Laboratory Services

"Laboratory Services" pp 328-361 from
Kirk's Fire Investigation, 4/E by John D. DeHaan, ©1997.
Reproduced by permission of Prentice-Hall, Inc., Upper Saddle River, NJ.
Permission from Prentice-Hall is required for all other uses.

[interFIRE VR Note: Figures and Tables are not reproduced.]

There are instances where even an experienced fire investigator requires an analysis or interpretation of the physical evidence relating to fire causation or spread which entails the services of a laboratory specialist. In fact, it is the wise and experienced investigators who recognize the limitations of their own knowledge, beyond which they must take advantage of other experts to ensure that the investigation is complete and accurate. The less experienced may try to overextend their expertise and offer erroneous or misleading interpretations of evidence which can embarrass themselves and their clients. It is well worth the time and effort for the investigator to anticipate the need for such experts* and determine their availability before they are needed in a crucial case.

AVAILABILITY OF LABORATORY SERVICES

Most states in the United States have at least one state criminalistics laboratory that can provide most of the analytical services a fire investigator will need. In fact, many states have a system of regional laboratories to minimize travel time and expense. These laboratories, operating under the auspices of the state's department of justice, public safety, state prosecutor, state police, or similar law enforcement authority, work in cooperation with the city or county laboratories found in many areas to provide a wide range of services. On a national level, the laboratories of the Federal Bureau of Investigation and Bureau of Alcohol, Tobacco, and Firearms are well known. In Canada, in addition to city or provincial laboratories in Vancouver, Montreal, Ottawa, and the Centre of Forensic Sciences in Toronto, there is a system of excellent laboratories operated by the Royal Canadian Mounted Police. With some exceptions, the services from these public laboratories are available at no cost to personnel from public fire or police agencies and to investigators from some semiprivate agencies such as transit authorities, public utilities, railroads, and the like. Due to limited laboratory resources, there may be restrictions on which agencies they are permitted to serve. In addition, there may be a case priority scheme operating which relegates fire-related evidence to low-priority status. Since the priority is often set on the basis of whether there were deaths or injuries or on a court date having been set, that information should be provided to the lab when the evidence is brought in to ensure fastest possible service time. The investigator should contact the nearest laboratory before its services are needed, so that time is not wasted just in finding a laboratory.

Private investigators, whether retained by public agencies such as public defenders or by private or corporate clients in civil or criminal cases, have a more difficult and costly problem. Although a few public laboratories will service public defenders and even private-sector investigators on a contractual basis, for the most part, private-sector investigators must turn to private laboratories. Many large insurance and engineering firms, in addition to providing laboratory services to their own investigators, offer such services to outside clients on a fee basis. There are also small, private laboratories that can perform appropriate tests on a per-case, per-analysis, or hourly fee basis and provide expert testimony as to their results. Finally, there are qualified consultants who will evaluate the evidence and provide interpretation and expert testimony while contracting with others for the actual laboratory work. There are many more qualified, private-sector facilities available now than just a few years ago. Many also do work (on a fee basis) for public-sector agencies whose own labs cannot provide timely services. The most important consideration is not whether a lab has the necessary equipment but the experience necessary to provide accurate interpretation. The effects of fire, evaporation, exposure, and even sample collection and storage on physical evidence from fires must be well understood by the analyst if critical errors in interpretation are to be avoided. Simply having a chart or numerical reading does not do the investigator any good. Only a qualified analyst will be able to correctly interpret that data and guide the investigator in assaying the contribution of the evidence to the case. Unfortunately, judging someone's qualifications is much more difficult than dialing a phone number, and there have been instances of malfeasance and even fraud on the part of some laboratories. The International Association of Arson Investigators (IAAI) first addressed this problem by establishing a Forensic Science Committee and developing consensus guidelines on the analysis and interpretation of fire-related evidence. ¹ The American Society for Testing and Materials (ASTM) used that IAAI document as the foundation for what is now a series of test methods, guides, and practices covering many of the techniques used in fire debris analysis, including ASTM "E 1492: Practice for Receiving, Documenting, Storing, and Retrieving Evidence in a Forensic Science Laboratory," ² and "E 1387, Standard Test Method for Ignitable Liquid Residues in Extracts from Samples of Fire Debris by Gas Chromatography." ³ These methods were developed by consensus of the scientists involved and they can be used by any analyst. Adherence to them indicates a professional attitude on the part of the analyst. The American Society of Crime Laboratory Directors (ASCLD) has established a protocol for accreditation of forensic laboratories that calls for use of such standardized methods for analysis and interpretation, as well as record keeping. Finally, the American Board of Criminalistics (ABC) has established a certification program for individuals involved in fire debris analysis. Scientists who pass the General Knowledge Examination in forensic science and a specialty knowledge exam and who participate in a quality assurance testing program may be designated to be a Fellow of the ABC in that specialty. In addition, active membership in IAAI or one of the regional or national forensic science organizations is a hallmark of a professional forensic scientist. These qualifications should aid the investigator in finding a suitable laboratory and a qualified analyst. The selection of a qualified lab is not easy and there are too many variables of expertise, qualifications, analysis time, and costs to provide specific guidelines here. The laboratory services discussed here cover the entire range of those available from many sources. With the information offered on types of evidence, possible analyses, and strengths and weaknesses of evidence and techniques, the investigator should be able to confer knowledgeably with various experts and determine their suitability for the particular problem at hand.

GENERAL FIRE EVIDENCE

Identification of Charred or Burned Materials

One of the most common questions a fire investigator asks is, "What is (or was) this stuff-and does it belong at the scene?" In earlier chapters we discussed the importance of looking for things that appear to be out of place. In a fire, many items, both trivial and significant, can be sufficiently damaged that a casual examination will not result in their identification. Plastic, rubber, or metal objects having a relatively low melting point are typical problems. The identification may be based on the shape of the remaining portion, portions of labels or details cast into the object that survive, or a determination of the type of material present. Even though the plastic or metal can be chemically identified, if only minute amounts remain unburned this information may not be of use in establishing the specific item. Some materials, like polyethylene, polystyrene, or zinc diecast, seem to be universal in their applications. Some plastic bottles may be identified by their shape or casting marks, but the knowledge that a puddle of debris is melted polyethylene is of little use in figuring out which of thousands of polyethylene objects it may represent. Glass objects, whether broken by mechanical or thermal shock, can be pieced back together (as in Figure 14-1) if there has not been excessive melting of the pieces.

Laboratory identification may consist of visual examination, reassembly, cleaning, or microscopic, chemical, or instrumental tests of considerable complexity, depending on the nature of the material, the quantity recovered, and the extent of damage. By verifying the composition of even partial remains, these services may be crucial in detecting the substitution of less valuable items for heavily insured objects like jewelry, art, or clothes in cases of insurance fraud, or even in confirming their complete removal in cases of fraud or theft. If an ignition or time-delay device is suspected, the laboratory may be able to reconstruct the device. The nature or specific details of the device may then be of use in identifying the perpetrator, linking him or her with the scene, or in interrogating suspects.

Burned Documents

The reconstruction and identification services described earlier can extend to fire-damaged documents. Writing paper has an ignition temperature of approximately 230°C (450°F), and single sheets of paper exposed to fire in air will readily char and burn almost completely. However, most documents in files, stacks, or books will not burn completely but will char and, if intact, may be identified. Identification in the laboratory may employ visual examination under visible, ultraviolet, or infrared light; photographic techniques; or treatment with glycerine, mineral oil, or organic solvents to improve the contrast between the paper and the inks. Some clay-coated papers and writing papers with a high percentage of rag content are more resistant to fire and are more easily restored than cheaper wood-pulp papers like newsprint. The most critical problem is the fragility of the charred papers. Even fragmentary remains can sometimes be identified, but the more intact the document, the better the chances of identification. When charred paper is to be recovered, the investigator should take every precaution against physical destruction of the remains. Documents should be disturbed as little as possible and carefully eased, using a piece of stiff paper or thin cardboard, onto a cushion of loosely fluffed cotton wool. This cotton is placed into a rigid box of suitable size and a layer of fluffed cotton wool placed over the documents to keep them in place. (Rigid plastic containers like those used for cottage cheese are excellent for small items like matchbooks.) The box should then be hand-carried to the laboratory. Reconstruction is best done under controlled laboratory conditions by experienced personnel with proper photographic support in case characters are visible only fleetingly. For these reasons, the investigator should not attempt to perform any tests in the field. Virtually anything done to the document in the way of coating, spraying, or treating it with solvent will interfere with laboratory tests. Many general crime laboratories and qualified document examiners have the expertise and equipment needed to perform such tests properly.

