Residues

Gunshot residue consists of a variety of materials: particles of the projectile or the projectile’s jacket, unburnt particles of smokeless powder, partially burnt particles of powder, combustion products and particles of primer residue. These materials are projected from the muzzle of the weapon in a conical cloud. The particles are slowed down by air resistance, with the larger particles traveling the greater distances. Gunshot residue may also escape from various openings in the firearm: from the space between the chamber and the barrel in revolvers, from the ejection ports of self-loading or automatic firearms and even from the trigger hole. Gunshot residue escaping in this way may be deposited on the hands of the shooter and on the hands of someone who is grappling for possession of the firearm at the instant of its discharge. In the field of forensic science, the detection of gunshot residue becomes important in two aspects of the investigation of a shooting incident: the determination of the range from which a shot was fired and as an aid in the identification of the shooter. The first involves the examination of powder patterns on the surfaces of targets, and the second involves the analysis of residues removed from the hands of a suspected shooter.
The gross appearance of a gunshot wound can provide some insight as to the range from which it was fired. Distant gunshot wounds are fired from a sufficient distance that no powder residue reaches the body surface; only the bullet reaches the target. The injury in this case typically consists of a circular or elliptical defect surrounded by a contusion ring (a ring of bruising caused by the bullet’s stretching and tearing of the skin); the contusion ring may be overlain by a gray ring, a ring of material wiped from the surface of the bullet as it passed through the skin. Bullet wipe consists of powder residue, bullet lubricant and traces of metal and metal oxides from the surface of the bullet. A close range gunshot wound is one that is inflicted from a sufficiently short range that powder residue reaches the body surface. A close range gunshot wound will have the same features as a distant shot plus deposition of tattooing (also called stippling) and soot (finely divided combustion products from the burning of propellant). Tattooing or stippling consists of particles of unburned and partially burned propellant that are driven into the skin surface or the surface of the shooting victim’s clothing. As the range of fire shortens the pattern of tattooing and soot deposition becomes both smaller and denser. At near-contact range (less than about 1 in (2.5 cm) the hot gases comprising the muzzle flash will singe the target surface, burning hair and natural textile fibers and melting synthetic fibers. The muzzle flash can split woven fabrics along the warp and weft directions. Forensic pathologists distinguish between loose contact and tight contact gunshot wounds. Loose contact gunshot wounds may show gunshot residue blown along the body surface or between layers of clothing. In addition to singeing from the muzzle flash, loose contact gunshot wounds may show ironing, i.e. a stiffening of fabric due to contact with a hot surface, caused by the weapon’s muzzle. Tight contact wounds usually show no gunshot residue on the body surface. If a tight contact gunshot wound is inflicted over a boney plate such as the vault of the skull or the sternum, it may appear as a stellate (starlike) defect with a patterned contusion from the weapon’s muzzle. In this case, the hot gases issuing from the muzzle of the weapon separate the soft tissue from the underlying bone. The resulting pocket of hot gases pushes the soft tissue against the weapon’s muzzle; if gas pressure is high enough the soft tissue may tear, allowing the gas to escape. Blood may be blown back on to the hand or arm of the shooter; blood and tissue may also be blown into the muzzle of the weapon.
The determination of the range of fire from a powder pattern requires four things: the original powder pattern; the weapon that produced the pattern; ammunition from the same lot as that used to produce the original pattern; and knowledge of the weather conditions. If the original powder pattern is on clothing its preservation is straightforward. On the other hand, if the powder pattern is on the skin of a shooting victim, a scaled photograph should be taken using color film. Because skin blemishes (small moles, freckles and the like) may be mistaken for tattooing or stippling – even in a color photograph – the examiner who will make the range of fire determination should be able to examine the powder pattern himself. From time to time recommendations for the excision of powder patterns from the bodies of deceased have appeared in the forensic science literature. The skin is sutured to a metal hoop and then carefully cut away from the body. The excised skin is then treated with a preservative such as formaldehyde. As an alternative, the excised tissue may be preserved by freezing. Although excision of the powder pattern may be necessary for laboratory analysis, the firearms examiner needs to be aware that the actual removal of the powder pattern is fraught with problems. To begin with, the skin may be stretched or it may shrink, altering the size of the powder pattern. At trial, the judge may exclude the admission of the powder pattern as evidence on the ground that the prejudicial effect of the admission of such evidence outweighs its probative value. Finally, relatives of the deceased may regard the excision of the powder pattern as a desecration of the victim’s body.
