Fibre analysis techniques Flashcards
1
Q
Case work
A
- Fibres found at a scene will need to match a reference sample e.g suspect, location, object
- In some cases some analysis may take place prior to a possible reference sample becoming available however it is preferred to have a reference
- Possible matching fibres can be found rapidly using simple stereo microscope (10-40x) or using an automated ‘fibre finder’ in a process known as closed searching
- Fibres of interest can then be identified using a permanent marker on the acetate side (reverse) of the sample, using multiple colour markers allows rapid review
- Potential matches can then be subjected to more rigorous analysis
2
Q
Presentation of fibres- fibre de-mounting
A
- Once a target fibre has been found it then be viewed under a microscope to determine its morphology and other physical parameters
- fibre must be released from the acetate by cutting a flap using a scalpel blade being careful to only cut the tape layer
- applying a small amount of ethanol or xylene on forceps can help release fibre
- Once released, the fibre is often cleaned of adhesive using additional ethanol or xylene
- At this stage the fibre can either be examined instrumentally or using microscopy for which it must be appropriately mounted
3
Q
simple wet mounting
A
- Non-permanent mounting method
- The fibre is immersed in a small drop of medium such as water or a specialised medium such as apathy’s gum syrup, glycerol jelly, cargille oils or specialist media
4
Q
permanent or semi permanent mounting
A
- Canada balsam- not favoured due to colour case
- DPX new/ permount- common
- Entellan new- best option when MSP is used
- Meltmount- semi permanent heat flow medium which comes in a variety of variants and allows fibre demounting
5
Q
fibre microscopy
A
- Analysis will begin by simple brightfield microscopic techniques. Enable visualisation of principle features, hair type, striations
- The fibre can be measured- diameter using calibrated graticule
- Presence of delustrants can also be seen as can the gross colour of the dye in distribution of dye
- Production of cross sections can be determined which can be suggestive of a particular use of the fibre
6
Q
Gross features
A
- Reference to a fibre guide
- Is it scaled suggesting a hair or wool? Featureless could result in man made fibre or silk Etc
7
Q
fibre diameter
A
measured with a calibrated eyepiece gratiule
8
Q
is the fibre delustred
A
- Yes- man made
- No- man made or natural
9
Q
are scales a medulla cross marking or a lumen present
A
- Yes (scales)- animal fibre/hair (not silk)
- Yes (lumen or cross markings)- vegetable fibre
10
Q
is the fibre striated
A
- One or more centrally positioned lines- probably regular lobed (acrylic, modacrylic, polyamide or polyester most likely)
- Several lines- viscose, acetate, triacetate
11
Q
inclusions
A
- delustrant particles used to lower the reflectivity of man-fibres
- Another might be the presence of visible dye pigments (high magnification) which may be suggestive of certain fibre types e.g. man-made/melt spun
12
Q
cross section
A
- These are produced using a microtome or by the ‘sandwich and section’
- Generally the gross cross-sectional morphology is obvious
- Complex shapes such as trilobal suggest man-made (melt spun) fibres and in fact may suggest application (Carpet)
- Triangular may suggest silk if the diameter is also appropriate
- Crenulated is normally suggestive of man-made and is common in regenerated fibres
13
Q
polarised light microscopy
A
- potentially allowing the scientist to determine the type of fibre encountered and determine some physical attributes rapidly
- The PLM differs somewhat from the standard microscope in that 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
14
Q
first process in PLM is if the sample goes to extinction
A
- This involves placing the mounted sample under the microscope under crossed polarisers and rotating the stage and the sample
- Most fibres when rotated will appear dark at N-S and E-W positions and become bright at NW-SE and NE-SW positions
- Cotton is the only common fibre that does not become extinct and can thus be readily identified and its presence confirmed beyond reasonable doubt
- Once the extinction has been determined we can then move onto determining the sample’s birefringence
15
Q
PLM birefringence
A
- Most fibres are what is described as anisotropic having different refractive indices according to whether light passing through the fibre travels parallel or perpendicular to the fibre
- Birefringence the essentially the difference between these numbers
- B = n (parallel) – n (perpendicular)
- 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 recombing
- 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)
- When the waves recombine at the analyser of the PLM interference results which produces colours within the fibre which vary according to thickness
- Birefringence can be calculated by determining two RI values – time consuming
- Or, one of the most common methods 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
- 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
- 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
- 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
16
Q
PLM-sign of elongation
A
- 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
- By adding a first order retardation plate into the light path, we can determine which orientation allows light to travel fastest
- By orientating the fibre NW-SE noting the colour and then NE-SW we can determine the sign of elongation
- As you saw in the practical, 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)
17
Q
rapid fibre determination- birefringence
A
- 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 natiral fibres have a medium to high +ve birefringence
18
Q
the comparison microscope
A
- allows the scientist to directly compare reference and scene fibres within the same field and allows direct colour comparison
- This is important as although the human eye can differentiate wavelength differences of about 3nm, the brain cannot remember hues with any accuracy
- The device consists of two optically identical microscopes joined by an optical bridge and allows split imaging or side by side viewing
19
Q
the fluorescence microscope
A
- Many fibres, when irradiated with light of specific wavelength may fluoresce as a result when excited electrons fall back to ground state
- In most cases, this is due to optically active dyes or optical brighteners within their structure
20
Q
the hot stage
A
- An attachment which can be placed onto the stage of any microscope and allows even accurate heating of a sample
- Allows determination of melting point of fibre
- This is clearly a destructive technique so care should be taken
21
Q
microspectophotometry
A
- For fibre differentiation
- used to determine the colour of samples
- Provide dp in excess of 0.99 easily differentiating identical fibres
- Prior to MSP scientists would have to reply on calibrated colour charts to compare similar fibres by eye
- Devices designed to reflect or transmit light through a sample
- absorbance can be measured 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
- Modern systems operate in the visible and UV range 380-1000nm and also UV range 190-380nm
- The latter can increase discriminating power but may cause samples to photo bleach
- The instrument consists of a microscope with a stabilised light source
- 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 blank
