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Chemistry for the Analysis of Trace Evidence > Fibres > Flashcards

Fibres Flashcards

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1
Q

What are fibres?

A
  • basic unit of yarns and threads which then turn into fabrics, garments, textiles
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2
Q

Where are fibres found?

A
  • clothing
  • bedding
  • carpets
  • curtains
  • bandages
  • fibreglass
  • loft insulation
  • seatbelts
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3
Q

What are natural fibres?

A
  • can be plant, animal or mineral based
  • also known as staple fibres
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4
Q

What are some examples of natural plant fibres?

A
  • cotton
  • linen
  • hemp
  • coir
  • flax
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5
Q

What aresome examples of natural animal fibres?

A
  • silk
  • wool
  • cashmere
  • camel
  • angora
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6
Q

What is an example of mineral fibre?

A

asbestos

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7
Q

What is cotton made from?

A

cellulose (polymer)

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8
Q

What is silk made from?

A

sericin/fibroin (polymeric protein)

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9
Q

Why is silk shimmery?

A

due to prism-like structure

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10
Q

Asbestos

A
  • naturally occuring silicate material
  • carcinogenic
  • sound absorbing, strong, cheap, fire resistant, electrical insulating
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11
Q

What are synthetic fibres?

A
  • also called filament fibres
  • longer than natural fibres
  • polymer with very high length to diamter ratio
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12
Q

What are some examples synthetic fibres?

A
  • polyester
  • nylon
  • acrylic
  • polypropylene
  • glass fibre (made from silica which is natural but process of making it isnt)
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13
Q

How are synthetic fibres made?

A
  • extruded through a spinneret device - polymer is pushed through tiny holes that are differently shaped
  • fibres spun into bundles called filaments
  • can alter the characteristics with different spinning techniques
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14
Q

What are the recovery considerations for fibres?

A
  • may become dislodged quickly after deposition
  • air dry wet clothing in controlled environment
  • store in paper bags to prevent mould growth
  • double package
  • submit whole item
  • druggists fold for small fibres
  • never package with debris from the scene
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15
Q

What is the analytical workflow for fibres?

A
  1. gross examination, recovery and collection
  2. preliminary evaluation of physical characteristics
  3. physical fit assessment - most probative value
  4. microscopic techniques
  5. microspectrophotometry (UV-Vis) - colour determination
  6. infrared spectroscopy - manufactured fibres
  7. Raman spectroscopy - dyes and pigments
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16
Q

What are some non-routine techniques that arent recommended and are only used after everything else has been exhausted?

A
  • thin layer chromatography
  • pyrolysis GS-MS
  • HPLC
  • melting point
  • microchemical tests
    ALL DESTRUCTIVE METHODS
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17
Q

What are some things to look for when analysising fibres?

A
  • surface treatments
  • dye penetration
  • diameter and length
  • cross-section
  • chemical composition (type, polymer)
  • texture
  • colourant/dye - colour, type, how applied, weathering
  • natural or synthetic and type
  • striations (lines down the fibre) and pitting (holes/damage)
  • direction of yarn twist
  • threat count
  • coatings
  • scale protrusion on animal fibres
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18
Q

What does IR/Raman look at on fibres?

A
  • colour - different dyes/pigments
  • chemical composition
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19
Q

What does dye uptake depend on with fibres?

A
  • when the fibre is dyed during the manufacturing process - before, after being spun or after garment construction
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20
Q

What does the transfer of fibres depend on?

A
  • nature of contact
  • multiple association mitigate coincidental transfer
  • new fabrics possess loosley adhering fibres
  • old/damaged fabrics may shed more
  • tightly fibres shed less than staple fibres
  • background and persistence considerations
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21
Q

What are the issues with fibres as trace evidence?

A
  • cant state that it is unique
  • few databases for origin
  • often overlooked as difficult to locate
  • expensive, time-consuming skilled analysis
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22
Q

What some microscopic techniques for fibres?

A
  • stereoscopic
  • comparison
  • polarised
  • fluorescence
  • brightfield
  • thermal (destructive)
  • SEM and TEM
  • crystallography and diffraction
23
Q

What are some physical fit and gross characteristics in fibres?

A
  • surface treatments
  • dye penetration
  • diameter
  • chemical composition
  • cross section
  • texture
  • colourant/dye - colour, type, how applied, weathering
24
Q

What is a microscopic technique of cotton?

A

twisting shape

25
Q

What are some microscopic characteristics of flax?

A
  • nodes
26
Q

What are some microscopic characteristics of hemp?

A

no nodes but irregularities

27
Q

What are some microscopic characteristics of jute?

A
  • nodes
  • tapering on the ends and is more narrow
  • colouration changes as it is rotated in polaried light
28
Q

What is a microscopic characteristic of silk?

