LECTURE 4: CHEMICAL CRIMINALISTICS AND TRACES AT THE SCENE

Question 1

Physical/trace evidence

Answer 1

• Manufactured materials & residues foreign to a crime scene/ victim that may be associated with a suspect or other crime scenes(s)

Question 2

Physical/trace evidence types

Answer 2

• Volatile compounds/traces – fire accelerants, explosives, personal defence sprays, cosmetics, lubricants
• Non-volatile compounds/traces – fibres, paint, rubber, plastic, metal, glass, soil
• Very small particles – gunshot residues
• Volatile organic chemicals – fire accelerants, explosives, pepper sprays, cosmetics and lotions, toxic chemicals (e.g. chemical warfare agents)

Question 3

Gas chromatography – mass spectrometry

Answer 3

• Separation of volatile organic compounds
• Volatile – when heated, VOCs undergo a phase transition into intact gas-phase species
• Separation occurs as a result of unique equilibria established between the solutes and the stationary phase (the GC column)
• An inert gas (N2, He) carries the solutes through the column

Question 4

Spectroscopy

Answer 4

• The study of the interaction of matter with the electromagnetic spectrum
• Electromagnetic radiation displays the properties of both particles and waves
  o The particle component is called a photon
  o Small, mass-less particle that contains a small wave-packet of EM radiation/light
• The energy (E) component of a photon is proportional to the frequency. Where h is Plancks constant and v is the frequency in Hertz:

E= hv

• Because the speed of light, c, is constant, the frequency, v, can complete in the same time, must be inversely proportional to how long the oscillation is, or wavelength

Question 5

Infrared spectroscopy

Answer 5

• As a covalent bond oscillates – due to the oscillation of the dipole of the molecules – a varying electromagnetic field is produced
• When a wave of infrared light encounters this oscillating EM field generated by the oscillating dipole of the same frequency, the two waves couple, and IR light is absorbed
• Each stretching and bending vibration occurs with a characteristic frequency as the atoms and changes involved are different for different bonds
• Primary use of the IR is to detect functional groups
• The x-axis of the IR spectrum is in units of wavelengths, v, which is the number of waves per centimetre in units of cm^-1

Question 6

Accelerant analysis

Answer 6

• Accelerant – anything used to intentionally start or spread a fire, often an ignitable liquid
• Ignitable liquid – and liquid capable of fuelling a fire; this includes flammable and combustible liquids, or any other material that can be liquefied and burned
  o Defined based on flash points
• Flash point – the minimum temperature to which a liquid must be heated for vapours emitted to ignite momentarily in the presence of a flame
  o Flammable liquid <38 degrees Celsius (petrol -43 degrees Celsius)
  o Combustible liquid >38 degrees Celsius (diesel 52 degrees Celsius)