The quantity of organic compounds left over from the burning of the propellant is obviously vast in comparison to those from the priming compound. As a result, early attempts at detecting GSRs were directed towards the recovery and identification of the organic components.
These included the identification of nitrites and nitrates in partially burnt propellant particles using chemical spot tests.
One very popular test was the Walker test (Walker, 1940), which used desensitized photographic paper as a medium to pick up and retain the particles. After picking up the particles, they were then visualized using a chemical spot test for the nitrites present. The technique was, however, mainly intended for recovery of GSR particles from clothes and was of little use in discovering whether a person had fired a weapon.
The technique was quite long and was conducted as follows:
• Desensitize a sheet of photographic paper by removing the silver halides with 'hypo' photographic fixer.
• After drying, immerse the paper in a solution of 5% 2-naphthylamine 4,8 disulphonic acid and dry.
• Place a cloth wetted with 20% acetic acid under the object to be tested. Place the photographic paper on top and press with a hot iron. This softens up the gel coating of the photographic paper to enable it to pick up any GSR particles.
• Any bright red spots which appear are diazo compounds indicating the presence of nitrites.
Despite its complex nature, the test was very efficient at picking up partially burnt propellant particles from clothing for range of firing estimations.
The Greiss, Marshall and Tewari tests were merely variations on the Walker test using different chemicals to produce other coloured diazo compounds.
Probably the most infamous test was the dermal nitrate or paraffin test, which was first introduced by Teodoro Gonzalez of the Mexico City police laboratory in 1933. This involved the taking of a cast of the back of the suspect-s hand using hot paraffin wax. When cooled and set, the wax was peeled off along with any imbedded GSR particles. The cast was then sprayed with Lunge reagent, which is a 0.25% solution of diphenylbenzidine in concentrated sulphuric acid. Later variations of the test used diphenylamine in concentrated sulphuric acid. Both these reagents gave a deep blue colouration with nitrates from the partially burnt and unburnt propellant particles.
Unlike the Walker, Greiss and Marshall tests, which merely indicated the presence of these particles on the hands, the paraffin test gave a distribution pattern for the particles. As particles are only deposited on the back of the hand during firing, the palm being wrapped round the weapon 's grip and thus protected, the presence of these particles only on the back of the hands is highly indicative of a person having fired a weapon.
Whilst the test gave good information regarding the distribution of these particles, the test itself was only indicative of nitrates. Fertilizer, rust, face powder, sugar, paint, even urine were also found to give a positive reaction to the Lunge reagent.
In 1935, the FBI indicated that the test was not specific and cautioned against its further use (FBI Law Enforcement Bulletin 4, 1935; 9, 1940).
Thin layer and gas chromatography were also used at this time to detect the nitrocellulose component of propellants. Whilst these were quite successful, nitrocellulose is not a desirable analyte for GSR analysis due to its presence in many consumer products such as nail polish, wood finishes, paints and even the surface of playing cards.
Gas chromatography and high-pressure liquid chromatography have also been used (Andrasko, 1992) for the identification of propellant particles. As the identification of propellant particles is less specific than that of the primer discharge residues, such methods have found little favour.
6.4.2 Inorganic or metallic component identification
In 1959, Harrison and Gillroy developed a test for the identification of lead, barium and antimony, the main metallic components of primer discharge residues (see Section 2.5, primer compositions).
In this test, the back and palm of each hand (a total of four swabs) are vigorously rubbed with a swab moistened with dilute hydrochloric acid. This physically removes any GSR particles and places them into an acidic environment. The swab is then dried and treated with a solution of triphenylmethylarsonium iodide. Any orange spots would indicate the presence of antimony. After drying, a solution of sodium rhodizonate was added. Any red spots indicate barium which, if on the addition of dilute hydrochloric acid turn purple, indicate the presence of lead.
The great advantage of this test over the dermal nitrate test was the low incidence of false positives. Its shortcomings, however, included a relatively low sensitivity and the fact that it only showed the presence of the individual elements on the hands. What was required was to show the presence of all three elements in discrete particles as occurs in GSR particles. Merely identifying the presence of the individual elements leaves open the interpretation as to whether they originated from general environmental and occupational contaminants or from the discharge of a firearm.
In 1966, the use of neutron activation analysis (NAA) for the identification of GSR was reported (Ruch et al., 1964). In this, the samples are placed in a nuclear reactor and bombarded with neutrons making the various elements present radioactive. By analyzing the energy distribution and intensity of the radioactive emissions, it is possible to identify the elements present and the amount of each. This is a highly sensitive method of analysis for most elements, but it is, of course, not applicable to lead, the main component of GSR. Another problem is that not everybody has a convenient nuclear reactor.
In 1970, Bashinki, Davis and Young of the Oakland Police Laboratory, USA, reported on the use of sodium rhodizonate for the detection of GSR. This test is only useful for the identification of lead and barium, but because of its sensitivity and simplicity, it is still a commonly used test. Whilst it is of little use in determining whether a person has fired a gun, it is still very useful for range of firing estimations on dark clothing and identification of entry and exit holes in clothing. In this test, the area to be examined is rubbed with a swab in slightly acidic (pH 2.8) conditions. The swab is then partially dried (with hot air and not an infrared (IR) hot lamp as this destroys the test). The swab is then lightly sprayed with a saturated solution of sodium rhodizonate in water. Any deep purple-coloured spots indicate lead. The swab is then partially dried and lightly sprayed with dilute hydrochloric acid. Purple spots remaining confirm the presence of lead. If the swab is then placed into an alkaline condition, that is, by exposing it to the fumes of 880 ammonia solution, any pink/orange spots which develop indicate the presence of barium.
In 1972, a technique was reported for the analysis of GSR by atomic absorption spectroscopy (AAS) (Green and Sauve, 1973). Atomic absorbtion derives its name from the fact that the atoms of an element will absorb light at a wavelength which is particular to that element. Also, the quantity of light absorbed is proportional to the quantity of that element present.
Basically, a solution of the chemical under test is aspirated into a flame which is sufficiently hot to vapourize the element into its free atoms. If light of the appropriate wavelength is shone through the flame, a portion of the light will be absorbed by the free atoms present. It is the wavelength of the light absorbed which identifies the element present and the quantity of light absorbed which reveals the quantity of the element.
Heated graphite tubes were later used instead of a flame as this was found to give a greatly enhanced sensitivity. This technique was called flameless atomic absorption spectroscopy (FAAS).
Whilst this technique is an extremely sensitive and accurate analytical technique for lead, barium and antimony, it still lacks the specificity required. The results only show that the three elements are present; it cannot show that they are all in a single particle. As such, the elements could be environmental contaminants picked up separately, that is, antimony as a surficant on most fibres to give them lustre, barium from face make-up powders and lead from battery terminals, or a hundred and one other sources.
Many other techniques have been tried, including proton-induced X-ray emission (Panigahi et al., 1982), anodic stripping voltammetry (Brihaye, Machiroux and Gillain, 1982) and Auger electron microscopy (Hellmiss, 1987). For one reason or another, none ever gained any great deal of credibility.
The most which can be said for any of the above tests is that they provide presumptive evidence for the presence of GSR particles.
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