Background interference from car carpets—the evidential value of petrol residues in cases of suspected vehicle arson

Abstract

This study was undertaken to test the theory that there is a natural occurrence of petrol on the carpet or carpet mats of motor vehicles that would interfere with the potential determination of arson residues. Part of this study was also to determine what levels of background interference due to the carpet matrix are likely to be encountered in the general population of motor vehicles. The study was conducted in three parts. 1. A persistence study was conducted to determine the evaporation and persistence of unleaded petrol on carpet. The results indicate that small volumes of petrol (less than 100 μl) are unlikely to be detected on carpet after a 24 h period. Larger volumes will be detected after this period, but will generally not be detectable after 1 week. 2. A known history study was conducted by the insertion of carpet mats, for varying lengths of time, into the driver area of separate vehicles, with subsequent analysis for the presence of petrol. A ‘history’ sheet was completed by the occupants of the vehicles during the insertion periods to record the frequency of contact with petrol, and general usage of the vehicles. The results indicate that petrol will not normally be found on previously uncontaminated carpet mats after a 6-week period of use in a vehicle, however, the occupation and behaviour of the occupants can affect the types of compounds deposited onto the carpet. 3. An unknown history study was conducted via the collection of carpet or carpet mat samples from vehicles with an unknown history, and subsequent analysis for the presence of petrol. Six of the 150 samples examined contained petrol. These results indicate that only a small proportion of motor vehicles will exhibit the presence of petrol on carpet or carpet mats, and then only as evaporated petrol. All of the above findings increase the evidential value of finding significant volumes of fresh or slightly evaporated petrol on carpet products in motor vehicles.