analysis of GSR Flashcards
firearms chemistry
- Firearms chemistry however is unique and both source and activity level analysis requires significant assessment and interpretative skill when compared to other forms of forensic science
- Your knowledge of firearms and ammunition manufacture and use needs to be excellent hence the long introduction
- Your Involvement in even comparatively simple cases
• Can be considerable
Initially you may be asked to advise on submission and sampling strategy
- How should police ensure potential GSR is not lost?
- How should they sample a suspect
- How should police package the evidence to maintain value
- This is generally informed by case circumstances – You also need to know what these are…
- Was a gun actually fired or are we dealing with contact with a firearm?
- Were both firearm and ammunition recovered?
sampling strategy
- Sampling should make the most effective use of intelligence
Prioritise testing areas consistent with the alleged behaviour
- What type of firearm was used?
- How could this affect GSR distribution?
- Which hand was used to fire the weapon?
- Where was the weapon holstered?
All of these points can help in deciding the best strategy - However be aware that GSR will be lost and redistributed rapidly
Consider the competing requirements of other examinations - Fibres, FP, DNA
source level considerations
- Is this material GSR?
- Does this combination of materials come from firing of a weapon?
- Is morphology consistent with GSR?
- Is the size consistent with GSR?
- Is the elemental composition consistent with GSR
- If the answer to all of these questions is YES, then it could be GSR
further considerations of source level considerations
What type of GSR is present? – Type I-IX
• What is the evidential weight of this kind of GSR?
• Is this type Indicative or Characteristic?
• How much of it is there? (perhaps more of an activity level consideration!)
• What does this tell us?
• Could there be an alternative source?
characteristic of GSR
- Morphology – Spherical
- Size – 0.5µm to 10µm (Many smaller than 2µm)
- Elemental composition
- Almost all types contain Pb and or Ba
what type of GSR is present
- Type I-IX
- Type 1 Pb-Ba-Sb (Characteristic)
- Possible Type IX Zn-Ti (Sr)
significance
- Indicative or Characteristic?
- At what level?
Significance of p-GSR evidence
3 classifications
unique
characteristic
indicative
unique
No longer used in UK parlance
• Particulate once believed to be found only from the detonation of a primer
• Generally of Pb-Ba-Sb type (Type I)
• Studies have shown some alternate sources for this type of GSR – Brake linings
characteristic
Strongest term used in UK parlance
• Particulate extremely strongly associated with firearm discharge
• This is almost definitely GSR
• In general uncommon in the environment
• May have alternate sources but it’s probably GSR
• Evidential strength depends upon amounts present and any contra-indications but these are perhaps activity level considerations
indicative
Consistent with…
• Particulate fairly strongly associated with firearm discharge
• Its probably GSR but might not be
• Particulate has alternate sources
• Not very common environmentally
• It is probably GSR, but it could be something else and therefore
• Not always reported, but may lend supporting evidence
• At high levels
• Pb and Pb-Sb could be bullet/pellet debris, but it could be solder!
• Ba-Al, Ba-Ca-Si, Ba-Sb and Sn-Sb could be GSR, but some combinations are produced by fireworks!
• That said, size and morphology of firework residue generally inconsistent with p-GSR
challenges to the result
• Defence assertion – The residue is not GSR
Could there be an alternative non-firearms source?
• What is the suspect’s occupation?
• Do they have contact with GSR like substances as a result?
• Are there any hobbies that might be of relevance?
• Do they have contact with fireworks or other pyrotechnics?
• Is there any contact with particulate of a similar nature to that found?
challenges to the result
Characteristic GSR has fewer alternative sources
- Therefore the possibility of an alternative source is less likely
- Environmental sampling may be appropriate to test the hypothesis
challenges to the result
Indicative GSR has more potential alternative sources
• Therefore the possibility of an alternative source becomes more likely
Analysis- in the absence of contra-indicators
It may be reasonable to suggest that the residue present is GSR
- Especially if it is characteristic and at a reasonable level
- This doesn’t necessarily indicate the suspect having fired a weapon, just that samples obtained contain what is believed to be GSR
Apply extreme caution if indicative or at low level
- Such results may not be reported in some cases
- Barry George GSR evidence has dented confidence in GSR evidence
- Re-evaluate as further information becomes available
activity level analysis
Given the assertion that the particulate found is GSR
- There are numerous innocent possibilities to explain its presence
Firing of a weapon or blank firing device
- Not necessarily the weapon implicated in an offence!
- This is a serious complication
Aerosol transfer
- Present during or shortly after firing of a weapon
- GSR can remain at high concentration minutes after a weapon has been fired
Contact with a fired weapon
Secondary or even tertiary transfer
Does the suspect have access to a legally held weapon?
- This could be their own weapon – over 500,000 firearms owners in the UK
- A borrowed/hired weapon – Possibly the property of a gun club
- Involvement in clay, rough shooting or beating
- This would be a quite legitimate source of GSR
Does the suspect have access to a blank firing device?
