Development of a searchable major and trace element database for use in forensic soil comparisons.

Forensic soil comparisons are normally undertaken on the basis of several physical, chemical and biological properties, but in all cases the interpretation of results is dependent on the availability of relevant contextual information. This paper summarises the results of major and trace element analyses performed using inductively coupled plasma atomic emission spectrometry and inductively coupled plasma-mass spectrometry on the < 150 microm size fraction of 1896 soil samples collected in connection with forensic investigations in England and Wales between 1999 and 2007. A number of new methods are described which facilitate inter-sample comparison. Although the available data do not provide uniform geographical coverage they do provide useful information which can assist police search investigations and they provide valuable contextual information which aids the evidential assessment of soil evidence when used in court. There is considerable scope to improve the database by increasing the geographical coverage and increasing the number of soil attributes which are included.

Discrimination between sediment and soil samples for forensic purposes using elemental data: an investigation of particle size effects.

This paper reports the results of an investigation to quantify variations in elemental concentrations amongst different particle size fractions obtained from a suite of sediment samples collected from the River Avon, UK. Concentrations of 49 elements determined by inductively coupled plasma spectrometry (ICP-AES and ICP-MS) were compared using conventional and multivariate statistical methods in order to assess compositional differences between different size fractions and between different samples. The results showed significant differences between different size fractions and the bulk sediments, but indicated that all of the size fractions considered (<150, 63-150, 20-63 and <20microm) provided an adequate basis on which to identify associations and differences between samples. It was concluded that, while in certain specific circumstances it may be most appropriate to make forensic comparisons based on a very narrowly defined particle size range, for the majority of purposes the <150microm fraction provides the best compromise between sample size requirements and data resolution.

Forensic analysis of soil and sediment traces by scanning electron microscopy and energy-dispersive X-ray analysis: an experimental investigation.

This paper reports the results of a series of experiments carried out to determine the precision of soil trace comparisons based on elemental peak height ratios determined by energy-dispersive X-ray analysis (EDXRA) in a variable pressure scanning electron microscope (VP-SEM). Experiments were conducted on 'bulk' soil aggregates, ground powders prepared from the <150 microm soil fractions and on smears of both the bulk soil and <150 microm material placed on cotton cloth. X-ray count data were obtained using area scans and spot analyses at different magnifications. The effects on elemental peak height ratios of varying the SEM chamber pressure, beam spot size, emission current and accelerating voltage were also examined. The peak height ratios for oxygen, silicon, aluminium, potassium, calcium and iron were found to show little variation as a function of chamber pressure, spot size and emission current over the ranges examined, but a strong dependency on accelerating voltage was observed. Within-sample variation in results, expressed by the percentage coefficient of variation, was found to be lowest for area scan analyses of the ground <150 microm fractions and greatest for the spot analyses of the bulk soil aggregates and the <150 microm fractions. We conclude that comparison of elemental peak height ratios determined by EDXRA can be a useful tool for rapid screening of soil samples, especially when combined with investigation of other attributes of the soil traces such as colour, fabric and the composition, shapes and surface textures of individual particles or aggregates within the soil traces. If sufficient material is available and can be readily separated without contamination or loss, higher resolution and more precise elemental data should be obtained by methods such as inductively coupled plasma atomic-emission spectrometry (ICP-AES) or mass-spectrometry (ICP-MS).

Elemental analysis of soil samples for forensic purposes by inductively coupled plasma spectrometry--precision considerations.

Major and trace elemental composition provides a powerful basis for forensic comparison of soils, sediments and rocks. However, it is important that the potential 'errors' associated with the procedures are fully understood and quantified, and that standard protocols are applied for sample preparation and analysis. This paper describes such a standard procedure and reports results both for instrumental measurement precision (repeatability) and overall 'method' precision (reproducibility). Results obtained both for certified reference materials and example soils show that the instrumental measurement precision (defined by the coefficient of variation, CV) for most elements is better than 2-3%. When different solutions were prepared from the same sample powder, and from different sub-sample powders prepared from the same parent sample, the CV increased to c. 5-6% for many elements. The largest variation was found in results for certified reference materials generated from 23 instrument runs over an 18 month period (mean CV=c. 11%). Some elements were more variable than others. W was found to be the most variable and the elements V, Cr, Co, Cu, Ni and Pb also showed higher than average variability. SiO2, CaO, Al2O3 and Fe2O3, Rb, Sr, La, Ce, Nd and Sm generally showed lower than average variability, and therefore provided the most reliable basis for inter-sample comparison. It is recommended that, whenever possible, samples relating to the same investigation should be analysed in the same sample run, or at least sequential runs.

Particle size analysis of sediments, soils and related particulate materials for forensic purposes using laser granulometry.

Particle size is a fundamental property of any sediment, soil or dust deposit which can provide important clues to nature and provenance. For forensic work, the particle size distribution of sometimes very small samples requires precise determination using a rapid and reliable method with a high resolution. The Coulter trade mark LS230 laser granulometer offers rapid and accurate sizing of particles in the range 0.04-2000 microm for a variety of sample types, including soils, unconsolidated sediments, dusts, powders and other particulate materials. Reliable results are possible for sample weights of just 50 mg. Discrimination between samples is performed on the basis of the shape of the particle size curves and statistical measures of the size distributions. In routine forensic work laser granulometry data can rarely be used in isolation and should be considered in combination with results from other techniques to reach an overall conclusion.

The potential use of continuous-flow isotope-ratio mass spectrometry as a tool in forensic soil analysis: a preliminary report.

The use of continuous-flow isotope-ratio mass spectrometry (CF-IRMS) as a tool in soil analysis has been assessed as part of a larger study using a number of geological techniques applied in a forensic context. Carbon and nitrogen isotopic ratios, delta13C and delta15N, have been analysed to investigate situations which have arisen from crime casework. Three questions have been addressed: the role of spatial variation found over the short-scale (less than 20 m), temporal variation over a period of almost 2 years, and the variation found between source soils and soil transferred to footwear soles during a simple one-stage transfer process. Results are presented for the three experiments. The use of carbon and nitrogen isotopes has been shown to be useful in discriminating between soil types and sample locations, even when sampling occurs at a different time (as might be the case with a crime scene). In cases of primary transfer (from a source soil by a one-stage transfer to another surface, in this case, shoes and boots), the combination of carbon and nitrogen isotope ratios is a valuable tool in discriminating between sites and in showing the relationship of the transferred samples to the relevant source soils. Used in combination with other analytical techniques, isotopic analysis may prove to be a useful tool in a forensic context.