fibre analysis techniques Flashcards
how can possible matching fibres be found rapidly
using a simple stereomicroscope (10-40x) or using an automated fibre finder in a process known as closed searching
how are fibres of interest identified when found
using a permanent marker on the acetate side (reverse) of the sample, using multiple colour markers
fibre de-mounting
Once a target fibre has been found it will be viewed under a microscope to determine its morphology and other physical parameters
how is the target fibre released from the acetate
cut a flap using a scalpel blade being careful to only cut the tape layer
fibre can be released by applying small amount of ethanol or xylene on forceps
fibre is cleaned of adhesive using additional ethanol or xylene
simple wet mounting
Simple non-permanent mounting method
Fibre is immersed in small drop of medium such as water or a specialised medium such as apathy’s gum syrup, glycerol jelly, cargille oils
permanent or semi-permanent mounting
Canada balsam
- Not favoured due to colour cast
DPX new/Permount
- Common
Entellan new
- Best option when MSP to be used
Meltmount
- Semi-permanent heat flow medium which comes in a variety of variant and allows fibre demounting
how does analysis of fibres begin
use of simple brightfield microscopic techniques which will enable visualisation of principle features such as any scaled indicating a hair type fibre, striations or other obvious features
how can fibre diameter be measured
calibrated eyepiece graticule
gross features
Reference to a fibre guide
Scaled suggesting hair or wool
Featureless suggesting silk or man-made
rapid guide to fibre determination
Is the fibre delustred?
- Yes – Man-Made
- No – Man-made or Natural
Are scales, a medulla, cross-markings or a lumen present?
- Yes (Scales) – Animal Fibre/hair (not silk)
- Yes (Lumen or cross markings) – Vegetable Fibre
Is the fibre striated?
- One or more centrally positioned lines – Probably regular lobed (Acrylic, Modacrylic, polyamide or polyester most likely)
- Several lines (often discontinuous/irregular) – Viscose, acetate, triacetate
inclusions
Delustrant particles used to lower the reflectivity of man-made fibres
Presence of visible dye pigments
how is cross section produced
produced using a microtome or by the ‘sandwich and section’
what do tribal cross sections indicate
man-made fibres
what do triangular cross sections suggest
silk
what do crenulated cross sections suggest
man-made and regenerated fibres
what is polarising light microscopy
Powerful techniques allowing scientists to determine the type of fibre encountered and determine some physical attributes rapidly
how does polarising light microscopy differ from standard microscopy
it includes two polarising plates, the polariser and the analyser
These along with a retardation plate allow us to determine the sign of elongation and the birefringence of the sample which may allow us to determine the fibre type
what is one of the first processes of polarising light microscopy
determining if the sample goes to what is called extinction
involves placing the mounted sample under the microscope under crossed polarisers and rotating the state and the sample
what positions do fibres appear dark and bright
Most fibres when rotated will appear dark at N-S and E-W positions and become bright at NW-SE and NE-SW positions
what is the only fibre that does not become extinct and can thus be readily identified and its presence confirmed beyond reasonable doubt
cotton
what can we do once the extinction has been determined
determine sample birefringence
what does it mean by fibres being described as anisotropic
having different refractive indices according to whether light passing through the fibre travels parallel or perpendicular to the fibre
what is birefringence
Birefringence the essentially the difference between these numbers
B = n (parallel) – n (perpendicular)
polarising light microscopy - birefringence
Under the PLM, the effects of birefringence can be seen when a fibre is placed diagonally on the stage by the production of interference colours
These occur when the light passing through the fibre (which is split into two rays one running parallel to the fibre one perpendicular) recombine
When the waves recombine at the analyser of the PLM interference results which produces colours within the fibre which vary according to thickness
polarising light microscopy - sign of elongation
When a fibre is orientated diagonally, polarised light entering the fibre is split into two waves which due to the differences in refractive index travel at different speeds one being retarded
