What four components are needed to superglue fume fingerprints

The Oklahoma City Bombing

It was the biggest act of mass murder in U.S. history. On a sunny spring morning in April 1995, a Ryder rental truck pulled into the parking area of the Alfred P. Murrah federal building in Oklahoma City. The driver stepped down from the truck’s cab and casually walked away. Minutes later, the truck exploded into a fireball, unleashing enough energy to destroy the building and kill 168 people, including 19 children and infants in the building’s day care center.

Later that morning, an Oklahoma Highway Patrol officer pulled over a beat-up 1977 Mercury Marquis being driven without a license plate. On further investigation, the driver, Timothy McVeigh, was found to be in possession of a loaded firearm and charged with transporting a firearm. At the explosion site, remnants of the Ryder truck were located and the truck was quickly traced to a renter—Robert Kling, an alias for Timothy McVeigh. Coincidentally, the rental agreement and McVeigh’s driver’s license both used the address of McVeigh’s friend, Terry Nichols.

Investigators later recovered McVeigh’s fingerprint on a receipt for 2,000 pounds of ammonium nitrate, a basic

explosive ingredient. Forensic analysts also located PETN residues on the clothing McVeigh wore on the day of his arrest. PETN is a component of detonating cord.

After three days of deliberation, a jury declared McVeigh guilty of the bombing and sentenced him to die by lethal injection.

headline news

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After studying this chapter you should be able to: • Understand how explosives are classified

• List some common commercial, homemade, and military explosives

• Describe how to collect physical evidence at the scene of an explosion

• Describe laboratory procedures used to detect and identify explosive residues.

forensic investigation of explosions

black powder deflagration detonating cord detonation explosion high explosive low explosive oxidizing agent primary explosive safety fuse secondary explosive smokeless powder

(double-base) smokeless powder

(single-base)

KEY TERMS

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oxidizing agent A substance that supplies oxygen to a chemical reaction

explosion A chemical or mechanical action caused by combustion, accompanied by creation of heat and rapid expansion of gases

374 CHAPTER 15

Explosions and Explosives The ready accessibility of potentially explosive laboratory chemicals, dynamite, and, in some countries, an assortment of military explosives has provided the criminal element of society with a lethal weapon. Unfortunately for society, explosives have become an attractive weapon to crim- inals bent on revenge, destruction of commercial operations, or just plain mischief.

Although politically motivated bombings have received considerable publicity worldwide, in the United States most bombing incidents are perpetrated by isolated individuals rather than by organized terrorists. These incidents typically involve homemade explosives and incendiary devices. The design of such weapons is limited only by the imagination and ingenuity of the bomber.

Like arson investigation, bomb investigation requires close cooperation of a group of highly specialized individuals trained and experienced in bomb disposal, bomb-site investigation, foren- sic analysis, and criminal investigation. The criminalist must detect and identify explosive chem- icals recovered from the crime scene as well as identify the detonating mechanisms. This special responsibility concerns us for the remainder of this chapter.

The Chemistry of Explosions Like fire, an explosion is the product of combustion accompanied by the creation of gases and heat. However, the distinguishing characteristic of an explosion is the rapid rate of the reaction. The sudden buildup of expanding gas pressure at the origin of the explosion produces the violent physical disruption of the surrounding environment.

Our previous discussion of the chemistry of fire referred only to oxidation reactions that rely on air as the sole source of oxygen. However, we need not restrict ourselves to this type of situa- tion. For example, explosives are substances that undergo a rapid exothermic oxidation reaction, producing large quantities of gases. This sudden buildup of gas pressure constitutes an explosion. Detonation occurs so rapidly that oxygen in the air cannot participate in the reaction; thus, many explosives must have their own source of oxygen.

