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FIRE INVESTIGATION | Laboratory
922 FIRE INVESTIGATION/Laboratory Munday JW (1994) Safety at Scenes of Fire and Related Incidents. London: Fire Protection Association. Noon R (1995) The Engineering Analysis of Fires and Explosions. Boca Raton: CRC Press. White PC (ed.) (1998) Crime scene to court. The Essentials of Forensic Science. London: Royal Society of Chemistry.
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Laboratory P J Thatcher, Forensic Services, Northern Territory Police Fire and Emergency Services, Darwin, Northern Territory, Australia J Kelleher, Fire and Explosion Investigation Section, Victoria Forensic Science Centre, Melbourne, Victoria, Australia
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Copyright # 2000 Academic Press doi:10.1006/rwfs.2000.0530
Introduction The effectiveness of any laboratory examination conducted on items taken from a crime (or other) scene is fundamentally linked to the quality of the scene examination. Included in this examination should be the recording of the scene, the interpretation of events through scene reconstruction, sample selection and sample packaging. The premise applies particularly to the laboratory examination of items taken from fire scenes. In addition to diagnosing a possible fire origin and cause, a competent fire-scene examination can provide evidence of point of entry and other offenses. As a consequence, a thorough scene examination could require the attendance of a team of experts with a range of different skills, who have the potential to generate a range of samples requiring different scientific and technical examinations. In order that the maximum scientific value is obtained from these samples, the forensic science laboratory must provide scientific skills that complement the scene skills of the investigative team. Therefore, although this article will specifically refer to laboratory skills and techniques applied to fire-cause investigation, it is necessary to consider briefly the range of fire-scene examination skills provided by an investigative team. Scene Expertise
. Fire-cause investigator A fire-cause investigator will attempt to determine the fire origin and cause. Supplementary information concerning ignition,
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fire spread, combustion properties, etc. will then be provided by the laboratory examination of samples taken from the scene. Fingerprints examiner A fingerprint expert can locate and recover evidence at fire scenes pertaining to the identity of victims and offenders. The expert often submits liftings of latent fingerprints and other items to the laboratory for further examination. Crime scene examiner A trained crime scene examiner will locate evidence that assists in the reconstruction of the incident and collect evidence accordingly, for example, castings of jemmy marks from the point of entry, shoe impressions, tire impressions, burnt documents, biological samples, etc. Crime scene recorder Although all members of the investigative team will meticulously record relevant aspects of the scene, occasionally specialists are required to take photographs, record videos and draft accurate plans. Other experts Some fires require the attendance of specialists from other disciplines. Examples of this include electricians, engineers and, in the case of fires in clandestine laboratories, drug analysis experts.
Laboratory Expertise
If it is expected that a single laboratory will provide full scientific support to the investigative team, a wide range of forensic science skills will be required in that laboratory. These will include the following. . Forensic biology The examination of items containing biological evidence (blood, saliva, semen, etc.) can assist in the identification of an offender and/or victim through DNA profiling. . Fingerprints Specialized techniques for the visualization of latent fingerprints can be conducted in the laboratory. Also, any fingerprints located can be classified and compared with fingerprint files for the purpose of identifying an offender or victim. . Criminalistics The comparison of casts, photographs of toolmarks, etc. with items taken from suspects and offenders are conducted in the laboratory to assist with the identification of offenders. . Document examination Document examiners can use specialized methods to restore, conserve and decipher fire-damaged documents. These skills can assist investigators examining the circumstances surrounding the fire. Notwithstanding these important investigative requirements, by far the largest scientific effort in the investigation of fire scenes is the examination of items
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collected for their potential to provide evidence concerning the fire cause. Although the majority of these examinations concern the extraction and identification of flammable liquids and flammable liquid residues from debris, clothing, soil and other samples, there are other less common analytical procedures that require consideration. The remainder of this article will focus on all these issues.
Flammable Liquids The identification of areas of a fire scene possibly affected by the presence of flammable or combustible liquids (see below), and of areas where flammable or combustible liquid residues may remain, are clearly relevant to a fire scene examination. Identification of these liquids can be of importance in determining fire spread and can be of critical importance in the interpretation of the fire cause. It should be noted that flammable liquids, such as petrol, have flash points at or below 618C. Combustible liquids, such as automotive diesel fuel, have flash points above 618C. The term `flammable liquid' generally includes combustible liquids unless otherwise specified.
Flammable Liquids of Petroleum Origin In a commonly encountered fire-scene situation, most flammable liquids present are derived from petroleum products, typically petrol, kerosene, mineral turpentine or automotive diesel fuel. These liquids are not water-soluble, and they are absorbed by, or adsorbed onto, soil, furniture, floor coverings, fittings and building materials. They all evaporate at temperatures readily achieved in fires and some even exert significant vapor pressures at room temperature. A flammable liquid layer can be consumed if the substrate itself is consumed. However, because of modern fire-fighting techniques, in many cases substantial unburnt absorbent or adsorbent material remains, containing identifiable flammable liquid residues. There are three steps in the identification process. The flammable liquid must be extracted, it may require clean-up, and finally, identification through analysis. Extraction
Direct sampling This technique can be used for samples of flammable liquid, including relatively dilute samples. It can be used for both liquid and gaseous substances. With liquid samples, dilution with an appropriate solvent may be required, because
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most analytical methods are designed for low flammable liquid concentrations. Gaseous samples can also be collected directly. With sample containers suspected of containing large amounts of flammable liquid vapor, the headspace vapor can be collected using a syringe. The syringe will necessarily be of greater volume than that used for liquids, possibly 2± 5 ml compared to the 2±5 ml used for liquid samples. The direct methods are simple and relatively rapid, although they have limited application. They require minimal preparation time, and allow almost immediate analysis. The obvious disadvantage is their relatively low sensitivity, and also, in the case of vapor samples, direct gaseous injection tends to favor collection of lower boiling point components. Headspace adsorption This is the recovery technique most widely used at present. In all adsorption techniques, the flammable liquid vapor is adsorbed onto a suitable medium, often activated charcoal or a proprietary adsorbent such as `Tenax'. The adsorption process usually requires the sample to be heated and may be passive or dynamic. In the former, an adsorption medium is placed in the sample container and allowed to stand for some time, and in the latter, the headspace vapor is extracted through the adsorption medium by a pump. Headspace adsorption is an extremely sensitive method, simpler and more rapid than steam or vacuum distillation and does not require expensive glassware or extensive equipment cleaning operations. However, the extreme sensitivity increases the potential for contamination and interpretation to become issues, and headspace techniques cannot easily be related to the total amount of flammable liquid in the sample. Headspace adsorption techniques generally favor recovery of lower boiling point hydrocarbon components. The adsorbent is often enclosed in a glass tube that can be prepared as required or can be purchased prepacked. There are other forms of support in regular use, such as adsorbent mounted on a wire or paper strip. These are essentially minor variations, and the same considerations apply to these as to methods using glass tubes. Solid phase microextraction (SPME) SPME is a recent development that is similar to headspace adsorption but without the necessity for the solvent extraction step. A fused silica fiber with a suitable adsorbent coating is inserted into the sample container to collect flammable liquid vapor from the sample headspace. The collected material is loaded onto a gas chromatograph column by a rapid thermal desorption process.
