Note
Studied by 29 people
Lecture 10 - Gunshot Residue Analysis
Aims
Understand how gunshot residues (GSRs) are created, including the specific chemical reactions and physical processes involved, and detail their general compositions and morphologies at a microscopic level.
Appreciate potential contamination issues related to GSR during collection, storage, and analysis, emphasizing chain of custody and environmental controls.
Comprehensively understand how GSR is analyzed by SEM-EDX, covering sample preparation techniques, instrument settings, data acquisition, and spectral interpretation.
Understand a ‘standard’ GSR classification system according to particle composition, including the rationale behind different categories and their statistical significance.
Recognize sources of particles similar to GSR and how they can be differentiated based on morphology, elemental composition, associated elements, and statistical analysis.
Appreciate the limitations of GSR evidence when presenting to a court as an expert witness, including statistical probabilities, potential sources of error, and the importance of clear, unbiased testimony.
What is Gunshot Residue (GSR)?
GSR is any particles or residues discharged from a firearm after the trigger has been pulled, including those propelled forward from the barrel and those ejected from the breech.
These may include chemicals from the primer (e.g., lead styphnate, barium nitrate, antimony sulfide), propellant (e.g., nitrocellulose, nitroglycerin), oxidisers, reducing agents, sensitisers and binders.
Gunshot residue (GSR) is also often referred to as:
Firearm Discharge Residue (FDR)
Cartridge Discharge Residue (CDR)
Gunfire Residue (GFR)
Primer Discharge Residue (PDR)
Sources of GSR
Gunshot residue can be created from any combination of the following inside a firearm:
Residues formed through the explosive reaction of the primer compounds, detailing the chemical transformations and by-products created during ignition.
Material originating from the bullet and bullet jacket or coating, including lead, copper, and other alloys that may be transferred during firing.
Material eroded from the cartridge case, primer cup, and other cartridge components due to the high-pressure and temperature conditions.
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.
How is GSR Formed?
GSR particles form due to rapid cooling of the discharge gases and solid matter, originating from partially reacted components of the primer and propellant, including condensation and nucleation processes.
Some of the gases condense in the form of spheres, but they also interact with solid residue materials to form complex mixtures and aggregate forms, describing the physical interactions and chemical reactions involved.
Some residue material can be ejected with little or no physical or chemical modification, but most residue particles show evidence of exposure to or formation at extremely high temperatures and pressure, such as melting, vaporization, and recrystallization.
General GSR Appearance
GSR may show as spheroidal particles ranging in size from sub-micrometres (\mum) to several hundred micrometres in diameter. These spheres often exhibit a distinct surface texture and density.
ORIrregular 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. These aggregates can be complex mixtures of various materials.
Collecting GSR Samples
Contamination must be avoided when collecting GSR evidence to ensure an accurate measure of what is genuinely on the surface being sampled. Control samples should also be taken to account for background levels.
Contamination avoidance is best achieved by taking appropriate steps at all stages in the process to avoid transfer of unwanted material and by designing procedures that minimise the opportunity, and so the risk, of transfer of material. This includes using proper PPE, cleaning tools, and designated sampling areas.
Think of GSR like chalk dust. It exists within the area of the chalk board and on its accessories. Once contacted, some of the dust particles can be transferred to the new surface but as activity continues, they will progressively diminish in amount. The same is true for GSR, emphasizing the importance of timely collection.
Collecting GSR – Human Sampling (1)
The subject’s use of their hands should be minimised before collection. Wherever possible:
Sample from the subject prior to handcuffing to avoid potential contamination from the cuffs themselves.
Subject should be under visual observation before sample collection to ensure they do not tamper with their hands.
Do not allow subject to wash/wipe hands, as this can remove or redistribute GSR particles.
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 to prevent contamination from pocket lint or other materials.
Do not remove subject’s clothing before GSR sample collection to avoid dislodging GSR particles.
Do not fingerprint the subject before GSR collection, as the fingerprinting process can contaminate the sample.
Internal Swabs
Medical practitioners can swab nasal passages with the subject’s permission. This can be particularly useful in cases where the subject may have inhaled GSR.
There is no general power in law to take this type of sample by force, emphasizing the need for consent.
Pathologists can swab sinuses and recover mucous post-mortem for GSR analysis.
All biological GSR samples to be refrigerated immediately to prevent biological action degrading any nitrate compounds and to preserve the integrity of the sample.
Collecting GSR – Human Sampling (2)
Evidence collection teams typically will have two or four collection ‘stubs’ in their GSR kits to sample the following areas of hand with separate stubs for each hand:
The back of each hand including the thumb- forefinger web as well as all digits.
