Topic 10 - Gunshot residue analysis

Question 1

what is gunshot residue?

Answer 1

“Any particles or residues discharged from a firearm after the trigger has been pulled. These may include chemicals from the primer, propellant,
oxidisers, reducing agents, sensitisers and binders.”

Question 2

what are the other names for gunshot residue (GSR)?

Answer 2

Gunshot residue (GSR) is also often referred to as:

• Firearm Discharge Residue (FDR)
• Cartridge Discharge Residue (CDR)
• Gunfire Residue (GFR)
• Primer Discharge Residue (PDR)

Question 3

what are the sources of GSR?

Answer 3

Gunshot residue can be created from any combination of the following inside a firearm:

• Residues formed through the explosive reaction of the primer compounds.
• Material originating from the bullet and bullet jacket or coating.
• Material eroded from the cartridge case, primer cup and other cartridge components.
• Materials originating from the interior of the firearm chamber and barrel, including residues from previous discharges of the firearm and foreign materials, such as metal oxidation/corrosion, soil, and debris, including
  biological material.

Question 4

how is GSR formed?

Answer 4

• GSR particles form as a result of rapid cooling of the discharge gases and solid matter, originating from partially reacted components of the primer and propellant, as well as from the metallic components of the ammunition and firearm.

Question 5

what does GSR look like?

Answer 5

GSR may show as spheroidal particles ranging in size from sub-micrometres (μm) to several hundred micrometres in diameter.

or

Irregular and aggregate particles, which generally
constitute the majority of larger GSR particles produced, ranging in size typically from a few micrometres to several hundred micrometres.

Question 6

how do you collect GSR?

Answer 6

The subject’s use of their hands should be minimised before collection.

Wherever possible:
* Sample from the subject prior to handcuffing.
* Subject should be under visual observation before sample collection.
* Do not allow subject to wash/wipe hands.
* Do not allow subject to use the bathroom before collection. If the urge is irresistible and unavoidable, the subject should be supervised to ensure that he does not wash his hands.
* Do not allow subject to place hands in pockets.
* Do not remove subject’s clothing before GSR sample collection.
* Do not fingerprint the subject before GSR collection.

Question 7

what is police contamination?

Answer 7

• Beware asking Police Firearms Officers
  (FAO) to safe a weapon.
• It’s the right thing to do for safety, but bad for
  contamination.
• All FAOs will be contaminated with GSR.
• Consider the implications of the arrest of an
  armed suspect by FAO.
• If unavoidable take comprehensive control samples
  from the FAO.
• There will be questions over contamination at
  court (see Jill Dando case)

Question 8

how do you recover close range GSR?

Answer 8

• GSR on surfaces other than human tissue is best recovered by
  removing as large a section as possible (if we can’t take the whole
  item).
• The overall residue pattern can be just as important as the chemical
  composition of the particles.
• Dyes can be used to stain the GSR for pattern analysis.
• Any samples for chemical analysis should be taken prior to staining.
• If unsure, swab from one area then recover the remainder of the
  sample.

Question 9

how do you recover long range GSR?

Answer 9

• GSR will not be present much beyond a few metres from point of shot.
• However, important residue deposits will be present on the bullet and
  these will deposit onto any surface it interacts with. The bullet will
  leave a “wipe ring” if it passes through the impacted target.
• The deposits on the bullet and in the wipe ring can be analysed.
• These may also contain contaminants collected prior to impact.
• The collection process should ideally be conducted in a laboratory: no
  on-scene analysis or recovery should be attempted unless it’s
  unavoidable.
• Instead, recover the entire sample to a laboratory if possible.

Question 10

what are the patterns of GSR?

Answer 10

• GSR discharge patterns can be analysed for any firearm and
  ammunition combination and will be relatively consistent at any
  given range.
• A test pattern can be fired using the subject firearm and
  ammunition and the results compared to a distribution found
  deposited on a scene surface.
• This method is relatively accurate as long as most conditions
  are replicated.
• The ammunition is key though, since different manufacturers
  use different propellant designs.