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The limiting factor with solid phase microextraction is the efficiency of the adsorption process, as is the case with conventional headspace adsorption. This is a relatively new method which, in view of its simplicity and speed, may become more common. The sensitivity is comparable to that of headspace adsorption methods. Distillation The oldest extraction technique is distillation. Steam distillation, where steam is passed through the debris to vaporize and remove the flammable liquid, is most common, although vacuum distillation has also been used. It is a relatively insensitive method, but it has the advantage that it can provide a guide to the amount of flammable liquid originally present. It requires relatively large samples but is less sensitive to contamination than some other methods. Distillation techniques tend to favor recovery of higher boiling point petroleum components. A major concern with steam distillation is the recovery of water-miscible flammable liquids. Ethanol, in the form of methylated spirits, is a water-miscible solvent commonly used as a solvent and cleaning agent. The boiling point (788C) of ethanol is low compared to most flammable liquids, and losses by evaporation can be substantial. More importantly, ethanol is miscible with the water distilled in the process, and, as a consequence, as the distillation progresses, the concentration of ethanol will decrease. Therefore, in the steam distillation operation, where a water-miscible substance is known or suspected to be present, the first few milliliters of water distilled should be analyzed. At this stage of the process, the flammable liquid concentration will be at a maximum, and the distillate will not be significantly diluted. The difficulty of identifying these low-boiling-point, water-miscible substances is a significant argument in favor of alternative methods of sample treatment. Solvent extraction This is a basic chemical technique that can be applied to the analysis of samples of fire debris. It is conveniently applied to small samples, and requires the sample to be immersed in a suitable volatile solvent. Any flammable liquid present is absorbed by the solvent, which is then removed and, if necessary, concentrated through evaporation. The technique is at least as sensitive as distillation and is relatively simple. The major disadvantage of solvent extraction is that many pyrolysis products, dyes and other chemicals are also extracted, in some cases presenting major difficulties for the subsequent analysis. Solvent extraction can be used for solid or liquid samples. It has particular uses in small samples and in the separation of small amounts of flammable liquid remaining in containers.
Clean-up
Clean-up refers to a process of cleaning a sample or extract to prepare it for analysis. Direct injection of headspace vapor and solid-phase microextraction techniques, by their nature, render clean-up unnecessary or impractical. However, in extraction techniques, which result in the preparation of a small liquid aliquot for analysis, clean-up can be useful and occasionally essential. A simple clean-up procedure is filtration. This can be used to remove suspended solids from a neat liquid sample, or those resulting from a solvent extraction procedure. More complex clean-up techniques involve processes such as acid stripping. Acid stripping is used to remove oxygenated or nitrogenated compounds that arise from combustion of synthetic materials. These compounds can interfere with analysis of flammable and combustible liquids and addition of a small amount of strong mineral acid removes these compounds by extraction into the acid. The immiscible hydrocarbon layer can be analyzed in the normal manner. Chromatographic techniques, such as solid-phase extraction and thin layer chromatography, can be useful for clean-up of larger samples but are difficult to adapt to smaller quantities of flammable liquid. With solid-phase extraction, for example, the adsorption extract can be separated on suitable columns into aliphatic and aromatic hydrocarbon fractions. These fractions can be analyzed separately, thus minimizing the interference caused by pyrolysis products. Any procedure conducted on a sample increases the possibility that the sample will be contaminated or otherwise adversely affected. Although this can be monitored by appropriate use of control samples, the most effective method of reducing clean-up contamination problems is by the minimal application of clean-up procedures. There is a further advantage in minimal clean-up; the overall method remains relatively simple. Addition of a clean-up step is a significant increment in the complexity of the analysis, and the use of that step is only justified by a significant increase in the information recovered. Analysis
Essentially the same analytical procedures are adopted for the various classes of flammable and combustible liquids. The liquids can be derived from petroleum, and indeed, the most common flammable or combustible liquids detected at fire scenes are petrol (gasoline), kerosene, automotive diesel fuel (distillate) and mineral turpentine. Despite the obvious differences in volatility, the same extraction, clean-up and analysis schemes serve for identification
FIRE INVESTIGATION/Laboratory
of all these products, and for the identification of similar products such as white spirit and various naphthas. However, there are other flammable and combustible products, not of petroleum origin, such as pure or wood turpentine, essential oils, ethanol (methylated spirit), natural oils and waxes. Gas chromatography (GC) GC is the principal tool for the identification of these products. Most petroleum products are composed of many chemical components. Because there are relatively strict specifications which apply to petroleum products, in each different product the components will be present in a ratio which is characteristic of that product. Analysis by gas chromatography separates the individual components, presents the components according to volatility and identifies the amount of each component. This produces a `fingerprint' of the product that can be compared with known standards for identification purposes. Several liquids manufactured from petroleum, and many other commercial products (e.g. solvents such as methyl ethyl ketone or methyl isobutyl ketone, ethanol, etc.) have a single component, and it is not possible to identify a product on the basis of the chromatogram retention time alone. Products such as gasoline, kerosene and automotive diesel fuel, and even nonfuel solvents such as mineral turpentine or white spirit, can usually be identified in gas-liquid chromatography by a `fingerprint'-type comparison of a complex mixture of features. The features considered in any such comparison will include relative retention times, peak heights and peak height ratios. A single component substance obviously does not allow relative peak heights or peak height ratios to be compared, leaving only comparison of retention times. It is necessary, in these cases, to adopt a procedure that will allow the identity to be confirmed. This may consist of repeating the analysis with a `spiked' sample, to determine whether the sample and standard co-elute, that is, present a single peak. More commonly, the analysis is completed on a gas chromatograph/mass spectrometer. Modern laboratories use a technique known as capillary gas chromatography, with a specialized detection system called a flame ionization detector (FID), to easily detect microgram and even nanogram amounts of hydrocarbon products. Gas chromatography/mass spectrometry (GC/MS) GC/MS uses the same component separation procedure as gas chromatography but, as the various components are detected, they are individually analyzed and compared with a library of standards to positively identify the individual components. In the
