Define fibres
Describe the different between yard and threads
- fibres define the basic unit of yarns and threads, which then turn into fabrics, garments and textiles etc.
- for something to be a fibre, length must significantly exceed width and they must be flexible
- yarns are used to make fabrics
- threads stitched together
What are all fibres based on bar a few?
all fibres bar a few are based on polymers
Where are fibres found?
- clothing
- bedding
- carpets
- curtains
- car upholstery
- medical bandages
- garden twine
- conveyer belts
- boat sails
- fibreglass
- seatbelts
- parachutes
- loft insulation
- bullet proof vests
- cuddly toy stuffing
- get fibres from a lot of places so a high chance will be found at scene
What are the two classifications of fibres?
What are other names for these
natural or synthetic
natural - staple
synthetic - filament
What are three categories of natural fibres?
Give examples of each category
- plant, animal, mineral based
- plant (cotton, linen, hemp, jute, flax, sisal, coir)
- can also be categorised based on where they originate from within plant (seed, stem, leaf, fruit)
- animal fibres are either guard hairs or fur (silk, wool, cashmere, angora, camel)
- mineral (asbestos only - only one kind of mineral fibres)
What is overlap relating to animal derived natural fibres and animal hairs?
- there is a huge overlap of fibre theory relating to animal derived natural fibres with animal hairs – animals fibres are an animals hair
How are synthetic fibres made?
- raw polymer converted into fibres via spinning
- spinning takes place via spinneret device that is added onto manufacturing processes
- spinneret has different shaped holes so can get differences and characteristics
- various spinning techniques = alter characteristics
- end up with fine/infinitely long fibres coming out of spinneret
- fibres spin into bundles called filaments
When analysing synthetic fibres forensically, what is more useful?
- characteristics and properties more forensically useful than chemical composition - looked at using microscopy
What are the recovery considerations for fibres?
- fibres may become dislodged quickly after deposition (e.g. if outside and wind carries away, suspect/victim with fibres on that need obtaining is moving)
- if collecting wet clothing from scene, must be air dried in controlled environment and collect loose fibres that dislodge as it dries (no good waiting at scene as when dries, they will fall off) before packaging
- store in paper bags after sufficiently dried to prevent mould growth
- never package item of evidence with debris from scene - might not draw correct inference when analysed in lab
- emergency personnel intervention priority is to safe life so if they cut through garment this may dislodge fibres
- Druggist’s fold to package tiny fibres, then place this in evidence bag
- preferable to submit entire item to lab as location is important
- take control samples and package these separately
What is suggested 8-point analytical workflow for fibres suggested by SWGMAT:
- gross examination, recovery and collection
- preliminary evaluation of physical characteristics
- physical fit assessment - most probative value (if can show 2 pieces of fabric or bundle of fibres originated as one and have been cut, torn or separated - can match back together)
- all microscopic techniques
- Microspectrophotometry/MSP (Uv-vis) for colour determination (this is better than subjective visual colour determination)
- infrared spectroscopy (gives info on sample)
- particularly recommended if have manufactured fibres as these are carbon heavy polymers and IR loves carbon
- raman spectroscopy - good for dyes and pigments (not so common in forensic labs)
- once done non-destructive recommended techniques, need to do non-routine destructive ones (only do if really necessary as will destroy samples
- thin layer chromatography
pyrolysis gas chromatography mass spectrometry - high performance liquid chromatography
- melting point,
- microchemical tests, e.g. solubility (dissolving in acids/alkalis)
What 15 things are we looking for in fibre analysis?
1 - colour (say visually in preliminary, then use MSP to give values, then dyes/pigments using FTIR/raman - some D/P have same colour visually and via MSP but different chemical compositions)
2 - natural or synthetic and type (animal, plant or mineral)
3 - length and diameter
4 - assess cross-sectional profile (round, flat, trilobal, dumbbell) - come from spinneret
5 - striations (lines down fibre) and pitting (holes along fibre - probably damage)
6 - dyes and pigments
7 - assess conditions of fibres (fading/discolouration = old garment)
8 - direction of yarn twist (Z or S)
9 - thread count (indicates better quality, more threads per SA = more dense material = better quality)
10 - has it been subject to mercerisation technique (cotton and flax mainly) - if chemically alter cotton/flax it avoids shrinkage or fabric, impart lustrous sheen and improve durability
11 - coatings. usually polymers e.g. TEFLON
12 - Delustrants e.g. TiO2 (take off lustre (shimmer/sheen) and make everything look matte)
12 - scale protrusion on animal fibres (as they are hair)
13 - when fabric was dyed as dye uptake will depend on when dyed in manufacturing process (before spin into yarn, after spinning, after garment construction (white sections)
14 - surface printing on fabric
15 - look at different chemical composition of fibre itself (with IR and raman to extent)
- 8 different types of nylon produce different spectra
Describe the difference between class and individual characteristics in fibres?
