fibre analysis techniques Flashcards

Question

polarising light microscopy - sign of elongation

Answer 1

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

Answer 2

* 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

Answer 3

Allows scientist to directly compare reference and scene fibres within the same field and allows direct colour comparison

Answer 4

can differentiate wavelength differences of about 3nm, the brain cannot remember hues with any accuracy

Answer 5

Device consists of two optically identical microscopes joined by an optical bridge and allows spilt imaging or side by side viewing

Answer 6

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

Answer 7

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

Answer 8

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

Answer 9

Prior to MSP, scientist would have to rely on calibrated colour charts to compare similar fibres, fibre by eye with obvious potential consequences

Answer 10

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

Answer 11

The instrument consists of a microscope with a stabilised light source

Answer 12

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

Answer 13

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

Answer 14

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

Answer 15

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

Answer 16

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

Answer 17

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

Answer 18

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

Answer 19

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

Answer 20

Simple Cheap Non destructive Excellent results for larger samples of fibre material

Answer 21

not appropriate for individual fibres

Answer 22

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

Answer 23

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

Answer 24

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

Answer 25

Further confirm any colour result after FTIR Simple to perform Inexpensive Very discriminating

Answer 26

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

Answer 27

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

Answer 28

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

Answer 29

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

Answer 30

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

Answer 31

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

Answer 32

``` Fibre type and variant Shape and cross section Dimensions Colour and dye Delustrant type and distribution ```

Answer 33

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

Answer 34

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

Answer 35

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

Answer 36

Red dyed Octalobal Nylon 6 fibres or bilobal polypropylene fibre

Answer 37

round blue cotton fibres from jeans

Answer 38

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

Answer 39

- May be more consistent with prosecution hypothesis and not a typical secondary transfer scenario as may be alleged by the defence

Answer 40

two way transfer or cross transfer

Answer 41

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

Answer 42

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)

Answer 43

Birefringence can be calculated by determining two RI values of the fibre, however, this process is time consuming

Answer 44

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

Answer 45

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

Answer 46

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

Answer 47

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

Answer 48

By adding a first order retardation plate into the light path, we can determine which orientation allows light to travel fastest

Answer 49

By orientating the fibre NW-SE noting the colour and then NE-SW we can determine the sign of elongation

Answer 50

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)

Answer 51

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

Answer 52

The technique is however destructive and therefore should be used with care and consideration

Answer 53

The differences using PyrGC are clear and allow rapid solid results to be produced where fibre identification is tentative