Effect of activated charcoal on preservation of volatile hydrocarbons in arson samples

Abstract

The intention of this study has been to establish the effectiveness and practicality of several preservation techniques to minimise microbial degradation of volatile organic hydrocarbons that constitute arson evidence. Several typical arson samples have been tested: used carpet, wood and foam with and without additions of known hydrocarbon degrading bacteria (Pseudomonas pudia and P. flourescens). Samples were either unburnt or subject to a simulated fire and then all the samples were stored for periods of up to 16 weeks prior to analyses. The hydrocarbons in arson samples were detected by GC-Mass Spectroscopy using six key compounds to monitor degradation: heptane, decanol, methylbenzene, 1,3-dimethylbenzene, 1,2,3-11 trimethylbenzene, and 1,2,3,4-tetramethylbenzene. The numbers of bacteria in samples were also monitored. The techniques used to reduce microbial degradation included removal of oxygen by oxidation of iron or by substitution with nitrogen gas, reducing water availability from the samples using a desiccant and by the addition of activated charcoal bags at the time of sample collection. The effect that storage at both low and high temperatures was also investigated. The accelerant, petroleum distillate, degraded in all test scenarios except storage at -20C. After 4 weeks of storage at room temperature, the natural microbial populations in both burnt and unburnt carpet samples degraded the accelerant's six marker compounds by 22% and 32% respectively. By 8 weeks both had degraded by over 40%. The degradation was more rapid if bacterial cultures were added with a decrease of 48% in the unburnt samples and by 25% in the burnt samples after 4 weeks. Similar trends were seen with the painted and unpainted wood and furniture foam samples. The removal of oxygen and water was not sufficient to stop microbial growth and there was no decrease in the degradation of hydrocarbons. The use of activated charcoal in comparison greatly reduced degradation with a decrease of hydrocarbons of only 7.5% after 4 weeks in unburnt samples and 5.5% in burnt samples. The activated charcoal had no effect on microbial growth so the effect is most likely due to the volatile organic compounds being adsorbed by the charcoal and inaccessible for microbial degradation. While storage at low temperatures is the best method of storage to maintain sample integrity the use of activated charcoal added at the time of sample collection provides a cheap and practical alternative that is amenable with current practice.

Date of Award	2010
Original language	English