Question 11

what is GSR discharge plotting?

Answer 11

• Discharge plotting is a fairly simple
  physical test.
• If possible, use the subject firearm or if
  not then the same make and model.
• If possible, use the same ammunition. * Ideally from the same production batch,
  although this may take some research.
• Undergo test firings onto sterile white 0.5
  mm thick card.
• Vary the range between tests, initially
  using 0 to 2 metres in 20 cm increments.

Question 12

what is a Greiss test?

Answer 12

• The Greiss test is a presumptive chemical test
  which suggests the presence of organic nitrite
  compounds.
• It is used to test for traces of explosive materials
  and propellants and turns brown/orange in their
  presence.
• It is presumptive, since it can can give a false
  positive results in some circumstances.
• See ‘The Birmingham Six’ case.
• Greiss reagent is a solution of equal volumes of:
• 0.2% naphthylenediamine dihydrochloride
• 2% sulphanilamide in 5% Phosphoric acid
• This is always performed as the first presumptive
  test since it will not affect subsequent metal
  deposition tests.

Question 13

what is a Sodium Rhodizonate test?

Answer 13

• The sodium rhodizonate test is a chemical metal
  deposition test which suggests the presence of lead.
• It can be performed after the Greiss test and involves
  spraying a 1% solution of the reagent in distilled water
  onto the test surface.
• This produces a red/pink stain suggesting the presence
  of lead.
• The area is then treated with dilute HCl and if the stain
  turns blue then the presence of lead is confirmed.
• A positive result from this test and the Greiss test is
  strong evidence that a firearm has been discharged.
• Independently, the two tests do not represent such
  strong evidence.

Question 14

how can we concluisvely analyse GSR?

Answer 14

• Currently, chemical analysis of GSR is almost exclusively restricted to
  the inorganic (metallic) residues, which are most strongly linked to the
  primer-related residues.
• Inorganic residues can be interrogated using:
  o Scanning Electron Microscopy-Energy Dispersive X-ray analysis (SEM-EDX)
  o X-ray Fluorescence (XRF)
  o Atomic Absorption Spectroscopy (AAS)
• Far less work is done on the organic components, which come largely
  from the propellant composition (NC/NG) but can be analysed by:
  o FTIR, Raman, GC-MS

Question 15

what are primer compositions?

Answer 15

Priming compositions vary significantly depending on application and
manufacturer but two types of mixtures that can be encountered are:
* Mercuric (rather dated now):
o Mercury Fulminate
o Barium Nitrate
o Antimony Sulphide
* “Sinoxid” formulation (most common):
o Lead Styphnate (possibly with other lead compounds)
o Barium Nitrate
o Antimony Sulphide
These may contain other additives too (binders, sensitisers, frictionators etc.).
* “SINTOX” and other ‘less harmful’, green primers also exist.

Question 16

what are the main functions of primer components?

Answer 16

• Barium nitrate (BaNO3) acts as an oxidiser to increase
  the heat of ignition.
• Antimony Sulphide (Sb2S3) acts as the fuel in the ignition
  process. It also acts a “frictionator” during ignition.
• Mercury Fulminate (Hg(CNO)2) or Lead Styphnate
  (C6HN3O8Pb) acts as the explosive to drive the ignition
  process.

Question 17

what is the method of SEM-EDX??

Answer 17

• The electron gun (a heated tungsten
  filament) fires a beam of electrons at a high
  voltage through a vacuum.
• Electromagnetic lenses focus the beam onto
  the surface of an object being studied.
• The finely focused beam is scanned across
  a sample to cover the area of interest.
• Depending on the object’s orientation and its
  composition, electrons will be scattered by
  different amounts.
• Those scattered electrons hit an electron
  detector and are recorded.
• This information is then used to build up the
  picture of the object we are studying.

Question 18

why is SEM-EDX an optimum technique for examining GSR?