These waves recombine at the analyser of the PLM producing an interference colour
what is the level of birefringence
- High(+ve) – Polyester
- Medium (+ve) – Viscose, Cupro, Modal, Polyamide, Polyethylene, Polypropylene, Lyocell
- Low (-ve) – Acrylic
- Low (+/-ve) – Modacrylic, Triacetate
- Low (+ve) – Chlorofibre, Acetate
- None (isotropic) – Glass fibres
- Most natural fibres have a medium to high +ve birefringence
comparison microscope
Allows scientist to directly compare reference and scene fibres within the same field and allows direct colour comparison
how can the human eye differentiate wavelength differences
can differentiate wavelength differences of about 3nm, the brain cannot remember hues with any accuracy
structure of comparison microscope
Device consists of two optically identical microscopes joined by an optical bridge and allows spilt imaging or side by side viewing
fluorescence microscope
Many fibres when irradiated with light of specific wavelength may fluoresce as a result when excited electrons fall back to ground state
Due to optically active dyes or optical brighteners within their structure
the hot-stage
an attachment which can be placed onto the stage of any microscope and allows even accurate heating of a sample
Allows accurate determination of melting point of any given fibre which is a property which can be discriminatory
Clearly destructive method techniques so case should be taken
microspectophotometry
for fibre differentiation
MSP is a form of spectroscopy used to determine the colour (absorbance/reflectance) of microscopic samples
Devices designed to either reflect or transmit light through a sample and absorbance can be measured simply against a reference beam or suitable white reflectance standard
The light wavelength is changed during a scan in nm or cm-1 steps allowing the production of a total spectral curve
what can MSP provide dp in excess of
0.99
what did scientists do before MSP
Prior to MSP, scientist would have to rely on calibrated colour charts to compare similar fibres, fibre by eye with obvious potential consequences
what do modern MSP systems operate in
visible and UV range 380-1000nm and also UV range 190-380nm
The latter can increase discriminating power but may cause some samples to photo bleach
most operate in transmission and reflectance modes
what does the MSP instrument consist of
The instrument consists of a microscope with a stabilised light source
how does MSP work
Once the scan begins the wavelength of the light is varied step by step and the absorbance can be measured by reference to a standard or to a blank
Once complete the device can produce a full spectral response curve and CIE colour value which can be used for comparison purposes
advantages of MSP
Non destructive
Simple to operate
Produces high quality discriminating results
Data analysis simple and achieved by reference to the sample spectra
The spectra can be easily overlaid and examined by examination of bandwidth, peak shapes and definition as well as absorbance
spectral shape interpretation
The overall shape of the recovered spectrum must correspond in addition to the position of each maxima (peak), and each minima (trough)
The shape of each peak, and each minima, in the recovered spectrum must also match
absorbance values
The absorbance values of the recovered spectrum should correspond to those in the control spectra although differences of a few nm may be acceptable depending upon the tightness of absorbance within other fibres within the group
Where the recovered spectrum contains a number of peaks, the relative intensities of peaks should correspond
infrared spectroscopy of fibres
FTIR may play an extremely important role in determining the nature of a given fibre, allowing determination of chemical structure and thus class
FTIR may be used to confirm a microscopic or optical identification of a fibre or in some cases be used as a discriminatory tool
It can also be used to examine dye type in some cases by subtraction of fibre spectrum
FTIR variants
Transmission FTIR – The least useful of the 3 techniques
FTIR-ATR – Attenuated Total Reflectance – Sensitive and rapid
FTIR-Microscopy – Able to produce results from a single fibre
transmission FTIR
Requires sample to be placed between two KBr disks normally after being flattened using a diamond press
Method id extremely insensitive, as very little light of the IR light path actually interacts with the sample, the detector mostly receiving stray light
