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Trace Evidence > Lecture 9&10 - Fibres > Flashcards

Lecture 9&10 - Fibres Flashcards

1
Q

what is a fibre

A

the smallest unit of yarn or thread used to turn into fabrics and textiles

yarns make fabrics and threads stitch them together

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2
Q

what other evidence type we have looked at are fibres an example of

A

polymer

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3
Q

what does the length of a fibre normally exceed

A

the width

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4
Q

why do fibres become easily dislodged

A

because they are flexible

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5
Q

name some sources fibres may come from

A

clothes, bedding, carpet, curtains, car upholstery, bandages, fibreglass, seatbelts, parachute, loft insulation, cuddly toys, make up brushes or paint brushes

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6
Q

what are the two main categories of fibres

A

natural and synthetic

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7
Q

what is the probability that fibres will be found at a crime scene

A

high probability but because of this their relevance to the investigation will be context dependent

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8
Q

what three sources can natural fibres come from

A

plants
animals
minerals

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9
Q

what is another word for a natural fibre

A

a staple fibre

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10
Q

what is another word for a synthetic fibre

A

a filament fibre

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11
Q

where do plan based natural fibres originate from and
name some sources of them

A

from seeds, stems, leaves or fruit
e.g
cotton
linen
hemp
jute
flax
sisal (agave plant)
coir (around a coconut)

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12
Q

name some sources of animal based natural fibres

A

silk
wool
cashmere
angora (bunnies)
camel

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13
Q

what can animal fibres also be known as (another class of trace evidence)

A

animal hairs

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14
Q

what is the only source of mineral fibre, it is not used anymore

A

asbestos

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15
Q

what is the most common fibre encountered in forensic labs

A

cotton - a natural plant based fibre

cellulose based polymer
plant has pink flowers
raw cotton harvested

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16
Q

what evidence collection equipment also uses cotton so we can’t assume this definitely came from the scene

A

the tip of swabs

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17
Q

what is another type of fibre that is cellulose based but from the stem of the plant under the bark

A

hemp - from the cannabis sativa plant

low THC high CBD content

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18
Q

why is silk different from the other animal fibre types

A

it is made of sericin and fibroin and come from the cocoons of larvae

can also be found in spider webs or from silkworms

doesn’t actually come from the animal fur but it associated with animals

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19
Q

why is silk shimmery

A

because it has a prism like structure which can be useful in forensics and light bounces off prisms and separates light characteristically

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20
Q

what are the benefits of asbestos and why is it now banned

A

benefits
strong, cheap, sound absorbing, fire resistant, insulating

why now banned
CARCINOGENIC
also made of thin fibrous crystals (fibrils) which are thin and brittle and can be inhaled

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21
Q

where may asbestos still be found in a forensic sense

A

in old buildings or building sites

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22
Q

what distinguishes synthetic fibres from natural

A

synthetic have a very high length to diameter ratio as they are human made

they are also likely to have a characteristics end shape due to the process used to make the fibre

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23
Q

name some examples of synthetic fibres

A

polyester
nylon
polypropylene
acrylic
glass fibre
mineral wool

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24
Q

name a type of fibre that is semi-synthetic

A

Rayon - it is derived from wood so has a natural source but the way it is made makes it synthetic

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25
Q

how is a raw polymer converted into fibres

A

via a man made process called spinning where the fibre is pushed through a spinneret

spinneret = has a hole the fibre is pushed through which gives it a characteristic shape

26
Q

with fibres are physical characteristics or chemical composition more useful

how are these properties analysed

A

physical characteristics (fibre shape), if their are faults in the process e.g in the spinneret these show even better individual characteristics good for comparisons

analysed using microscopy

27
Q

what are things to consider when packaging fibres at crime scenes (4)

A

use paper bags to prevent mould growth

don’t package with other scene debris

if possible submit the entire item to the lab - context dependent

remember to take control samples but package these separately

28
Q

fibres can become dislodged quickly after deposition why might this be

A

they can be small and light

emergency service intervention

if outside - wind or other animal interference

29
Q

if an evidence item associated with fibres (clothing for example) if wet, what must be done before the item can be bagged

A

it must be air dried in a controlled environment e.g with a tray underneath to catch anything if it falls

30
Q

for small fibres at crime scenes what method is used to collect them

A

a druggist fold with a label then put into an evidence bag

31
Q

what is the analytical workflow for fibre analysis (generally)

A
  1. gross examination, recovery, collection
  2. preliminary evaluation of physical characteristics
  3. physical fit assessment
  4. microscopic techniques
  5. MSP - microspectrophotometry
  6. IR spectroscopy
  7. Raman spectroscopy
32
Q

what part of the fibre analytical workflow has the most probative value

A

the physical fit assessment - showing two were once one being cut with a certain tool or in a certain way