The palm of each hand including web of hand as well as all digits.
Dab the hand sampling areas in a ‘line search pattern’ up and down the palm, fingers and webbing with an SEM stub in its holder to ensure thorough collection.
Separate stubs for at least front and back of hand to prevent cross-contamination.
Anti-Contamination Procedures (1)
The following protocols should be carried out by the collector and incorporated into the instructions and collection data sheets developed by laboratories for inclusion with GSR kits:
All equipment to be used for the collection of GSR should be stored in a clean environment isolated from potential contamination. This includes storage in sealed containers away from firearms and ammunition.
When undertaking an examination, the equipment (writing materials, sampling equipment, cases etc.) should be protected from exposure to potential contamination. Use disposable or regularly cleaned materials.
Carry out an initial scene assessment. Note that it may be necessary to don PPE prior to this assessment to protect yourself and the integrity of the sample.
Avoid dealing with other items or evidence that can be heavily contaminated such as guns, spent cartridges, and other firearms-related items before sampling for GSR. This prevents cross-contamination.
Identify a collection area that is isolated from areas that may be contaminated. Designate a clean workspace.
Clean the collection area (cleaning protocols should be established by the laboratory). Use appropriate cleaning agents and techniques.
Wash hands and put on any PPE before sampling (the minimum PPE used for the collection of GSR is fresh disposable gloves). Minimise or avoid direct contact with the areas to be sampled (e.g. hands of a human subject, clothing or other object) prior to collection. Use tools to handle items when possible.
Anti-Contamination Procedures (2)
Only open one sampling container at a time to minimize exposure to the environment.
Take samples as soon as possible to avoid loss of possible evidence due to handling or environmental factors.
Properly seal and label the sample tube to preserve the integrity and continuity of the sample, including date, time, location, and collector’s initials.
When the sampling has been completed remove and discard the gloves appropriately in a designated waste container.
Fill out the collection data sheet after the samples have been taken to prevent contamination of the writing materials.
Document any deviations from the instructions and checklist provided, as well as any unusual circumstances.
Note any other information that could be relevant on the kit data / information sheet, such as weather conditions or potential sources of contamination.
Seal the samples and keep the data sheet separate from samples in case it has become contaminated: consider double bagging of the evidence kits as an extra anti-contamination precaution.
Store the evidence (used) kits in a clean area to avoid contaminating the surface of the packaging: isolate the kits from firearms and ammunition.
Consider using separate lockers dedicated to GSR Kits and other items for GSR analysis to ensure segregation of evidence.
Monitor storage areas to show cleaning and procedures are effective, including regular cleaning schedules and environmental controls.
Police Contamination (1)
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, especially after handling firearms.
Consider the implications of the arrest of an armed suspect by FAO, as this can lead to significant GSR transfer.
If unavoidable take comprehensive control samples from the FAO to account for their potential contribution to the GSR.
There will be questions over contamination at court (see Jill Dando case), highlighting the importance of proper procedures and documentation.
GSR: Close Range
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). This provides a larger sample area and more representative results.
The overall residue pattern can be just as important as the chemical composition of the particles, providing information about the distance and angle of the shot.
Dyes can be used to stain the GSR for pattern analysis, enhancing visualization and documentation.
Any samples for chemical analysis should be taken prior to staining to prevent interference from the dyes.
If unsure, swab from one area then recover the remainder of the sample to ensure both pattern and chemical analyses are possible.
GSR: Long Range
GSR directly from the gun will not be present much beyond a few metres from point of shot due to dispersion and environmental factors.
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 to provide information about the ammunition and potential contaminants.
These may also contain contaminants collected prior to impact, affecting the interpretation of the results.
The collection process should ideally be conducted in a laboratory: no on-scene analysis or recovery should be attempted unless it’s unavoidable due to the risk of contamination.
Instead, recover the entire sample to a laboratory if possible to maintain chain of custody and prevent contamination.
GSR Patterns
GSR discharge patterns can be analysed for any firearm and ammunition combination and will be ‘relatively’ consistent at any given range, allowing for estimations of distance.
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 requires careful replication of conditions.
This method is relatively accurate if most conditions are replicated, including firearm, ammunition, and environmental factors.
The ammunition is key though, since different manufacturers use different propellant designs which can affect the pattern.
GSR Discharge Plotting
Discharge plotting is a simple physical test used to create reference patterns.
If possible, use the subject firearm or if not then the same make and model to ensure consistency.
If possible, use the same ammunition from the same manufacturer and with the same specifications.
Ideally from the same production batch, although this may take some research, as batch variations can affect results.