Chemicals that supply oxygen are known as oxidizing agents. One such agent is found in black powder, a low explosive, which is composed of a mixture of the following chemical ingredients:

75 percent potassium nitrate (KNO3) 15 percent charcoal (C) 10 percent sulfur (S)

In this combination, oxygen containing potassium nitrate acts as an oxidizing agent for the char- coal and sulfur fuels. As heat is applied to black powder, oxygen is liberated from potassium nitrate and simultaneously combines with charcoal and sulfur to produce heat and gases (symbolized by �), as represented in the following chemical equation:

3C � S � 2KNO3 � carbon sulfur potassium nitrate yields

3CO2� � N2� � K2S carbon dioxide nitrogen potassium sulfide

Some explosives have their oxygen and fuel components combined within one molecule. For example, the chemical structure of nitroglycerin, the major constituent of dynamite, combines carbon, hydrogen, nitrogen, and oxygen:

HHH ƒ ƒ ƒ

H ¬ C ¬ C ¬ C ¬ H ƒ ƒ ƒ

NO2 NO2 NO2

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FORENSIC INVESTIGATION OF EXPLOSIONS 375

When nitroglycerin detonates, large quantities of energy are released as the molecule decomposes, and the oxygen recombines to produce large volumes of carbon dioxide, nitrogen, and water.

Consider, for example, the effect of confining an explosive charge to a relatively small, closed container. On detonation, the explosive almost instantaneously produces large volumes of gases that exert enormously high pressures on the interior walls of the container. In addition, the heat energy released by the explosion expands the gases, causing them to push on the walls with an even greater force. If we could observe the effects of an exploding lead pipe in slow motion, we would first see the pipe’s walls stretch and balloon under pressures as high as several hundred tons per square inch. Finally, the walls would fragment and fly outward in all directions. This flying debris or shrapnel constitutes a great danger to life and limb in the immediate vicinity.

On release from confinement, the gaseous products of the explosion suddenly expand and compress layers of surrounding air as they move outward from the origin of the explosion. This blast effect, or outward rush of gases, at a rate that may be as high as 7,000 miles per hour cre- ates an artificial gale that can overthrow walls, collapse roofs, and disturb any object in its path. If a bomb is sufficiently powerful, more serious damage will be inflicted by the blast effect than by fragmentation debris (see Figure 15–1).

Types of Explosives The speed at which explosives decompose varies greatly from one to another and permits their clas- sification as high and low explosives. In a low explosive, this speed is called the speed of deflagration (burning). It is characterized by very rapid oxidation that produces heat, light, and a subsonic pressure wave. In a high explosive, it is called the speed of detonation. Detonation refers to the creation of a supersonic shock wave within the explosive charge. This shock wave breaks the chemical bonds of the explosive charge, leading to the new instantaneous buildup of heat and gases.

LOW EXPLOSIVES Low explosives, such as black and smokeless powders, decompose rela- tively slowly at rates up to 1,000 meters per second. Because of their slow burning rates, they pro- duce a propelling or throwing action that makes them suitable as propellants for ammunition or skyrockets. However, the danger of this group of explosives must not be underestimated because, when any one of them is confined to a relatively small container, it can explode with a force as lethal as that of any known explosive.

Black Powder and Smokeless Powder The most widely used explosives in the low-explosive group are black powder and smokeless powder. The popularity of these two explosives is en- hanced by their accessibility to the public. Both are available in any gun store, and black powder can easily be made from ingredients purchased at any chemical supply house as well.

Black powder is a relatively stable mixture of potassium nitrate or sodium nitrate, charcoal, and sulfur. Unconfined, it merely burns; thus it commonly is used in safety fuses that carry a flame to an explosive charge. A safety fuse usually consists of black powder wrapped in a fabric or plastic casing. When ignited, a sufficient length of fuse will burn at a rate slow enough to allow a person adequate time to leave the site of the pending explosion. Black powder, like any other low explosive, becomes explosive and lethal only when it is confined.

The safest and most powerful low explosive is smokeless powder. This explosive usually consists of nitrated cotton or nitrocellulose (single-base powder) or nitroglycerin mixed with nitrocellulose (double-base powder). The powder is manufactured in a variety of grain sizes and shapes, depending on the desired application (see Figure 15–2).

Chlorate Mixtures The only ingredients required for a low explosive are fuel and a good oxidizing agent. The oxidizing agent potassium chlorate, for example, when mixed with sugar, produces a pop- ular and accessible explosive mix. When confined to a small container—for example, a pipe—and ignited, this mixture can explode with a force equivalent to a stick of 40 percent dynamite.