A questioned powder pattern on clothing or other textile may require special treatment to render it visible, particularly if the fabric is dark colored or soaked with blood. The soot in a powder pattern strongly absorbs infrared radiation. Therefore, it may be possible to photograph a powder pattern on the surface of a dark garment using infrared film and special camera filters that exclude visible light. The hemoglobin in blood does not strongly absorb infrared radiation; therefore, infrared photography can be used to visualize powder patterns on bloodstained garments. Forensic light sources (e.g. Polilight® or Omnichrome® 9000) have also proved useful for the visualization of powder patterns: partially burnt nitrocellulose particles fluoresce when illuminated with 415 nm light. Many forensic science laboratories use chemical methods to visualize powder patterns. First, the powder pattern is sprayed with a dilute solution ofsodium rhodizonate, which reacts with the residues of lead primer compounds to form a blue lead rhodizonate complex; the color of the lead complex is changed fromblue to red by spraying with an aqueous tartrate buffer solution. The primer residue pattern is photographed before additional tests are carried out.
Users of the sodiumrhodizonate test have noted several problems with it. An aqueous solution of sodiumrhodizonate decomposes very rapidly; and the final blue color may be subject to unpredictable rapid fading. A study of the chemistry of the reaction of sodiumrhodizonate with lead has also revealed that if the complexation reaction takes place at neutral pH rather than at pH 2.8 (the pH of the tartrate buffer) the formation of tetrahyroquinone will be favored over the formation of the scarlet lead-rhodi-zonate complex. Tartrate ion seems to participate directly in some way in the complexation reaction. To deal with these problems a modification of the sodiumrhodizonate test has been proposed. In this new procedure the powder residue is transferred to a sheet of filter paper. The filter paper is then sprayed first with tartrate buffer then with a saturated aqueous sodiumrhodizonate solution or with a tartrate-buffered saturated sodiumrhodizonate solution (which has a half-life at roomtemperature of about ten hours). If lead is present the scarlet lead-rhodizo-nate complex is formed. The scarlet complex is decomposed by spraying the pattern with 5% aqueous hydrochloric acid until the blue color reaches maximum intensity. The hydrochloric acid is removed by drying the filter paper with a hair drier. The blue pattern will be indefinitely stable.
After the sodiumrhodizonate test the combustion products of smokeless powder are visualized with the Griess test. A sheet of unexposed photographic paper that has been treated with developer and fixer and carefully washed is impregnated with Griess reagent. The questioned powder pattern is placed over the impregnated photographic paper in contact with the gelatin-coated surface and covered with a dampened towel. An electric clothes iron is used to iron the powder pattern on to the photographic paper where nitrites in the pattern react with the Griess reagent to produce a rose-colored azo dye through a diazo coupling reaction. The reaction of the powder pattern with Griess reagent can be enhanced by spraying the pattern with aqueous sodiumhydroxide solution and heating it in a laboratory oven. Under alkaline conditions nitrocellulose disproportionates to yield nitrite ions. The interpretation of the powder patterns visualized by the Griess reaction can be complicated by fabric dyes with similar colors to the Griess diazo reaction product: these dyes may leach out of fibers and bleed into the gelatin layer of the photographic paper.