Failure Analysis--Forensic Engineers

If a mechanical system has failed and has caused a fire, it may be necessary to determine the causation of the original mechanical fault. A registered mechanical or materials engineer is often able to tell if a driveshaft or bearing failed as a result of misuse, overloading, poor maintenance, or design error. The failure of hydraulic systems may produce leaks of flammable hydraulic fluid that can contribute to the initiation or spread of a fire, and the experience of a hydraulic engineer may be required. Because the civil liability of the manufacturer or user of a mechanical system may be in the millions of dollars, it is very important to have an accurate estimation of the condition of that system. Such determinations are beyond the capabilities of most crime laboratory personnel since they require specialized knowledge of the materials or processes involved. A registry of qualified professional engineers is offered in many states to assist investigators.

Evaluation of Appliances and Wiring

The guidelines offered in Chapter 10 will assist the investigator in making an accurate assessment of the condition of wiring and the contribution it may have made to a fire. Many diagnostic signs, however, require microscopic examinations, metallurgical tests, or continuity and conductance tests that require laboratory examination and interpretation by a specialist. For instance, whether a switch was on or off at the time of the fire requires careful dismantling and examination of the contacts, housing, and switch mechanism. Due to fire damage, such dissection may easily result in movement of the components and destruction of their precise relationship. X-ray analysis may be used if the contacts and housing are arranged to permit cross-sectional viewing. Twibell and Lomas described a novel resin-casting technique that allows cross-sectioning of the switch unit while ensuring that the components have not moved.⁴ Soft x-rays (of the energies used for medical procedures, 75­85 kV) have also been demonstrated to be of value in examining thermal cutoffs (TCOs) used to protect coffee makers and other similar appliances from overheating, and cartridge-type fuses. Such x-rays can be used to determine their continuity and often to establish the mechanism of their operation or cause of failure. In the case of fuses, those that failed due to moderate overcurrents were often differentiable from those that failed in a short circuit.⁵ Under some conditions, whether a wire was melted by overheating due to excessive current or by the heat of the fire may be determined by the degree of crystallization within the conductor, because the entire cross section of a conductor will be heated by an overcurrent, where the surface of a conductor is likely to be affected to a greater degree than the core if the heat is applied from the exterior. It has been suggested that analysis of the elemental contaminants trapped within a copper conductor should reveal a record of the environment which surrounded it. If the analysis can be carried out to examine elemental content as a function of depth within a melted copper or aluminum bead, a sequence of events might be reconstructed.^6,7 Auger electron spectroscopy (AES) and electron spectroscopy for chemical analysis (ESCA) both allow elemental analysis of a surface followed by etching away of the surface (using argon atoms) so that analysis at various depths can be carried out. Scanning electron spectroscopy with x-ray analysis (SEM/EDX) is also capable of elemental mapping. It has been suggested that if a conductor is subject to a direct fault, the absence of insulation in the vicinity is reflected by carbon, chlorine, and calcium (from PVC insulation) and oxygen (from the normal air environment) being found in the outermost layer of the bead. If the fire has created the fault by charring the insulation and the wire together, the carbon, chlorine, and calcium should be found at greater depths into the conductor, while oxygen concentration should be diminished due to the surrounding flames.^6,7 Metallurgists can conduct crystallographic tests but, unfortunately, the elemental analysis (AES or ESCA) requires expertise and equipment available only in fairly sophisticated materials- or surface-science laboratories. This method is presently awaiting confirmation by SEM/EDX. It should be kept in mind that, although the laboratory specialist can perform tests far beyond the capability of the investigator, there are practical limits. It is possible for a fire to be sufficiently hot and prolonged to destroy enough of the diagnostic signs that may have once been present that the required determination cannot be made with any degree of certainty. Every laboratory has its quota of miracles that it can perform!

Miscellaneous Laboratory Tests

There are a number of other determinations having an impact on the assessment of a fire that the laboratory is well-suited to perform. The temperatures produced during the fire may give an indication of the use of an incendiary ignition device or an unusually high fuel load. These temperatures may be indicated by the melting (or melted state) of various materials in the vicinity. Since small variations in composition may result in considerable differences in melting points, the objects of interest should be identified in the laboratory and their melting points measured directly if at all possible. Scanning electron microscopy (SEM) has been shown to be of value in establishing the nature of interactions between aluminum and copper in forming eutectic alloys during a fire.⁸ SEM can show differences in crystal structure, and when teamed with elemental analysis (SEM/EDX) it can demonstrate mixing of various elements.

Flash Point. The flash points of materials found at the fire scene, either as part of the scene itself or as part of an incendiary device, should also be determined in a laboratory. Flash point determinations are made using one of several types of apparatus: Tagliabue (Tag) Closed Cup (FP below 175°F), the Pensky-Martens (FP above 175°F), Tag Open Cup, or Cleveland Open Cup. The American Society for Testing and Materials (ASTM) has established standard methods for the use of these devices (previously described) and the reader is referred there for more details.⁹ Unfortunately, many laboratories cannot measure the flash point of a flammable liquid if that point is below ambient (room) temperature. In addition, the flash point apparatus most commonly found in analytical labs requires at least 50 milliliters (2 fluid ounces) for each determination. This is far more than is normally recovered as residue from an incendiary device and precludes such analysis. However, a flash point apparatus, called the Setaflash Tester, has found acceptance in forensic laboratories because it will accept samples as small as 2 milliliters as well as permit subambient measurements.¹⁰

Mechanical Condition. In reconstructing the scene, visual examination is often adequate to ascertain whether a door lock mechanism is locked or unlocked at the time of the fire (see Figure 14-2). The lock mechanisms are usually of brass or zinc diecast, which have relatively low melting points. As a result, they will often seize at temperatures produced in normal structure fires. When recovered at a scene, they should be photographed in place and then worked to clearly indicate their orientation. If it has not been exposed to too much direct fire, it is possible to ascertain whether a door was open or closed at the time of the fire by an examination of its hinges. In a closed door, the plates of a typical butt hinge are protected from fire by the door jamb and the edge of the door. The spine of the hinge will be exposed to the fire and will, therefore, be more heavily damaged than the more protected plates, as in Figure 14-3. In an open door, the spine and plates of the hinges will be exposed to roughly the same amount of heat, and damage will be uniform on both. If the fire is not too extreme, discoloration of the metal and remnants of paint on the hinge may indicate the relative positions of the two plates. It is possible for the fire damage to be severe enough to erase the diagnostic signs for this determination. Since such fire damage is more likely near the ceiling of a structure, if it is important to know the position of a door, be sure that all (usually three) hinges from a door are recovered and labeled as to top, bottom, and middle. Photos of the hinges in place may also be helpful (along with photos of any protected area on the floor or carpet). These determinations, like the others in this section, are within the capabilities of many experienced laboratory analysts.

Fire and Smoke Hazards. In accidental fires, the specialist laboratory can be of assistance in assessing the contribution made by various materials to the fuel load. Carpets, particularly early polyester materials, can represent a hazard because of their flammability. Although furnishings sold in some states must pass flammability tests before they can be sold, this is not the case nationwide. Furnishings with synthetic or cotton coverings and cotton or polyurethane fillings can, therefore, present a likely starting point for an accidental fire. If any of the coverings or fillings remain unburned, it may be possible to identify them, duplicate them, and test the combination for susceptibility to accidental ignition (see Chapter 11). Such testing, although it can be done informally by most laboratories, requires considerable experience if the results are to be used in civil litigation. Some states have agencies, like California's Department of Consumer Affairs, Bureau of Home Furnishings, which will test such hazards on a contract basis; otherwise, a private consumer or materials laboratory will have to be contacted.

As part of a reconstruction of the cause and manner of death in fatal fires it may be necessary to determine the origin of toxic gases that caused the death or heavy smoke which prevented the escape of still-conscious victims from a small fire. Although the production of toxic gases and dense smoke is largely a variable of the temperatures and ventilation during the actual fire, some laboratory tests can be carried out to evaluate potential hazards. Work of this type has been done to test upholstery materials for closed environments such as airplane interiors, but it is costly and time consuming and may not be available to most fire investigators. The NFPA has developed standard tests for evaluating the fire hazards of interior finishes and the reader is referred to its publications for specific guidance.¹¹

IDENTIFICATION OF VOLATILE ACCELERANTS

By far the predominant service provided by laboratories to fire investigators is the analysis of fire debris for suspected volatile accelerants. In one laboratory study, volatile accelerants were found in 49% of all arson cases submitted to a major crime laboratory over a three-year period.¹² The use of volatile accelerants, most commonly petroleum products, has been common throughout the history of twentieth-century incendiarism. Many accelerants are used in excess and their residues remain for ready detection by instrumental analysis. Unfortunately, they can be used in moderation or on flammable substrates like paper or plastic, and the resulting fire can evaporate all traces of the accelerant. The author has participated in a number of structure fire experiments where a petroleum distillate such as gasoline had been used to ignite a fire and the debris, even though collected directly after the fire, packaged properly, and analyzed by the best available methods, failed to reveal any accelerant. A negative laboratory finding is not proof that an accelerant was not used, it merely fails to establish its presence after the fire. In addition, petroleum products change in their physical and chemical properties as they burn or evaporate during (and after) a fire, and their specific identification can be a challenge even to experienced laboratory analysts.