A

ribbon shape

29
Q

What is a microscopic characteristic of wool?

A

scales

30
Q

What is a microscopic characteristic of synthetic fibres?

A
  • longtiudinal appearance - texture, crimping, pigment (can be unique to manufacturing process or machine)
  • cross-sectional appearance - cut and observe ((can be unique to manufacturing process or specifically engineered)
  • dye penetration
  • presence of crystalline regions - will refract light
  • gas voids
31
Q

What is the most common shape in nylon synthetic fibres?

A

tri-lobal

32
Q

How do we characterise nanostructures of the surface or bulk?

A
  • surface: microscopy (SEM, TEM, atomic force)
  • bulk: diffraction (X-ray power, optical)
33
Q

What are the benefits of electron microscopy for analysing fibres?

A
  • non-destructive analysis - beam damage can occur for sensitive samples
  • rapid accumulation of results
  • can give elemental composition
  • higher resolutions are achieved by use of electrons instead of light
34
Q

Light vs SEM vs TEM

A
  • light: low depth of focus, good field of view, easy and rapid, cheap, worst resolution
  • SEM: high depth of focus, good field of view, easy and rapid, quite expensive
  • TEM: medium depth of focus, limited field of view, skilled and slow, very expensive, best resolution
35
Q

What does SEM do for fibre analysis?

A
  • elemental analysis
  • surface features like scales
  • fibre-end fracture morphology
  • even more expensive
36
Q

What are the two types of microscopic analysis?

A
  • refractive index
  • birefringence
37
Q

What is refractive index?

A
  • when two refractive indices are equal, the light passing through the particle does not deviate at all and the particle remains invisible
  • when the refractive indicies are far apart, the light passing through will change diretion substantially
  • if refracted sufficiently, they miss the objective lens and these areas of the particle become dark, resulting in high contrast
38
Q

How is the refractive index measured?

A
  • Becke Line Test
39
Q

What is the Becke Line Test?

A
  • refracting light depends on the relative RI values of the particle and mounting medium
  • a particle with a higher RI mounted on a medium or lower RI, will focus axial illuminating rays towards a point above the particle
  • lower RI particle in a higher RI medium will direct light in opposite direction, moving the line outside the particle
40
Q

How are Becke Line immersions measurements made?

A
  • mounting the substance in a media of varying RI’s until little change is observed
41
Q

What is the limitation of the Becke Line Test?

A

will only be true for one wavelength of light at a time - wavelength dependent
* need a more variation method for a precise measurement

42
Q

Normal light vs linearly polarised light

A
  • normal: waves vibrating in every direction perpendicular to the direction of travel
  • polarised: waves vibrating in one direction
43
Q

What is anisotropic?

A

different physical properties in different planes

43
Q

Why and how is a polarised light microscope used?

A
  • light will go through a polariser to become plane polarised light
  • as the plane polarised light passes through the sample if it is crystalline or anisotropic it will change the light as only particular polarisations of light will be transmitted
44
Q

What is a uniaxial material?

A

allows rays to vibrate in two axes, ω (blue) and ε′ (red) depending on it’s orientation

45
Q

How to measure refractive index in uniaxial materials when the light has been polarised so that it is vibrating in the E-W plane?

A
  • when the epsilon (ε′) plane is aligned with the E-W polarised light use Becke line or variation methods- RI that we measure will correspond to that physical feature of that crystalline structure
  • when the omega (ω) plane is aligned with the E-W plane use Becke line or variation methods
  • in between the two orientations there is an intermediate where there is a linear response or where they look very similar in terms of the RI
46
Q

What is pleochroism?

A
  • property in anisotropic materials
  • causes the sample to show different absorption colours when exposed to polarised light coming from different directions
  • can see the difference of the omega plane and epsilon plane
47
Q

What is retardation?

A

when full white light passes through a particle, it wont pass through at the same speed, the slow ray is said to be more retarded than the other
* the exact distance that the slow ray falls behind is the retardation

48
Q

What is birefringence?

A
  • full white light is shone at a particle
  • one light ray will be more retarded than the other (slower)
  • this will cause a change in colour from the white light
49
Q

How are the velocities of the two rays determined in birefringence?

A

by the values of ω and ε′

50
Q

What is the equation to work out the birefringence of a material?

A

R (nm) = B x T (um) x 1000 (nm/um)
* R = retardation
* B = birefringence
* T = thickness

51
Q

How is the retardation measured?

A

by rotating the analyser relative to the polariser

52
Q

How many polarised rays does light split into when passing through a birefringent material?

A

2

Chemistry for the Analysis of Trace Evidence (16 decks)

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