How Science Catches Criminals

JOHN HOLDENtalks to Dr John O'Shaughnessy from the chemistry section of the Forensic Science Laboratory about five ways chemistry helps catch criminals

‘EVERY CONTACT leaves a trace.” Who’d be a criminal? The level of forensic evidence now being used by An Garda Síochána to help solve crimes should leave any bad guy reconsidering his career options.

First some background. The forensics lab is divided into four sections – chemistry, biology, DNA and drugs. Chemistry deals with crimes against property, DNA/biology tend to work on crimes against the person, and the drugs section analyse controlled substances.

Dr John O’Shaughnessy gives us a few different ways forensic scientists link the suspect, the crime scene and the injured party.

GLASS

“When a window is broken, approximately 80 per cent of the glass will fall away from the person, as in the direction of the blow. But about 20 per cent falls back in the opposite direction. Tiny, minute fragments of glass can deposit on their clothing so when a suspect is arrested after a crime, where glass has been broken from a car or from a house window, the suspect’s outer clothing will be seized, packaged and sealed and after submitting to the lab is brushed down to recover microscopic fragments of glass. A control sample of the glass taken from the actual windowpane will also be taken. The fragments of glass are recovered under a microscope.

“The refractive index (RI) – a physical property of the glass – of the recovered glass fragments is measured. It’s a numerical measurement of the angle by which light is bent as it passes through glass. We can measure the RI of recovered glass from garments and compare it to the control samples.”

PAINT

“There are many different types of paint and case samples come from vehicles, buildings, graffiti and so on. Paint is often used as evidence in instances such as hit and run car accidents. If a car hits somebody, you can get a transfer of paint onto the injured party clothing and this can be recovered to try and link it to a vehicle. The transfer will depend on a number of things though – degree of force, the condition of the paint, the duration of contact between two objects and the nature of contact. But we can work with tiny samples. Back in the lab we carry out a comparison between the control samples and the case samples. We use a comparison microscope so that we can look at both samples at once.”

FIREARM RESIDUE

“This comes from the discharge of a weapon – a shotgun, rifle, handgun and so on. When you pull the trigger of a gun, the firing pin strikes the base of the bullet and an explosive mixture is ignited, which in turn ignites the propellant powder in the cartridge case and very quickly and at very high temperatures generates a large volume of gas which propels the bullet down the firearm barrel. After the gas expands, it cools and you get molten metal particles invisible to the naked eye that condense, which deposit on the weapon and surrounding surfaces – the shooter’s hands and clothing. This gives rise to classic ‘3-component’ particles, characteristic of firearm residue – usually containing lead, antimony and barium. In the lab we have a scanning electron microscope to examine firearm residue. It has two main parts – an X-ray detector and a microscope. The microscope gives us a picture – the ideal particle would be spherical in shape – while the X-ray detector tells us its elemental composition.”

EXPLOSIVES

“We’re not ordinance experts in the lab but we are experts in the chemical analysis of the explosive ingredients from viable devices. The EOD (bomb squad) will make the device safe and – once they’ve done that – An Garda Síochána submits it to the lab for examination. Typically explosive compositions consist of either an organic explosive like TNT or Semtex, or inorganic substances such as potassium perchlorate or ammonium nitrate and can be mixed with other additives such as fuel oil or sulphur.

“All devices are then sent for ‘minitape lifting’. A small piece of acetate with a sticky end is run along the device to try and gather up any sources of DNA (for example, skin cells). The device is then forwarded to the fingerprint section in the Garda Technical Bureau, while the minitape lift is forwarded to the DNA section.”

DNA

“DNA is unique to each individual (except identical twins), regardless of the source (blood or skin) so they can be useful in identifying persons of interest. Exhibits typically processed for DNA profiling come from a variety of sources across different types of casework. Skin cells from the trigger of a gun, blood from the injured party on a knife found in possession of a suspect, and cells from saliva on a cigarette butt left at a scene are good sources of DNA.

“The purpose of DNA work is to generate DNA profiles from items left at crime scenes that can then be compared to reference samples from suspects. For example, during a burglary a suspect may cut themselves while breaking a window. A swab of blood left at the scene or gloves found near the scene can be sampled for DNA. DNA profiles generated can then be used to implicate or exonerate someone during an investigation.

“Forensic science is important but it’s just one part of the investigation. It’s never just one piece of evidence that solves a crime. It’s the combination of CCTV, DNA and all sorts of different forensic evidence types and good police investigative work. Sometimes though, forensic science can provide that crucial piece of the puzzle that makes an investigation.”