IS IT AN ACCIDENTAL FIRE OR ARSON?

by Tony Cafe and Ass. Prof. Wal Stern


Reprinted from Chemistry in Australia Magazine April, 1989


Introduction

Damage from fire is Australia's most costly public safety problem. Losses due to fire, in life and injury, are exceeded only by those due to traffic accidents. The cost of fire damage to the Australian community has been estimated to be $600 million per year. Our own experience over recent years suggests that this may be a conservative estimate. Arson, the wilful and malicious burning of property, accounts for approximately 30 percent of this figure.

The most effective way to try and reduce this appalling cost, and to reduce the damage caused by fire, is effective fire investigation. It should then be possible to reduce the number of accidental fires by improving building codes, and by identifying and eliminating dangerous products. Arson is said to be the easiest crime to commit (even young children can do it), but the most difficult to detect and prove. It needs to be combated by finding and prosecuting those responsible.

Fire Investigation

A fire investigation is an unenviable task. The devastation, charred debris, collapsed structures, water soaked ashes, together with the smoke and stench, makes the task uninviting and seemingly impossible. In the past many investigators appear to have come to the task with inherent biases; fire brigade members have decided that all unexplained fires were due to electrical faults, whilst police and insurance investigators leaned towards "arson, by person or persons unknown".

There are different types of fires; in homes or factories, in the bush or a forest. The best investigation would use a team of trained personnel; fire brigade staff, with their experience of fires at first hand, police and insurance investigators, with their skills for determining motive and opportunities. An electrical engineer or electrician is required to investigate electrical systems. The scientist also has a most valuable role to play. The scientist should he able to arrive at a fire scene without any predetermined ideas. An analytical approach, using patient, thorough and systematic techniques should reveal critical and vital information. The knowledge of a chemist is invaluable. A chemist should understand the properties of fuels and building materials, and have an understanding of the combustion process. In addition an analytical chemist should also be able to identify in the laboratory materials found at the fire scene, even if they are only present as trace amounts.

Methodology

The basic role of an investigator at fire scene is twofold; firstly to determine the origin of the fire (the site where the fire began), and secondly to examine closely the site of origin to try and determine what it was that caused a fire to start at or around that location. An examination would typically begin by trying to gain an overall impression of the site and the fire damage; this could be done at ground level or from an elevated position. From this one might proceed to an examination of the materials present, the fuel load, and the state of the debris at various places. The search for the fire's origin should be based on elementary rules such as:

A knowledge of the colour and state of various materials at elevated temperatures is required to help determine the temperature of the fire in different locations. An examination is also carried out of structural deformations, char depths, smoke patterns. It is important to try and discover if the fire started at floor level, as from a cigarette butt, or at elevated level, as for a gas explosion. This summary attempts only to indicate some of the steps typically undertaken. A more detailed list can be obtained from a number of texts (see, for example, references 1,2).

These procedures are designed to locate the site of origin of the fire. Multiple sites of origin suggest a deliberately lit fire. Assuming that the site of origin has been found a thorough examination of the debris in this area is then necessary. All electrical appliances in the vicinity should be examined. The presence of any flammable liquids, trails, or spalling of concrete or intense burn-marks in the floor should be checked. No fire can commence without an ignition source. One should therefore be on the lookout for matches, lighters, sources of sparks, hot objects, chemicals, gas and electrical lines, cigarettes, fireplaces and chimneys.

A knowledge of spontaneous combustion, and its likely sources, is needed. It may be necessary to collect samples and carry out experiments in the laboratory (it is not difficult to show that loose rags with linseed oil on them cause spontaneous combustion). The collection of samples requires a chemist's knowledge of sampling procedures and the need to obtain uncontaminated materials.

Provided the investigation has been patiently and scientifically carried out, when combined with the evidence of eyewitnesses or fire officers, it may be possible at this stage to draw a conclusion about the fire. Typical causes of accidental fires are cooking accidents, overheated or short circuited electrical connections, spontaneous combustion of oils, welding sparks, burst gas lines, sparks from fireplaces, lightning, cigarette butts, left-on appliances, reacting chemicals. The list of all the possible causes is very long.