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case of a single component product, this may be regarded as a positive identification. With specialty products such as solvents or thinners, it may still be necessary to run comparison standards. Specialty products are a major challenge in terms of brand identification. Most product types are similar, and examination of the components that make up the product may enable it to be identified in a generic sense. Positive brand identification is much more difficult to achieve, as it requires elimination of other possibilities. The identification of such a product is likely then to be as `. . . a toluene/xylene based thinner . . .' rather than as . . . Acme VM&P Naphtha . . .'. However, for evidentiary purposes, it may be appropriate to also describe the toluene/xylene based thinner as `. . . consistent with the composition of Acme VM&P Naphtha . . .'. Techniques in GC/MS, such as selected ion monitoring (SIM) allow identification to be made on the basis of the ratio of selected ions characteristic of the product/s of interest. This can reduce, but not eliminate, the effects of contamination from various sources, including pyrolysis products. SIM reduces the complexity of the comparison to a manageable level, although there are now pattern matching and library programs available that can considerably alleviate the difficulty of identifying an unknown. Infra-red spectrometry/Fourier transform infrared spectrometry (IR/FTIR) IR/FTIR is an analytical instrumental technique used widely in chemical analysis to identify the chemical `fingerprint'. An infrared spectrum is obtained that can be particularly useful for the identification of plastics and synthetic materials. The spectrum derived from the infrared analysis reflects the chemical bonding of the molecule, providing some information in relation to the structure of the unknown substance. The particular advantage of FTIR is that extremely small samples can be analyzed rapidly and accurately. As with GC and GC/MS, there are computer-based pattern matching programs to match sample spectra to library spectra that considerably reduce the difficulty of interpretation of complex spectra. In using spectral libraries, care must be taken not to limit the search to the library contents, because even the largest reference libraries do not cover every possible product. Because infrared spectrometry analyzes all the chemical groups present in the sample, only samples that are essentially unadulterated can be identified using IR/FTIR. Microscopy/scanning electron microscopy These techniques are applied in the analysis of debris and
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residues from fire and explosion scenes. An unidentified object or unknown material can be identified by examination using a relatively low power microscope. This may permit recognition of components of an incendiary device, of details on paper or material used as fuel, or a physical match of evidence found at the scene to an item found at a suspect's house. Particles or crystals may be selected for more detailed examination, under a higher power microscope or even a scanning electron microscope (SEM). The SEM, with the capability of magnification to 300 0006, can be used to study the morphology of materials and for X-ray analysis. The SEM can provide a rapid elemental analysis of microscopic samples, detailing the amount and distribution of the medium to heavier elements. The actual shape or surface characteristics of the material being examined may be of significance in some cases but, more commonly, the microscope is used to locate materials requiring analysis. This can provide a starting point for further investigations, possibly using additional instrumental techniques. The presence of barium, for example, may suggest the presence of sparkler composition, whereas the presence of strontium may suggest the presence of pyrotechnic material from a red handheld flare. In each case, further analysis would be conducted, such as ion chromatography to identify the nitrate or other oxidizing agent present.
Electrical Components and Electrical Wiring The diagnosis of electrical sources of ignition is one of the most challenging aspects of fire investigation. The scene examination process may result in the identification of one or more possible sources of ignition, which may or may not be electrical. A laboratory examination might be required to determine whether, say, an appliance was a possible source of ignition, whether a timing device was operating a light switch or an incendiary device, etc. Examination of individual electronic components with respect to their intrinsic properties, for example, measurement of the exact resistance or capacitance, is often beyond the capabilities of the forensic science laboratory. This is a specialized examination, requiring electronic engineering expertise, which is likely to be found only in universities or major telecommunications companies. The same may be said for integrated circuits and programmable microprocessors. A forensic scientist might identify them as such, but to determine exactly how the microprocessor operates is almost certainly a task for an electronic engineer. Examinations requiring laboratory expertise are often based on the application of simple physical
science. For example, a burnt and melted mixture of components and plastic might be examined using infrared or ultraviolet light, or X-rays. By means of X-ray examination in particular, it is sometimes possible to trace the circuit outlines clearly and identify the construction and means of operation of the components. Similarly, with individual components, the laboratory examiner is often able to identify a clock, wires, a battery and a short length of resistance wire. Obviously, these are the components required for a crude time-delay ignition mechanism. It is possible to calculate the current that can be drawn, and the heat output of the resistance wire, to determine whether this is a functional system. Laboratory examinations can enable appliances such as power boards or double adaptors to be identified. Careful cleaning, removal of melted material and microscopic examination can enable determination of the position of switches, or whether appliance pins were present in power points, or whether a thermal cut-out switch has operated. These examinations are technically straightforward, albeit they require some practical experience and theoretical knowledge. Clearly though, they can be of great significance since they can result in a definite answer to a potentially crucial question, such as whether a suspect appliance was connected to power or turned on. Formulating and addressing these basic questions, and interpreting the results of consulting electronic engineers on more complex questions, is an appropriate role for a forensic science laboratory. There are many proponents of theories relating to electrical arcing. Arcing is a well-known and understood phenomenon, which occurs under specified conditions and which can cause ignition of combustible material. When evidence of arcing is produced, there is the difficulty of determining whether it occurred prior to or as a result of the fire. Any microscopic examination of electrical wiring to determine whether arcing has occurred should be combined with scene observations to determine whether the origin of the arcing is consistent with the apparent point of origin of the fire. Related theories propose that the chemical composition of copper wires, that is, the relative oxygen and/ or chlorine content of electrical wires, as determined by Auger electron spectroscopy or scanning electron microscopy, can be used to ascertain the atmospheric conditions to which the melting wire was subjected. The basis of this theory is that high oxygen and/or low chlorine levels are consistent with arcing prior to the fire; low oxygen and/or high chlorine levels are consistent with arcing caused by the fire. Indeed, in controlled laboratory conditions, these theories can hold true.
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However, in the wildly fluctuating atmospheric conditions existing in structure fires, and the many other factors such as varying temperatures, exposure to firefighting water/chemicals, residues from household/industrial chemicals, variations in original wire and pre-existing damage from age or exposure, simple laboratory principles are found to be inadequate. These theories are not transferable from the static laboratory environment to the dynamic environment of an actual fire scene and this type of scene/laboratory examination is neither well established nor practical.
