Zubaidah A*, Chia Poh Ling, Lim Chin Chin, Michael Tay Ming Kiong
International Association of Forensic Sciences 17th Triennial Meeting, Hong Kong
21-26 August 2005. (poster presentation)
Background: In the analysis of suspected arson samples, analysts look for the presence of a series of normal alkanes, or certain profiles of aromatic peaks as an indication of ignitable liquid. Identification is based on the classification standard ASTM 1387 for gas chromatography (GC) and ASTM 1618 for gas chromatography/mass spectrometry (GC/MS). In the interpretation of GC results, analysts have to be careful of the possibility of two types of interference products. The first type arises from the actual liquid petroleum products originally present in substrate materials. One good example is the kerosene fraction present in printing inks for newspapers. Analysis of such newspapers would indicate the presence of an alkane profile consistent with that of kerosene. The second type of interference is due to the emission of pyrolysis products during the combustion of materials such as carpet tiles, adhesives and plastics. This second type of interference is easily recognizable and shall not be discussed here.
Lentini et al (J. Forensic Sci 2000;45(5):968-989) studied volatile components detectable in several kinds of otherwise uncontaminated substrates, including clothing, shoes, household products, building materials, paper products, cardboard and adhesives. The results showed that due to the use of petroleum-derived liquids in the manufacture of these materials, it is frequently possible to detect the liquids, even when the products are several years old.
Burnt and unburnt plastic bottles and containers are often found at or near fire scenes. Plastic bottles are commonly used for containing mineral water, drinks and other liquids in everyday domestic and work environments. However, these containers are also commonly used by arsonists to convey ignitable liquids to the scene, and are therefore submitted by fire investigators to forensic laboratories for the examination of ignitable liquid residues.
Methods and results: This study surveys a wide variety of household plastic containers to determine the type and concentration of the alkanes and/or aromatic peaks that are present and may potentially be mistaken for ignitable liquids carried in the container by an arsonist. Materials of the plastic containers were first analyzed using Fourier Transform Infrared Spectrometry (FT-IR) to determine the type of polymer used. Sampling was then done on both the burnt and unburnt samples using passive headspace carbon strip adsorption technique (ASTM 1412). Analysis was carried out using GC/MS as described in ASTM 1618. Both the total ion chromatograms and extracted ion chromatograms were used to look for the presence of alkanes and aromatic peaks. It has been observed in our study that traces of alkanes and aromatic compounds could indeed be detected in some plastic bottles.
Conclusion: This paper demonstrates the problem of background interference in plastic bottles and how this may complicate the interpretation of chromatograms in trace level arson analysis.