Answer 18

The combination of Scanning Electron Microscopy and Energy Dispersive
X-ray Spectrometry (SEM-EDX) has widespread scientific acceptance as
the optimum technique for the examination and analysis of GSR for the
following reasons:
* The technique is non-destructive.
* Minimal sample preparation is necessary.
* Individual particles can be analysed.
* The morphology of particles can be examined.
* GSR can be identified with a high level of confidence based on the
elemental composition and morphology.
* Highly effective and fairly rapid automated systems have been
developed for particle detection and classification.

Question 19

why is SEM-EDX good for particle searching?

Answer 19

• SEM-EDX can be applied either manually or in an automated mode to
  search for and analyse potential GSR particles. In both search modes,
  the sample surface is scanned in a systematic, defined sequence.
• Automated search is the preferred method due to increased efficiency.
• In both manual and automated systems, the search targets particles of
  high mean atomic number.
• These are visualised using a Back Scattered Electron (BSE) detector
  set at a threshold for high atomic number or calibrated to provide a
  correlation of atomic number with gray scale.
• Once a particle with a bright BSE image is detected, it is analysed by
  EDX.

Question 20

how can GSR particles be classified?

Answer 20

1. Particles classified as ‘characteristic of GSR’ have compositions rarely found
   in particles from any other source.
2. Particles classified as ‘consistent with GSR’ have compositions that are also
   found in particles from a number of relatively common, non-firearm sources.
   Particles within this group are produced through the operation of a variety of
   processes, equipment or devices and can be found in the environment with
   varying levels of frequency.
3. Particles classified as ‘commonly associated with GSR’ have compositions
   that are also commonly found in environmental particles from numerous
   sources. However, when present, in addition to particles that are
   characteristic of, and/or consistent with GSR, these particles can be of
   significance in the interpretation of a population of particles and,
   consequently, the likelihood that that population is GSR. In isolation,
   however, such particles have little significance in examinations for GSR.

Question 21

what must characterisitc GSR particles contain?

Answer 21

• “Characteristic” particles must contain:
  o Lead
  o Antimony
  o Barium
• Additional elements often found in major, minor or trace amounts may be: Silicon, Calcium, Aluminium, Copper, Tin
• Additional elements often found in minor or trace amounts may be: Iron, Sulphur, Zinc, Potassium, Chlorine, Phosphorus, Nickel

Question 22

what are “consistent with” GSR particles and what compositions do they have?

Answer 22

• “Consistent with” GSR particles will have one of the
  following compositions:

o Barium, Calcium, Silicon (with no more than a trace of Sulphur)

o Antimony, Barium (usually with no more than a trace of Iron or Sulphur)

o Lead with levels of Antimony greater than trace
amounts

o Barium, Aluminium

o Lead, Barium

Question 23

what are “commonly associated with” GSR particles?

Answer 23

• “Commonly associated with” GSR particles could include:
  o Lead with only trace levels of Antimony
  o Lead
  o Antimony
  o Barium (in the absence of Sulphur)
• These particles may also contain one or more of only the
  following other elements:
  o Silicon, Calcium, Aluminium, Copper, and trace amounts of Iron,
  Sulphur, Phosphorus, Zinc, Nickel (in conjunction with Copper and
  Zinc), Potassium, Chlorine and Tin.

Question 24

what are false positives in GSR analysis?

Answer 24

There are a small number of non-firearm sources of particles having compositions similar to GSR derived from Sinoxid type primers.

car air bags - Many vehicle air bags incorporate a primer device that is mounted in the passenger side dashboard of a vehicle or the steering wheel. Most of the air bag primer residue particles obtained from these deployed passenger side devices contain elements unusual to GSR as well as lead, antimony, and barium.

fireworks and pyrotechnics - Lead, antimony, and barium compounds are rarely found together in
fireworks and pyrotechnics.

car brake pads - Car brake pads containing compounds of lead, antimony, and barium have been used by some vehicle manufacturers in the past, although
not so common in modern times. The friction caused by the application of the brakes results in the
shedding of fragments and particles from the brake pads.