Machinery The principles for the examination of machinery in the forensic science laboratory are essentially the same as for examination of electrical or electronic appliances or components. The initial role of the forensic scientist is to formulate the questions that need to be addressed in the examination of the machinery. Some of these may be within the province of the forensic scientist, such as whether the machinery can cause sparks in some manner, what is the normal operating temperature and does partially blocking the exhaust markedly increase manifold temperatures? All these are questions that can be resolved by careful measurement and/or observation. With machinery which is alleged to have malfunctioned, and which has been operated incorrectly or which has been severely damaged, it may be necessary to consult with the manufacturer or some more specialized agency, for example universities and consulting engineers. More complex questions, or questions relating to efficiency, ergonomics or other design factors may be addressed by specialist investigators. This may be under the supervision of a forensic scientist or police officer for continuity purposes, as a consultant may not be aware of the requirements for security and continuity of exhibits.
Summary This article has briefly reviewed the wide range of scientific expertise and techniques necessary to support the fire scene investigator in a scientific examination and interpretation of a fire scene. However, it must be remembered that the quality of the fire-scene interpretation and of the samples taken will be directly responsible for the subsequent quality and usefulness of the laboratory results. Furthermore, these laboratory results will only rarely provide any proof of the fire cause. They will, in fact, provide supporting evidence to the scene interpretation so that possible fire causes can be identified and, perhaps, eliminated or upgraded to probable causes.
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See also: Analytical Techniques: Separation Techniques; Mass Spectrometry. Crime-scene Investigation and Examination: Recording; Collection and Chain of Evidence; Recovery of Human Remains; Packaging; Preservation; Contamination.
Further Reading Almirall JR, Bruna J and Furton KG (1996) The recovery of accelerants in aqueous samples from fire debris using solid-phase microextraction. Science and Justice 36:283±288. ASTM Standard E1385-95, Standard Practice for Separation and Concentration of Flammable or Combustible Flammable Liquid Residues from Fire Debris Samples by Steam Distillation. ASTM Standard E1386-95. Standard Practice for Separation and Concentration of Combustible Liquid Residues from Fire Debris Samples by Solvent Extraction. ASTM Standard E1387-95. Standard Test Methods for Flammable or Combustible Liquid Residues in Extracts from Samples of Fire Debris by Gas Chromatography. ASTM Standard E1388-95. Standard Practice for Sampling of Headspace Vapours from Fire Debris Samples. ASTM Standard E1389-95. Standard Practice for Cleanup of Fire Debris Sample Extracts by Acid Stripping. ASTM Standard E1412-95. Standard Practice for Separation and Concentration of Flammable or Combustible Liquid Residues from Fire Debris Samples by Passive Headspace Concentration. ASTM Standard E1413-95. Standard Practice for Separation and Concentration of Ignitable Liquid Residues from Fire Debris Samples by Dynamic Headspace Concentration. Brackett JW (1955) Separation of flammable material of petroleum origin from evidence submitted in cases involving fires and suspected arson. Journal of Criminal Law, Criminology and Police Science 46:554±561. Gilbert MW (1998) The use of individual extracted ion profiles versus summed ion profiles in fire debris analysis. Journal of Forensic Science 43:871±876. Juhala JA (1979) Determination of fire debris vapours using an acid stripping procedure with subsequent gas chromatographic and gas chromatography mass spectrometry analysis. Arson Analysis Newsletter 3(4):1±19. Karkkainen M, Ilkka S and Himberg K (1994) Detection of trace levels of gasoline in arson cases by gas chromatography±mass spectrometry with an automatic on-line thermal desorber. Journal of Forensic Science 39:186± 193. Lennard C (1995) Fire (determination of causes) a review 1992 to 1995. 11th Interpol Forensic Science Symposium, Lyon, France. Mickiff C (1978) Recovering accelerants from debris. Arson Analysis Newsletter 2(6):8±20. Twibell JD, Home JM and Smalldon KW (1980) A comparison of the relative sensitivities of the adsorption wire and other methods for the detection of accelerant residues in fire debris. HOCRE Report no. 368.
928 FIRE INVESTIGATION/Physics/Thermodynamics Waters LV and Palmer LA (1993) Multiple analysis of fire debris samples using passive headspace concentration. Journal of Forensic Science 38:165±183. Woycheshin S and De Han J (1978) An evaluation of some distillation techniques. Arson Analysis Newsletter 2(5):1±16. Yip IH and Clair EG (1976) A rapid analysis of accelerants in fire debris. Canadian Forensic Science Society Journal 9(2):75±80.
Physics/Thermodynamics J H C Martin and R S Pepler, Institut de Police Scientifique et de Criminologie, Universite de Lausanne, Lausanne, Switzerland Copyright # 2000 Academic Press doi:10.1006/rwfs.2000.0525
Introduction and Overview Within the various domains of criminalistics, the investigator compares traces found at the scene of a crime with those recovered from a person, tool or place. By contrast, the work of the fire investigator is not purely comparative in nature; extensive practical experience of the examination of as many fire scenes as possible combined with a sound scientific knowledge, especially in the areas of physics and thermodynamics, is paramount to the success of the investigations. These two criteria enable him or her to identify the origin of the fire and determine its cause despite having to work on sites that have been destroyed by the fire and further disrupted during the extinguishing process. This article explores: the relevant background areas of physical thermodynamics; the role of thermodynamics in fire investigation; fire ignition and propagation; thermodynamic classification of ignition sources; and the phenomena of smouldering and flaming combustion.