Arson

If a fire is not the result of an accident, it must have been deliberately-lit; arson. The motives to commit arson include vandalism, fraud, revenge, sabotage and pyromania. A major objective in any suspected case of arson would be to search for, locate, sample and analyse residual accelerants. Most, though certainly not all incendiary fires involve the use of an accelerant to speed the ignition and rate of spread of fire. A rapid and intense fire, inconsistent with the natural fuel loading is indicative of an accelerated fire. Such a fire is likely to be initiated at ground level, possibly in a number of sites and may produce trail marks, burn-throughs or spalling of concrete.

The accelerants most-commonly used, on account of their flammability and ready availability are petrol, kerosene, mineral turpentine and diesel. Other accelerants such as alcohols, acetone and industrial solvents are less commonly used. It might be thought (certainly many arsonists assume) that after an intense fire there will be negligible amounts of such accelerants remaining. Given our current sophistication of analytical techniques, this is not true. The amount of accelerant remaining after a fire will depend on factors such as the quantity and type of compound used, but also on the nature of material it is poured on, the elapsed time since the fire, and the severity of the fire. Chemists have been able to locate and detect trace amounts of liquid hydrocarbons in soil beneath a gutted house several months after a fire.

Detection of trace quantities of materials requires careful attention to sampling techniques and analysis. The most frequently sampled material is flooring material such as wood, carpet, soil and linoleum. Porous material is best. There is a need to take control samples in some cases, away from the area where the accelerant is suspected, but preferably of the same material as the sample.

Some investigators use "sniffers" at fire scenes. These portable detectors usually note changes in oxygen level on a semiconductor. They are not specific for liquid hydrocarbons, responding to a variety of vapours, and need to be used with caution. They can be used as a guide as to the best place from which to collect samples, for removal to, and analysis in, the chemical laboratory.

Sampling

The materials found to give the most positive analyses for accelerants are porous samples; carpet and underlay, cardboard, paper, felt, cloth and soil. At all stages, because of the sensitivity of the analysis, care must be taken to avoid contamination. In our experience unlined metal cans have been found to be the best containers.

Lined cans may have a coating which contains volatile components and should not be used. Plastic bags may allow diffusion of volatile components either into or out of the sample and are not recommended. Glass containers may be used, but the cleanliness of lids needs to be assured. Cans need to be clean and well sealed, and clearly labelled, for transport to the laboratory. At the laboratory they need to be documented and kept secure prior to analysis.

(see also: Sampling Debris at the Fire Scene)

Extraction

The methods of extraction most commonly used for fire debris samples, are distillation, solvent extraction, and headspace analysis. The distillation techniques used have included steam distillation, ethylene glycol distillation, ethanol distillation and vacuum distillation. Of these, steam distillation has been the most widely used, and is still used, particularly where reasonably large quantities of accelerant are suspected to be present. Solvent extraction is not used except in special cases. Both static and dynamic headspace analysis are now in common use, in both cases at and above room temperatures. In the former case a needle of a gas syringe is placed into a container containing fire debris, and a volume of vapour is withdrawn for analysis.

Figure 1.

Figure 1.

Schematic diagram of Dynamic Headspace Extraction Equipment

Our own preferred method, on the basis of experience and experimentation, is for dynamic headspace extraction, as represented in Figure 1. The fire debris, in its original container is placed into an oven and heated at 150° C for approximately one hour, whilst at the same time a continuous flow of filtered nitrogen gas flushes the headspace and sweeps any volatile components through a water trap onto an absorbent. This method in effect samples 3000 times more gas than does a static headspace sample. It has the advantage that the can is always vented, so that pressure does not build up in the can. Water present will volatilise, and essentially steam distil the sample.