Gas Chromatography

Gas chromatography is used both for the screening of samples to determine which contain adequate volatile ignitable liquids for identification and for the identification itself once the volatile has been isolated. Although gas chromatographs (called GCs) vary considerably in their complexity, their basic principles of operation are quite simple. All types of chromatography are means of separating mixtures of materials on the basis of slight differences in their physical or chemical properties. Gas chromatography uses a stream of gas (nitrogen or helium) as a carrier to move a mixture of gaseous materials along a long column or tube filled or coated with a separating compound. The components in the mixture interact with the separating compound by alternately dissolving in it and then volatilizing to be swept farther along the column by the carrier. To ensure that all the compounds in the unknown mixture remain in the gaseous state, the whole column is kept in an oven to maintain it at a precisely controlled temperature. The mixture is injected onto the column at one end and at the other there is a detector, usually a small flame whose electrical properties, monitored by electronic circuits, change whenever a compound comes off the column. The whole process may be likened to the analogy of a stream of vehicle traffic being routed through a forest. The motorcycles speed right through, zigzagging around the trees and come out the other side of the forest first to encounter a radar speed trap. The small cars have more trouble finding a path and come out shortly afterward to encounter the same radar. The big cars take a long time to find spaces big enough to allow passage and they exit after a longer delay, followed by the trucks that take the longest of all.

In the case of volatile hydrocarbon analysis, our "traffic" would be our unknown hydrocarbon mixture and our "forest" would be the GC column. We can adjust the spacing of our "trees" by choosing the correct chemicals for the separating compound and carefully controlling the temperature and carrier flow so that each type of "traffic" comes out in a neat, readily identifiable group. To extend the "forest-traffic" analogy further, the proper choice of GC column and conditions for the hydrocarbons found in fire debris allow the lab to identify the type of "traffic" introduced by analysis of the groups of "vehicles" that result. Gas chromatography as an accepted laboratory procedure has been in use for the last 35 years or so. From simple, insulated box ovens that kept the column at a set temperature, we have progressed to ovens that are programmed to provide steady or increasing temperatures at preset time intervals to maximize the separation of similar compounds. From simple thermal detectors, technology has progressed to flame detectors that can be made sensitive only to compounds containing specific elements.

Although the column can be selected to separate a mixture of compounds on the basis of their chemical properties, most separations in volatile accelerant analysis are made on the basis of differences in the boiling points of the various components. Nearly all the components of petroleum distillates of interest are either aliphatic (straight-chain) or aromatic (ring) in structure, and thus will have very similar chemical properties within those two groups. A typical separation of a series of aliphatic hydrocarbons is shown in Figure 14-4. In this typical chromatogram, one can see that the heavier a molecule is, the less volatile it is, and the later it will appear on the chromatogram. Columns for GC analysis of suspected arson residues are usually set up to offer the best separation of components having volatilities between those of pentane (C5H12) and triacontane (C30H62), since that range includes all of the commonly encountered petroleum products. Most examiners prefer a silicone-based stationary phase, like OV-1, OV-101, or SP-2100, on an inert filler or support medium. Although the mainstay of columns was for many years a packed tubular column, averaging 1/8 inch (3 millimeters) in diameter and 10 feet (3 meters) in length, this has now been supplanted by a new generation of very narrow (approximately 0.1 millimeter in diameter) capillary columns of various lengths.^13,14 Early capillary columns required very long lengths (50 meters or more) to achieve good separations, but these extreme lengths usually required long analysis times. Such columns were used primarily for research or development where short analysis times could be sacrificed for sensitivity and selectivity. The newest columns provide extremely high performance on short columns (10­25 m) using high temperature-programming rates. The result is analysis times that are shorter than those possible with packed columns and faster turnaround times. Capillary columns also use smaller sample sizes than was ever possible with packed columns, an advantage of much merit in forensic work where the sample size is often critically small. Sensitivity is now in the nanoliter range (10^­9 liter).¹⁵ Examples of capillary chromatographs are seen in Figure 14-5.

Some analysts prefer to repeat the GC analysis on another column using a stationary phase that discriminates by chemical properties. A packing such as Bentone 34 affects aromatics (in gasoline these would be benzene, toluene, xylenes, and alkylbenzenes) differently than it does the aliphatics. As a result, the aromatics show up on the chromatogram in a discrete "envelope" of peaks which is readily identifiable even at very low concentrations, thereby improving the detectability of gasoline in complex or "dirty" debris. Other columns can separate polar (oxygenated) species from nonpolar (hydrocarbon) ones.

In addition to the flame ionization detector, which has been standard for analysis of hydrocarbons for a number of years, there are now specialized detectors that can be used in arson work. Electron capture detectors, which use a minute radioactive source as a monitor, are very sensitive to particular chemical species such as organometallics and halogenated hydrocarbons. They can be used to characterize a petroleum distillate more precisely than is possible with flame ionization alone. These detectors can be used to characterize the lead additive package sometimes still found in gasolines as an octane booster.¹⁶ Naturally such additives are not found in unleaded gasolines, and some petroleum companies tend to share the same additive package, making brand discrimination uncertain; however, there are occasions when such information can be of use. There are nitrogen-phosphorus detectors that are not sensitive to "normal" hydrocarbons but will select those compounds in a complex mixture that contain nitrogen or phosphorus. By monitoring the color of the detector flame, specialized detectors can be made specific for compounds containing sulfur. Since the crude oil feedstocks for gasoline vary considerably in their nitrogen-, phosphorus-, and sulfur-containing components, it is very likely that such techniques may allow discrimination between gasolines from different origins. Such discrimination has been greatly needed in the past when comparing the gasoline from an incendiary device to gasoline from a suspected source.

In addition, progress in microelectronics has dramatically changed the data collection and handling in gas chromatography. Runs (and irreplaceable samples) are no longer lost when the recording conditions are not set just right, since the data system collects all the data and then displays whatever portion the analyst wants. The displayed data can expand or condense portions of the chromatogram to make identification easier. Collected chromatograms are also filed on minicomputer discs and can be compared against a library of stored chromatograms of known products. Such libraries can be swapped between instruments and even between labs, permitting ready intercomparison.

Gas Chromatography/Mass Spectrometry (GC/MS)

Flame ionization detectors (FIDs) are sensitive and stable detectors but they yield little chemical information about the compounds that pass through to produce a signal. Mass spectrometry allows the analyst to break apart each compound into small submolecular pieces and, by counting those pieces, establish the chemical structure of the original molecule. Mass spectrometry alone is an analytical technique that needs to be fed one compound at a time, while gas chromatography is a separation technique that is good at separating things into groups. Since most of the volatile residues in fire debris involve complex mixtures, coupling the two together gives the best tool. While GC/MS has been available for more than 20 years, only recently has it been made small, inexpensive, and user-friendly enough to make it convenient for fire debris analysis. In addition to the capability of displaying a mass spectrum for each peak, it can be asked to scan for selected ions, those that are characteristic for a particular chemical species of interest. This means that it can display a chromatogram just for aromatics, for instance, ignoring all the peaks that do not yield that specific ion. This can make it possible to pick out characteristic patterns even amidst complex chromatograms with high backgrounds. The technique is well established and there is an ASTM method for its use (ASTM E 1618).^17,18

As of this writing most forensic gas chromatographic analyses for suspected acclerants are being carried out using a capillary column of moderate length, using SP-2100 (or OV-1 or an equivalent bonded phase), in a programmed temperature range of 50°C to 250°C, and employing a flame ionization detector. The discussions that follow are based on the results of such analyses.

Sample Handling and Isolation of Volatile Residues

As described in previous chapters, the best containers for debris suspected of containing volatile ignitable liquids are clean metal paint cans, sealable glass jars, or bags of nylon or a suitable copolymer (nylon/polyester/polyolefin or nylon/polyethylene). The first step in the normal examination protocol of such debris is to briefly open the container (in the case of bags, this will usually not be necessary) and to inspect the contents. The nature of the contents (wood, carpet, soil, etc.) may provide guidance in selecting an analytical technique or predicting what interferences are likely to be encountered. The contents should be checked against the labeled or indicated contents to ensure there have been no errors in packaging or labeling that could compromise the value of the evidence. The examination must be brief to minimize losses of volatiles that may be present only at trace levels. Very strong odors of a volatile detected during this examination may indicate that a particular test protocol should be used. The major problem with fire debris analysis is having a small quantity of product mixed with a great deal of solids or liquids that may provide contamination. Each of the techniques described here has advantages and disadvantages that must be evaluated in light of the character of the sample.

