Separation and concentration of trace evidence from soils using a hydropneumatic elutriation trace evidence concentrator (TEC)

Abstract

Trace evidence is a forensic science term that describes any evidence that occurs in very small quantities and which usually requires a microscope for visualisation and analysis. Common types include hairs, textile fibres, paint fragments, glass, bomb parts, plastics, metal fragments and other similar materials. Methods of analysis for trace evidence are well accepted and validated. The major challenge has been, and continues to be, the discovery and isolation of fragments of trace evidence from a matrix such as soil or similar debris. Evidence in outdoor crimes such as bombings and fires is especially susceptible to difficulties in evidence collection. Traditional methods for trace evidence collection from soils include hand sorting under a low power microscope and the use of nested sieves of decreasing mesh size. Both of these methods suffer from being extremely time consuming and recovery rates are generally very low owing to the difficulty in picking out soil coated fragments. The Trace Evidence Concentrator (TEC) system utilises the concept of density gradient separation by combining centrifugal forces which separate more dense materials from lower density materials. The lighter density materials of trace evidence are carried by surface tension forces at the air-water interface of small bubbles to a stack of multiple sized screens which retain the trace evidence. Additionally, the TEC cleans glass fragments, vehicle headlamp filaments, paint chips, and unexploded ordnance lost at the scenes of violent accidents and crimes. The result is that recovery rates can be dramatically increased and search times significantly reduced.