Some other commonly encountered ingredients that may be combined with chlorate to produce an explosive are carbon, sulfur, starch, phosphorus, and magnesium filings. Chlorate mixtures may also be ignited by the heat generated from a chemical reaction. For instance, sufficient heat can be generated to initiate combustion when concentrated sulfuric acid comes in contact with a sugar–chlorate mix.

FIGURE 15–1 A violent explosion. © Stefan Zaklin/Corbis. All Rights Reserved

deflagration A very rapid oxidation reaction accompanied by the generation of a low-intensity pressure wave that can disrupt the surroundings

detonation An extremely rapid oxidation reaction accompanied by a violent disruptive effect and an intense, high-speed shock wave

low explosive An explosive with a velocity of detonation less than 1,000 meters per second

black powder Normally, a mixture of potassium nitrate, carbon, and sulfur in the ratio 75/15/10

smokeless powder (double-base) An explosive consisting of a mixture of nitrocellulose and nitroglycerin

safety fuse A cord containing a core of black powder, used to carry a flame at a uniform rate to an explosive charge

smokeless powder (single-base) An explosive consisting of nitrocellulose

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high explosive An explosive with a velocity of detonation greater than 1,000 meters per second

376 CHAPTER 15

Gas–Air Mixtures Another form of low explosive is created when a considerable quan- tity of natural gas escapes into a confined area and mixes with a sufficient amount of air. If ignited, this mixture results in simultaneous combustion and sudden production of large volumes of gases and heat. In a building, walls are forced outward by the expanding gases, causing the roof to fall into the interiors, and objects are thrown outward and scattered in erratic directions with no semblance of pattern.

Mixtures of air and a gaseous fuel explode or burn only within a limited concen- tration range. For example, the concentration limits for methane in air range from 5.3 to 13.9 percent. In the presence of too much air, the fuel becomes too diluted and does not ignite. On the other hand, if the fuel becomes too concentrated, ignition is prevented because there is not enough oxygen to support the combustion.

Mixtures at or near the upper concentration limit (“rich” mixtures) explode; how- ever, some gas remains unconsumed because there is not enough oxygen to complete

the combustion. As air rushes back into the origin of the explosion, it combines with the residual hot gas, producing a fire that is characterized by a whoosh sound. This fire is often more destruc- tive than the explosion that preceded it. Mixtures near the lower end of the limit (“lean” mixtures) generally cause an explosion without accompanying damage due to fire.

HIGH EXPLOSIVES High explosives include dynamite, TNT, PETN, and RDX. They detonate almost instantaneously at rates of 1,000–8,500 meters per second, producing a smashing or shat- tering effect on their target. High explosives are classified into two groups—primary and secondary explosives—based on their sensitivity to heat, shock, or friction.

Primary explosives are ultrasensitive to heat, shock, or friction, and under normal conditions they detonate violently instead of burning. For this reason, they are used to detonate other explo- sives through a chain reaction and are often referred to as primers. Primary explosives provide the major ingredients of blasting caps and include lead azide, lead styphnate, and diazodinitrophenol (see Figure 15–3). Because of their extreme sensitivity, these explosives are rarely used as the main charge of a homemade bomb.

Secondary explosives are relatively insensitive to heat, shock, or friction, and they normally burn rather than detonate when ignited in small quantities in open air. This group comprises most high explosives used for commercial and military blasting. Some common examples of second- ary explosives are dynamite, TNT (trinitrotoluene), PETN (pentaerythritol tetranitrate), RDX (cyclotrimethylenetrinitramine), and tetryl (2,4,6-trinitrophenylmethylnitramine).

Dynamite It is an irony of history that the prize most symbolic of humanity’s search for peace—the Nobel Peace Prize—should bear the name of the developer of one of our most lethal discoveries—dynamite. In 1867, the Swedish chemist Alfred Nobel, searching for a method to desensitize nitroglycerin, found that when kieselguhr, a variety of diatomaceous earth, absorbed a large portion of nitroglycerin, it became far less sensitive but still retained its explosive force. Nobel later decided to use pulp as an absorbent because kieselguhr was a heat- absorbing material.