Heated Headspace. Since any volatiles present will reach a vapor equilibrium in a sealed container, we can take advantage of the vapors in the air cavity (headspace) of the container. Heating the sample container to 50­60°C (120­140°F) will ensure at least a partial vaporization of heavier hydrocarbons prior to sampling. Some analysts prefer heating the container above 100°C to force even higher boiling materials (such as fuel oil) into the vapor stage. It is the experience of most other analysts, however, that even the heavy ends of fuel oil will be detectable in samples heated to moderate temperatures and, more importantly, that such excessive temperatures encourage the degradation of synthetics (carpet, upholstery, etc.) in the debris. These degradation products add to the complexity of the chromatogram, possibly masking low levels of real accelerants, while not contributing significantly to the sensitivity of the method.

Once the sample container has equilibrated at a moderate temperature, it is punctured and a small (0.1­3 milliliter) sample of the headspace vapor is drawn off using a gas-tight singe. This vapor is then injected directly into the gas chromatograph. For many mixtures, positive results of this injection may be adequate to permit characterization of the volatiles present (as described in ASTM "E 1388: Standard Practice for Sampling of Headspace Vapors from Fire Debris Samples"). It also signals the presence of very volatile residues that may be lost be careless handling, or alcohols or ketones that require special handling. Since this technique is not as sensitive as others to be described, a negative result here merely indicates that no high concentration of volatile exists, and that another isolation technique is suggested. It is fast, convenient, and requires virtually no sample handling, while providing useful information.

Passive Headspace Diffusion (Charcoal Sampling). Another technique that capitalizes on the vapor equilibrium in a sealed container while concentrating some of the vapors in a more condensed form is passive adsorption. This actually constitutes several similar techniques that rely on the adsorptivity of activated charcoal. Originally developed by forensic scientists in England using a charcoal-coated wire, this technique requires no manipulation of the sample or its original container. As originally described, the method called for a coated wire to be inserted in the air space of the sample container (as shown in Figure 14-6) and allowed to equilibrate for at least 2 hours. The wire is then removed from the container and inserted into the pyrolysis unit of a GC, where it is raised instantly to a very high temperature, driving the absorbed hydrocarbons off into the column.¹⁹ This technique has been adapted in a variety of methods described in ASTM "E 1412: Practice for Separation and Concentration of Ignitable Liquid Residues from Fire Debris Samples by Passive Headspace Concentration." These adaptations include the insertion of a plastic or glass bead coated with activated charcoal or a sampling tube or bag containing charcoal into the airspace of the container. The container is resealed and allowed to equilibriate, usually at room temperature for 12-15 hours or at 50°C-60°C for 2 hours. The sampling device is removed and adsorbed volatiles are extracted from the charcoal with a small quantity of solvent such as pentane, diethyl ether, or carbon disulfide for injection into the GC. These techniques are very sensitive to all types of volatiles, require no heating or manipulation of the samples, and can be repeated again and again since they take up only a small fraction of the volatiles each time.^20,21 They are suitable for all kinds of debris, and are especially valuable for debris that will be examined for trace evidence or latent fingerprints.

A variant of this technique has become the single most widely used sampling and concentration method for fire debris. In this form, a small strip of carbon fiber mat is suspended in the container, at room temperature or at moderately elevated temperatures. It is then extracted with carbon disulfide or diethyl ether. It has excellent sensitivity (0.1 microliter under some conditions) and no interferences. It is described in ASTM E 1412.

A new variant, called solid-phase microextraction, uses a solid phase absorbent bonded to a fiber that is inserted into the can of fire debris through a hollow needle. After it has been exposed to the heated contents of the can for 5 to 15 minutes, it is withdrawn and then inserted directly into the injection port of a GC. The heat of the injection port then thermally desorbs the volatiles directly into the GC column. The technique offers extreme simplicity (with no manipulation of the sample or any extract) and low cost, since it involves no solvents and no modifications to existing gas chromatographs.

Dynamic Headspace (Swept Headspace).  This technique, sometimes called the charcoal trap method, employs a charcoal or polymeric matrix through which the air from the evidence container is drawn. It was adapted from methods developed for trapping ultralow concentrations of hydrocarbons for environmental monitoring.²² The sample container is fitted with a modified lid or a septum that allows for the introduction of a heated carrier gas (purified room air or nitrogen). The container is warmed while vapors are drawn off by a low vaccuum through a cartridge filter containing activated charcoal or a molecular trapping agent such as Tenax^TM (see Figure 14-7). Once extraction is complete (20-45 minutes), the charcoal can be washed with carbon disulfide or a like solvent, and the resulting solutions is ready for GC analysis. Tenax^TM or charcoal may be thermally desorbed to avoid the handling of toxic solvents, contamination, or any further dilution. This technique requires the use of traps (commercially prepared or laboratory prepared) and specially built apparatus, described in ASTM "E 1413, Standard Practice for Separation and Concentration of Ignitable Liquid Residues from Fire Debris Samples by Dynamic Headspace Concentration." It is ver useful because of its sensitivity (less than 1 microliter) and its applicability to all ignitable liquid residues (from gasoline to fuel oil) including alcohols and ketones. The GC analysis of Tenax^TM traps, which are thermally desorbed, has been automated, making it more suitable for laboratories with a large caseload. It does have the drawbacks that all of the volatiles in a sample can be swept out (and pulled through the trap) if the time and temperature conditions are not monitored closely and are too severe for the sample size. Excessive extraction produces loss of the lighter fractions and the process can generally be carried out only once on a sample.

Steam Distillation.  The oldest technique, originating from classical chemistry, requires only modest equipment. The specimen is removed from its container and boiled with water in a glass flask (as in Figure 14-8). The steam, carrying with it the volatiles, is condensed and trapped. It is suitable for volatiles that form an azeotropic (nonmixing) layer with water; therefore, it cannot be used to recover alcohol or acetone. It is suitable for petroleum distillates as heavy as paint thinner; heavier products (e.g., fuel oil) require the use of ethylene glycol, which boils at a higher temperature.²³ Light hydrocarbons may be lost in recovery. Detection limits are on the order of 50 microliters (for gasoline), so it is not as sensitive as charcoal-trapping methods, but it has the advantage of providing a neat liquid sample with no extraneous solvents.²⁴ The method is described in ASTM "E 1385, Standard Practice for Separation and Concentration of Ignitable Liquid Residues from Fire Debris Samples by Steam Distillation."

Vacuum Distillation.  This well-known technique common to organic chemistry requires only modest equipment (see Figure 14-9) and a supply of refrigerant such as dry ice or liquid nitrogen. It is suitable for the recovery of any volatile hydrocarbon, but excessive water in the specimen may interfere with recovery. It is suitable for debris that is susceptible to water damage, including fragile, charred documents. Recovery yield is on the order of 60% for gasoline, while detection limits are on the same order as steam distillation.²⁵

Solvent Extraction.  This very direct and simple method for extracting volatiles has been in use for many years. Debris is extracted or washed with a small quantity of n-pentane, n-hexane, dichloromethane, or carbon disulfide that has been checked to ensure its purity. The extract is then concentrated by evaporation in a stream of warm air until a small quantity remains for testing. It is especially suitable for treating small, nonabsorbent specimens (glass, rock, metal) or washing the interior of containers used to transport ignitable liquid accelerants. It provides good recovery for most petroleum products, especially the heavier ends of kerosene and diesel fuel, but it is not suitable for isolating the light petroleum distillates due to evaporative losses. Its overall sensitivity is about the same as for steam distillation with the main drawback that it will extract residues of partially pyrolyzed carpet or foam rubber, which often interfere with the detection of low levels of hydrocarbons of evidentiary significance. This technique requires only simple glassware and high-purity solvents. It is described in detail in ASTM "E 1386, Standard Practice for Separnitable Liquid Residues from Fire Debris Samples by Solvent Extraction."

It should be noted that most of the newer isolation techniques, especially when combined with capillary column gas chromatography, are more sensitive than the techniques used less than a decade ago. In many cases they are more sensitive than the human nose, often relied upon to screen for the best debris samples at the scene. This means that sample collection may not depend on just the detection of suspicious odors but also on the presence of suspicious burn patterns and on aids such as canine detection. Comparison samples of floor coverings or other fuels involved are more desirable now than ever before because of the contributions made by the pyrolysis of synthetic materials, binders, and adhesives, which can interfere.^25,26

Identification of Volatile Residues

Once the suspected ignitable liquid residues have been isolated, attention can turn toward identifying them as precisely as possible. Because recoveries are typically on the order of several microliters, gas chromatography will be the technique of first choice because of the quantity of information it yields while consuming 1 microliter or less of the sample. The identification of the volatiles (usually a petroleum distillate or product) present is then usually based on a comparison of the pattern of peaks generated by a flame ionization detector (FID) in the chromatogram of the unknown against a library of chromatograms of known materials made under the same conditions. A standardized approach to the classification and characterization of volatiles is described in ASTM "E 1387: Standard Test Method for Ignitable Liquid Residues in Extracts from Samples of Fire Debris by Gas Chromatography." Classification as to whether the volatile fits one of six categories--light, medium, or heavy petroleum distillates, gasoline, kerosene, or miscellaneous--is based on the presence of particular sequences of hydrocarbons or oxygenated compounds. This classification is intended to make description and comparisons easier for the laboratory analyst, but because consumer products may represent different classes of distillates, and a single distillate may have more than one product designation, the system is not always well understood by the investigator. For this reason, it is recommended that when an analyst describes an ignitable liquid residue as a particular classification, examples of some common consumer products should be included, as in Table 14-1.

