Submitted (September 2012) in partial fulfilment of the requirement for the degree of Masters of Science in Fire Investigation - School of Forensic and Investigative Sciences - University of Central Lancashire (UCLan).

The integrity of evidence is extremely important in forensic science and in the judicial system to avoid miscarriages of justice. Contamination and cross-contamination must be prevented. Evidence should be packed in a container that prevents loss or addition to the material. Nylon bags are the most used container for fire debris evidence in the UK and are also used in several European countries. Cross-contamination of nylon bags in normal casework conditions was studied, using heavy loaded petrol fire debris and using automotive topcoat thinner (oxygenated solvent). Samples were sealed with swan-neck with plastic cable ties in both ends and were stored in crates, typically used in the UK to transport evidence from the fire scene to the laboratories. The analyses were performed using gas chromatography / mass spectrometry. Cross-contamination was found for petrol in the two brands of nylon bags used, after significantly different periods of time. In both brands the same aromatic compounds were identified. Cross-contamination was also found using automotive topcoat thinner and using only one brand of bags. Oxygenated compounds and one aromatic compound were identified.