Bloodstains can interfere with the Griess test. Traces of blood can be transferred to the test paper, either partially or completely obscuring the color of the reaction product. The Maiti test was developed to avoid this masking problem. In the Maiti test sheets of photographic paper are prepared by fixation in so-diumthiosulfate solution, washed and then dried. Then the sheets are soaked in a solution of p-nitroani-line, a-naphthol and magnesium sulfate (0.25% of each in 1:1 aqueous alcohol). The impregnated paper is now dried and preserved for later use. When a powder pattern is to be visualized, a sheet of impregnated photographic paper is placed on the laboratory bench emulsion side up. The item of clothing bearing the powder pattern is then placed on top of the photographic paper with the surface bearing the powder pattern placed in contact with the emulsion layer of the photographic paper. A cloth moistened with 10% acetic acid is placed on top of the itemof clothing and the whole stack is pressed with a hot iron to transfer powder particles to the photographic paper. Finally, the surface of the emulsion is swabbed with a 10% sodiumhydroxide solution. The resulting powder pattern consists of blue flecks on a pale yellow background.
The weapon that produced the original powder pattern is test fired into pieces of white cotton sheeting at various ranges until a test-fired pattern with the same size and density as the original is produced. The original weapon must be used because its condition(e.g. degree of erosion of the bore) can affect the powder pattern. Likewise ammunition from the same lot as that used to produce the original powder pattern must also be used to obtain the test-fired patterns. Different lots of the same brand of ammunition may contain smokeless powder from different lots of propellant. Ammunition manufacturers often change lot numbers when a sublot of one component (projectile, casing, propellant or primer) has been exhausted and the use of a new sublot of that component is begun. Propellants from different sublots may differ in the quantity of gunshot residue that they produce. To insure that the firearms examiners have ammunition from the same lot as that used to fire the original powder pattern, investigators should seize as evidence any remaining boxes of ammunition in the suspect’s possession.
Knowledge of the weather conditions at the time of the shooting is essential to the estimation of the range from which a powder pattern was fired. Wind and rain will disperse the gunshot residue. The ambient temperature may also affect the burning rate of the propellant and thus influence the appearance of the powder pattern.
The range from which a shotgun pellet pattern was fired can be estimated from the pattern of deposition of gunshot residue as discussed above or from the size and density of the pellet pattern. The making of such range estimates from pellet patterns is complicated by the existence of different shotgun chokes. Many shotgun barrels are produced with constrictions at the muzzle whose purpose it is to concentrate the shot pattern. Common designations of the degree of choke of a shotgun barrel are the following: cylinder-bore (no choke), improved cylinder (slight choke), modified choke and full choke. The choke of a shotgun barrel can be determined by firing pellet patterns at a range of 40 yards (37m): a full-choke barrel will place 65-75% of the pellets within a 30-inch (75 cm) circle; a modified choke barrel will place 45-65% within the circle; an improved cylinder barrel will place 35-45% within the circle; and the cylinder bore barrel will place 25-35% within the circle. The two barrels of a double-barreled shotgun frequently have different chokes. The choke of a shotgun can also be changed with barrel inserts or adjustable compensators. Obviously the choke of a shotgun affects the size and density of the pellet pattern it fires at any given range. Several approaches have been used to estimate the range of fire from the size and density of a pellet pattern. Some firearms examiners have used the rule of thumb that a shotgun pellet pattern spreads about one inch (2.5 cm) for each yard (meter) that the shot string travels down range. Another approach is to test fire the weapon that fired the questioned pellet pattern at various ranges into paper or cardboard targets until a pellet pattern of the same size and shot density as that of the questioned pellet pattern is obtained. The test-fired patterns must be obtained using shot shells from the same lot as that used to fire the questioned pattern. Studies have shown that the sizes of shotgun pellet patterns produced by different lots of the same brand of ammunition are statistically different. Knowledge of the weather conditions at the time the questioned pattern was fired is also important. The ambient temperature has been shown to affect the spread of shotgun pellets. Attempts have also been made to apply regression analysis to the estimation of the range of fire from the size of a pellet pattern. However, the application of regression analysis to the estimation of the range of fire of a shotgun pellet pattern requires that a large number of test-fired pellet patterns (more than twenty if the confidence limits of the estimate are to be forensically useful). Rarely will sufficient shot-shells from the same batch as that used to fire the questioned pattern be available to the firearms examiner for use to be made of regression analysis.
