The Interference Of Plastics On The Identification Of Gasoline Combustion Debris

As one of the most common devastating disasters,arson is a problem that goes much beyond property crime.It also takes the lives of many people and animals every year and produces feelings of insecurity to citizens.Nevertheless,for such a crime,it is difficult to prosecute because,by its nature,it destroys evidence.Owning to the contribution to a judgement,ignitable liquid debris is qualified as an important evidence for the trial of suspects and the investigation of fire cause.However,some common substrates will release the target characteristic components of ignitable liquids,and therefore affect the data interpretation.As one of the preferred accelerators for arsonists,gasoline is the most common ignitable liquids in every country around the world.However,plastic combustion products exist some gasoline characteristic components,which may interfere with the correct identification of gasoline combustion debris.In this paper,we choose gasoline and plastics as the research objects.By carriing out a series of combustion experiments,the composition variation laws of gasoline combustion debris were studied.Furthermore,in combination with ASTM E1618-14 and GBT 16840.5-12 identification standards,we have established a more scientific method for the identification of gasoline combustion debris.Through the comprehensive analysis of plastic combustion products,the combustion markers and interference components were summarized.Based on the combustion products of some common plastics,we have deduced the reaction mechanisms.On this basis,we have summarized the structural characteristics of plastics which tend to produce the gasoline characteristic components and established a set of prediction methods for interference components.We have explored the changing rules of the interference components under different conditions,which would provide a scientific basis for extracting the interference components in the pyrolysis reference samples.Finally,the feasibility of the methods was verified by practical cases.The main conclusions in this work are as follows:1.At present,international and domestic institutions have established ASTM E1618-14 and GBT 16840.5-12 identification standards respectively.However,these two standards were designed for all ignitable liquids,not specifically for gasoline.Through a series of combustion experiments,we have studied the composition variation laws of gasoline combustion debris.On the basis of these studies,we have worked out a scientific and effective identification scheme for gasoline combustion debris.During the combustion process,gasoline accord with the changing rule that the relative content of light components decreases and the relative content of heavy components increasies.In the face of samples,the appraisers should make a preliminary determination of the type of the ignitable liquids and combustion degree according to total ion chromatograms.After determining ignitable liquid as gasoline,the appraisers should select the appropriate target characteristic components for accurate discrimination.2.Due to the advantages of light weight,low cost,non-corrosive and so on,plastics have been widely applied to daily necessities,and found in large quantities at fire sites.Firstly,we have improved the conventional detection method for volatile materials.Then,combining with the target characteristic components of gasoline,we have comprehensively analyzed the combustion products of common plastics,and summarized the combustion markers and interference components.The results show that the conventional detection method can't detect some low-content components.For this reason,the pretreatment has been improved to overcome for its shortcomings.Among the common plastics,polyvinyl chloride combustion residues contain a large number of gasoline target characteristic components,which seriously interfere with the identification of gasoline.Polystyrene,polyvinyl alcohol,polyurethane,polycarbonate and acrylonitrile-butadiene-styrene have slightly weaker interference.The combustion residues of polyethylene,polypropylene,polyformaldehyde,polymethyl methacrylate,phenolic resin,polyethylene terephthalate and polyamide do not contain gasoline target characteristic components and cannot interfere with the identification.3.In addition to combustion products,plastic pyrolysis products can also interfere with the identification of gasoline combustion debris,and gradually attract the attention of researchers.By simulating the pyrolysis conditions by heating,we have studied the influencing factors of pyrolysis and summarized the interference components.On this basis,the differences between combustion and pyrolysis products have been explored.It is found that plastic pyrolysis products are affected by many factors such as degree of pyrolysis,temperature,and substrate type.Largely carbonized samples have many pyrolysis products and they seriously interfere with the identification of gasoline combustion residues.However slightly and completely carbonized samples have much weaker interference.At low temperatures,plastics release less pyrolysis products and have limited interference with the identification of gasoline combustion residue.At high temperatures,the plastic pyrolysis products increase,which may interfere with the identification.The pyrolysis products of polyvinyl chloride contain a large number of gasoline target characteristic components,which seriously interfere with the identification of gasoline combustion residues.In comparison with polyvinyl chloride,the pyrolysis products of polyvinyl alcohol,polystyrene,polycarbonate and other plastics have smller amount of gasoline target characteristic components.The plastics with simple reaction mechanisms and few products have the similar kinds of combustion and pyrolysis products.Plastics with complex reaction mechanisms and many products have slightly more pyrolysis products than combustion products,but the components with higher relative contents are the same.4.The study of matrix interference mechanism can not only explore the combustion reaction process,but also provide references for the prediction of interference components in other combustion debris.Based on the combustion products of common plastics,we have deduced the reaction mechanisms and summarized the structural characteristics of plastics which tend to produce the gasoline target characteristic components.On this basis,we established a set of prediction methods for interference components.The experimental results indicate that there are two kinds of plastics that tend to produce gasoline target characteristic components.The first one is the plastics consisting of C-C chain,which is connected with a substituent every other carbon atom.This kind of plastics mainly undergoes side group elimination and cyclization reactions,resulting in a large number of aromatic hydrocarbons such as benzene,toluene,C2-alkylbenzene,naphthalene and methyl naphthalene.The other one has benzene ring in the molecular structure.This kind of plastics mainly occurs chain breaking,which usually produces alkylbenzene compounds such as toluene,C2-alkylbenzene and so on.Facing with the samples without relevant interference data,the appraisers can determine the most possible reaction pathes according to the chemical structures and the average bond dissociation energies,and then infer the combustion products and interference components.From the aspects of combustion products,reaction mechanisms and influencing factors,we have comprehensively analyzed the interference of plastics on the identification of gasoline combustion residues.It was hoped that our research results could provide reference value for appraisers to get accurate and scientific appraisal opinions.