This can be rectified to some extent with the use of masks, but even so the signal to noise ratio and sensitivity issues make this technique all but obsolete in most labs
FTIR-ATR - attenuated total reflectance
Sample is brought into intimate contact with the ATR crystal which is generally composed of diamond etc
Pressure is enables by use of some kind of press which compacts the sample resenting a larger surface area and increasing sensitivity without the need for use of a diamond press
A beam of IR then passes through the ATR crystal forming an evanescent wave which extends into the sample absorbing at characteristic wavelengths
These absorptions can be detected fairly simply and plotted in the form of a spectrum
advantages of ATR
Simple
Cheap
Non destructive
Excellent results for larger samples of fibre material
disadvantage of ATR
not appropriate for individual fibres
FTIR microscopy
Device consists of digital or optical microscope which can be used to visualise an individual fibre mounted above an aperture or on a reflective gold-plated slide
Microscope not only allows the operator to view the fibre, but also define which part of the fibre they wish to examine thereby improving flexibility of the technique and allowing specific point spectra to be obtained with relative ease
The instrument can be used in reflective or transmission modes according to specific needs
FTIR
Regardless of the variant of FTIR used, all techniques work to the same principle, that being that all fibres will interact with IR radiation
In doing so, energy is absorbed at specific wavelengths as atoms within the molecule absorbing energy move relative to each other
It is this absorption that allows us to identify the fibre type in question, with certain functional groups absorbing at very specific wavenumbers
By considering these absorptions, it is a comparatively trivial task to determine the fibre in question in most cases
Interpretation of the FTIR spectrum can be done by eye, by reference to a simple database or by electronic search
FTIR - rapid interpretation
Maybe fibre chemists will use a table allowing rapid rule out of non matches
Once identified either manually or by automated database, it may be necessary to compare against a reference sample to confirm
advantages if TLC
Further confirm any colour result after FTIR
Simple to perform
Inexpensive
Very discriminating
disadvantages of TLC
Difficulty lies in the extraction of the dye from small quantities of fibre material, which is not only difficult but destructive, therefore TLC one of last stages of analysis
Dye extraction is often difficult and relies on knowing what type of dye is being used
Alternatively, the scientist must determine this themselves by testing solubility using various solvents, this is a depletive process and may not be suitable if little simple is available
other techniques
SEM-EDX – scanning electron microscopy
- High magnification microscopy and elemental composition
PyrGC-MS – pyrolysis gas chromatography with MS
Hyper-DSC – high speed differential calorimetry
- Used to examine the calorimetric properties of a material
Burning tests
SEM-EDX - scanning electron microscopy
SEM is principally a useful high-resolution microscope which enables the scientist to visualise a sample at high magnification and resolution
EDX allows Acrylic to be differentiated from Modacrylic (the latter designed to be fire retardant and thus normally containing Chlorine or Bromine atoms) or Chlorofibre
PyrGC-MS
It can play a vital part in fibre differentiation, allowing variant fibres to be readily identified
Nylon 6 and Nylon 6,6 have very similar properties and are not easily differentiated even using FTIR
burning tests - presumptive testing of bulk samples
The test involved taking a number of fibres and subjecting them to an open flame
The ‘scientist’ will then remove the flame and observes whether the fibres burn or not
If they do, the scientist will then determine whether they self-extinguish, the smell of the fumes produced and the nature of the charred or melted material produced
interpretation and reporting
The strength of fibre evidence can vary according to case circumstances and fibre type
Unlike DNA technology, fibre analysis has not traditionally relied upon statistics derived from large ‘ultra-reliable’ databases and therefore relies much more on the individual expertise of the reporting scientist
Although semi-bayesian type approaches to analysis and interpretation are not necessarily uncommon, results are not typically expressed in this way
analysis should allow the RO to define what 5 things?