Although many of the same procedures may be used both to visualize powder patterns and to detect gunshot residue on the hands of a suspected shooter, it must be strongly emphasized that the purposes of these two types of analysis are quite different. In the first case, the test results are used to estimate the range from which a gunshot was fired, and in the second case the test results are used to link a suspect with the discharge of a firearm. The first test for the presence of gunshot residue on the hands of a suspect was the dermal nitrate test (also called the paraffin test, the Gonzalez test or the diphenylamine test) which was introduced in the 1930s. The test was developed by Tomas Gonzalez, chief of police of Mexico City and later Chief Medical Examiner of New York City. Gunshot residue was removed from the suspect’s hand using a paraffin glove. A layer of melted paraffin was carefully ‘painted’ on to the hand and then while it was still soft the paraffin was reinforced with a layer of surgical gauze. Further layers of melted paraffin and gauze would be applied to produce a thick glove that could be handled without danger of disintegration. The glove would be allowed to cool and then it would be cut from the hand. Drops of diphenylamine reagent (typical formula: 0.25 g diphenylamine and 0.25 g N,N’-diphenylbenzidine in 70 ml concentrated sulfuric acid) would be applied to the interior surface of the glove. Gunshot residue would be revealed by the presence of blue flecks whose blue color streamed off into the reagent solution. The diphenylamine reagent produces a blue color when it reacts with partially burnt particles of smokeless powder. The diphenylamine reagent also reacts with unburned and partially burned particles of smokeless powder. Because the dermal nitrate test is actually a test for oxidizing agents a variety of materials may interfere with it: household bleaches (sodiumand calcium hypochlorites); water treatment chemicals (calcium hypochlorite); fertilizers (ammonium nitrate); and explosives. Even the nitrates in urine could produce a false positive test. Cosmetics and tobacco have also been found to interfere with the test. Because of these problems with the dermal nitrate test an Interpol seminar unanimously recommended in 1963 that the dermal nitrate test no longer be used for either investigative or evidentiary purposes.
In 1959 Harrison and Gilroy published a chromo-genic analysis scheme for the detection of primer residues. The suspect’s hand were first swabbed with a small square of clean cotton cloth which had been dampened with 0.1 N hydrochloric acid. A drop of a 10% alcoholic solution of methyltriphenylarso-niumiodide was placed on the cloth. An orange ring indicated the presence of antimony. After drying, the cloth was next tested with two drops of 5% aqueous sodiumrhodizonate solution. A red color developing within the orange ring indicated the presence of bar-iumor lead or both. The cloth swab was again dried and then one or two drops of 1:20 hydrochloric acid were added to the red colored area. A blue color indicated the presence of lead; if the red color remained inside the blue-colored ring barium was also present. The addition of the 1:20 hydrochloric acid was important because mercurous ion, ferrous ion and thiosulfate ion give a red or orange color with sodiumrhodizonate, but hydrochloric acid destroys the colored complexes with these ions. The detection limits for the three metal ions were determined by Harrison and Gilroy to be 4 ug of antimony in the presence of 1.5 mg of lead and 10 mg of barium, 10 ug of bariumin the presence of 1.5 mg of lead (higher quantities of lead masking the color of the barium rhodizonate complex) and 2.5 ug of lead.
