Chemical tests for range of firing estimations

In cases where a SEM is not available, the tests described below can still be used. Great caution should be used with the interpretation of any results obtained from these tests as none of them is specific. At best, the results could be presumptive and at worst, only indicative.

Sodium rhodizonate test. The most valuable of the available spot tests is the sodium rhodizonate test for lead. This test is a rapid and very cheap method for determining, in those cases where a microscopic examination is ambiguous, the entry and exit holes.

The test relies on the specificity of this reagent, in acidic conditions, to give a positive reaction to lead. It also relies on the bullet being either plain lead or, if it is a jacketed bullet, having picked up some lead primer residues on its passage through the bore.

When passing through cloth, the residues or some of the bullet lead will be transferred from the bullet to the impact side of the target. Thus, if we are dealing with a bullet which has completely penetrated a body, the outside surface of the bullet entry hole on the outer garment will give a positive reaction to lead as will the inside surface of the bullet exit hole.

The test is carried out by firmly pressing a clean filter paper, which has been lightly moistened with 0.1 N hydrochloric acid (HCl), over the bullet hole. The filter paper is then dried using a hot air blower and carefully spotted with a saturated solution of sodium rhodizonate in water. The filter paper will eventually take on an orange colour from the sodium rhodizonate. The filter paper is then warmed once again with the air blower, but not dried. The solution of 0.1 N HCl is then lightly spotted, or preferably sprayed, onto the paper until the orange colour disappears. If there are any lead particles present, they will remain as a purple colouration.

The sodium rhodizonate test can also be used for the detection of barium, although it is not as sensitive as when used for lead.

This test can be used in conjunction with the test for lead, giving a more specific identifier of GSR.

Method. After spotting with the 0.1 N HCl solution and noting or photographing any purple-coloured spots, hold the filter paper over a solution of 880 ammonia solution. This will, with care, place the filter paper into a mildly alkaline condition (preferably about pH 8). This will remove the purple colouration due to lead and place it in a condition where the sodium rhodizonate will react with any barium present to give a red/brown colouration.

Alternatively, the filter paper can be spotted with dilute ammonia solution, but this tends to dilute the result, leading to difficulties in identifying the colour change.

Walker test for nitrites. This is used for the detection of nitrites in the partially burnt and unburnt propellants. On darkly coloured clothing, the test can indicate the distribution of such particles, thus enabling the range of firing to be estimated.

The test uses the slightly sticky, gelatinous surface layer of desensitized photographic paper to pick up the particles from the cloth. Any nitrites present are then converted to a diazo dye compound. These diazo compounds are, depending on what chemicals are used, brightly coloured red/orange dyes which can easily be seen and photographed. The technique is as follows:

• Desensitize a sheet of photographic paper in hypo fixer.

• Immerse the paper in a solution of 5% 2-naphthylamine-4,8-disulphonic acid. Dry in air.

• Place a cloth wetted with 20% acetic acid under the clothing under test. Place the photographic paper on top and cover this with a piece of dry cloth. Apply pressure with a hot iron set at 'warm' for 5 min.

• Any bright red spots which appear indicate the presence of nitrite compounds.

It should be noted that many compounds other than nitrocellulose propellants can give a positive reaction, for example, urine, face powder, fertilizers.

Greiss test. This is identical to the Walker test except for the main reagent which is naphthylamine instead of 2-naphthylamine-4,8-disulphonic acid. This reagent gives orange spots. Once again, this test is mainly used for detection of propellant particles in range of firing estimations.

Marshall test. Using the desensitized photographic paper, soak it in 0.5% solution sulphanilic acid for 10 min, then dry. Soak this in a 0.5% solution of N-a naphthyl-ethylenediamine hydrochloride in methanol for 2 min then dry. Place a cloth wetted with 20% acetic acid under the clothing under test. Place the photographic paper on top and cover this with a piece of dry cloth. Apply pressure with a hot iron set at 'warm' for 5 min. A positive for nitrites will appear as purple spots on a purple background. Rinse in warm water to remove background colour. If this is then rinsed in methanol, the spots will turn orange. Once again, this test is mainly used for detection of propellant particles in range of firing estimations.

