Lecture 9&10 - Fibres Flashcards
what is a fibre
the smallest unit of yarn or thread used to turn into fabrics and textiles
yarns make fabrics and threads stitch them together
what other evidence type we have looked at are fibres an example of
polymer
what does the length of a fibre normally exceed
the width
why do fibres become easily dislodged
because they are flexible
name some sources fibres may come from
clothes, bedding, carpet, curtains, car upholstery, bandages, fibreglass, seatbelts, parachute, loft insulation, cuddly toys, make up brushes or paint brushes
what are the two main categories of fibres
natural and synthetic
what is the probability that fibres will be found at a crime scene
high probability but because of this their relevance to the investigation will be context dependent
what three sources can natural fibres come from
plants
animals
minerals
what is another word for a natural fibre
a staple fibre
what is another word for a synthetic fibre
a filament fibre
where do plan based natural fibres originate from and
name some sources of them
from seeds, stems, leaves or fruit
e.g
cotton
linen
hemp
jute
flax
sisal (agave plant)
coir (around a coconut)
name some sources of animal based natural fibres
silk
wool
cashmere
angora (bunnies)
camel
what can animal fibres also be known as (another class of trace evidence)
animal hairs
what is the only source of mineral fibre, it is not used anymore
asbestos
what is the most common fibre encountered in forensic labs
cotton - a natural plant based fibre
cellulose based polymer
plant has pink flowers
raw cotton harvested
what evidence collection equipment also uses cotton so we can’t assume this definitely came from the scene
the tip of swabs
what is another type of fibre that is cellulose based but from the stem of the plant under the bark
hemp - from the cannabis sativa plant
low THC high CBD content
why is silk different from the other animal fibre types
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
why is silk shimmery
because it has a prism like structure which can be useful in forensics and light bounces off prisms and separates light characteristically
what are the benefits of asbestos and why is it now banned
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
where may asbestos still be found in a forensic sense
in old buildings or building sites
what distinguishes synthetic fibres from natural
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
name some examples of synthetic fibres
polyester
nylon
polypropylene
acrylic
glass fibre
mineral wool
name a type of fibre that is semi-synthetic
Rayon - it is derived from wood so has a natural source but the way it is made makes it synthetic
how is a raw polymer converted into fibres
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
with fibres are physical characteristics or chemical composition more useful
how are these properties analysed
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
what are things to consider when packaging fibres at crime scenes (4)
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
fibres can become dislodged quickly after deposition why might this be
they can be small and light
emergency service intervention
if outside - wind or other animal interference
if an evidence item associated with fibres (clothing for example) if wet, what must be done before the item can be bagged
it must be air dried in a controlled environment e.g with a tray underneath to catch anything if it falls
for small fibres at crime scenes what method is used to collect them
a druggist fold with a label then put into an evidence bag
what is the analytical workflow for fibre analysis (generally)
- gross examination, recovery, collection
- preliminary evaluation of physical characteristics
- physical fit assessment
- microscopic techniques
- MSP - microspectrophotometry
- IR spectroscopy
- Raman spectroscopy
what part of the fibre analytical workflow has the most probative value
the physical fit assessment - showing two were once one being cut with a certain tool or in a certain way
when is the use of IR spectroscopy recommended in the analysis of fibres
when manufactured fibres are found
what is raman spectroscopy suggested for in the analysis of fibres
looking at the dyes of pigments of fibres
what is MSP (microspectrophotometry) suggest for is fibre analysis
colour determination - the use of UV-Vis light and a value is obtained
are the microscopic methods suggested for fibre analysis in the general analytical workflow destructive or non destructive
non destructive
when should destructive methods of analysis be used in fibre analysis - suggest by SWGMAT
if 100% necessary and non destructive method have all been used
Name some fibre features that are important to note during fibre analysis (13)
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
how should the colour of fibres initially be noted
then how after this and why?
visually
using MSP, FTIR or raman (despite two fibres visually having the same colour their chemical composition may be different)
what might fading in colour of fibre suggest
the garment it came from is older or has been washed more regularly
how can similarities and differences between spectra be compared
by overlaying the two spectra you want to compare
or by using chemometrics
what are things that need to be considered when interpreting fibre evidence before saying there is a match
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
what types of fibre are likely to shed more
newer fabrics - loosely adhering fibres
old and damaged fabrics
what fibres are likely to shed less
tightly knit/woven fabric
filament fibres (synthetic) compared to (staple) natural fibres
what two types of microscopy are particularly good in fibre analysis
comparison microscopy = to compare known and unknown fibres
polarised light microscopy = studying the structure
what other types of microscopy can also be beneficial in fibre analysis
stereoscopic
fluorescence
brightfield
SEM and TEM
crystallography and XRD
Thermal - destructive though
tri-lobal is a shape often seen in fibres, why is this
gives fibres strength
often seen in nylon synthetic fibres
what is a good way of numerically designating tri-lobal fires to compare them
the modification ratio
apart from modification ratio, what are three other key microscopic characteristics identified in microscope analysis
gas voids - defects on surface or within
dye penetration
crystalline regions - likely to interact with light more and refract it in a certain way
what type of microscopy gives higher resolution than light
electron
(SEM and TEM)
but takes longer
why is SEM good for fibre analysis
can reveal surface features not seen in optical microscopy as it scans the surface of the fibre
what is something to be wary off when using electron microscopy in fibre analysis
beam damage as it can destroy the sample
what addition to SEM can give elemental analysis of fibres
the use of backspatter
what are the two main considerations when deciding what methods to use for fibre analysis
time and cost
SEM is good but is it necessary when physical fit has the highest probabtive value
using microscopy what two features are useful in identifying fibres
refractive index and birefringence
how can polarised light microscopy be used in fibre analysis
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
what is pleochroism
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
how can pleochroism be useful in investigations
the pleochroism or RI of an unknown fibre can be compared to that of a known fibre giving links between suspects and scenes
what is the retardation and how can it be measured
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
how can we calculate the birefringence of a fibre
using polarised microscopy - find the retardation and thickness of the fibre and then calculate the birefringence
what two features measured using polarised microscopy of fibres can be used to identify and compare fibre samples
retardation and birefringence
comparisons can also be made using the Michel-Levy chart