Instrumental methods of analysis have replaced the chromogenic methods discussed above in modern crime laboratories. Atomic absorption (AA) spectro-photometry has become the method of choice for many forensic science laboratories for the analysis of suspected gunshot residue. AA is much less expensive than neutron activation analysis (NAA), (see below) and it can detect lead, as well as antimony and barium. The detection limits of AA for these elements are higher than those of NAA but are adequate for most gunshot residue samples. A typical AA spectrophotometer consists of a light source, a monochromator, a sample atomizer, a photomultiplier detector and some type of read-out device. The light sources used in AA are hollow cathode lamps, which consist of glass envelopes with windows through which the radiation generated at the hollow cathode passes. The hollow cathode is a cylinder made of or coated with the element for whose analysis the lamp is to be used. The lamp is filled with a low pressure of argon gas. When a high voltage (5000 V) is applied between the hollow cathode lamp’s anode and cathode the argon fill gas is ionized. The positively charged argon ions are accelerated toward the hollow cathode whose surface they bombard. Atoms are knocked fromthe surface of the hollow cathode in excited electronic states. The excited atoms emit the wavelengths of light characteristic of the element comprising or coating the hollow cathode. The light emitted by the hollow cathode lamp passes through a monochromator which isolates the wavelength of light which has been selected as the analytical wavelength. The emission profile of the analytical wavelength is much narrower than the absorption profile of gas phase atoms at the same wavelength. This is a necessary requirement for quantitative elemental analysis by AA because Beer’s Law is valid only when the absorptivity (or extinction coefficient) of the absorbing species is constant over the range of wavelengths passing through the sample.
The analytical wavelength next passes through the sample atomization region of the spectrophotometer where the analytical sample is vaporized and atomized. The analytical wavelength passes through the resulting cloud of gas phase atoms. Flame atomization has been a mainstay of the AA technique. The analytical sample (in the form of a liquid) is aspirated into a gas burner (burning acetylene in oxygen or nitrous oxide) where the heat of the flame first vaporizes it and then dissociates it into its component atoms. Flame AA is not appropriate for the analysis of gunshot residue. Lead, antimony and barium all form refractory oxides in the flame and so are unavailable to absorb the analytical wavelength. The use of carbon rod atomizers has also been explored; however, it was found that bariumcould not be determined by this method because it forms refractory barium carbide. Tantalumstrip atomizers and graphite furnaces have been successfully used as atomizers for the analysis of gunshot residue.
AA is compatible with a wide variety of sampling techniques. Gunshot residue can be removed from the hands by swabbing or by dipping themin to plastic bags containing a dilute acid solution. Tape lifts and filmlifts can also be used to collect gunshot residue samples for AA analysis. The lifts must be ashed in an oxygen plasma; the primer residues are dissolved in an acid solution for AA analysis. Regardless of the method used to collect the gunshot residue, control samples of the materials used in the collection (e.g. acid solution, swab, lifting tape or lifting film) must also be submitted for analysis to verify that these materials do not contain the elements of interest.
Scanning electron microscopy (SEM) has been used by some forensic science laboratories for the detection of gunshot residue. In the SEM, an electron beamis swept raster-fashion over the surface of the specimen. A detector collects electrons being produced by the surface of the specimens; an image is formed by converting the detector current into the intensity of a spot on a cathode ray tube whose scan is synchronized with the scan of the electron beamover the specimen surface. Two types of electrons are commonly detected: secondary electrons (electrons ‘knocked’ off the surface of the specimen by the electron beam) and back-scattered electrons (electrons fromthe electron beam that are deflected backwards by collisions with the electron clouds surrounding atoms in the specimen). Backscattered electron detectors have proven to be particularly useful in the analysis of primer residues because heavy atoms such as lead, antimony and bariumare very efficient backscatterers. Particles containing these elements appear brighter on the SEM image display. This allows the SEM operator to pick out the primer particles more quickly. Primer particles take a number of forms; they may be single spheres or clusters of globules. The majority of primer residue particles are spheres ranging from0.5 umto 5.0 umin diameter.
