Analytical Techniques for the Comparison of Soil Samples
The forensic technique commonly used for the analysis of soil samples is the density gradient tube technique. Glass tubes measuring 6 to 10 millimetres in diameter and 25 to 40 centimetres in length are filled with several layers of two liquids mixed in varying proportions such that each layer has a different density. An example is the mixture of tetrabromoethane, which has a density of 2.96 gmL-1 with ethane, with a density of 0.789 gmL-1. Using varying proportions of each, from pure tetrabromoethane (bottom of the tube) to pure ethanol (top of the tube) provides a density gradient into which the soil specimen is added. The soil components then sink to the layers corresponding to their own density values and the distribution of particles can be compared between soil specimens.

Inductively coupled plasma coupled to mass spectrometry (ICP-MS) analyses the elemental composition of the soil. The plasma is formed by argon gas flowing through a radiofrequency field where it is kept in a state of partial ionisation, i.e. the gas consists partly of electrically charged particles. This allows it to reach very high temperatures of up to approx. 10,000ºC. The sample being analysed is introduced into the plasma as a fine droplet aerosol. ICP-MS is the combination of an ICP with a mass spectrometer (MS). The ions generated by the ICP are directed into the MS, which separates the ions according to their mass-to-charge ratio. Thus, ions of a selected mass-to-charge ratio can be detected and quantitated.

The coupling of ICP-MS shows many advantages over other analysis techniques. Benefits of the ICP over other radiation sources include improvement in excitation and ionisation efficiencies and the reduction or elimination of many of the chemical interferences found in flames or furnaces. Mass spectrometry generates a large amount of information, has high throughput capabilities, high sensitivity and low limits of detection. ICP-MS is capable of multi-elemental detection which reduces analysis time and therefore increases sample throughput. ICP-MS is one of the few analytical techniques that permits that quantitation of elemental isotopic concentrations and ratios. It can achieve very low limits of detection (e.g. 1-10 ng L-1)

The geographical location of soil samples gives rise to variation in their elemental composition. Thus, soil samples can be compared to samples collected from their suspected sources of origin.