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)