Absorption and Desorption

The absorbents in most common use are activated charcoal, Tenax G.C. and Porapak Q. All three used absorb accelerant components, but do not absorb water or nitrogen. Tenax is stable up to 350C and is ideal for rapid thermal desorption. It is used for most fire samples by the London Metropolitan Police Laboratories (Scotland Yard). An advantage of thermal desorption is that the material to be analysed can proceed directly from the absorbent into a gas chromatogram. One disadvantage is that when this happens the sample is used up, and the evidence is no longer present.

In the case of solvent desorption one obtains a liquid sample which can be re-analysed many times and retained as evidence. A variety of solvents have been used for desorption, but we have found carbon disulfide to be the best, because of its high desorption efficiency for the components commonly found, its low detector response and its high volatility. We use 1 mL A.R. grade carbon disulfide for extraction, and store liquid samples in a glass vial to which we add 1 mL water to prevent loss by evaporation.

Detection and Identification

Ultraviolet, infrared and nuclear magnetic resonance spectroscopy have all been used for identifying accelerant components, but by far the most widely used technique is gas liquid chromatography. It is able to separate and detect trace amounts of volatile hydrocarbons in complex mixtures. The flame ionisation detector has been widely used because of its great sensitivity for these components. The introduction of capillary columns allows for smaller samples and produces sharper peaks and greater resolution. The number of different columns now available is quite large, but we have found that a 25m. BP-1 capillary column, 0.33mm. i.d. to be widely applicable. In our laboratory we run unknown samples on a dual plotter against standard samples, so that a comparison can be made of samples run under similar conditions.

The four most commonly found accelerants (petrol, kerosene, mineral turpentine and diesel) are all highly complex mixtures of many components, in very different ratios. Most forensic laboratories feel confident in identifying these compounds on the basis of their gas chromatograms alone, even if the samples are evaporated and contaminated. Typical gas chromatograms of these four mixtures are given in Figure 2.

In order to make a positive identification it is necessary to identify a large number of the components present, and to note that their ratios are very similar to that of a standard. The use of evaporated and burnt standards may aid this comparison. To make absolutely sure of the identity of any component we have relied on gas chromatography/mass spectrometry (GC/MS). We have been fortunate to have available an AEI MS902, fitted with a Pye Unicam GC, and more recently a Hewlett Packard GC/MS, the 5970 MSD. This latter instrument contains a data library of some 42,000 compounds. It is possible to conduct searches for particular fragments, groups of fragments or molecular ions, and is particularly well suited to identify aliphatic and aromatic hydrocarbon peaks and mixtures.

We have had a number of honours students working on finding the best experimental conditions, and on identifying as many as possible of the straight and branched chain aliphatic hydrocarbons, the xylenes, tri and tetramethylbenzenes, naphthalenes and methylnaphthalenes, styrene and indanes. In the case of petrol it is also possible to detect and identify, under different conditions, the organo-lead additives. At the same time consideration has also been given to the effects of evaporating and burning accelerants. A study has also been made of likely contaminants, particularly the pyrolysis products from various plastics, carpet, wood, tiles, glues and other adhesives, lacquers, thinners, vegetable oils. We have built up a library of possible contaminants. The techniques described are capable of detecting 1 uL of accelerant. In fact it is possible to detect 0.1 uL, but we have set a minimum level of 1 uL, because of the possibility of background material which may be present. At this level one must be very careful about contamination and pyrolysis compounds, so that they are not confused with accelerant. It is necessary to clean all equipment before use and to run blanks at regular intervals to ensure that there is no contamination present.

Conclusion

Fires present a major social and economic problem. A thorough investigation of any large-scale fire, be it accidental or deliberate, is warranted. Chemists have expertise which can be used in an on-the-spot investigation, and in the analytical laboratory. This is not an area for which many scientists in Australia have been specifically trained but is an area where the chemist's skills and expertise can be of great benefit.

References

  1. John D. Dehaan - Kirk's Fire Investigation (Second Edition), John Wiley. 1983.
  2. Roy A. Cooke and Rodger Fl. Ide - Principles of Fire Investigation. The Institution of Fire Engineers, 1985.

Some figures discussed in this report are not available.