- Blank firing grenades, bird scarers or blank firers will produce GSR
Does the suspect have contact with those who do?
- In the UK on average around 1/120 people own a firearm and this number increases significantly in rural areas
- This contact may be overt or otherwise
- I would imagine that after a visit to my local pub there is GSR present on my clothing
Has the suspect had contact with a firearm?
- Clearly GSR is likely to be present on a firearm and is rapidly and easily transferred between a weapon and clothing
- Handling a firearm may not be something the suspect will easily admit to
Was the suspect present at the scene at any point?
- Aerosol transfer is a distinct possibility and the presence of GSR does not mean that the suspect is the firer of a weapon
How was the arrest (if relevant) made?
- Was a bang-flash or similar pyrotechnics used?
- Were armed officers involved – Clearly firearms officers may well be covered in GSR?
- Is contamination a possibility?
assessing possible scenarios
What sort of factors do we need to consider?
Amount of GSR
- The quantity of GSR present may well lend support particular hypotheses
- This needs to be considered within the context of the elapsed time since firearm discharge (discussed shortly)
Quantity
- Very High/High – May indicate recently fired weapon, in vicinity of recently fired weapon, contact with weapon
- Moderate/High – As above but less recent
- Low/Moderate – May indicate secondary/tertiary transfer, contamination
levels of GSR - LEARN THESE NUMBERS
Generally accepted significance based on levels of p-GSR
- Low: 1-2 Primer particles
- Moderate: 4-12 primer particles
- High:13-50 primer particles
- Very High: 50+ particles
levels of GSR - LEARN THESE NUMBERS
• Generally accepted significance based on levels of o-GSR
- Low: 2-20ng of propellant
- Moderate: 21-200ng of propellant
- High: 201-2000ng of propellant
- Very High: Greater than 2000ng of propellant
persistence
- Hands ~4hrs
- Face ~6hrs
- Hair ~12 hrs
- Clothing up to 24 hrs
- Residue in pockets can persist far longer… possibly years (Barry George)
These figures vary considerably
factors depending persistence
All GSR may be lost in much less time
- Particularly if the initial amounts were low
- Common in cases of handling not firing
factors depending persistence
GSR found after typical persistence times is more likely to be the result of another event
- Clearly GSR found on hands 24hrs after discharge is odd so think again
- Possibly redistribution of residue from another surface
- Secondary or tertiary transfer
factors depending persistence
GSR found in pockets
- May be problematic due to lengthy persistence
- GSR may remain in pockets for years under some circumstances even after washing
- Treat such evidence with extreme caution
factors depending persistence
Location of GSR
- The location of GSR can in theory provide some support to particular hypotheses, but caution must be exercised – GSR moves about
- GSR found on the back of a garment may indicate proximity to firing and could be used to help understand a suspect’s role and relative position in some cases
- GSR on the waistband may be the result of holstering a weapon in trousers but this material is lost rapidly due to skin/material friction
- GSR in pockets is potentially significant and is probably less subject to secondary transfer or contamination
- Danger in some cases of overstressing significance
Interpretation of GSR distribution
Although we can use intelligence information to inform GSR sampling strategy
- We should however not rely on GSR distribution to provide us with intelligence on an unknown scenario
- This approach is quite inaccurate
- “Very little significance should be attached to the distribution of GSR on the surface of an item especially clothing”
We cannot even state which hand was used to fire the weapon
- There is little correlation between firing hand and GSR levels
- In fact firing hand will often contain less GSR than non-firing hand
- Dominant hand is used more and will therefore shed GSR more rapidly
range and proximity
Reconstruction of the scene
May not be possible with any level of accuracy using GSR distribution
- Logically residue deposition increases as we get closer to the place of firing
- Primer residue tends to travel no more than 3m from the barrel
- Unburned propellant may travel somewhat further
Each firearm/cartridge combination will vary
- Depends upon construction and action
- Therefore very little credence should be given
secondary transfer
Every contact leaves a trace
- Contact with an object or person contaminated with GSR may result in transference – Secondary transfer
- There is even limited evidence of tertiary transfer
- In both scenarios, the level of transfer is logically likely to be low
- Higher GSR levels are unlikely the result of secondary or tertiary transfer
Contact with GSR
- Firer of a weapon – Legitimate or otherwise
- Those in the vicinity of weapons discharge
- Objects which were in the vicinity of weapons discharge
- Objects handled by the firer of a weapon
The suspect may be unaware of a persons contact
- How many people or objects have you come into contact with today?
police
Due to the nature of the crimes involved
• Arrests are more likely to have been made by firearms officers
• These individuals are trained in use of firearms
• Likely to be a source of potential GSR particulate
Studies in the US show that secondary transfer between firearms officers and suspect is extremely common
• Firearms chemists will always ask if firearms officers were involved
• Analysis of GSR present on officers may not match that found on the suspect
• Type VII found on suspect, police use type I ammunition
• This eliminates the police as a potential source in most cases