As discussed in a previous chapter, isoparaffinic hydrocarbon products are being used in many consumer products from copier toners to insect sprays. Since they are not true petroleum distillates, they should not be classified as such, although in some instances they display a boiling point range that fits classes 3 or 4. Examples of these products should give the investigator a clue as to what product may have been used to start the fire or that a product normally at the scene may have become involved accidentally in the fire. Since the contents of many products are not always described accurately on the labels, containers of any product in the vicinity of the fire's origin should be collected for analysis if there is a chance they could be involved in the fire.

While the GC/FID analysis described here is adequate for most products, there is a growing number of ignitable liquids that are not petroleum distillates and, therefore, do not lend themselves to a classification scheme based on peak pattern; for instance, there are aromatic specialty solvents used in insecticides, enamel reducers, and pavement sealers, and naphthenic/paraffinic petroleum products used in lamp oils. The GC/FID patterns of these products are not enough to permit their identification and GC/MS is required.

Special detectors may also be used to gain more information about a volatile. Each requires injection on a separate GC column with a specific detector in place of the FID, but some GCs have the ability to split a single injection onto two separate columns, each with its own detector. Photoionization detection (PID), for example, can be used in parallel with FID since it can be made specific to aromatic, olefinic, or aliphatic hydrocarbons. Another detector, originally intended for the detection of nitrogen- and phosphorus-containing compounds, has been modified so it will detect specifically the oxygenated volatiles (ketones, alcohols, and ethers) that have become widely used as fuel additives to minimize air pollution.²⁷

When distillation is used to isolate a neat volatile, infrared spectroscopy can yield considerable information about its chemical structure. About 20 microliters of sample are needed for conventional liquid-cell spectrophotometry but a specialized FTIR microcell has been designed for use as a detector on a GC column when smaller quantities are recovered. While its sensitivity is limited, it can aid in the identification of gasoline, complex pyrolysis products, and some natural products (terpenes).²⁸

If sufficient quantities of the accelerant are recovered, physical properties like flash point and refractive index can be measured. Refractive index can be measured on samples as small as 20 microliters, but it is generally of little value in characterizing a volatile petroleum distillate. The criminal charges against a suspected arsonist may depend on the flammability of the accelerant in his or her possession, and a flash point determination may be required. A precise determination requires a considerable sample and specialized equipment, but a rough estimate can be made by placing several drops of the liquid on a watch glass and placing it in a freezer. On removal from the freezer, the glass will warm up gradually and a small ignition source passed over the specimen will cause a flash when the flash point is exceeded. If the liquid has a flash point above ambient (usually 25°C, 77°F), the glass can be placed on a hot plate and the sample tested in the same manner. Recent advances in ultraviolet (UV)/visible fluorescence technique permit extensive characterization of petroleum distillates and this technique, along with high-pressure liquid chromatography, may be useful in special circumstances because it provides information that supplements the gas chromatography.²⁹

Petroleum products behave in a fairly complex but predictable manner when burned or simply exposed to air to evaporate. Due to their higher vapor pressures, the most volatile components evaporate more quickly than those less volatile, and this changes the peak profile of the product when it is subjected to gas chromatography. As we can see from Figure 14-5, a gasoline profile changes dramatically as it evaporates, complicating its identification. When the gasoline is burned, this same process occurs, only much more rapidly. It is not possible to determine whether a petroleum product burned or merely evaporated at room temperature by examination of its chromatogram.³⁰ In the case of gasoline, there are characteristic aromatic compounds that are found in residues of gasoline that have evaporated down to less than 3% of its original volume. Therefore, evaporated or "weathered" gasoline can still be identified if those key or target compounds are identified in the debris, as in Figure 14-10. Unfortunately, this is not true for simple petroleum distillates like mineral spirits (paint thinners). As they evaporate, the residues assume more and more of the characteristics of the next heavier class of petroleum distillate. Thus, it is not always possible to conclusively establish the exact nature of the original product. In addition, petroleum products when exposed to moist garden soil can undergo microbiological degradation. This degradation involves the consumption of both aliphatic and aromatic compounds, so the peak profile of gasoline, for instance, can be significantly changed.³¹ These effects combined with contamination from pyrolysis products in the debris can make the identification of a specific petroleum product from postfire debris difficult. In most cases, the laboratory can be expected to characterize the product or its general family--for example, light (petroleum ether), medium (kerosene), or heavy (fuel oil) petroleum distillates; lacquer thinners; and so on--from residues left in the debris unless the accelerant has been nearly or totally consumed. Any petroleum distillate, when used as an accelerant, can be burned away completely, leaving no readily detectable traces, if the fire is hot enough and long enough, and the accelerant is exposed directly to its effects. Fortunately, in most arson-caused fires, accelerants are used in excess and the resulting fires do not burn with sufficient intensity to completely destroy them.

Laboratories are often asked to determine the presence of petroleum distillate accelerants from the soot accumulated on window glass taken from the scene. As we discussed in Chapter 7, the appearance of the soot is most directly linked to the state of combustion in the room and the amount of ventilation. Fuel-rich fires produce heavy soot; lean, or well-ventilated fires produce little or none. The synthetic fibers and rubbers that constitute a large percentage of the fuel load in modern structures produce an oily soot very similar chemically to that produced by petroleum distillate accelerants. It has been suggested that there should be chemical species in the soot produced by gasoline which are characteristic for that fuel and are not produced by any "normal" fuel. It was once thought that the combination of bromine and lead in leaded automotive gasoline would produce a soot with a unique elemental profile. Unfortunately, some carpets contain both elements so there is a potential for background interference.³² More importantly, lead and its related additives are being phased out of modern fuels, making them less likely to be detected. There are several organic molecules, chiefly polynuclear aromatic hydrocarbons, that appear to be produced by burning gasoline but are not produced by burning synthetic materials.³³ The National Bureau of Standards conducted preliminary evaluations of this idea, but the work was never completed and was not suitable for adaptation to real casework.³⁴ The most extensive work done in this area is that of Pintorini et al., who used gas chromatography, combined with elemental analysis, x-ray diffraction, and SEM to characterize soot from various ignitable liquids and that from synthetic fibers and materials.³⁵ They discovered that under controlled conditions, soot from the liquids could be distinguished from that produced by solid fuels, using multiple techniques. Unfortunately, the presence of water from suppression produced nonreproducible condensation of the soot, and limited the discrimination value of even the combination of techniques. Some analysts have reported that freshly made carbon soot will absorb petroleum distillate fumes from the room air just as activated charcoal absorbs them in laboratory extractions. If a fire occurs and a too-rich mixture of fuel is present, the soot formed during the early stages may retain enough of the accelerant to permit identification upon extraction of the soot.

In spite of partial evaporation, most common petroleum distillates have a distinctive GC pattern that can be compared with the patterns of known compounds. The experienced analyst will know what they effective of fire and evaporation can be and will take them into account when comparing chromatograms. All petroleum distillates are overlapping "cuts" or fractions of a continuous spectrum of compounds. Therefore, it is possible for a complex hydrocarbon mixture to be sufficiently affected by fire that its chromatogram cannot be readily identified. It could, for instance, represent a heavily evaporated product such as kerosene, or a relatively undamaged product like a heavy solvent.

As to the identification of a specific source for a product, the range of techniques available now enables the analyst to rule out many potential sources and thereby shorten the list of "possibles." The sophistication of chromatography and today's data handling systems makes it possible to recognize delicate shifts in the patterns of GC peaks between products that were thought to be indistinguishable. Determining the significance of these shifts is entirely another matter, requiring extensive field sampling and validation of the reproducibility of patterns detected. As with many mass-produced materials that start from different feedstocks in different manufacturing facilities, changes can be produced by many factors. To complicate matters, when gasoline, for instance, is distributed, it mixes with other gasolines already in storage and transportation facilities and its composition changes. Under some circumstances, such as having a liquid gasoline sample that has not undergone significant burning or evaporation, and a small number of potential sources, it is possible to state with some confidence that certain sources can be completely excluded and that another represents a source which is indistinguishable by any of its individual features.³⁶ Rather than give the investigator a false lead with a "possible" identification the laboratory will usually suggest several alternative identifications in its report. The investigator can evaluate the suggestions and determine which, if any, fit the circumstances best. If the investigator does not clearly understand the identification or characterization, the problem should be discussed with the laboratory before proceeding.

CHEMICAL INCENDIARIES

Volatile acclerants are not the only arson evidence that is amenable to laboratory testing. A small but significant percentage of all detected arsons use a chemical incendiary as an initiator or primary accelerant.³⁷ Fortunately, most chemical arson sets leave residues with distinctive chemical properties, although physically the residues may be nondescript and easily overlooked.