This so-called pulp dynamite was the beginning of what is now known as the straight dyna- mite series. These dynamites are used when a quick shattering action is desired. In addition to nitroglycerine and pulp, present-day straight dynamites also include sodium nitrate (which fur- nishes oxygen for complete combustion) and a small percentage of a stabilizer, such as calcium carbonate.

All straight dynamite is rated by strength; the strength rating is determined by the weight percentage of nitroglycerin in the formula. Thus, a 40 percent straight dynamite contains 40 percent nitroglycerin, a 60 percent grade contains 60 percent nitroglycerin, and so forth. However, the rel- ative blasting power of different strengths of dynamite is not directly proportional to their strength ratings. A 60 percent straight dynamite, rather than being three times as strong as a 20 percent, is only one and one-half times as strong (see Figure 15–4).

Ammonium Nitrate Explosives In recent years, nitroglycerin-based dynamite has all but disap- peared from the industrial explosives market. Commercially, these explosives have been replaced mainly by ammonium nitrate–based explosives, that is, water gels, emulsions, and ANFO explo- sives. These explosives mix oxygen-rich ammonium nitrate with a fuel to form a low-cost, stable explosive.

FIGURE 15–2 Samples of smokeless powders. Courtesy of ATF (Bureau of Alcohol, Tobacco, Firearms & Explosives)

primary explosive A high explosive that is easily detonated by heat, shock, or friction

secondary explosive A high explosive that is relatively insensitive to heat, shock, or friction

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FORENSIC INVESTIGATION OF EXPLOSIONS 377

Typically, water gels have a consistency resembling that of set gelatin or gel-type toothpaste. They are characterized by their water-resistant nature and are employed for all types of blasting under wet conditions. These explosives are based on formulations of ammonium nitrate and sodium nitrate gelled with a natural polysaccharide such as guar gum. Commonly, a combustible material such as aluminum is mixed into the gel to serve as the explosive’s fuel.

Emulsion explosives differ from gels in that they consist of two distinct phases, an oil phase and a water phase. In these emulsions, a droplet of a supersaturated solution of ammonium nitrate is surrounded by a hydrocarbon serving as a fuel. A typical emulsion consists of water, one or more inorganic nitrate oxidizers, oil, and emulsifying agents. Commonly, emulsions contain micron-sized glass, resin, or ceramic spheres known as microspheres or microballoons. The size of these spheres controls the explosive’s sensitivity and detonation velocity.

Ammonium nitrate soaked in fuel oil is an explosive known as ANFO. Such commercial explo- sives are inexpensive and safe to handle and have found wide applications in blasting operations in the mining industry. Ammonium nitrate in the form of fertilizer makes a readily obtainable ingredi- ent for homemade explosives. Indeed, in an incident related to the 1993 bombing of New York City’s World Trade Center, the FBI arrested five men during a raid on their hideout in New York City, where they were mixing a “witches’ brew” of fuel oil and an ammonium nitrate–based fertilizer.

TATP Triacetone triperoxide (TATP) is a homemade explosive that has been used as an impro- vised explosive by terrorist organizations in Israel and other Middle Eastern countries. It is prepared by reacting the common ingredients of acetone and hydrogen peroxide in the presence of an acid catalyst such as hydrochloric acid.

FIGURE 15–3 Blasting caps. The left and center caps are initiated by an electrical current; the right cap is initiated by a safety fuse.

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378 CHAPTER 15

TATP is a friction- and impact-sensitive explosive that is extremely potent when confined in a container such as a pipe. The 2005 London transit bombings were caused by TATP-based ex- plosives and provide ample evidence that terrorist cells have moved TATP outside the Middle East. A London bus destroyed by one of the TATP bombs is shown in Figure 15–5.