Tewari test. Dissolve 1 g of antazoline hydrochloride (2-N-benzylanilinomethy-liminazoline hydrochloride) in 50 mL of water. Slowly add 45 mL of concentrated HCl. Stir until the white precipitate dissolves. Soak a filter paper in acetone and press on target. Air dry and spray heavily with the prepared anta-zoline solution. Nitrite compounds will register a positive reaction as deep yellow spots. This test is mainly used for range of firing estimations by visualization of propellant particles.

Lunge reagent. This was the original 'dermal nitrite test' used for the proof of firing a weapon.

Originally, the reagent consisted of a 0.25% solution of diphenybenzidine in concentrated sulphuric acid. This reagent was sprayed onto paraffin casts of the suspect's hands, and any nitrite particles present, which include nitrocellulose, would give a deep blue colouration.

Diphenybenzidine is, however, very carcinogenic and has been replaced with diphenyamine, which also gives a deep blue colouration to nitrites.

The problem with this test is that so many everyday chemicals, for example, fertilizer, urine, make-up, etc., can give a positive reaction with this reagent that it is no longer used for the identification of GSRs on hands.

Harrison and giliroy teagent. Whilst this reagent is really intended for the identification of GSR on hand swabs, it can be used just as well for range of firing estimations once the particles have been removed from the garment.

Use the photographic paper method of lifting the residues from the clothing, but use dilute HCl instead of acetic acid. Dry, then spray with a 10% solution of triphenylmethylarsonium iodide in alcohol. An orange colouration is positive for antimony. Dry and spray with a saturated solution of sodium rhodizonate. Red spots are positive for barium or lead. Dry and spray with dilute HCl. Purple spots are positive for lead. If the spots are then exposed to a 35% ammonia solution, any particles containing barium will give a red colouration.

Dithiooxamide (DTO) test. As previously explained, sodium rhodizonate can be successfully utilized for the identification of bullet entry and exit holes caused by plain lead or semi-jacketed bullets. However, in the case of fully jacketed bullets, the only lead that is likely to be present around the bullet entry hole is from the priming compound. This is often of an insufficient quantity to produce a positive result.

DTO will, however, detect copper and nickel, and can be utilized for the determination of bullet entry and exit holes for fully jacketed gilding metal (Cu/Zn) and cupro nickel (Cu/Ni) bullets. It can also detect the presence of cobalt, although this is currently of no significance in forensic firearms examination.

With DTO, copper produces a very dark-green colouration, nickel a pink to violet colouration and cobalt a brown colouration. The presence of blood can, however, give a false negative result.

The reagent must be freshly prepared from 0.2 g of DTO in 100 mL of ethanol. The ammonium hydroxide is 20 mL of ammonium hydroxide in 50 mL of distilled water.

A filter paper moistened with the ammonium hydroxide is pressed onto the bullet hole. Gentle heat via a hair dryer can be applied, which will enhance the transfer.

Place three drops of the DTO solution to the area in contact with the bullet hole.

A dark greenish/gray reaction constitutes a positive reaction for copper.

A blue/violet colouration constitutes a positive reaction for nickel.

Was this article helpful?

0 0


  • liya
    What chemicals do you use to test a bullet hole?
    9 years ago
    What metals turn purple when doing sodium rhodizonate test in ballstics?
    8 years ago
  • tim wei
    How to dilute hcl to swab hand sfor gsr?
    8 years ago
  • jana
    How does rhodizonate detect lead?
    6 years ago
  • Steven
    Which chemical test for the presence of GSR forms blue specks on a yellow background?
    10 months ago
  • billie
    How does the uffer remove background colour in sodium rhodizonate test?
    10 months ago

Post a comment