Safety flares or fusees are the most commonly used chemical incendiary, presumably because they are effective, predictable, and readily available. Red-burning flares contain strontium nitrate, potassium perchlorate, sulfur, wax, and sawdust.³⁸ Their postfire residue consists of a lumpy inert mass, white, gray, or greenish-white in color. It is not soluble in water, but prolonged exposure to water turns it to a pasty mush.

The residue contains oxides of strontium and various sulfides and sulfates. Although strontium is found in all natural formations of calcium, only flare residues contain strontium salts in nearly pure form. Elemental analysis by emission spectroscopy, atomic absorption, or x-ray analysis will readily reveal these high concentrations of strontium in suspected debris. The igniter-striker button on the cap and the wooden plug in the base of the stick are resistant to burning and may be found in the fire debris and may be identified as to manufacturer by their physical features.³⁹

The solid chlorine tablets or powder used for swimming pool purification will react with organic liquids such as the glycols in automotive brake fluid or some hair dressings to produce a very hot flame that lasts for several seconds. This occurs after a time delay that varies with the concentration of the chlorine-based components and the degree of mixing between the solid and liquid phases. This reaction has been studied and a mechanism for the production of ethylene, acetaldehyde, and formaldehyde by the action of the hypochlorite on the ethylene glycol has been suggested and verified.⁴⁰ The production of a cloud of readily ignitable vapors would certainly account for the dramatic yellow-orange fireball of flames that occurs in these reactions. The starting solid compounds are usually not completely consumed and are not immediately water soluble and may be visibly detected after the fire. The presence of hypochlorite salts can be detected by infrared spectrometry or chemical spot tests to confirm the identity of the oxidizer.

Potassium permanganate is used in a variety of chemical incendiary mixtures, including a hypergolic (self-igniting) mixture with glycerine. It is soluble in water and can be washed away. Even a weak water solution of it, however, will have a pronounced red, green, or brown color, depending on the oxidation state of the manganese ion. Such chemical ions are readily identified by chemical test or by infrared if even a minute chip of the solid chemical is recovered.

Flashpowders contain a fine metallic powder, usually aluminum, and an oxidizer such as potassium perchlorate or barium nitrate. Elemental analysis of the powdery residue left after ignition will reveal the aluminum, chlorine, potassium, or barium. The perchlorate or nitrate residues are detectable y chemical test if any of the mixture remains unreacted.⁴¹

Potassium chlorate is easily identified by chemical or instrumental tests if it can be recovered in its unreacted state. When it reacts in an incendiary mixture, however, its residues are almost entirely chloride salts, which are innocuous and which, if recovered from debris, could not be readily identified as having come from a device.

Sugar reacts with sulfuric acid to leave a puffy brown or black ash of elemental carbon. It may have a faint smell of burning marshmallows, but for the most part it is innocuous in appearance and is easily overlooked. The acid present may char wood, paper, or cloth in its concentrated form. It is very soluble in water, but mineral acids like sulfuric (H2SO4) do not evaporate and remain active and corrosive for some time. A burning sensation in the skin on contact with debris indicates that the debris should be checked for acids. The pH (acidity) can be easily measured, and chemical tests for sulfate or nitrate ions carried out. Reactive metals such as potassium or sodium can be used as an ignition device because they generate great amounts of heat and gaseous hydrogen on contact with water. The residues will contain potassium hydroxide or sodium hydroxide (lye)--both very strong caustics that can cause skin irritation and burns. The pH of water containing such residues will be very alkaline (basic), and chemical or spectrophotometric tests will reveal the metal present. It should be noted that the residues from these and similar devices are very corrosive to metal. Debris suspected of containing such corrosives should be placed in glass jars with plastic or phenolic lids, or in nylon or polyester/polyolefin bags--not in metal cans, paper bags, or polyethylene bags.

Since white phosphorus ignites on contact with air, it has been used as an incendiary device in the past. (Feenian fire is a suspension of crushed white phosphorus in an organic solvent. When the container is smashed, the solvent evaporates, exposing the phosphorus to air with immediate ignition.) Elemental analysis of the debris may reveal an elevated concentration of phosphorus, and there are chemical spot tests, although it tends to be a difficult element to detect and confirm.

A common source of phosphorus in incendiary mixtures is the household match. Characterization of the components in match heads can be carried out by elemental analysis alone or combined with microscopic analysis using SEM. ⁴²

Many improvised chemical incendiaries are described in the literature (too many to describe here). ⁴³ Whenever such an incendiary is suspected, samples of nearby materials should be collected for comparison purposes. Sometimes it is not the presence of a particular element or ion but an elevated concentration with respect to that found in the background that betrays the presence of the incendiary.

NON-FIRE-RELATED CRIMINAL EVIDENCE

The most frequent mistake made by arson investigators is a single-minded focus on the fire itself and its origin, while ignoring other typs of physical evidence. It is up to the arson investigator to construct an entire scenario of a fire setter's activities at the scene. These activities are not necessarily limited to the setting of the fire itself. Since arson may be used to destroy evidence of other criminal acts, the careful investigator must be always alert to the existence of evidence of "normal" criminal activity.

Fingerprints

Many investigators assume, as do arsonists, that a fire destroys all fingerprints. That is not true. While itt is true that many fingerprints will be lost during a fire, many will survive, even on ignition sources, as in Figure 14-11. Plastic (three-dimensional) impressions in window putty or patent (visible) impressions in paint or blood may remain even after diirect exposure to fire. In fact, they may be permanently fixed in place by the heat. Plastic containers used to carry or hold gasoline in an arson fire have been seen to be softened sufficiently by the gasoline to allow the plastic to take on the fingerprint impressions of the person holding the container. If such a container is not exposed directly to the flames, these plastic impressions may survive. The plastic window attacked with a butane cigarette lighter in Figure 14-12 bore both plastic impressions in the softened plastic and patent impressions in the soot.

Latent fingerprints, those requiring some sort of physical or chemical treatment to make them visible, consist basically of five components: water, skin oils, proteins, salts, and contaminants. The water evaporates rapidly or soaks into absorbent surfaces and is usually of little value. Skin oils are the residues normally detected by dusting with a soft brush and powdered carbon. These oils will remain on hard surfaces for some time or will soak into absorbent materials, like paper. They can be evaporated by elevated temperatures and such "dried out" prints may not respond to dusting. The soot produced by a smoky fire (such as a gasoline pool) may condense on a glass or metal surface to protect the latent prints, however, needing only to be gently brushed or washed in a stream of tap water to reveal the impressions. Proteins degrade to amino acids that are detected by reaction with the chemical ninhydrin. They are easiest to detect when the latent impressions are on clean, light-colored, porous surfaces like paper or cardboard. They can be denatured by high temperatures so that they no longer react, but if the paper has not been charred by the fire it may be worth testing. However, exposure to water (from suppression, condensation of water vapor from the fire, or from wet weather) will dissolve the proteins and amino acids and blur the fingerprints. When this occurs the fatty deposits in the latents (skin oils and cosmetics) are not affected by the water. They can be detected by the physical developer method. Nonporous surfaces that are or have been wet can be examined using small particle reagent (SPR). The salts are the most fire resistant of all latent fingerprint residues, resisting very high temperatures. Like amino acids, however, they are susceptible to water damage. Salts may react with the inks or coatings on paper or with the metal surfaces of cans. In doing so, they may leave an identifiable patent (visible) print in corrosion. This effect may be enhanced by the heat and moisture of the fire, leaving a visible print requiring no further treatment. Other contaminants that can produce patent prints include cosmetics, food residues, grease, motor oil, blood, and other products.

In the last 15 years, there have been dozens of techniques developed for the detection of latent prints that may aid the inspection of fire-related exhibits. Examination using lasers and high-intensity tunable light sources (including ultraviolet and infrared wavelengths) makes it possible to penetrate contaminants and make prints visible against a variety of backgrounds. ⁴⁴ Cyanoacrylate ester fuming and a variety of fluorescent dye stains and powders further expand the possibilities of recovery of latent print. ^45,46 All these developments permit the development of latent finger prints on many surfaces thought previously to be incapable of retaining such impressions. These include surfaces such as leather, wood, vinyl plastics, smooth fabrics, even charred paper and metal. Latent prints have even been recovered from human skin. Recovering such prints requires appropriate skin conditions (smooth, dry, free of hair) and the contact has to be of adequate duration. Such prints lose detail after a few hours even if the victim is deceased. If a fire victim has not been dead more than a few hours and the skin has not been damaged by the fire, a search for such latents should be considered. ⁴⁷ There is no more conclusive proof of identity than a fingerprint, making it worth the time and effort to search for it. Because of the rapidity with which these new techniques are being put into service, the investigator should never conclude that latents are impossible to recover before consulting with a knowledgeable specialist in the field.