Physical Thermodynamics: the Relevant Background Before considering the role of thermodynamics in fire investigation, it is essential to remember that the fundamental subject under scrutiny is concerned with the transformation of a macroscopic system which is dependent on one of the basic elements of physics: temperature. It is also important to under-
stand that an intensive thermodynamic variable is one which is dependent on the amount of a chemical substance in the system, whereas an extensive variable does not. Physical systems
A physical system is defined as that part of the universe under the influence of thermodynamics which has been identified for study. It can be: . isolated: no exchange of heat, work or matter with the surroundings is possible. The system is said to be ideal and can be assumed in various situations; . closed: exchange of heat and work, but not matter, is possible with the surroundings; . open: exchange of heat, work, and matter is possible with the surroundings. Thermodynamic principles
Two of the fundamental laws of thermodynamics can be described in the following manner: The first law of thermodynamics All systems possess an internal energy U, which is a state variable, signifying that U is independent of the history of the system. When a system undergoes a change from state 1 to state 2, the difference in thermodynamic energy, DU is: U U2
U1
While respecting the principle of the conservation of energy, the DU of a closed system can be expressed as: U Q W where DQ and DW are, respectively, the applied heat and work. For an isolated system U is constant; in keeping with the conservation of energy, U1 is therefore equal to U2. There is no energy change between the system and its surroundings therefore: Q W 0 The second law of thermodynamics This law concerns change and entropy. Entropy, S, is an extensive state variable which denotes the degree of disorder of the system. During reversible changes, the system and surroundings are in constant equilibrium. When considering such a change from a state 1 to a state 2, the entropy of a system is: Z S dQ=T irrespective of the pathway taken. However, energy dissipated in the form of heat is not an ideal process; it is spontaneous and irreversible
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Laboratory P J Thatcher, Forensic Services, Northern Territory Police Fire and Emergency Services, Darwin, Northern Territory, Australia J Kelleher, Fire and Explosion Investigation Section, Victoria Forensic Science Centre, Melbourne, Victoria, Australia
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Copyright # 2000 Academic Press doi:10.1006/rwfs.2000.0530
Introduction The effectiveness of any laboratory examination conducted on items taken from a crime (or other) scene is fundamentally linked to the quality of the scene examination. Included in this examination should be the recording of the scene, the interpretation of events through scene reconstruction, sample selection and sample packaging. The premise applies particularly to the laboratory examination of items taken from fire scenes. In addition to diagnosing a possible fire origin and cause, a competent fire-scene examination can provide evidence of point of entry and other offenses. As a consequence, a thorough scene examination could require the attendance of a team of experts with a range of different skills, who have the potential to generate a range of samples requiring different scientific and technical examinations. In order that the maximum scientific value is obtained from these samples, the forensic science laboratory must provide scientific skills that complement the scene skills of the investigative team. Therefore, although this article will specifically refer to laboratory skills and techniques applied to fire-cause investigation, it is necessary to consider briefly the range of fire-scene examination skills provided by an investigative team. Scene Expertise
. Fire-cause investigator A fire-cause investigator will attempt to determine the fire origin and cause. Supplementary information concerning ignition,
.
fire spread, combustion properties, etc. will then be provided by the laboratory examination of samples taken from the scene. Fingerprints examiner A fingerprint expert can locate and recover evidence at fire scenes pertaining to the identity of victims and offenders. The expert often submits liftings of latent fingerprints and other items to the laboratory for further examination. Crime scene examiner A trained crime scene examiner will locate evidence that assists in the reconstruction of the incident and collect evidence accordingly, for example, castings of jemmy marks from the point of entry, shoe impressions, tire impressions, burnt documents, biological samples, etc. Crime scene recorder Although all members of the investigative team will meticulously record relevant aspects of the scene, occasionally specialists are required to take photographs, record videos and draft accurate plans. Other experts Some fires require the attendance of specialists from other disciplines. Examples of this include electricians, engineers and, in the case of fires in clandestine laboratories, drug analysis experts.
Laboratory Expertise
If it is expected that a single laboratory will provide full scientific support to the investigative team, a wide range of forensic science skills will be required in that laboratory. These will include the following. . Forensic biology The examination of items containing biological evidence (blood, saliva, semen, etc.) can assist in the identification of an offender and/or victim through DNA profiling. . Fingerprints Specialized techniques for the visualization of latent fingerprints can be conducted in the laboratory. Also, any fingerprints located can be classified and compared with fingerprint files for the purpose of identifying an offender or victim. . Criminalistics The comparison of casts, photographs of toolmarks, etc. with items taken from suspects and offenders are conducted in the laboratory to assist with the identification of offenders. . Document examination Document examiners can use specialized methods to restore, conserve and decipher fire-damaged documents. These skills can assist investigators examining the circumstances surrounding the fire. Notwithstanding these important investigative requirements, by far the largest scientific effort in the investigation of fire scenes is the examination of items
FIRE INVESTIGATION/Laboratory
collected for their potential to provide evidence concerning the fire cause. Although the majority of these examinations concern the extraction and identification of flammable liquids and flammable liquid residues from debris, clothing, soil and other samples, there are other less common analytical procedures that require consideration. The remainder of this article will focus on all these issues.
Flammable Liquids The identification of areas of a fire scene possibly affected by the presence of flammable or combustible liquids (see below), and of areas where flammable or combustible liquid residues may remain, are clearly relevant to a fire scene examination. Identification of these liquids can be of importance in determining fire spread and can be of critical importance in the interpretation of the fire cause. It should be noted that flammable liquids, such as petrol, have flash points at or below 618C. Combustible liquids, such as automotive diesel fuel, have flash points above 618C. The term `flammable liquid' generally includes combustible liquids unless otherwise specified.
Flammable Liquids of Petroleum Origin In a commonly encountered fire-scene situation, most flammable liquids present are derived from petroleum products, typically petrol, kerosene, mineral turpentine or automotive diesel fuel. These liquids are not water-soluble, and they are absorbed by, or adsorbed onto, soil, furniture, floor coverings, fittings and building materials. They all evaporate at temperatures readily achieved in fires and some even exert significant vapor pressures at room temperature. A flammable liquid layer can be consumed if the substrate itself is consumed. However, because of modern fire-fighting techniques, in many cases substantial unburnt absorbent or adsorbent material remains, containing identifiable flammable liquid residues. There are three steps in the identification process. The flammable liquid must be extracted, it may require clean-up, and finally, identification through analysis. Extraction
Direct sampling This technique can be used for samples of flammable liquid, including relatively dilute samples. It can be used for both liquid and gaseous substances. With liquid samples, dilution with an appropriate solvent may be required, because
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most analytical methods are designed for low flammable liquid concentrations. Gaseous samples can also be collected directly. With sample containers suspected of containing large amounts of flammable liquid vapor, the headspace vapor can be collected using a syringe. The syringe will necessarily be of greater volume than that used for liquids, possibly 2± 5 ml compared to the 2±5 ml used for liquid samples. The direct methods are simple and relatively rapid, although they have limited application. They require minimal preparation time, and allow almost immediate analysis. The obvious disadvantage is their relatively low sensitivity, and also, in the case of vapor samples, direct gaseous injection tends to favor collection of lower boiling point components. Headspace adsorption This is the recovery technique most widely used at present. In all adsorption techniques, the flammable liquid vapor is adsorbed onto a suitable medium, often activated charcoal or a proprietary adsorbent such as `Tenax'. The adsorption process usually requires the sample to be heated and may be passive or dynamic. In the former, an adsorption medium is placed in the sample container and allowed to stand for some time, and in the latter, the headspace vapor is extracted through the adsorption medium by a pump. Headspace adsorption is an extremely sensitive method, simpler and more rapid than steam or vacuum distillation and does not require expensive glassware or extensive equipment cleaning operations. However, the extreme sensitivity increases the potential for contamination and interpretation to become issues, and headspace techniques cannot easily be related to the total amount of flammable liquid in the sample. Headspace adsorption techniques generally favor recovery of lower boiling point hydrocarbon components. The adsorbent is often enclosed in a glass tube that can be prepared as required or can be purchased prepacked. There are other forms of support in regular use, such as adsorbent mounted on a wire or paper strip. These are essentially minor variations, and the same considerations apply to these as to methods using glass tubes. Solid phase microextraction (SPME) SPME is a recent development that is similar to headspace adsorption but without the necessity for the solvent extraction step. A fused silica fiber with a suitable adsorbent coating is inserted into the sample container to collect flammable liquid vapor from the sample headspace. The collected material is loaded onto a gas chromatograph column by a rapid thermal desorption process.