The morphology of primer particles is highly characteristic; however, the elemental makeup of the particles must also be examined. If the primer residue particles are bombarded with high-energy electrons they can be made to produce X-rays. The high-energy electrons penetrate the electron shells of atoms at the surface of the primer residue particle. Collisions with inner shell electrons can lead to removal of an inner shell electron fromthe atom, producing a hole in the K shell. This hole can be filled by an outer electron falling into it; when it falls, the outer electron loses energy in the formof an X-ray photon. The emitted X-ray photon is detected by an energy-dispersive X-ray analyzer (hence the name SEM/EDX). The X-ray emission lines of antimony, barium and lead are given in Table 1.
In 1994 the ASTM (American Society for Testing and Materials) Committee E-30 on Forensic Science adopted guidelines (Standard E 1588) for SEM/EDX analysis of gunshot residue, including guidelines for the interpretation of the results of the elemental analysis. The recommended operating parameters for the SEM/EDX systemwere: (1) that the SEM when operating in the backscattered mode be capable of detecting potential gunshot residue particles down to a diameter of 0.5 um; (2) that the instrument be capable of producing a 3:1 signal-to-noise ratio for the lead La emission line from a lead particle no larger than 1 um in diameter; and (3) that the instrument be capable of resolving the La1,Lp1 and Lp2 emission lines. To fulfill these requirements the SEM/EDX must be capable of operating at a 20 KeV accelerating potential or higher.


Table 1 X-ray emission lines (KeV)

No. Element Ka-, Ka2 Kf>1 La1 La2 LP2
51 Sb 26.3591 26.1108 29.7256 3.60472 3.59532 3.84357 4.10078 4.34779
56 Ba 32.1936 31.8171 36.3782 4.46626 4.45090 4.82753 5.1565 5.5311
82 Pb 74.9694 72.8042 84.936 10.5515 10.4495 12.6137 12.6226 14.7644

The following combinations of elements have been observed only in primer residue particles: lead, antimony and barium; and antimony and barium. Other combinations are consistent with primer residue but might derive from other sources: barium, calcium, silicon with a trace of sulfur; lead and antimony; lead and barium; lead; barium.
Samples for SEM analysis may be collected in a variety of ways. Both tape lifts and filmlifts have been used. The lifts are sputter-coated with carbon to insure electrical conductivity and prevent charging of the tape or film surfaces. Metal SEM sample stubs coated with adhesive can also be used. Carbon is used to sputter-coat the samples because its X-ray emissions are blocked by the berylliumwindow of the SEM’s X-ray analyzer. Because observation of the morphology of the particles is essential to the identification of primer residue, sample collection methods (e.g. swabbing with dilute acid or dipping in dilute acid) in which the primer residue particles are dissolved cannot be used.
Some forensic science laboratories have begun to use X-ray microfluorescence for the analysis of gunshot residue. In this technique a narrow beam of X-rays is focused on the sample. The X-rays ionize some of the atoms in the sample by removing inner shell electrons; outer shell electrons fall into the resulting vacancies, emitting fluorescent X-rays as they do so. The emitted X-rays are then analyzed by an energy-dispersive X-ray spectrometer. Instruments such as the Kevex Omnicron energy dispersive X-ray micro-fluorescence spectrometer are capable of scanning a large area and generating maps of the intensities of the fluorescent X-rays. Consequently, X-ray microfluorescence has the capability not only of determining the presence of lead, antimony and barium in primer residue but also of showing their distribution pattern on the target surface.