Blood

Bloodstains left at a crime scene have much greater evidentiary value now than ever before because of great strides made in determining subtle variations in the proteins and enzymes of human blood. Not long ago typing was limited to ABO, MN, and Rh types. They can be detected by the physical developer method. Nonporous surfaces that are or have been wet can be examined using small particle reagent (SPR). The salts are the most fire resistant of all latent fingerprint residues, resisting very high temperrred to exclude witnesses or bystanders as sources of bloodstains at a scene or on a suspect's clothes or vehicle. All 16 commonly used systems can be done on a fresh, dried stain the size of a dime, making it possible to get conclusive results even from small stains. These variants are, however, susceptible to deterioration by age, bacteria, or heat. If the temperature of a bloodstain does not exceed 50°C (120°F), almost all typings can be carried out on a fresh stain. Between 50° and 100°C (212°F), many off the enzymes degrade and can no longer be identified. Between 100° and 200°C (425°F), the proteins degrade and the species and ABO blood group can no longer be determined. Above 200°C, the stain degrades and probably cannot be even identified as blood. If the pros and probably cannot be even identified as blood. If the pros and probably cannot be even identified as blood. If the proteins in a blood or semen stain (or tissue fragment) have not degraded, and the evidence is critical enough, the stain can be subjected to DNA typing. This new approach is only recently tested in criminal cases and not all forensic labs are equipped or prepared to take it on. DNA typable material is more resistant to thermal and biological degradation than are the enzymes and protein factors. DNA does not result in a unique identification but it does promise a means of associating a stain or tissue fragment with an individual with considerable certainty. If you are in doubt about a suspected stain, collect it, keep it dry and as cool as possible, and submit it to your laboratory as quickly as possible.

Impression Evidence

Often overlooked, impressions of tools on windows or doors can confirm the fact of forced entry (Figures 14-13 and 14-14), identify points of entry (or attempted entry), and, if tools are recovered, permit identification of the tools responsible. Such tool marks may also be found on desk drawers, filing cabinets, locks, or chains at a scene, confirming that a burglary took place before the fire. The investigator should learn to recognize signs of forcible entry and be able to account for all such damage at a fire scene. The best kind of tool-mark evidence is the impression itself. After it has been sketched and photographed, the object (or suitable portion) bearing the impression should be collected. If its size makes recovery impractical, a replica may be cast in silicone rubber or dental impression material. Photos of a striated impression are of no use in a laboratory comparison other than to establish the location and orientation of the mark. The submitted tools are tested on a variety of surfaces to duplicate the manner of use and the qualities of mark-bearing material as closely as possible without risking damage to the tool. The evidence and test marks are examined under 40 to 400X magnification to try to match the striations or contours present. See Figure 14-15 for a typical comparison.

Impressions of footwear and vehicle tires may be found on the periphery of the fire scene. Shoe prints have also been linked to forcible entry, as shown in Figure 14-16. Although prints in soil cannot generally be collected themselves, they are worth photographing in place and possibly casting with plaster of Paris. Photographs must be taken using low-angle (oblique) lighting to highlight the three-dimensional details; a scale or ruler must be included in the photo, near (not on!) and parallel to the impression; and the film of the camera must be parallel to the ground to prevent distortion. See Figure 14-17 for a good example. Although particular tires can sometimes be matched back to their impressions, it is more common to establish only a correspondence of size and tread pattern. Footwear, due to its characteristic details and slower rate of change, is more amenable to identification and offers a means of placing the subject right at the fire.

Physical Matches

One of the few conclusive identifications possible with typical evidence is the physical comparison of torn, cut, or broken edges or surfaces with each other to establish a "jigsaw" fit between them. Pieces of glass from shoes or garments have been matched back to windows, vehicle lamps, or bottles to connect a victim or suspect with a scene. Wrappings from incendiary or explosive devices have been matched to source materials in the possession of the suspect, as have pieces of tape, wire, wood, and rope. The evidentiary value of such positive identifications is very high and the investigator must be aware of the possibilities when searching a scene for anything that looks out of place or when searching a suspect's property or vehicle. Close cooperation and communication between the lab and the investigator has produced some excellent evidence (see Figure 14-18 for an example).

Trace Evidence

Trace or transfer evidence such as paint, fibers, soil, or glass can be used to link a suspect with a crime scene even in arson cases. Paint can be transferred to the clothing or tools when a forcible entry is attempted. Flakes as small as 1 millimeter square are adequte for color or layer sequence comparisions or complete analysis by infrared, spectrography, or x-ray analysis. Such properties are generally class characteristics; that is, even a complete correspondence between the chemical properties of known and questioned paints does not imply that the two paints must have come from the same object. Instead, it confirms that they came from the same class of objects painted with the same paint. Multilayered paint chips, however, may be linked to a particular source if they have enough layers to demonstrate a unique origin.

Glass is found in nearly all structures and can tell a good deal about the events of the crime. It can be broken by either mechanical force or thermally induced stress. Curved fracture ridges on the edges of glass fragments reveal that the breaking force was mechanical. The shape of these conchoidal fracture lines can help establish from which side of the window the force came. The absence of these lines usually means the fracture was caused by thermal stress. The presence or absence of soot, char, or fire debris on the broken edges can establish whether breakage occurred before or after the fire. When a glass window is broken, minute chips of glass are scattered as much as 3 meters (10 feet) away, especially back in the direction of the breaking force. If a subject is standing nearby, minute glass chips may be found in his or her hair, clothes, hat, pockets, or cuffs. Even very small (less than 1 millimeter) fragments can be compared to a suspected origin, such as a bottle or window. Once again, this usually results in a class characteristics comparison. Glass is made in large batches that vary little in their physical or chemical properties. But such transfer evidence does not require direct or prolonged contact with the source to make it valuable to the investigator.

Fibers can be transferred from the furnishings of the fire scene to the subject or from the subject's clothes to the scene, particularly at the point of entry. Such two-way transfers can be very useful in linking suspect and scene, even though fibers are usually another type of class characteristic evidence. Fibers are compared by microscopic, chemical, elemental, or spectrophotometric tests. Even the most sophisticated analyses must deal with the fact that synthetic fibers are produced in such large quantities that they are used mainly as corroborative or circumstantial evidence.

In a similar way, soils from shoes or clothing can be compared to soil from a fire scene. This may be especially helpful in a wildlands fire where a distinctive or unusual type of natural soil is found in the vicinity of the fire's origin or along an access road. In urban environments, artificial contaminants such as slag, cinders, metal filings, or paint chips may contribute to the uniqueness of a soil. Contact with the soil near a workshop or industrial site may result in the transfers of soils that are much more unique than the natural soils nearby. Normally, soil comparisons are of marginal evidential value, but in instances such as those offered, individual soils, especially artificial ones, do occur and should not be overlooked.

SUMMARY: THE EXPERT WITNESS

The forensic scientists or criminalists who perform the examinations discussed in this chapter are a breed apart from scientists in research or industrial laboratories. Although they may share the same science or engineering backgrounds as the others, forensic scientists relate their work directly to the real world and its criminal activities. They are uniquely capable of applying scientific tests to evidence, interpreting the results, and reconstructing a sequence of events that led to the production of that evidence. They cannot work in a vacuum, however. To do their job properly, they must interact with the investigators, exchanging information and testing ideas and hypotheses. Investigators should encourage this kind of communication; only in this way can we obtain better evidence through fuller use of all that the scientific expert witness has to offer.

REFERENCES

¹IAAI Forensic Science and Engineering Committee. "Guidelines for Laboratories Performing Chemical and Instrumental Analysis of Fire Debris Samples." Fire and Arson Investigator, 38, June 1988.

²ASTM, "E 1492, Practice for Receiving, Documenting, Storing, and Retrieving Evidence in a Forensic Science Laboratory." ASTM, Philadelphia, PA, 1992.

³ASTM, "E 1387, Standard Test Method for Ignitable Liquid Residues i Extracts from Samples of Fire Debris by Gas Chromatography." ASTM, Philadelphia, PA, 1995.

⁴Twibell, J. D. and Lomas, S. C. "The Examination of Fire-Damaged Electrical Switches." Science and Justice, 35, 2, 1995, pp. 113­116.

⁵Twibell, J. D. and Christie, C. C. "The Forensic Examination of Fuses." Science and Justice, 35, 2, pp. 141­149.

⁶Anderson, R. N. "Surface Analysis of Electrical Arc Residues in Fire Investigation." Journal of Forensic Sciences, 34, May 1989.

⁷Beland, B. "Examination of Arc Beads." Fire and Arson Investigator, June 1994.

⁸McVicar, M. J. "The Perforation of a Copper Pipe During a Fire." The/Le Journal, Canadian Association of Fire Investigators, June 1991, pp. 14­16.

⁹Sly, O. M., Jr. "Flammable and Combustible Liquids." Fire Protection Handbook, 16th ed., Section 3, Chapter 5, NFPA, Quincy, MA, 1991, pp. 3-43­3-53.