Undergo test firings onto sterile white 0.5 mm thick card placed at varying distances from the firearm.
Vary the range between tests, initially using 0 to 2 metres in 20 cm increments to create a detailed reference set.
Greiss Test
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, indicating a positive result.
It is presumptive, since it can can give a false positive results in some circumstances, such as the presence of other oxidizing agents.
See the case of ‘The Birmingham Six’ case, which highlights the dangers of relying solely on presumptive tests.
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, ensuring accurate results from subsequent analyses.
Sodium Rhodizonate Test
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, indicating a positive result.
The area is then treated with dilute HCl and if the stain turns blue then the presence of lead is confirmed, providing further evidence.
A positive result from this test AND the Greiss test is strong evidence that a firearm has been discharged, particularly when combined with other evidence.
Independently, the two tests do not represent such strong evidence and should be interpreted with caution.
How do we analyse GSR more conclusively?
Currently, chemical analysis of GSR is almost exclusively restricted to the inorganic (metallic) residues, which are most strongly linked to the primer-related residues and are more stable and easily detected.
Inorganic residues can be interrogated using:
Scanning Electron Microscopy-Energy Dispersive X-ray analysis (SEM-EDX), the most widely used and reliable technique.
X-ray Fluorescence (XRF), a non-destructive technique that can provide elemental composition.
Atomic Absorption Spectroscopy (AAS), a sensitive technique for quantifying specific elements.
Far less work is done on the organic components, which come largely from the propellant composition (NC/NG) but can be analysed by:
FTIR, Raman, GC-MS. These techniques are used to identify and characterize organic compounds.
Primer Compositions
Priming compositions vary significantly depending on application and manufacturer but two types of mixtures that can be encountered are:
Mercuric (rather dated now):
Mercury Fulminate, a highly sensitive explosive.
Barium Nitrate, an oxidizer.
Antimony Sulphide, a fuel and frictionator.
“Sinoxid” formulation (most common):
Lead Styphnate (possibly with other lead compounds), the primary explosive.
Barium Nitrate, an oxidizer.
Antimony Sulphide, a fuel and frictionator.
These may contain other additives too (binders, sensitisers, frictionators etc.).
“SINTOX” and other ‘less harmful’, green primers also exist, which use alternative compounds to reduce lead exposure.
Primer Component Functions
Barium nitrate (BaNO_3) acts as an oxidiser to increase the heat of ignition, promoting rapid combustion.
Antimony Sulphide (Sb2S3) acts as the fuel in the ignition process. It also acts a “frictionator” during ignition, enhancing sensitivity.
Mercury Fulminate (Hg(CNO)2) or Lead Styphnate (C6HN3O8Pb) acts as the explosive to drive the ignition process, initiating the chain reaction.
SEM-EDX (1)
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, preserving the sample for potential further analysis.
Minimal sample preparation is necessary, reducing the risk of contamination or alteration.
Individual particles can be analysed, providing detailed information about their composition and morphology.
The morphology of particles can be examined, aiding in differentiation from other materials.
GSR can be identified with a high level of confidence based on the elemental composition and morphology, particularly when characteristic elements are present.
Highly effective and fairly rapid automated systems have been developed for particle detection and classification, improving efficiency and throughput.
SEM-EDX (2)
The electron gun (a heated tungsten filament) fires a beam of electrons at a high voltage through a vacuum, generating a focused electron beam.
Electromagnetic lenses focus the beam onto the surface of an object being studied, allowing for high-resolution imaging.
The finely focused beam is scanned across a sample to cover the area of interest, systematically analyzing the surface.
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, providing information about the sample's surface and composition.
This information is then used to build up the picture of the object we are studying, creating a detailed image and elemental map.
SEM-EDX – Particle Searching
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, allowing for the analysis of larger areas and more particles.
In both manual and automated systems, the search targets particles of high mean atomic number, which are more likely to contain heavy elements associated with GSR.
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 grayscale. This enhances the contrast of heavy elements.
Once a particle with a bright BSE image is detected, it is analysed by EDX to determine its elemental composition.
SEM-EDX – Automated Search
In an automated search, the user sets several parameters that determine the minimum particle size to be detected as well as the area of the sample surface to be searched. This may also include a particular composition for targeted analysis.
The automated search software classifies the composition of each particle analysed and saves the coordinates or position on the sample stub for later review. This allows for efficient data analysis and validation.
Multiple sample analysis is possible with current, automated GSR search systems but be careful of cross-contamination between samples. Implement rigorous cleaning procedures between samples.
Classification of GSR Particles (1)
Particles classified as ‘characteristic of GSR’ have compositions rarely found in particles from any other source. These are highly indicative of a firearm discharge.