The use of neutron activation analysis (NAA) for the detection of primer residues was developed during the heyday of the exploitation of NAA in forensic science. In NAA the sample is exposed to bombardment by thermal neutrons in the core of a nuclear reactor. Stable isotopes of lead, antimony and barium (Table 2) capture neutrons and are converted into radioactive isotopes. After the sample is removed from the reactor the induced radioactivity is measured with a y-ray spectrometer. The y-ray energies measured identify the radioactive isotopes present in the sample (Table 3) and the number of y-rays indicates the number of radioactive nuclei present. Although NAA has a number of attributes that commend it for the detection of primer residues, such as very low detection limits and compatibility with a number of sampling methods, it also suffers from a number of flaws. The most obvious is the cost of this method of analysis. Another is the procedure’s inability to detect lead. Although the common stable isotopes of lead may be converted into radioactive isotopes by neutron bombardment, none of these isotopes emits y-rays. Low-resolution y-ray detectors have also posed problems. When such detectors are used the most intense y-ray emission of 139Ba can be obscured by a y-ray emission of 24Na. Perspiration will produce very high levels of sodiumin samples taken fromthe hands. Initially, the interference fromsodiumwas removed by radiochemical separation. However, Krishnan has developed a NAA procedure using a series of irradiations followed by cool down periods to alleviate the interference from 24Na. For example, the sample is irradiated at a neutron flux of 5 x 1012 neutrons cm-2 sec-1 for 40 mins. The induced barium radioactivity is counted immediately. The sample is returned to the nuclear reactor for irradiation at the same flux for five hours. After a two-day cool down period the induced antimony radioactivity is counted. The long cool-down period permits most of the 24Na radioactivity to disappear.

Table 2 Stable isotopes of barium and antimony

Isotope % Natural abundance
134Ba 2.42
135Ba 6.59
136Ba 7.81
137Ba 11.32
138Ba 71.66
121Sb 57.25
123Sb 42.75

Table 3 Radioactive isotopes of barium, sodium and antimony


Radioactive isotope Half-life y-Ray emissions (MeV)
139Ba 82.9 minutes 0.166
1.270 1.430
24Na 14.96hours 1.369 (100%) 2.754 (100%)
122Sb 2.80 days 0.564 (66%)
1.14 (1%) 1.26(1%)
124Sb 60.4 days 0.603 (97%) 0.644 (7%)
0.72 (14%)
0.967 (2%) 1.048 (2%) 1.31 (3%)
1.37 (5%) 1.45 (2%)
1.692 (50%) 2.088 (7%)

Samples for NAA analysis can be obtained in a variety of ways. The paraffin glove method used in the dermal nitrate test was the first to be used. Later it was deemed easier to remove the primer residues by dipping the hands into a plastic bag containing a dilute nitric acid solution or by swabbing the hands with cotton-tipped swabs dampened with a dilute acid solution. The swabbing method of sample collection is used in commercially packaged NAA gunshot residue collection kits. Swabbing is now the collection method of choice. Swabs can be collected from several areas on the firing and non-firing hands (e.g. web between thumb and forefinger, back of the hand and palmof the hand). High levels of antimony and bar-iumin the web area of the right hand of a suspect with low levels elsewhere would be consistent with the suspect having discharged a firearm. On the other hand, elevated levels of antimony and barium on the palmof one or both hands would be consistent with the suspect merely handling a firearm. Regardless of the method of sample collection chosen, the low detection limits of NAA require that negative control samples (reagent solutions, swabs and the like) also be submitted for analysis so that the absence of antimony and barium from the sampling media can be demonstrated. Early in the development of the swabbing technique some samples of cotton swabs were found to have high levels of bariumin their wooden shafts.
A 1992 survey of forensic science laboratories in the United States and in two Canadian provinces found that 44% of the respondent laboratories used AA by itself. About 26% of the laboratories used SEM/EDX by itself and 29% combined AA and SEM/EDX. Only 2% of the respondent laboratories were still using NAA for gunshot residue detection. The laboratories reported a wide range of threshold values for reporting a positive result: for Pb 0.1-2.0 ugml-1; for Ba 0.1-1.0ugml-1; and for Sb 0.02-0.2ugml-1.