FIGURE 15–4 Sticks of dynamite. U.S. Department of Justice\AP Wide World Photos

FIGURE 15–5 A London bus destroyed by a TATP-based bomb. Courtesy AP Wide World Photos IS

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FORENSIC INVESTIGATION OF EXPLOSIONS 379

A plot to blow up ten international plane flights leaving Britain for the United States with a “liquid explosive” apparently involved plans to smuggle the peroxide-based TATP explosive onto the planes. This plot has prompted authorities to prohibit airline passengers from carrying liquids and gels onto planes.

Military High Explosives No discussion of high explosives would be complete without a men- tion of military high explosives. In many countries outside the United States, the accessibility of high explosives to terrorist organizations makes them common constituents of homemade bombs. RDX, the most popular and powerful military explosive, is often encountered in the form of a pliable plastic of doughlike consistency known as composition C–4 (a U.S. military designation).

TNT was produced and used on an enormous scale during World War II and may be consid- ered the most important military bursting charge explosive. Alone or in combination with other explosives, it has found wide application in shells, bombs, grenades, demolition explosives, and propellant compositions (see Figure 15–6). Interestingly, military “dynamite” contains no nitro- glycerin but is actually composed of a mixture of RDX and TNT. Like other military explosives, TNT is rarely encountered in bombings in the United States.

PETN is used by the military in TNT mixtures for small-caliber projectiles and grenades. Commercially, the chemical is used as the explosive core in a detonating cord or primacord. Instead of the slower-burning safety fuse, a detonating cord is often used to connect a series of explosive charges so that they will detonate simultaneously.

Detonators Unlike low explosives, bombs made of high explosives must be detonated by an ini- tiating explosion. In most cases, detonators are blasting caps composed of copper or aluminum cases filled with lead azide as an initiating charge and PETN or RDX as a detonating charge. Blasting caps can be initiated by means of a burning safety fuse or by an electrical current.

Homemade bombs camouflaged in packages, suitcases, and the like, are usually initiated with an electrical blasting cap wired to a battery. An unlimited number of switching-mechanism designs have been devised for setting off these devices; clocks and mercury switches are favored. Bombers sometimes prefer to employ outside electrical sources. For instance, most automobile bombs are detonated when the ignition switch of a car is turned on.

detonating cord A cordlike explosive containing a core of high-explosive material, usually PETN; also called primacord

FIGURE 15–6 Military explosives in combat use. Courtesy Getty Images/Time Life Pictures

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Liquid Explosives

In 2006, security agencies in the United States and Great Britain uncovered a terrorist plot to use liquid explosives to destroy commercial airlines operating between the two countries. Of the hundreds of types of explosives, most are solid. Only about a dozen are liquid. But some of those liquid explosives can be readily purchased and others can be made from hun- dreds of different kinds of chemicals that are not dif- ficult to obtain. After the September 11 attacks, worries about solid explosives became the primary concern. In 2001, Richard Reid was arrested for at- tempting to destroy an American Airlines flight out of Paris. Authorities later found a high explosive with a TATP detonator hidden in the lining of his shoe. It is therefore not surprising that terrorists turned to liq- uids in this latest plot. A memo issued by federal se- curity officials about the plot to blow up ten international planes highlighted a type of liquid ex- plosive based on peroxide.

forensics at work

Gels discarded by airline passengers before boarding. Stefano Paltera, AP Wide World Photos

380 CHAPTER 15

Collection and Analysis of Evidence of Explosives The most important step in the detection and analysis of explosive residues is the collection of appropriate samples from the explosion scene. Invariably, undetonated residues of the explosive remain at the site of the explosion. The detection and identification of these explosives in the lab- oratory depends on the bomb-scene investigator’s skill and ability to recognize and sample the areas most likely to contain such materials.

