Forensic Science - Chemical Evidence

What is ink mainly made of?

Mainly made up of dye/pigment, solvent and resin.

How is ink collected?

• It is collected from the crime scene in a tamper evidence bag.

• In the lab, punch a small area with ink on.

• Solvent dissolving ink is added.

• Carry out TLC.

What is TLC? And how is it carried out?

• Thin layer chromatography.

• A small spot of ink is placed on a plate coated in silica gel.

• The plate is then placed upright in a jar with a solvent at the bottom.

• The solvent moves up the plate through capillary action, carrying the dyes with it.

• Different dyes move at different speeds and so they separate into distinct coloured spots.

• The retention factor then needs to be calculated.

What is the retention factor?

How far the component moved from the spotted point divided by how far the solvent moved up the plate.

Each substance has its own Rf value under the same conditions. 

If a spot in a mixture has the same Rf as a known sample, they are likely the same substance.

What is gunshot residue?

A cloud of particles given off when a gun is fired. It can sometimes be microscopic. 

GSR can tell us firing distance, bullet entrance & exit wounds, whether a suspect fired a gun and if there’s a link between a weapon and a gunshot entry wound.

What is GSR composed of?

Propellant, lubricants, metal from the bullet/cartridge case, primer particles.

How do you recover GSR?

It can be recovered using tape lifts, vacuum lifts, swabbing or glue lifts.

It can be found on skin, hair, clothes, vehicles, windows etc.

How do you analyse GSR?

Colour or spot tests such as the Griess Test (nitroglycerine).

Separation techniques can be used such as: gas chromatography, high-performance liquid chromatography or capillary electrophoresis.

Scanning electron microscopy with energy dispersion x-ray detection can also be used (SEM-EDX).

What is SEM-EDX?

• Scanning electron microscopy with energy dispersion x-ray detection.

• It is the most reliable analysis method.

• Examines morphology and chemical composition.

• Analysing GSR consists of: elemental composition and microscopically examined for their shape.

• Studying their shape and composition can confirm the particles came from a fired gun, and uniquely identify them as GSR.

What factors can affect the recovery of GSR?

• Suspect’s activity after the shooting.

• Quality & efficiency of method.

• Distance from the firearm at the point of discharge.

• Human skin on hands.

What are explosives? And name a few types.

Explosives are a very high speed chemical reaction.

Examples are:

• Pyrotechnics (fireworks, flares).

• Blasting explosives (TNT, dynamite).

• Initiators.

Where can you find explosive residue?

• On the hands of people handling them.

• In places where they’ve been prepared, manufactured or stored.

• On items recovered from an explosion site.

What are some key precautions for handling bulk explosives?

• Prevent sparks & dust (treat the area as electrostatic and sensitive to ignition).

• Earthing & grounding (bench, tools and personnel should be grounded to dissipate static).

• Protective bench surfaces (rubber bench coverings to reduce friction & static).

How do you collect trace explosives?

• Swab hands, clothing, surfaces and components from crime scene to collect residue.

• Process samples in a clean, controlled environment to prevent contamination.

• Place swabs in clean, sealed containers.

• Document where each swab came from.

Name some recovery techniques for explosive residue.

• Swabbing.

• Solvent washing.

• Contact heater.

• Vacuuming.

Name some analytical techniques used for explosive evidence.

• Colorimetric tests (presumptive).

• Ion mobility spectrometry (IMS).

• HPLC or GC-MS (gas chromatography-mass spectrometry).

• Microscopy (light and SEM).

• Infrared spectroscopy.

Why is it important to take control swabs from a crime scene?

It helps prevent contamination from:

• The swabs themselves.

• Lab surfaces.

• Reagents.

• Analysts PPE (gloves, disposable suits).

What happened in the Birmingham Six case?

• November 21st 1974.

• Two IRA bombs exploded in 2 separate pubs in Birmingham.

• 21 people killed.

• 6 Irish men were arrested and charged based on forensic evidence and confessions obtained under police brutality.

• The IRA denied the 6 men were members.

• The Griess test was used to show the men handled the explosives.

• This test was then found to be unreliable and not specific for nitroglycerine.

• Poor analysis records weakened the forensic evidence and after 16 years their convictions were overturned.

When examining a fire scene, what needs to be done?

• Treat it as a crime scene.

• Gather intelligence.

• Secure, preserve and record appropriately.

• Recover items of interest and analysis for lab.

• Find the seat of the fire.

• Establish causes.

How can we tell a fire was intentionally started using a firestarter?

• Evidence of the flammable liquid can be found.

• Localised burn patterns (often on carpet with sharp, curved edges).

• Can smell the liquid.

Name 4 common accelerants.

• Petrol.

• Paraffin.

• White spirit.

• Lighter fluid.

Where can accelerants be found at a crime scene?

• On fire debris from the scene.

• On suspect’s clothes.

• In a container containing the accelerant.

What method is used to analyse accelerants?

GC-MS (gas chromatography - mass spectrometry).

Different accelerants can produce different characteristic patterns of peaks on chromatogram.

Should a control be taken at a fire scene?

Yes, it’s important to collect a control sample. 

Initially a sample with liquid on it will be taken, but it’s important to take one with no liquid on it.

This gives confirmation that the peaks on the chromatogram have come from an accelerant not just the carpet.

Why is it important to know if a vehicle’s lights were on or off in a collision?

It can affect how the crash is interpreted in an investigation.

What metal is used in traditional headlight bulbs?

Tungsten.

What happens to tungsten when it gets hot?

It becomes brittle.

How can you tell if a bulb was on during a crash?

The filament shows oxidation.

How can you tell if a bulb was off during a crash?

The filament looks cleanly snapped with no oxidation.

Why are LED lights harder to examine?

They don’t have tungsten filaments.

Name 3 class A drugs.

• Cocaine.

• Heroin.

• MDMA.

Name 3 class B drugs.

• Cannabis.

• Ketamine.

• Powdered amphetamine.

Name 3 class C drugs.

• Anabolic steroids.

• Buprenorphine.

• Counterfeit Viagra.

How should plant-based drugs be collected?

In a brown paper evidence bag.

How should non-plant drugs be collected?

In a tamper-evident bag.

How are biohazardous drug samples stored?

In a tamper-evident bag, clearly labelled as a biohazard.

What details should be recorded about the drugs?

• Appearance (powder, waxy, crystalline, colour).

What are the 2 main stages of drug testing?

• Screening (spot tests).

• Confirmation (instrumental analysis).

Why is screening done before analysis?

To help guide which confirmatory tests are needed.

What drug does the Cobalt Thiocyanate test detect?

Cocaine.

What drugs does the Marquis test detect?

Amphetamines and opioids.

What is Ehrlich’s test used for?

LSD detection.

Which part of the plant is examined for cannabis identification?

The female flowering head (bud).

What are the curved, hair-like structures on the surface of cannabis called?

Cystolithic trichomes.

What technique is used to confirm drug identity?

GC-MS (gas chromatography-mass spectrometry).

What does GC-MS do?

Separates and identifies drugs.