- Once complete the device can produce a full spectral response curve and CIE colour value
22
Q
advantages of microspectrophotometry
A
- Non-destructive
- simple to operate
- produces high quality discriminating results
23
Q
spectral shape interpretation
A
- the overall shape of the recovered spectrum must correspond in addition to the position of each maxima and each minima
- the shape of each peaks and each minima in the recovered spectrum must also match
24
Q
absorbance values
A
- 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
25
infra red spectroscopy fibres
- allows 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 spctrum
- Three main forms of the technique can be used for the analysis of fibre evidence
26
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
27
Transmission FTIR
- requires the sample to be placed between two KBr disks normally after being flattened using a diamond press
- This method is extremely insensitive, as very little 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
28
FTIR-ATR- attenuated total reflectance
- In practice, the sample if brought into intimate contact with the ATR crystal which is generally composed of diamond, ZnSe or similar high RI material
- Pressure is ensured by use of some kind of press or similar which compacts the sample presenting 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
- ATR is simple, cheap, non-destructive and provides excellent results for larger samples of fibre material
- ATR is probably not appropriate for individual fibres
29
FTIR microscopy
- The device consists of a digital or optical microscope which can be used to visualise an individual fibre mounted above an aperture or on a reflective gold plated slide
- The 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 need
30
FTIR
- 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
31
FTIR rapid interpretation
- many 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
32
Thin-Layer chromatography
- Although MSP is an extremely discriminating technique, it is often necessary to further confirm any colour result (after FTIR) using chromatography
- TLC simple to perform, inexpensive and potentially very discriminating
- The principal difficulty lies in the extraction of the dye from small quantities of fibre material which is not only difficult but also destructive, which is why, TLC is often one of the last stages of analysis
- ,dye extraction is often difficult and relies on knowing what type of dye is being used – This can sometimes be surmised
- Alternatively, the scientist must determine this themselves by testing solubility using various solvents
- Scientists may typically follow a standard extraction protocol which can help identify the type of dye present within the fibre as well as extract it
- extraction is carried out on a micro scale in most cases, requiring comparatively few individual fibres but it should be noted that not all dyes will extract
- Pigment dye is insoluble and reactive dyes require more complex extraction
- Once extracted, the dye can be run on a silica TLC plate and compared to extract from a reference/control sample
- Eluent solution must be carefully considered to ensure separation of individual dyes and 12 common types are in use and can be chosen according to dye and fibre type
- very effective technique and can show remarkable discriminatory power, but is very sensitive to poor technique
- Solvent should always be made up extremely carefully and stored no longer than is necessary
- The TLC chamber should always be allowed to equilibrate before the samples are run – Failure to do show results in a ‘smiley’ run!
- Once the TLC is complete, the solvent front is immediately marked and the solute spots can be marked or developed chemically or under UV
- The VSC is perfect for this process
- The RF values can be calculated and then compared
33
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 Scanning Calorimetry
Used to examine the calorimetric properties of a material
- Burning Tests
34
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
- 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
- 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
35
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
- The differences using PyrGC are clear and allow rapid solid results to be produced where fibre identification is tentative
- The technique is however destructive and therefore should be used with care and consideration
36
Burning tests- presumptive testing of bulk samples
- The most common and least useful (still in use by customs officials in some regions and is taught in textile courses) is that of burn tests which are used with 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
- This can help to rapidly determine fibre type
37
interpretation and reporting
- The strength of fibre evidence can vary enormously according to case circumstances and fibre type and a trivial change to the defence case may require extensive re-evaluation and research!
- 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
- Source level propositions such as does our fibre match? Could the fibre be from this particular item? How common is it? What does that mean?
- Activity level propositions such as how did our fibre get there? Could the fibre be present just by chance?
38
source level- how common is it?
Fibres are mass produced and therefore a given fibre cannot be definitively identified as coming from one particular source
Blue or white cotton fibres are not always very evidentially compelling; they are generally just too common within the environmen. By contrast, some fibres are uncommon not just as a result of their type but by virtue of damage, fault or other factor
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
- As with all branches of forensic science, 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 combinations
- All of these can help to give some statistical information to show the relative commonality of a given fibre but experience can be equally valuable
- There are very few sources of Red dyed Octalobal Nylon 6 fibres or bilobal polypropylene fibres meaning matches are so much more significant
- By contrast Round blue cotton fibres are considerably more common being used in many garments such as jeans
39
activity level- could the fibres be present just by chance
In other words the fibre is present on my suspect for reasons unrelated to any criminal offence having occurred… No suspect/victim transfer
Where we are dealing with a common fibre, then the answer is a firm yes our red fibre may well have come from a source other than our victim
Clearly as the rarity of the fibre increases, such random events become more and more unlikely, however we must always consider other possibilities…
• We could be dealing with secondary or tertiary transfer and in some cases contamination
• Only full understanding of case circumstances can truly reveal how likely these events may be
• It could be that our suspect and victim are close friends or car share for example
40
activity level- could the fibre be present just by chance?
| factors that suggest secondary transfer was less likely
- Number of fibres
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
May be more consistent with prosecution hypothesis and not a typical secondary transfer scenario as may be alleged by the defence
- Two Way Transfer or Cross-Transfer
A two way transfer can be extremely compelling!
41
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 (if presented) 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