class characteristics - traits common to a group e.g. if looking at cotton, can say they all have finite length
individual characteristics - traits that define and identify an item as different to others in the class
What things are important to note in the interpretation of fibres? (rarity, number/location, substrate considerations, filament fibres compared to staple fibres multiple associations, nature of contact)
- rarity of fibre increases probative value (much more unusual to find cerise wool than blue denim fibres)
- number and location of fibres found will have probative value given the context
- substrate considerations: absence of fibres doesn’t mean fibres are absent - may have fallen off
- fibres will cling onto wool jumper more than silk dress
- new fabrics often have loosely adhering fibres so will shed more easily
- old/damaged fabrics may also shed more fibres
- tightly knit/woven fabrics shed less than those that are loosely woven
- filament fibres shed less than staple fibres as they are longer
- multiple associations mitigate coincidental transfer
- the more fibres of different types you can link between suspect/victim/scene/object, less likely you can say it was accidental transfer
- nature of contact - damage increases transfer
- more fibres transferred if assault someone than if you hug them gently
What are the caveats with fibre interpretation?
- can never state a fibre is unique (don’t actually know if unique as cannot disclose whole population of whatever that sample is
- can use chemometrics to say how likely something might be, how similar/dissimilar something is
- can always categorically exclude a piece of evidence
- very few databases of fibre origin (databases still unlikely in future as always changing
- fibre evidence is often overlooked at scenes as difficult to locate
- very few CSI’s will really look for tiny fibres – hard graft
- when get tiny fibres back to lab it is expensive, time-consuming, skilled analysis
- most people tend to just want to swab everything, put in DNA analyser and get answer
- fibre evidence can be that association that is important
What microscopic techniques can be used for fibres?
- stereoscopic
- comparison
- fluorescence
- brightfield
- polarised
- thermal
- scanning and transmission electron microscopy
- crystallography and diffraction
- darkfield can also be used - cellulose/biology based fibres
What is starting point in fibre analysis to look at gross characteristics and physical fit?
What can we match between two fibres?
- brightfield, stereoscopic and comparison
- potential things we can link between two fibres:
- talk about anything on surface (surface treatments - delustering agents, texture, colourant/dye - colour, type, how applied and weathering)
- dye penetration
- understanding chemical composition (type, polymer)
- cross-section
- diameter (pure optical microscopy)
What are 2 microscopic characteristics looked at in synthetic fibres?
- longitudinal appearance (texture, crimping, pigment)
- related to a manufacturing process or particular machine
- cross-sectional appearance (cut and observe)
- if already damaged just look at end
- can be unique to manufacturing process or specifically engineered
What is most common cross sectional shape in nylon synthetic fibres?
What is this shape like this?
How can you numerically designate this shape fibres? Why would you want to do this?
- tri-lobal shape is most common in nylon synthetic fibres
- additional strength
- modification ratio is a good way to numerically designate tri-lobal fibres in order to identify and compare them
- long axis radius / short axis radius
What are other microscopic characteristics to be observed in fibres (synthetic?)?
- dye penetration
- understand how far dye has penetrated into fibre
- presence of crystalline region
- if something has crystalline region, will be interesting with light - will interact/refract with light
- more you stretch fibre = more likely to align crystalline regions
- amount of crystalline region changes depending on manufacturing process
- presence of gas voids
What can SEM for fibre analysis reveal?
- can reveal features unseen in optical microscopy e.g. scale-like features on the outer surface or fibre-end fracture morphology
- elemental analysis of fibres reveals even more information
- can pick out what fibre is made of in non-destructive way (can do without having to go into spectroscopic techniques but it is more expensive as it is an add on to an electron microscope)
Describe pleochroism
How is this a useful property in forensics?
- a property of anisotropic materials that causes it to show different absorption colours when it is exposed to polarised light coming from different directions
- compare the pleochroism of fibre found at crime scene to fibre taken from suspect - suggesting/disproving link between suspect and scene
Describe retardation
How is this calculated?
- approach sample with full white light
- two components of light travel through the crystal at two different velocities (determined by omega and epsilon)
- as velocities are different, one way travels faster than other white ray inside crystal
- end up with light that comes out as a different colour (no longer white light)
- slower ray is said to be retarded behind the fast ray and exact distance the slow ray falls behind is called the retardation (R)
- R = B x T x 1000 (nm/um)
- B = birefringence
- T = thickness in um
define birefringence
how can this be calculated?
- birefringence is difference between refractive indices of the two components of light travel through the crystal at two different velocities
- if know thickness (measured optically) and retardation (measured by rotating analyser relative to polariser to see how much light comes through)
What does retardation, thickness and birefringence of a fibre allow for?
- allows us to identify it and compare it to a known sample
- Michael-Levy chart - plot for all potential thickness and retardation to get birefringence
- can compare this birefringence to known sample