33
Q

when is the use of IR spectroscopy recommended in the analysis of fibres

A

when manufactured fibres are found

34
Q

what is raman spectroscopy suggested for in the analysis of fibres

A

looking at the dyes of pigments of fibres

35
Q

what is MSP (microspectrophotometry) suggest for is fibre analysis

A

colour determination - the use of UV-Vis light and a value is obtained

36
Q

are the microscopic methods suggested for fibre analysis in the general analytical workflow destructive or non destructive

A

non destructive

37
Q
A
38
Q

when should destructive methods of analysis be used in fibre analysis - suggest by SWGMAT

A

if 100% necessary and non destructive method have all been used

39
Q

Name some fibre features that are important to note during fibre analysis (13)

A

colour
cross section
diameter
texture
chemical composition (polymer type, monomers)
surface treatments (e.g pitting = holes)
dye penetration
length
fading/discolouration
thread count
coating
delustrant (make it appear matte)
yarn twist direction

40
Q

how should the colour of fibres initially be noted

then how after this and why?

A

visually

using MSP, FTIR or raman (despite two fibres visually having the same colour their chemical composition may be different)

41
Q

what might fading in colour of fibre suggest

A

the garment it came from is older or has been washed more regularly

42
Q

how can similarities and differences between spectra be compared

A

by overlaying the two spectra you want to compare

or by using chemometrics

43
Q

what are things that need to be considered when interpreting fibre evidence before saying there is a match

A

class and individual characteristics

fibre rarity

number and location of fibres found

substrate type - affects if fibres adhere well or not

coincidental transfer - secondary/tertiary

nature of contact

background and persistence at the scene

44
Q

what types of fibre are likely to shed more

A

newer fabrics - loosely adhering fibres

old and damaged fabrics

45
Q

what fibres are likely to shed less

A

tightly knit/woven fabric
filament fibres (synthetic) compared to (staple) natural fibres

46
Q

what two types of microscopy are particularly good in fibre analysis

A

comparison microscopy = to compare known and unknown fibres

polarised light microscopy = studying the structure

47
Q

what other types of microscopy can also be beneficial in fibre analysis

A

stereoscopic
fluorescence
brightfield
SEM and TEM
crystallography and XRD
Thermal - destructive though

48
Q

tri-lobal is a shape often seen in fibres, why is this

A

gives fibres strength

often seen in nylon synthetic fibres

49
Q

what is a good way of numerically designating tri-lobal fires to compare them

A

the modification ratio

50
Q

apart from modification ratio, what are three other key microscopic characteristics identified in microscope analysis

A

gas voids - defects on surface or within
dye penetration
crystalline regions - likely to interact with light more and refract it in a certain way

51
Q

what type of microscopy gives higher resolution than light

A

electron

(SEM and TEM)
but takes longer

52
Q

why is SEM good for fibre analysis

A

can reveal surface features not seen in optical microscopy as it scans the surface of the fibre

53
Q

what is something to be wary off when using electron microscopy in fibre analysis

A

beam damage as it can destroy the sample

54
Q

what addition to SEM can give elemental analysis of fibres

A

the use of backspatter

55
Q

what are the two main considerations when deciding what methods to use for fibre analysis

A

time and cost

SEM is good but is it necessary when physical fit has the highest probabtive value

56
Q

using microscopy what two features are useful in identifying fibres

A

refractive index and birefringence

57
Q

how can polarised light microscopy be used in fibre analysis

A

rotating the sample and it will sometimes ‘disappear’ shows what polarisation of light can be transmitted through - likely to be different for different fibre types

58
Q

what is pleochroism

A

a property in anisotropic (different properties in different directions) materials resulting in different absorption colours when the material is exposed to polarised light coming from different directions

59
Q

how can pleochroism be useful in investigations

A

the pleochroism or RI of an unknown fibre can be compared to that of a known fibre giving links between suspects and scenes

60
Q

what is the retardation and how can it be measured

A

when two components of light travel through one material at two different velocities - so one travels faster than the other - the exact distance the slower ray is = RETARDATION

can be measured by rotating the analyser relative to the polariser in polarised microscopy

61
Q

how can we calculate the birefringence of a fibre

A

using polarised microscopy - find the retardation and thickness of the fibre and then calculate the birefringence

62
Q

what two features measured using polarised microscopy of fibres can be used to identify and compare fibre samples

A

retardation and birefringence

comparisons can also be made using the Michel-Levy chart

Trace Evidence (16 decks)

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