Chemistry:

Forensic Science Ireland
Eolaíocht Fhóiréinseach Éireann

// Science Supporting Justice

Chemistry Analysis:

The chemistry section typically deals with crimes against property and is divided into two teams - the Complex Chemistry Team and the Analytical Chemistry Team. The types of cases encountered fall into a large range of categories such as shootings, hit and runs, arson attacks, burglaries, armed robberies, explosives and cases of criminal damage.

Several types of evidence are recovered from the scenes of these crimes, including:
Paint Analysis

Paint analysis can be very useful in hit and run traffic accidents. The paint on vehicles is made up of several layers starting with a basecoat or primer, followed by several layers of colour and then finally a clear lacquer which is applied to protect the surface. On impact, paint can transfer from one surface to another as flakes or smears.

In a hit and run involving a pedestrian, paint and clothing fibres may even fuse together providing there is enough energy in the impact between a vehicle and the person. Samples are collected from a suspected vehicle by scraping the full depth of layers with a blade. These samples are initially compared to samples taken from a victims clothing using a comparison microscope to determine physical features. Further analysis is carried out by FT-IR.

Brushing for glass

Glass evidence is encountered in cases such as burglaries and hit and run traffic accidents. For example, if a window is smashed to gain entry to a building the majority of the glass breaks inwards but a certain percentage will fragment back towards the burglar. In such cases, the suspect's clothing may be submitted to the laboratory and brushed to collect any glass fragments caught in the clothing.

The fragments are collected (left) and examined under high power magnification. These fragments are then compared to control pieces taken from the window at source by measuring the Refractive Index (RI) of both. The RI is a numerical value based on the angle through which light is refracted on passing through the glass and is based on the chemical composition of the glass itself.

Firearm Residue

Firearm residue (FAR), often referred to as gunshot residue (GSR), is the residue that gets deposited on the hands of a shooter after a bullet or cartridge has been fired. When a gun is fired a series of reactions occur in the barrel releasing a large amount of energy as heat. This heat causes the numerous metallic elements present in the weapon such as the primer, cartridge, bullet and the weapon itself to be vapourised into a cloud. On cooling, several small spherical particles are formed which are invisible to the naked eye. These particles are generally composed of antimony, barium and lead.

The shower of particles deposits on any surface which is close to the weapon at the time of firing such as the hands, face, clothing of the shooter and on the weapon itself. The FAR is lost from the skin within hours but persists on clothing and other surfaces for a lot longer.

Firearm Residue

Samples are collected from the suspect's skin and clothing and are submitted to the laboratory for analysis. The examination of samples for FAR is carried out by scanning electron microscopy – energy dispersive X-ray analysis (SEM-EDX) and is well established and used internationally in forensic science laboratories. A characteristic speherical particle can be seen on the left.

Fibres

Fibre analysis is carried out in cases such as murder, assault, sexual assault, hit and run traffic accidents and burglaries. Fibres can be transferred from a fabric source, such as clothes, to another item. Examination of fibres involves comparison microscopy and analysis using MSP and FT-IR.

Explosive Evidence

The Explosives area in the Chemistry section of the Laboratory has a wide array of instrumentation at its disposal in order to carry out a chemical analysis of a suspect material.

Such instrumentation includes: FT-IR, XRF, GC-MS, GC-ECD, TLC and Ion Chromatography.

Items submitted for analysis are examined for the presence of organic and inorganic explosive substances and other materials related to explosives. Typcial case work items include pipe bombs, hoax devices, suspicious powders, firework compositions and propellant powders.

Footwear Analysis

Impression evidence can be generally defined as “objects or materials that have retained the characteristics of other objects or materials through direct physical contact”. Many forms of impression evidence are encountered in forensic science such as footwear impressions, tyre marks or tool marks.

Footwear impression evidence may be generated when footwear comes into contact with a surface while a person is walking or running, such as a floor or a flowerbed. These impressions are recorded by photography. They may also be removed by preparing a cast using dental stone and/or lifted (e.g. by gel lifting or by electrostatic means) and then compared with test impressions made with the suspect's footwear.

Tyre Cast Analysis

Tyre marks such as those found at the scene of a robbery or hit and run can be compared in a similar fashion. On the left you can see the cast of a tyre mark being overlaid with the print of a tyre from the suspect vehicle. All the grooves line up perfectly.

Analysis involves a wide range of microscopic and chromatographic techniques.