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. These require careful interpretation.
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. These should be considered with caution.
SEM-EDX Analysis Output for Single Particles
Modern software systems can direct the instrument to collect GSR-specific information for direct use in evidential reports, streamlining the reporting process.
The example here gives lots of useful information and also classifies specific particles according to their overall composition, aiding in accurate interpretation.
SEM-EDX Analysis Output – Particle Populations
The software will also facilitate the counting of GSR- related particles on a sample stub and display this information according to composition, providing a comprehensive overview of the sample.
This is extremely important so that the evidential importance of each particle and the overall sample can be assessed and presented to the court. Statistical analysis of particle populations can strengthen the evidence.
False Positives for GSR
There are a small number of non-firearm sources of particles having compositions similar to GSR derived from Sinoxid type primers, requiring careful differentiation.
Each of these sources produces a wide variety of particles that are characteristic of the source. Almost all of these particles can be clearly distinguished from residues originating from the discharge of a firearm by their morphology and/or composition. Detailed analysis is crucial.
An extremely small number of particles originating from such sources can be very similar to or indistinguishable from Sinoxid GSR. Statistical analysis of particle populations can help resolve these cases.
However, it is extremely unlikely that, out of all the particles generated by any of these devices, only particles having a similar composition to GSR would be detected – think about the overall population! The presence of additional, non-GSR particles can help identify false positives.
False Positives – Car Air Bags (1)
Many vehicle air bags incorporate a primer device that is mounted in the passenger side dashboard of a vehicle or the steering wheel.
They employ primer initiation to guarantee a sufficient rate of inflation, releasing primer residues upon deployment.
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. The presence of these unusual elements can help distinguish air bag residues from GSR.
Furthermore, many thousands of non-primer particles, characteristic of air bag residues but foreign to GSR, are also expelled from the air bag on deployment. The abundance of these non-primer particles can also aid in differentiation.
False Positives – 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, releasing these elements into the environment.
Some of these particles contain lead, antimony, and barium, usually with a variety of other elements. The presence of these elements can mimic GSR.
Most particles can be clearly distinguished from GSR due to the presence of elements rarely found in GSR, and/or elements present in levels not generally seen in GSR, such as elevated levels of iron. Careful elemental analysis is crucial.
False Positives – Fireworks & Pyrotechnics (1)
Lead, antimony, and barium compounds are rarely found together in fireworks and pyrotechnics, making instances of false positives less common.
However, few uncommon firework products, such as the “Crackering Ball”, can contain all three elements, posing a challenge for differentiation.
Most particles generated from the ignition of firework products contain elements not typically found in GSR, such as magnesium and/or other elements present in levels not generally seen in GSR (for example, elevated levels of chlorine and potassium). These elements can serve as markers for fireworks.
A very small proportion of particles generated, however, have simple three- component (lead, antimony, and barium) compositions. These require more detailed analysis.
False Positives – Fireworks & Pyrotechnics (2)
The morphology of firework particles commonly shows the characteristics of having been formed in very high temperatures. They often have irregular shapes and textures.
However, the rare instances of firework particles found with lead, antimony, and barium have not been found with spheroid morphology, aiding in differentiation from typical GSR.
Pyrotechnic mixtures that contain aluminium powder and barium nitrate (in addition to various other compounds) can give rise to some spherical particles containing barium and aluminium.
Such particles are also commonly generated from firearm ammunition primers rich in aluminium and barium nitrate. These can be particularly challenging to distinguish.
Occasionally, aluminium-barium particles created by pyrotechnics can be indistinguishable to those generated by certain ammunition. Contextual information and additional analyses are crucial in these cases.
Classification of GSR Particles (2)
In addition to main primer elemental components, a range of other elements may be present in ‘major’, ‘minor’ or ‘trace’ amounts, which is judged by how high the EDX spectrum lines are for these additional elements relative to the highest peak in the spectrum:
A MAJOR additional component has a peak height greater than 30% of the highest peak in the EDX spectrum.
A MINOR additional component has a peak height between 10% and 30% of the highest peak in the EDX spectrum.
A TRACE additional component has a peak height less than 10% of the highest peak in the EDX spectrum.
Just note that the above thresholds are usually based on the relative peak sizes for the L-transitions for Barium and any values should all be greater than the limit of detection for the instrument to be considered. Instrument calibration and sensitivity are critical.