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The limiting factor with solid phase microextraction is the efficiency of the adsorption process, as is the case with conventional headspace adsorption. This is a relatively new method which, in view of its simplicity and speed, may become more common. The sensitivity is comparable to that of headspace adsorption methods. Distillation The oldest extraction technique is distillation. Steam distillation, where steam is passed through the debris to vaporize and remove the flammable liquid, is most common, although vacuum distillation has also been used. It is a relatively insensitive method, but it has the advantage that it can provide a guide to the amount of flammable liquid originally present. It requires relatively large samples but is less sensitive to contamination than some other methods. Distillation techniques tend to favor recovery of higher boiling point petroleum components. A major concern with steam distillation is the recovery of water-miscible flammable liquids. Ethanol, in the form of methylated spirits, is a water-miscible solvent commonly used as a solvent and cleaning agent. The boiling point (788C) of ethanol is low compared to most flammable liquids, and losses by evaporation can be substantial. More importantly, ethanol is miscible with the water distilled in the process, and, as a consequence, as the distillation progresses, the concentration of ethanol will decrease. Therefore, in the steam distillation operation, where a water-miscible substance is known or suspected to be present, the first few milliliters of water distilled should be analyzed. At this stage of the process, the flammable liquid concentration will be at a maximum, and the distillate will not be significantly diluted. The difficulty of identifying these low-boiling-point, water-miscible substances is a significant argument in favor of alternative methods of sample treatment. Solvent extraction This is a basic chemical technique that can be applied to the analysis of samples of fire debris. It is conveniently applied to small samples, and requires the sample to be immersed in a suitable volatile solvent. Any flammable liquid present is absorbed by the solvent, which is then removed and, if necessary, concentrated through evaporation. The technique is at least as sensitive as distillation and is relatively simple. The major disadvantage of solvent extraction is that many pyrolysis products, dyes and other chemicals are also extracted, in some cases presenting major difficulties for the subsequent analysis. Solvent extraction can be used for solid or liquid samples. It has particular uses in small samples and in the separation of small amounts of flammable liquid remaining in containers.
Clean-up
Clean-up refers to a process of cleaning a sample or extract to prepare it for analysis. Direct injection of headspace vapor and solid-phase microextraction techniques, by their nature, render clean-up unnecessary or impractical. However, in extraction techniques, which result in the preparation of a small liquid aliquot for analysis, clean-up can be useful and occasionally essential. A simple clean-up procedure is filtration. This can be used to remove suspended solids from a neat liquid sample, or those resulting from a solvent extraction procedure. More complex clean-up techniques involve processes such as acid stripping. Acid stripping is used to remove oxygenated or nitrogenated compounds that arise from combustion of synthetic materials. These compounds can interfere with analysis of flammable and combustible liquids and addition of a small amount of strong mineral acid removes these compounds by extraction into the acid. The immiscible hydrocarbon layer can be analyzed in the normal manner. Chromatographic techniques, such as solid-phase extraction and thin layer chromatography, can be useful for clean-up of larger samples but are difficult to adapt to smaller quantities of flammable liquid. With solid-phase extraction, for example, the adsorption extract can be separated on suitable columns into aliphatic and aromatic hydrocarbon fractions. These fractions can be analyzed separately, thus minimizing the interference caused by pyrolysis products. Any procedure conducted on a sample increases the possibility that the sample will be contaminated or otherwise adversely affected. Although this can be monitored by appropriate use of control samples, the most effective method of reducing clean-up contamination problems is by the minimal application of clean-up procedures. There is a further advantage in minimal clean-up; the overall method remains relatively simple. Addition of a clean-up step is a significant increment in the complexity of the analysis, and the use of that step is only justified by a significant increase in the information recovered. Analysis
Essentially the same analytical procedures are adopted for the various classes of flammable and combustible liquids. The liquids can be derived from petroleum, and indeed, the most common flammable or combustible liquids detected at fire scenes are petrol (gasoline), kerosene, automotive diesel fuel (distillate) and mineral turpentine. Despite the obvious differences in volatility, the same extraction, clean-up and analysis schemes serve for identification
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of all these products, and for the identification of similar products such as white spirit and various naphthas. However, there are other flammable and combustible products, not of petroleum origin, such as pure or wood turpentine, essential oils, ethanol (methylated spirit), natural oils and waxes. Gas chromatography (GC) GC is the principal tool for the identification of these products. Most petroleum products are composed of many chemical components. Because there are relatively strict specifications which apply to petroleum products, in each different product the components will be present in a ratio which is characteristic of that product. Analysis by gas chromatography separates the individual components, presents the components according to volatility and identifies the amount of each component. This produces a `fingerprint' of the product that can be compared with known standards for identification purposes. Several liquids manufactured from petroleum, and many other commercial products (e.g. solvents such as methyl ethyl ketone or methyl isobutyl ketone, ethanol, etc.) have a single component, and it is not possible to identify a product on the basis of the chromatogram retention time alone. Products such as gasoline, kerosene and automotive diesel fuel, and even nonfuel solvents such as mineral turpentine or white spirit, can usually be identified in gas-liquid chromatography by a `fingerprint'-type comparison of a complex mixture of features. The features considered in any such comparison will include relative retention times, peak heights and peak height ratios. A single component substance obviously does not allow relative peak heights or peak height ratios to be compared, leaving only comparison of retention times. It is necessary, in these cases, to adopt a procedure that will allow the identity to be confirmed. This may consist of repeating the analysis with a `spiked' sample, to determine whether the sample and standard co-elute, that is, present a single peak. More commonly, the analysis is completed on a gas chromatograph/mass spectrometer. Modern laboratories use a technique known as capillary gas chromatography, with a specialized detection system called a flame ionization detector (FID), to easily detect microgram and even nanogram amounts of hydrocarbon products. Gas chromatography/mass spectrometry (GC/MS) GC/MS uses the same component separation procedure as gas chromatography but, as the various components are detected, they are individually analyzed and compared with a library of standards to positively identify the individual components. In the