Fibre type and variant Shape and cross section Dimensions Colour and dye Delustrant type and distribution
how common is a fibre
Fibres are mass produced and therefore a given fibre cannot be definitively identified as coming from one particular source
As a general rule, blue or white cotton fibres are not always very evidentially compelling; they are generally just too common within the environment to provide compelling support to any particular hypothesis
give 6 examples of uncommon fibres as a result of their type/damage/fault
Unusual polymer composition – Kevlar
Unusual colourant type – e.g. mismatch pigmented polyester
Manufacturing Faults – Many possibilities
Damage (Bleaching, melting, insect damage, photo bleaching effects)
Over-dyeing
Flame retardants
databases and scientific papers do exist with some high quality information of value in many cases
Frequency of morphological characteristics e.g. Pentalobal, octalobal etc.
Frequency of polymer composition… These show just how common cotton is!
Frequency of polymer type within a given population (UK)
Frequency of fibre type usage in textiles
Frequency of combination
give 2 examples of uncommon fibres
Red dyed Octalobal Nylon 6 fibres or bilobal polypropylene fibre
give an example of a common fibre
round blue cotton fibres from jeans
number of fibres - could my fibre be there by chance?
Large numbers of fibres would be suggestive of primary transfer
However, this is depending upon shedding and decay characteristics of our fibre/garment which should be considered
This is especially relevant with a large interval between alleged offence and collection of evidential materials
location of fibres - could my fibre be there by chance?
- May be more consistent with prosecution hypothesis and not a typical secondary transfer scenario as may be alleged by the defence
what can eliminate a fibre by chance
two way transfer or cross transfer
evaluation
It is your responsibility as the case RO to present a fair assessment of the significance of the evidence in the case, considering all contra-indicators by sensible evaluation of case circumstances and defence position
It can be difficult to be conservative in such cases, but nevertheless you must be… all figures should be transparent, justifiable, validated and rounded in favour of the defence where possible
You will have to defend your views against cross examination so everything you write must be defensible and pass scientific and legal scrutiny
what is optical path difference in birefringence
As the RI differs according to light path, the two rays are normally out of phase (the difference described as optical path difference or OPD)
how can birefringence be calculated and a negative of this?
Birefringence can be calculated by determining two RI values of the fibre, however, this process is time consuming
what is one of the most common methods of calculating birefringence
involves adding a quartz wedge into the light path slowing one of the rays of light before recombination – The more insertion the greater the effect
how does the quartz wedge method of calculating birefringence work?
As the compensator is inserted, the interference colours of the fibre may disappear (black) as the two waves come back into phase at which point the birefringence can be read off the device or inferred
what is a simpler method for calculating birefringence?
A simpler method can be used which involves comparing the interference colour of the fibre to a simple colour chart known as the Michel-levy chart which allows the user to determine approximate birefringence values in seconds and is useful for triage
how does the michel-levy chart work for calculating birefringence
Determine the fibre diameter using an eyepiece micrometer
Position the fibre in the NW-SE position and note the interference colour of the fibre considering colour orders carefully
Compare this to the colours in the Michel Levy chart and read off the birefringence value
what can be determined by adding a first order retardation plate in PLM - sign of elongation
By adding a first order retardation plate into the light path, we can determine which orientation allows light to travel fastest
how can you determine the sign of elongation by orientation in PLM
By orientating the fibre NW-SE noting the colour and then NE-SW we can determine the sign of elongation
how do you know if a fibre has a positive or negative sign of elongation
If the fibre ‘colour’ goes up the spectrum then the fibre has a positive sign of elongation (length slow), if it goes down the spectrum then it has a negative sign of elongation (length fast)
advantages of SEM-EDX
SEM however has an enormous depth of field and for this reason, visualisation of microstructure, striations and cross section is much simpler
The magnification may also allow the operator a greater view of delustrant particles and their distribution and can allow identification of dye or pigment using EDX
EDX may also play a part in differentiation of fibre type
negative of PyrGC-MS
The technique is however destructive and therefore should be used with care and consideration
advantage of PyrGC-MS
The differences using PyrGC are clear and allow rapid solid results to be produced where fibre identification is tentative