Ammunition manufacturers have begun to introduce lead-free primers. Not only do these primers lack the lead azide or lead styphnate primary high explosives they also do not contain antimony sulfide or bariumnitrate. The primary high explosives used in lead-free primers include diazodinitrophenol and tet-racene. Zinc peroxide or strontiumnitrate may be added as oxidizers and fine titaniumparticles may also be found in the primer mixture. Elemental analysis of primer residues produced by lead-free primers reveals particles containing zinc and titaniumor particles containing strontium. Lead and antimony from the surfaces of lead-alloy bullets may also be incorporated in the primer residue particles. Firearms which have been previously used to fire ammunition having lead-based primers have been found to produce residue particles containing up to four elements. Primer residues produced by lead-free primers have similar morphologies to those produced by conventional primers: spheres and clusters of spheres. The mechanism by which primer residue particles are formed is basically the same: the metals condense froma hot vapor. The SEM/EDX therefore remains a viable method for identifying primer residues. However, residues fromfireworks and flares have been found to contain particles with a similar morphology to those produced by primers containing strontium.
A growing body of research has been focused on the analysis of the organic constituents of smokeless powder, which may also be detectable in gunshot residue. Smokeless powder consists of nitrocellulose to which a variety of compounds may have been added: the energetic plasticizers nitroglycerin and dinitroto-luene, stabilizers such as diphenylamine and ethyl centralite, and nonenergetic plasticizers such as dibu-tyl phthalate and triacetin. Gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC) have both been applied to the detection of the organic additives in smokeless powders; however, micellar electrokinetic capillary electrophoresis (MECE) has shown the greatest promise as a method for the detection of organic compounds in gunshot residue. In conventional capillary electrophoresis ionic species are separated on the basis of their electrophoretic mobilities (which are dependent on such factors as the effective sizes of the ions, their electrical charges and the viscosity of the medium in which the electrophoresis is taking place). If the wall of the capillary bears ionized groups (e.g. Si-OH in the case of fused silica capillaries) ions of opposite charge will be attracted to the wall of the capillary, leaving the solution in the capillary with a net electrical charge. Application of a voltage through the capillary results in a net flow of the solution through the capillary. This phenomenon is called electroendosmosis (EEO). In MECE an anionic detergent such as sodiumdocecyl sulfate (SDS) is added to a buffer solution; the detergent forms micelles, spherical aggregates in which the nonpolar tails of the detergent molecules project into the center of the aggregates and their electrically charged heads project out into the buffer solution. The anionic detergent micelles have a large net negative charge, which gives them a large anodic electro-phoretic mobility. If the electrophoresis buffer has a large EEO flow toward the cathode (as most do) the capillary will contain a fast moving aqueous phase and a slow-moving micellar phase. Mixtures of organic compounds can be separated based on the compound’s differential solubility in the micelles: compounds that are very soluble in the micelles migrate with the slow-moving micelles, whereas compounds that are more soluble in the buffer will migrate more rapidly. The eluted compounds are detected by their absorption of ultraviolet light. The advantage of MECE over GCMS is that MECE separations take place at roomtemperature so that temperature labile compounds (e.g. nitroglycerin) are less likely to undergo decomposition. Because of its flow profile MECE is capable of higher resolution than HPLC.
In some cases criminal defendants claim that a negative test for the presence of gunshot residue on their hands shows that they did not discharge a firearm. The interpretation of the results of tests for gunshot residues is not so simple. First of all, many firearms deposit little or no detectable residue on the shooter’s hands. Surveys of firearmsuicides in which handswabs were collected and submitted for gunshot residue have found gunshot residue detection rates which varied from62% to 38%. In the case of a living shooter the gunshot residue may be removed by washing the hands; it may also be rubbed off the hands on to clothing. Because of the possibility that gunshot residue may be deliberately removed or inadvertently lost froma shooter’s hands other sources of gunshot residue should be considered. Gunshot residue may be deposited on the face and hair of the shooter or on his clothing. Gunshot residue deposited in these areas will generally be retained longer than gunshot residue on the hands. Nasal mucus has also been explored as a source of gunshot residue samples. The problemwith these alternate sources of gunshot residue is that finding traces of gunshot residue on the face, in the hair, on clothing or in nasal mucus of a criminal suspect only establishes that the suspect was near a firearmwhen it was discharged.

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