¹⁰ASTM, "D 3278, Test Method for Flash Point of Liquids by Setaflash Closed Tester." ASTM, Philadelphia, PA, 1987.

¹¹DeHaan, N. R. "Interior Finish." Fire Protection Handbook, 16th ed., Section 6, Chapter 4, NFPA, Quincy, MA, pp. 6-37­6-47.

¹²DeHaan, J. D. "Laboratory Aspects of Arson: Accelerants, Devices, and Targets." Fire and Arson Investigator, 29, January-March 1979, 39-46.

¹³Armstrong, A. T., and Wittkower, R. S. "Identification of Accelerants in Fire Residues by Capillary Column Gas Chromatography." Journal of Forensic Science, 23, October 1978.

¹⁴Willson, D. "A Unified Scheme for the Analysis of Light Petroleum Products Used as Fire Accelerants." Forensic Science, 10, 243­252, 1977.

¹⁵Jennings, W. Gas Chromatography with Glass Capillary Columns. Academic Press, New York, 1980.

¹⁶Frank, H. A. "Lead Alkyl Components as Discriminating Factors in the Comparison of Gasolines." Journal of the forensic Science Society, 20, 1980.

¹⁷Nowicki, J. "An Accelerant Classification Scheme Based on Analysis by Gas Chromatography/Mass Spectrometry." Journal of Forensic Sciences, 35, 5, September 1990.

¹⁸ASTM, "E 1618, Standard Guide for Ignitable Liquid Residues in Extracts from Fire Debris Samples by Gas Chromatography-Mass Spectrometry." ASTM, Philadelphia, PA, 1994.

¹⁹Twibell, J. D., and Home, J. M. "A Novel Method for the Selective Adsorption of Hydrocarbons from the Headspace of Arson Residues." Nature, 268, 1977.

²⁰Juhala, J. "A Method of Absorption of Flammable Vapors by Direct Insertion of Activated Charcoal into Debris." Bridgeport Crime Lab, Bridgeport MI. Reprinted in Arson Accelerant Detection Course Manual. U.S. Treasury Department, Rockville, Maryland. 1980.

²¹Twibell, J. D., Home, J. M., Smalldon, K. W. "A Comparison of Relative Sensitivities of the Adsorption Wire and Other Methods for the Detection of Accelerant Residues in Fire Debris." Journal of the Forensic Science Society, 22, April 1982.

²²Chrostowski, J. E., and Holmes, R. N. "Collection and Determination of Accelerant Vapors from Arson Debris." Arson Analysis Newsletter, 5, 1979.

²³Brackett, J. Journal of Criminal Law, Criminology and Police Science, 46, 554­558, 1955.

²⁴Woycheshin, S., and DeHaan, J. D. "An Evaluation of Some Arson Distillation Techniques." Arson Analysis Newsletter, 2, September 1978.

²⁵Ibid.

²⁶DeHaan, J. D. and Bonarius, K. "Pyrolysis Products of Structure Fires." Journal of the Forensic Science Society, 28, 1989.

²⁷Patterson, P. L. "Oxygenate Fingerprint of Gasolines." DET Report No. 18, October 1990, pp. 8­12.

²⁸Hipes, S. E. et al. "Evaluation of the GC-FTIR for the Analysis of Accelerants in the Presence of Background Matrix Materials." MAFS Newsletter, 20, 1, pp. 48­76.

²⁹Alexander, J. et al. "Fluorescence of Petroleum Products II: Three-Dimensional Fluorescence Plots of Gasolines." Journal of Forensic Sciences, 32, January 1987.

³⁰Mann, D. C. "Comparison of Automotive Gasolines Using Capillary Gas Chromatography (I and II)." Journal of Forensic Science, 32, May 1987.

³¹Mann, D. C. and Gresham, W. R. "Microbial Degradation of Gasoline in Soil." Journal of Forensic Sciences, 35, 4, 1990.

³²Andrasko, J. et al. "Practical Experiences with Scanning Electron Microscopy in a Forensic Science Laboratory." Linkoping University, NCJRS-Microfiche Program, Report Number 2, 1979.

³³Andrasko, J. "Analysis of Polycyclic Aromatic Hydrocarbons in Soils and Its Application to Forensic Science." Linkoping University, NCJRS-Michofiche Program, Report Number 4, 1979.

³⁴National Bureau of Standards. Gaithersburg, MD, personal communication, 1986.

³⁵Pinorini, M. T., Lennard, C. J., Margot, P., Dustin, I., and Furrer, P. "Soot as an Indicator in Fire Investigations: Physical and Chemical Analysis." Journal of Forensic Sciences, 39, 4, July 1994.

³⁶Mann, (1990) op. cit.

³⁷DeHaan (1979) op. cit.

³⁸Ellern, H. Modern Pyrotechnics. Chemical Publishing Co., New York, 1961.

³⁹Dean, W. L. "Examination of Fire Debris for Flare (Fusee) Residues by Energy Dispersive X-Ray Spectrometry." Arson Analysis Newsletter, 7, 1984.

⁴⁰Kirkbride, K. P. and Kobus, H. J. "The Explosive Reaction Between Swimming Pool Chlorine and Brake Fluid." Journal of Forensic Sciences, 36, 3, May 1991.

⁴¹Meyers, R. E. "A Systematic Approach to the Forensic Examination of Flash Powders." Journal of Forensic Sciences, 23, October 1978.

⁴²Andrasko, J. "Identification of Burnt Matches by Scanning Electron Microscopy." Journal of Forensic Sciences, 23, October 1978.

⁴³U.S. Army. Unconventional Warfare Devices and Techniques. TM 3-201-1, May 1966.

⁴⁴Margot, P. and Lennard, C. J. Techniques for Latent Print Development. University of Lausanne, Lausanne, 1991.

⁴⁵Lee, H. C. and Gaensslen, R. E. "Cyanoacrylate 'Super Glue' Fuming for Latent Fingerprints." The Identification Officer, .Spring 1985.

⁴⁶Graham, D. "Some Technical Aspects of the Demonstration and Visualization of Fingerprints on Human Skin." Journal of Forensic Science, 14, 1­12, 1969.

⁴⁷Menzel, E. R., and Duff, J. M. "Laser Detection of Latent Fingerprints-Treatment with Flourescers." Journal of Forensic Science, 24, 96-100, 1979.

SUGGESTED READING

Bertsch, W., Zhang, Q. W., and Holzer, G. "Using the Tools of Chromatography, Mass Spectrometry and Automated Data Processing in the Detection of Arson. Journal of High Resolution Chromatography, 13, September 1990.

Caddy, B., Smith, F. P., and Macy, J. "Methods of Fire Debris Preparation for Detection of Accelerant." Forensic Science Review, 3, 1, June 1991.

Conkling, J. The Chemistry of Pyrotechnics. Marcel Dekker, New York, 1985

Coulombe, R. "Chemical Markers in Weathered Gasoline." Journal of Forensic Sciences, 40, 5, September 1995.

Fultz, M. L. and DeHaan, J. D. "Gas Chromatography in Arson and Explosives Analysis." Gas Chromatography in Forensic Science, I. Tebbett, ed., Ell Handbook, 16th ed., Section 3, Chapter 5, NFPA, Quincy, MA, 1991, pp. 3-43­3-53.

.Holzer, G. and Bertsch, W. "Recent Advances Toward the Detection of Accelerants in Arson Cases." American Laboratory, December 1988.

Lennard, C. J., Rochaix, V. T. and Margot, P. "A GC-MS Database of Target Compound Chromatograms for the Identification of Arson Acccelerants." Science and Justice, 35, 1, 1995.

Lennard, C. J. "Fire (Determination of Cause)--A Review: 1992­95." Presented at the 11th INTERPOL Forensic Science Symposium, Lyon, France, November 1995.

Metson, J. B. and Hobbis, C. M. "The Use of Auger Electron Spectroscopy in Fire Investigations." Chemistry in New Zealand, July 1994.

Phelps, J. L., Chasteen, C. E., and Render, M. M. "Extraction and Analysis of Low Molecular Weight Alcohols and Acetone from Fire Debris Using Passive Headspace Concentration," Journal of Forensic Sciences. 39, 1, January 1994.

Sheff, L. M. and Siegel, J. A. "Fluorescence of Petroleum Products V: Three-Dimensional Fluorescence Spectroscopy and Capillary Gas Chromatography of Neat and Evaporated Gasoline Samples." Journal of Forensic Sciences, 39, 5, September 1994.

*Many such experts may be described as forensic scientists--forensic chemists, forensic engineers, and the like. The term forensic means having to do with a court of law, so such terms refer to the application of a scientific discipline to problems having to do with the law.

Kirk's Fire Investigation, 4/E by John D. DeHaan, ©1997. Reproduced by permission of Prentice-Hall, Inc., Upper Saddle River, NJ. Permission from Prentice-Hall is required for all other uses.