; it alters the physiochemical properties within the soil. (Benton Jones, 2001)

2. Preparation of the Laboratory Samples.

There are two main ways to prepare a sample before analysing it, but some of the methods have a different way of preparing and this is stated in the methods section. First sample preparation is drying; this is to air-dry the soil samples at laboratory temperature 21 to 27⁰C before crushing and sieving. The drying procedure should be done in a timely manner to avoid mineralization. The texture, organic matter and moisture will determine how long it will take for the soil to get to the air-dried condition required. Soils that are high in clay and/or organic matter constant need a long air-drying time compared to that of the sandy-textured soil. When drying the samples the temperature shouldn’t exceed 38⁰C, because this can cause changes in the physiochemical properties within the soil. Drying of the sample can affect the determination of micronutrients Copper (Cu), Iron (Fe), Manganese (Mn), and Zinc (Zn), since drying caused significant changes in the soil there was a time in the laboratory which they would analysis the soil taken from the field as received using the slurry method. This slurry proved to be a lot more time-consuming for processing larger number of samples. The moisture of an air-dried sample is determined from the physiochemical properties of the soil and the humidity of air surrounding the sample. This variability has little effect on the soil analysis; the only effect is that the soil is measured in volume instead of weight. (Benton Jones, 2001)

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Second preparation procedure is Crushing/Grinding/Sieving, following drying the soil is crushed either by hand or by using a mechanical device and is then passed through a 10-mesh (2-mm). the grinding process can have an effect on the extracting of Copper (Cu), Iron (Fe), Manganese (Mn), and Zinc (Zn), Potassium (K) and Phosphorus (P). Removal of stones and any other substances are done when the soil is sieved through the 10-mesh, this leaves a sample that can be easily handled and stored in the laboratory. When preparing a sample for testing in the laboratory, the sample can be easily contaminated by coming in contact with surfaces or with dust residue from a previous sample. The crushing and sieving devices used the preparation should be free of elements that can be determined in the analysis. “For example, brass sieves should not be used if Cu and Zn are elements to be determined” (Benton Jones, 2001). For crushing and sieving sample reduction may be required and this takes precise care as the sample needs to be thoroughly mixed before it can be divided. Reduction of particle size can have an effect on determination of a number of elements such as Cu, Fe and Zn. (Kahn, 1979) and also for equilibrium extraction reagent procedures (Houba, 1993). Both authors agree that the reduction of particle size effects certain element determination but understands that it must be done to make the sample easier to handle.

Once the soil has been dried, crushed and screened it can be then stored in a dry environment indefinitely without any changes occurring within the soil test values.

3. ATR – FTIR Method

For the FTIR method, three samples were taken from the Canberra area and all of the samples were analysed individually. The soil was oven dried for 24-48 hours @47⁰C, a mortar and pestle were then used to crush the soil lightly. The dried soil was then sieved to get it to its finest particles. Each soil specimens were analysed “using a Smiths Detection Identify IR portable FTIR spectrometer which uses a diamond ATR sampling interface”. Each soil specimen was analysed in triplicate. The diamond cell detection window has a 1.3mm diameter; the soil specimen must cover this window. The sieved soil specimen was pressed down on the ATR crystal. The range of the spectra was 4000-650cm^-1, the soil was scanned 64 times to get a clear reading. (Woods et al. 2014)

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The results from the spectra were placed into Minitab16 (version 2.2) this is a statistical software, this constructing a plot of the first two components in the soil specimen. (Woods et al. 2014)

The three soil samples appeared to be the same as the soil in the Canberra area were rudosols and chromosols from volcanic mountains, alluvial fans, granitic material and metasediments. (Woods et al. 2014)

Variations present at peak 3700/3620cm-1 (Fig.1) and at 796/779/694cm-1 (Fig.2). The variations at 3700/3620cm-1 present can be a results of Kaolin Minerals and the variations at 796/779/694cm-1 present could be a results of quartz. (Woods et al, 2014)