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case of a single component product, this may be regarded as a positive identification. With specialty products such as solvents or thinners, it may still be necessary to run comparison standards. Specialty products are a major challenge in terms of brand identification. Most product types are similar, and examination of the components that make up the product may enable it to be identified in a generic sense. Positive brand identification is much more difficult to achieve, as it requires elimination of other possibilities. The identification of such a product is likely then to be as `. . . a toluene/xylene based thinner . . .' rather than as . . . Acme VM&P Naphtha . . .'. However, for evidentiary purposes, it may be appropriate to also describe the toluene/xylene based thinner as `. . . consistent with the composition of Acme VM&P Naphtha . . .'. Techniques in GC/MS, such as selected ion monitoring (SIM) allow identification to be made on the basis of the ratio of selected ions characteristic of the product/s of interest. This can reduce, but not eliminate, the effects of contamination from various sources, including pyrolysis products. SIM reduces the complexity of the comparison to a manageable level, although there are now pattern matching and library programs available that can considerably alleviate the difficulty of identifying an unknown. Infra-red spectrometry/Fourier transform infrared spectrometry (IR/FTIR) IR/FTIR is an analytical instrumental technique used widely in chemical analysis to identify the chemical `fingerprint'. An infrared spectrum is obtained that can be particularly useful for the identification of plastics and synthetic materials. The spectrum derived from the infrared analysis reflects the chemical bonding of the molecule, providing some information in relation to the structure of the unknown substance. The particular advantage of FTIR is that extremely small samples can be analyzed rapidly and accurately. As with GC and GC/MS, there are computer-based pattern matching programs to match sample spectra to library spectra that considerably reduce the difficulty of interpretation of complex spectra. In using spectral libraries, care must be taken not to limit the search to the library contents, because even the largest reference libraries do not cover every possible product. Because infrared spectrometry analyzes all the chemical groups present in the sample, only samples that are essentially unadulterated can be identified using IR/FTIR. Microscopy/scanning electron microscopy These techniques are applied in the analysis of debris and
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residues from fire and explosion scenes. An unidentified object or unknown material can be identified by examination using a relatively low power microscope. This may permit recognition of components of an incendiary device, of details on paper or material used as fuel, or a physical match of evidence found at the scene to an item found at a suspect's house. Particles or crystals may be selected for more detailed examination, under a higher power microscope or even a scanning electron microscope (SEM). The SEM, with the capability of magnification to 300 0006, can be used to study the morphology of materials and for X-ray analysis. The SEM can provide a rapid elemental analysis of microscopic samples, detailing the amount and distribution of the medium to heavier elements. The actual shape or surface characteristics of the material being examined may be of significance in some cases but, more commonly, the microscope is used to locate materials requiring analysis. This can provide a starting point for further investigations, possibly using additional instrumental techniques. The presence of barium, for example, may suggest the presence of sparkler composition, whereas the presence of strontium may suggest the presence of pyrotechnic material from a red handheld flare. In each case, further analysis would be conducted, such as ion chromatography to identify the nitrate or other oxidizing agent present.
Electrical Components and Electrical Wiring The diagnosis of electrical sources of ignition is one of the most challenging aspects of fire investigation. The scene examination process may result in the identification of one or more possible sources of ignition, which may or may not be electrical. A laboratory examination might be required to determine whether, say, an appliance was a possible source of ignition, whether a timing device was operating a light switch or an incendiary device, etc. Examination of individual electronic components with respect to their intrinsic properties, for example, measurement of the exact resistance or capacitance, is often beyond the capabilities of the forensic science laboratory. This is a specialized examination, requiring electronic engineering expertise, which is likely to be found only in universities or major telecommunications companies. The same may be said for integrated circuits and programmable microprocessors. A forensic scientist might identify them as such, but to determine exactly how the microprocessor operates is almost certainly a task for an electronic engineer. Examinations requiring laboratory expertise are often based on the application of simple physical
science. For example, a burnt and melted mixture of components and plastic might be examined using infrared or ultraviolet light, or X-rays. By means of X-ray examination in particular, it is sometimes possible to trace the circuit outlines clearly and identify the construction and means of operation of the components. Similarly, with individual components, the laboratory examiner is often able to identify a clock, wires, a battery and a short length of resistance wire. Obviously, these are the components required for a crude time-delay ignition mechanism. It is possible to calculate the current that can be drawn, and the heat output of the resistance wire, to determine whether this is a functional system. Laboratory examinations can enable appliances such as power boards or double adaptors to be identified. Careful cleaning, removal of melted material and microscopic examination can enable determination of the position of switches, or whether appliance pins were present in power points, or whether a thermal cut-out switch has operated. These examinations are technically straightforward, albeit they require some practical experience and theoretical knowledge. Clearly though, they can be of great significance since they can result in a definite answer to a potentially crucial question, such as whether a suspect appliance was connected to power or turned on. Formulating and addressing these basic questions, and interpreting the results of consulting electronic engineers on more complex questions, is an appropriate role for a forensic science laboratory. There are many proponents of theories relating to electrical arcing. Arcing is a well-known and understood phenomenon, which occurs under specified conditions and which can cause ignition of combustible material. When evidence of arcing is produced, there is the difficulty of determining whether it occurred prior to or as a result of the fire. Any microscopic examination of electrical wiring to determine whether arcing has occurred should be combined with scene observations to determine whether the origin of the arcing is consistent with the apparent point of origin of the fire. Related theories propose that the chemical composition of copper wires, that is, the relative oxygen and/ or chlorine content of electrical wires, as determined by Auger electron spectroscopy or scanning electron microscopy, can be used to ascertain the atmospheric conditions to which the melting wire was subjected. The basis of this theory is that high oxygen and/or low chlorine levels are consistent with arcing prior to the fire; low oxygen and/or high chlorine levels are consistent with arcing caused by the fire. Indeed, in controlled laboratory conditions, these theories can hold true.
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However, in the wildly fluctuating atmospheric conditions existing in structure fires, and the many other factors such as varying temperatures, exposure to firefighting water/chemicals, residues from household/industrial chemicals, variations in original wire and pre-existing damage from age or exposure, simple laboratory principles are found to be inadequate. These theories are not transferable from the static laboratory environment to the dynamic environment of an actual fire scene and this type of scene/laboratory examination is neither well established nor practical.
Machinery The principles for the examination of machinery in the forensic science laboratory are essentially the same as for examination of electrical or electronic appliances or components. The initial role of the forensic scientist is to formulate the questions that need to be addressed in the examination of the machinery. Some of these may be within the province of the forensic scientist, such as whether the machinery can cause sparks in some manner, what is the normal operating temperature and does partially blocking the exhaust markedly increase manifold temperatures? All these are questions that can be resolved by careful measurement and/or observation. With machinery which is alleged to have malfunctioned, and which has been operated incorrectly or which has been severely damaged, it may be necessary to consult with the manufacturer or some more specialized agency, for example universities and consulting engineers. More complex questions, or questions relating to efficiency, ergonomics or other design factors may be addressed by specialist investigators. This may be under the supervision of a forensic scientist or police officer for continuity purposes, as a consultant may not be aware of the requirements for security and continuity of exhibits.
