Lecture 21 - Advanced Evidence Types Part 4

Question 1

Fluorescence

gemstones

Answer 1

Gemstones are exposed to the short or long wave UV radiation, they emit the visible light.

Question 2

Phosphorescent

gemstones

Answer 2

If the luminescence continues after the exposure to UV radiation - Due to impurities and defect in the crystal lattice which is a key identifying feature.

Question 3

X-ray diffection is used

Answer 3

• X-ray Diffraction (XRD) is used to establish the arrangement of atoms within a crystal structure and how they stack together.
• or parallel planes of atoms, with a space (d) between the planes, constructive interference only occurs when Bragg’s law is satisfied.
• if there are planes in the crystalline lattice the reflectance and positions of peaks gives us information.
• We can control the wavelength, change the angle by titlying the sample or moving the
  we can see defects which gives identifying information about gemstones.

Question 4

X-ray diffrection can determine…

Answer 4

• Lattice parameters - By indexing the position of the peaks (Giving information on alloying, doping or even strain in the material)
• Phase composition of the sample - Given by the relative amounts of overlaid diffraction patterns (Giving compositional information)
• rystal structure - By refining the whole diffraction pattern (Giving texture and orientation of crystals in the bulk)
• Crystallite Size - By looking at peak broadening (Giving even more bulk structural information of the material)
• Able to identify impurities and defects in the crystal lattice of gemstones.
• Crystallites smaller than ~120nm create broadening in diffraction peaks.
• Scherrer equation enables the average size of nanocrystals to be calculated (if no microstrain)

Question 5

Birefringence

Gemstones

Answer 5

Most significant optical property used for the identification of gemstones.

Question 6

Limitation of XRD for gemstones

Answer 6

We can’t see gemstone inclusions

Question 7

Optical analysis

Isotropic substance

Answer 7

• If an isotropic substance is illuminated with normal incidence to the crystal face, all light passes through without deviation.
• Snell’s Law still applies as sin0 = 0
• Velocity changes but not the direction of light due to snells law.
• RI is measured in the usual way.
• Due to the structure it is aligned in any direction, it doesn’t change when the angle of light passes through it.

Question 8

Opticak analysis for isotropic substances

Snells law equation

Answer 8

(Sinθ₁ / Sinθ₂) = n₂₁ = (n₂ / n₁)

Question 9

Optical analysis - unaxial substance

Answer 9

• More complicated
• Light incident on the cleavage face of the crystal will break into two separate rays as the light travels through the crystal.
• The waves of the two rays vibrate in planes perpendicular to each other and travel through the crystal at different velocities.
• The faster ray emerges from the crystal slightly ahead and in a different location to the slower ray.
• The light divies into two directions some light changes direction and the rest of the light carries on passing through.
• The two different paths are going to have different velocity.
• There are two visions as a direct conseqeunce of the light splitting
• Quartz in uniaxia
• One ray of light obeys snell law but the other ray doesn’t.

Question 10

Optical analysis - Uniaxial substance

Rays

Answer 10

• The two rays are known as the ordinary ray (O) and the extraordinary ray (E) - the full optical physics explanation behind this phenomena is beyond this course but relates to the polarisability of ions in different directions in the crystal lattice.
• The two rays vibrate in different planes through the material and have different RIs
• Birefringence is the difference between these two refractive indices.
• As the polarisation of light changes through these materials, this can be measured.
• The faster ray emerges from the crystal slightly ahead and in a different location to the slower ray.
• Because they vibrate in different ways it gives it two different RIs.

Question 11

Retardation

Uniaxial substance

Answer 11

• In a uniaxial substance, the two components of light travel through the crystal at two different velocities (exact velocities determined by the values of ω and ε′).
• Birefringence (B) is the difference between these two refractive indices.
• the difference between the speed of light between the two RI is the retardation factor.
• as we can control the polarisation we can measure the polarisation coming out of the side to measure the birefringence.

Question 12

Examples of meetals as trace evidence

Answer 12

• Rust and metal shavings transferred to a person or clothing.
• Metal weapon used in an assault leaving trace in the wound.
• Metal filings on a pipe wrench after a twist attack on a doorknob.
• Metal from a crowbar used to force open a window or door.
• Gunshot residue
• Explosive residue
• Fireworks and sparklers
• Lightbulb filaments

Question 13

Primary techniques used for the analysis of metal trace evidence?

Answer 13

• SEM-EDX
• XRF
• HPLC and GC-MS
• LA-ICP-MS
• AAS
• Colour spot and presumptive chemical tests also regularly used (particularly for GSR).

Question 14

Inorganic GSR

Answer 14

• Mainly from the primer mixture, some from cartridge case.
• Historically heavy metal components, although toxic metal free primer more common now.

Question 15

Organic GSR

Answer 15

• Mainly from the smokeless powder, composed of primary explosives, stabilisers, plasticisers, sensitisers and flash inhibitors.
• Combustion products also found on spent cartridge and firearm muzzle.

Question 16

GSR Detection

Optical methods

Answer 16

Optical Methods - Organic compounds identified due to strong IR-luminescence. Methods non-destructive, rapid but low specificity and high LoD.

Question 17

GSR detection

Spectrometry

Answer 17

Spectrometry - Primarily identify inorganic components. SEM-EDX most common as it is quantitative, gives highest compositional and morphological detail, but takes time.

Question 18

GSR detection

Chemographic testing

Answer 18

Chemographic Testing - Invasive, but can be performed at crime scene. Most target inorganic elements with presumptive test. High selectivity, but only qualitative with risk of false positives.

Question 19

GSR Detection

Separation methods

Answer 19

Separation Methods - Particularly coupled with mass spectrometry e.g. LC/GC-MS/MS. Invasive method but reliable, specific and has a low LoD.

Question 20

Detecting metals in latent fingermarks

Answer 20

• Particularly of interest to link activity with specific metal objects e.g. guns or explosives
• X-ray Fluorescence Microscopy could enable this - needs large and expensive synchrotron to access this data with a high enough spacial and spectrographic resolution.
• Able to identify differences between handling gun barrel and ammunition cartridge above background activity.

Question 21

Explosives trace evidence

Answer 21

• Use derivatives of ion mobility spectrometry
• IMS - separates and identifies ionised molecules present in the the gas phase based on their mobility in a carrier buffer gas.
• Builds on the concepts of mass spectrometry - controlled ionisation and separation based on an ions interaction with an electric field to enable detection and identification.
• Measurements can occur in milliseconds, which makes it great for usage in situations where fast, reliable results are required e.g. airports!!
• Techniques including GC/LC, SEM, FTIR, Raman & other spectrometric methods used to elsewhere to analyse the organic and inorganic components.

Question 22

Fireworks as trace evidence

Answer 22

• Paper, plastic, explosive and metals all in one.
• Sparklers are particularly interesting as commonly used to initiate inorganic homemade explosives.
• Explosives analysis techniques used to great effect to detect and identify a sparkler, they aren’t enough to distinguish between brands of sparkler.