“Characteristic” of GSR
“Characteristic” particles must contain:
Lead
Antimony
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
“Consistent with” GSR Particles
“Consistent with” GSR particles will have one of the following compositions:
Barium, Calcium, Silicon (with no more than a trace of Sulphur)
Antimony, Barium (usually with no more than a trace of Iron or Sulphur)
Lead with levels of Antimony greater than trace amounts
Barium, Aluminium
Lead, Barium
“Commonly Associated With” GSR Particles
“Commonly associated with” GSR particles could include:
Lead with only trace levels of Antimony
Lead
Antimony
Barium (in the absence of Sulphur)
These particles may also contain one or more of only the following other elements:
Silicon, Calcium, Aluminium, Copper, and trace amounts of Iron, Sulphur, Phosphorus, Zinc, Nickel (in conjunction with Copper and Zinc), Potassium, Chlorine and Tin.
SWGGSR Guidelines
A good source of information on GSR analysis is set out by the “scientific working group for gunshot residue” or “SWGGSR”.
Their webpage can be found here, which contains many useful reports and guidelines on analysis and court reporting of evidence for forensic practitioners. These guidelines promote standardization and best practices.
Reporting GSR to the Courts (1)
There is no universal reporting format but it must contain clear results/ conclusions. The results must be scientifically accurate and should be written in terms understandable to a layperson. The results/conclusions typically state whether or not GSR was present.
“Two particles containing lead, antimony, and barium were found on the tabs from John Doe. Such particles are residue from a detonated primer of a discharged firearm.”
“Particles containing barium/antimony, lead/barium, or lead/ antimony were found on the stubs from John Doe. Such particles are found in primer residue, but also may originate from other sources.” Clear disclaimers are essential.
Reporting GSR to the Courts (2)
The criteria used to define GSR (e.g. elemental composition and morphology) must be included in the report. Transparency in methodology is crucial.
“The sampling devices were examined by scanning electron microscopy energy dispersive x-ray spectrometry and analysed for elemental composition and morphology of GSR particles.” Describing the analytical techniques used is important.
Reporting GSR to the Courts (3)
If gunshot residue is reported:
The word “unique” is excluded from use in the description of GSR particles, as GSR is not unique to firearms.
When reporting two component particles, it must be stated that these may have originated from a firearm discharge or non-firearms related sources. Proper context is essential.
“Particles containing barium/antimony, lead/barium, or lead/ antimony were found on the lifts from John Doe. Such particles are found in primer residue, but also may originate from other sources.” This statement provides necessary context.
Reporting GSR to the Courts (4)
Conclusions drawn from the identification of GSR on a sample from a person must include wording clarifying that the person discharged a firearm, was in the vicinity of a firearm discharge, or came in contact with something that had GSR on it. All possibilities must be addressed.
“Primer residue can be deposited on the hands by circumstances such as firing a weapon, handling a weapon, being in the proximity of the discharge of a weapon or coming into contact with an object that has primer residue on it. The examination itself cannot determine the relative likelihood of these listed circumstances.” This statement provides a balanced perspective.
Reporting GSR to the Courts (5)
Conclusions drawn from the identification of GSR on a sample from an inanimate object must address the potential that at some time in the history of the item it was in the vicinity of a firearm when it was discharged or came in contact with something that had GSR on it. Historical context is important.
“The presence of primer residue on an item is consistent with that item having been in the vicinity of a firearm when it was discharged or having come in contact with primer residue on another item.” This statement is clear and comprehensive.
If GSR is found on the negative control, this can be disclosed within the report to ensure transparency and account for potential contamination.
Reporting GSR to the Courts (6)
If GSR results are negative, a statement can be included indicating that a negative result could occur even if the subject discharged a firearm, was in the vicinity of a firearm when it was discharged, or came in contact with something with GSR on it. Negative results do not always indicate innocence.
“The absence of primer residue on the hands is consistent with an individual not having fired a weapon. A negative result could also occur from circumstances such as washing the hands, wiping the hands, wearing gloves, sweating profusely, environmental factors including wind and rain, bloody hands, excessive debris on the sample, normal physical activity within 4 to 6 hours passing between firing and sampling, or the weapon not producing primer residue on the hands when discharged.” This statement provides a comprehensive set of possible explanations.
Summary
There are discharge pattern tests and chemical presumptive tests that can be undertaken on scene samples to build general understanding from a scene shooting. These tests provide valuable initial information.
All standard GSR analysis is still undertaken by SEM-EDX to interrogate the inorganic components present, although more research is now being undertaken into the organic component. SEM-EDX remains the gold standard.
Contamination is the biggest issue in GSR analysis. Proper protocols and controls are essential.
You should always consider the particle population (and related chemical compositions) as a whole before considering overall significance of any given outputs from GSR analysis. A holistic approach is crucial for accurate interpretation.