Summary This article has briefly reviewed the wide range of scientific expertise and techniques necessary to support the fire scene investigator in a scientific examination and interpretation of a fire scene. However, it must be remembered that the quality of the fire-scene interpretation and of the samples taken will be directly responsible for the subsequent quality and usefulness of the laboratory results. Furthermore, these laboratory results will only rarely provide any proof of the fire cause. They will, in fact, provide supporting evidence to the scene interpretation so that possible fire causes can be identified and, perhaps, eliminated or upgraded to probable causes.
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See also: Analytical Techniques: Separation Techniques; Mass Spectrometry. Crime-scene Investigation and Examination: Recording; Collection and Chain of Evidence; Recovery of Human Remains; Packaging; Preservation; Contamination.
Further Reading Almirall JR, Bruna J and Furton KG (1996) The recovery of accelerants in aqueous samples from fire debris using solid-phase microextraction. Science and Justice 36:283±288. ASTM Standard E1385-95, Standard Practice for Separation and Concentration of Flammable or Combustible Flammable Liquid Residues from Fire Debris Samples by Steam Distillation. ASTM Standard E1386-95. Standard Practice for Separation and Concentration of Combustible Liquid Residues from Fire Debris Samples by Solvent Extraction. ASTM Standard E1387-95. Standard Test Methods for Flammable or Combustible Liquid Residues in Extracts from Samples of Fire Debris by Gas Chromatography. ASTM Standard E1388-95. Standard Practice for Sampling of Headspace Vapours from Fire Debris Samples. ASTM Standard E1389-95. Standard Practice for Cleanup of Fire Debris Sample Extracts by Acid Stripping. ASTM Standard E1412-95. Standard Practice for Separation and Concentration of Flammable or Combustible Liquid Residues from Fire Debris Samples by Passive Headspace Concentration. ASTM Standard E1413-95. Standard Practice for Separation and Concentration of Ignitable Liquid Residues from Fire Debris Samples by Dynamic Headspace Concentration. Brackett JW (1955) Separation of flammable material of petroleum origin from evidence submitted in cases involving fires and suspected arson. Journal of Criminal Law, Criminology and Police Science 46:554±561. Gilbert MW (1998) The use of individual extracted ion profiles versus summed ion profiles in fire debris analysis. Journal of Forensic Science 43:871±876. Juhala JA (1979) Determination of fire debris vapours using an acid stripping procedure with subsequent gas chromatographic and gas chromatography mass spectrometry analysis. Arson Analysis Newsletter 3(4):1±19. Karkkainen M, Ilkka S and Himberg K (1994) Detection of trace levels of gasoline in arson cases by gas chromatography±mass spectrometry with an automatic on-line thermal desorber. Journal of Forensic Science 39:186± 193. Lennard C (1995) Fire (determination of causes) a review 1992 to 1995. 11th Interpol Forensic Science Symposium, Lyon, France. Mickiff C (1978) Recovering accelerants from debris. Arson Analysis Newsletter 2(6):8±20. Twibell JD, Home JM and Smalldon KW (1980) A comparison of the relative sensitivities of the adsorption wire and other methods for the detection of accelerant residues in fire debris. HOCRE Report no. 368.
928 FIRE INVESTIGATION/Physics/Thermodynamics Waters LV and Palmer LA (1993) Multiple analysis of fire debris samples using passive headspace concentration. Journal of Forensic Science 38:165±183. Woycheshin S and De Han J (1978) An evaluation of some distillation techniques. Arson Analysis Newsletter 2(5):1±16. Yip IH and Clair EG (1976) A rapid analysis of accelerants in fire debris. Canadian Forensic Science Society Journal 9(2):75±80.
Physics/Thermodynamics J H C Martin and R S Pepler, Institut de Police Scientifique et de Criminologie, Universite de Lausanne, Lausanne, Switzerland Copyright # 2000 Academic Press doi:10.1006/rwfs.2000.0525
Introduction and Overview Within the various domains of criminalistics, the investigator compares traces found at the scene of a crime with those recovered from a person, tool or place. By contrast, the work of the fire investigator is not purely comparative in nature; extensive practical experience of the examination of as many fire scenes as possible combined with a sound scientific knowledge, especially in the areas of physics and thermodynamics, is paramount to the success of the investigations. These two criteria enable him or her to identify the origin of the fire and determine its cause despite having to work on sites that have been destroyed by the fire and further disrupted during the extinguishing process. This article explores: the relevant background areas of physical thermodynamics; the role of thermodynamics in fire investigation; fire ignition and propagation; thermodynamic classification of ignition sources; and the phenomena of smouldering and flaming combustion.
Physical Thermodynamics: the Relevant Background Before considering the role of thermodynamics in fire investigation, it is essential to remember that the fundamental subject under scrutiny is concerned with the transformation of a macroscopic system which is dependent on one of the basic elements of physics: temperature. It is also important to under-
stand that an intensive thermodynamic variable is one which is dependent on the amount of a chemical substance in the system, whereas an extensive variable does not. Physical systems
A physical system is defined as that part of the universe under the influence of thermodynamics which has been identified for study. It can be: . isolated: no exchange of heat, work or matter with the surroundings is possible. The system is said to be ideal and can be assumed in various situations; . closed: exchange of heat and work, but not matter, is possible with the surroundings; . open: exchange of heat, work, and matter is possible with the surroundings. Thermodynamic principles
Two of the fundamental laws of thermodynamics can be described in the following manner: The first law of thermodynamics All systems possess an internal energy U, which is a state variable, signifying that U is independent of the history of the system. When a system undergoes a change from state 1 to state 2, the difference in thermodynamic energy, DU is: U U2
U1
While respecting the principle of the conservation of energy, the DU of a closed system can be expressed as: U Q W where DQ and DW are, respectively, the applied heat and work. For an isolated system U is constant; in keeping with the conservation of energy, U1 is therefore equal to U2. There is no energy change between the system and its surroundings therefore: Q W 0 The second law of thermodynamics This law concerns change and entropy. Entropy, S, is an extensive state variable which denotes the degree of disorder of the system. During reversible changes, the system and surroundings are in constant equilibrium. When considering such a change from a state 1 to a state 2, the entropy of a system is: Z S dQ=T irrespective of the pathway taken. However, energy dissipated in the form of heat is not an ideal process; it is spontaneous and irreversible