Detecting and Recovering Evidence of Explosives The most obvious characteristic of a high or contained low explosive is the presence of a crater at the origin of the blast. Once the crater has been located, all loose soil and other debris must immediately be removed from the interior of the hole and preserved for laboratory analysis. Other good sources of explosive residues are objects located near the origin of detonation. Wood, insula- tion, rubber, and other soft materials that are readily penetrated often collect traces of the explosive. However, nonporous objects near the blast must not be overlooked. For instance, residues can be

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forensics at work

The most common peroxide-based explosive is TATP (triacetone triperoxide), which is made up of acetone and hydrogen peroxide, two widely available substances. TATP can be used as a detonator or a pri- mary explosive and has been used in Qaeda-related bomb plots and by Palestinian suicide bombers. TATP itself is a white powder made up of crystals that form when acetone and hydrogen peroxide are mixed together, usually with a catalyst added to speed the chemical reactions. Acetone is the main ingredient in nail polish remover, while hydrogen peroxide is a pop- ular antiseptic. When the two main ingredients are mixed, they form a white powder that can be easily det- onated using an electrical spark.

Commercially available hydrogen peroxide, however, is not concentrated enough to create TATP. The solution sold in stores contains about 3 percent hydrogen per- oxide, compared to the approximately 70 percent con- centration need for TATP. However, hydrogen peroxide solutions of up to 30 percent can be obtained from

chemical supply houses. According to explosives experts, a mixture of 30 percent hydrogen peroxide and acetone can create a fire hot enough to burn though the fuselage of an aircraft.

In theory, scientists know how to detect peroxide- based explosives. The challenge is to design machines that can perform scans quickly and efficiently on thou- sands of passengers passing through airport security checks. Current scanning machines at airports are designed to detect nitrogen containing chemicals and are not designed to detect peroxide-containing explo- sive ingredients. Since 9/11, security experts have worried about the possibility of liquid explosives in the form of liquids and gels getting onto airliners.

Without the luxury of waiting for newly designed scanning devices capable of ferreting out dangerous liquids to be in place at airports, the decision was made to use a common-sense approach—that is, to restrict the types and quantities of liquids that a passenger can carry onto a plane.

found on the surfaces of metal objects near the site of an explosion. Material blown away from the blast’s origin should also be recovered because it, too, may retain explosive residues.

The entire area must be systematically searched, with great care given to recovering any trace of a detonating mechanism or any other item foreign to the explosion site. Wire-mesh screens are best used for sifting through debris. All personnel involved in searching the bomb scene must take appropriate measures to avoid contaminating the scene, including dressing in disposable gloves, shoe covers, and overalls.

ION MOBILITY SPECTROMETER In pipe-bomb explosions, particles of the explosive are frequently found adhering to the pipe cap or to the pipe threads, as a result of either being impacted into the metal by the force of the explosion or being deposited in the threads during the construction of the bomb. One approach for screening objects for the presence of explosive residues in the field or the labora- tory is the ion mobility spectrometer (IMS).1 A portable IMS is shown in Figure 15–7.

1 T. Keller et al., “Application of Ion Mobility Spectrometry in Cases of Forensic Interest,” Forensic Science International 161 (2006): 130.

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This handheld detector uses a vacuum to collect explosive residues from suspect surfaces. Alternatively, the surface suspected of containing explosive residues is wiped down with a Teflon- coated fiberglass disc and the collected residues are then drawn into the spectrometer off the disc. Once in the IMS, the explosive residues are vaporized by the application of heat. These vapor- ized substances are exposed to a beam of electrons or beta rays emitted by radioactive nickel and converted into electrically charged molecules or ions. The ions are then allowed to move through a tube (drift region) under the influence of an electric field. A schematic diagram of an IMS is shown in Figure 15–8.

The preliminary identification of an explosive residue can be made by noting the time it takes the explosive to move through the tube. Because ions move at different speeds depending on their size and structure, they can be characterized by the speed at which they pass through the tube. Used as a screening tool, this method rapidly detects a full range of explosives, even at low detection levels. However, all results need to be verified through confirmatory tests.

The IMS can detect plastic explosives as well as commercial and military explosives. More than 10,000 portable and full-size IMS units are currently used at airport security checkpoints, and more than 50,000 handheld IMS analyzers have been deployed for chemical-weapons monitoring in various armed forces.

COLLECTION AND PACKAGING All materials collected for examination by the laboratory must be placed in airtight sealed containers and labeled with all pertinent information. Soil and other soft loose materials are best stored in metal airtight containers such as clean paint cans. Debris and articles collected from different areas are to be packaged in separate airtight containers. Plas- tic bags should not be used to store evidence suspected of containing explosive residues. Some explosives can actually escape through the plastic. Sharp-edged objects should not be allowed to pierce the sides of a plastic bag. It is best to place these types of items in metal containers.

Analysis of Evidence of Explosives When the bomb-scene debris and other materials arrive at the laboratory, everything is first examined microscopically to detect particles of unconsumed explosive. Portions of the recovered debris and detonating mechanism, if found, are carefully viewed under a low-power stereoscopic microscope in a painstaking effort to locate particles of the explosive. Black powder and smoke- less powder are relatively easy to locate in debris because of their characteristic shapes and colors

382 CHAPTER 15

FIGURE 15–7 A portable ion mobility spectrometer used to rapidly detect and tentatively identify trace quantities of explosives. Courtesy GE Ion Track, Wilmington, Mass. 01887

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FIGURE 15–8 Schematic diagram of an ion mobility spectrometer. A sample is introduced into an ionization chamber, where bombardment with radioactive particles emitted by an isotope of nickel converts the sample to ions. The ions move into a drift region where ion separation occurs based on the speed of the ions as they move through an electric field.

Ionization chamber

Drift rings

Sample is bombarded by radioactive particles emitted by an isotope of nickel to form ionsSample is

drawn into ionization chamber

(a) Drift region

Collection electrode

Shutter

63Ni

Ionization chamber (b)

Drift rings Sample is converted into ions of different sizes and structures

Drift region

Collection electrode

Shutter

Ionization chamber

(c)

Drift rings

Explosive substances can be characterized by the speed at which they move through the electric field

Drift region Ions separate as they move through an electric field

Collection electrode

Shutter

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384 CHAPTER 15

TABLE 15–1 Color Spot Tests for Common Explosives

Reagent

Substance Greissa Diphenylamineb Alcoholic KOHc

Chlorate No color Blue No color Nitrate Pink to red Blue No color Nitrocellulose Pink Blue-black No color Nitroglycerin Pink to red Blue No color PETN Pink to red Blue No color RDX Pink to red Blue No color TNT No color No color Red Tetryl Pink to red Blue Red-violet

a Greiss reagent: Solution 1—Dissolve 1 g sulfanilic acid in 100 mL 30% acetic acid. Solution 2—Dissolve 0.5 g N-(1-napthyl) ethylenediamine in 100 mL methyl alcohol. Add solutions 1 and 2 and a few milligrams of zinc dust to the suspect extract.

b Diphenylamine reagent: Dissolve 1 g diphenylamine in 100 mL concentrated sulfuric acid. c Alcoholic KOH reagent: Dissolve 10 g of potassium hydroxide in 100 mL absolute alcohol.

(see Figure 15–2). However, dynamite and other high explosives present the microscopist with a much more difficult task and often must be detected by other means.

Following microscopic examination, the recovered debris is thoroughly rinsed with acetone. The high solubility of most explosives in acetone ensures their quick removal from the debris. When a water-gel explosive containing ammonium nitrate or a low explosive is suspected, the debris should be rinsed with water so that water-soluble substances (such as nitrates and chlorates) will be extracted. Table 15–1 lists a number of simple color tests the examiner can perform on the acetone and water extracts to screen for the presence of organic and inorganic explosives, respectively.

SCREENING AND CONFIRMATION TESTS Once collected, the acetone extract is concentrated and analyzed using color spot tests, thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC; see pages 127–128), and gas chromatography/mass spectrometry. The presence of an explosive is indicated by a well-defined spot on a TLC plate with an Rf value corresponding to a known explosive—for example, nitroglycerin, RDX, or PETN.

The high sensitivity of HPLC also makes it useful for analyzing trace evidence of explosives. HPLC operates at room temperature and hence does not cause explosives, many of which are tem- perature sensitive, to decompose during their analysis. When a water-gel explosive containing ammonium nitrate or a low explosive is suspected, the debris should be rinsed with water so that water-soluble substances (such as nitrates and chlorates) will be extracted.