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Materials Identification > fibre analysis techniques > Flashcards

fibre analysis techniques Flashcards

1
Q

how can possible matching fibres be found rapidly

A

using a simple stereomicroscope (10-40x) or using an automated fibre finder in a process known as closed searching

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2
Q

how are fibres of interest identified when found

A

using a permanent marker on the acetate side (reverse) of the sample, using multiple colour markers

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3
Q

fibre de-mounting

A

Once a target fibre has been found it will be viewed under a microscope to determine its morphology and other physical parameters

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4
Q

how is the target fibre released from the acetate

A

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

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5
Q

simple wet mounting

A

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

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6
Q

permanent or semi-permanent mounting

A

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

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7
Q

how does analysis of fibres begin

A

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

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8
Q

how can fibre diameter be measured

A

calibrated eyepiece graticule

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9
Q

gross features

A

Reference to a fibre guide

Scaled suggesting hair or wool

Featureless suggesting silk or man-made

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10
Q

rapid guide to fibre determination

A

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
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11
Q

inclusions

A

Delustrant particles used to lower the reflectivity of man-made fibres

Presence of visible dye pigments

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12
Q

how is cross section produced

A

produced using a microtome or by the ‘sandwich and section’

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13
Q

what do tribal cross sections indicate

A

man-made fibres

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14
Q

what do triangular cross sections suggest

A

silk

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15
Q

what do crenulated cross sections suggest

A

man-made and regenerated fibres

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16
Q

what is polarising light microscopy

A

Powerful techniques allowing scientists to determine the type of fibre encountered and determine some physical attributes rapidly

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17
Q

how does polarising light microscopy differ from standard microscopy

A

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

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18
Q

what is one of the first processes of polarising light microscopy

A

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

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19
Q

what positions do fibres appear dark and bright

A

Most fibres when rotated will appear dark at N-S and E-W positions and become bright at NW-SE and NE-SW positions

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20
Q

what is the only fibre that does not become extinct and can thus be readily identified and its presence confirmed beyond reasonable doubt

A

cotton

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21
Q

what can we do once the extinction has been determined

A

determine sample birefringence

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22
Q

what does it mean by fibres being described as anisotropic

A

having different refractive indices according to whether light passing through the fibre travels parallel or perpendicular to the fibre

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23
Q

what is birefringence

A

Birefringence the essentially the difference between these numbers

B = n (parallel) – n (perpendicular)

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24
Q

polarising light microscopy - birefringence

A

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

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25
Q

polarising light microscopy - 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

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26
Q

what is the level of 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 natural fibres have a medium to high +ve birefringence
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27
Q

comparison microscope

A

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

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28
Q

how can the human eye differentiate wavelength differences

A

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

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29
Q

structure of comparison microscope

A

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

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30
Q

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

Due to optically active dyes or optical brighteners within their structure

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31
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 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

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32
Q

microspectophotometry

A

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

33
Q

what can MSP provide dp in excess of

A

0.99

34
Q

what did scientists do before MSP

A

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

35
Q

what do modern MSP systems operate in

A

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

36
Q

what does the MSP instrument consist of

A

The instrument consists of a microscope with a stabilised light source

37
Q

how does MSP work

A

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

38
Q

advantages of MSP

A

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

39
Q

spectral shape interpretation

A

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

40
Q

absorbance values

A

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

41
Q

infrared spectroscopy of fibres

A

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

42
Q

FTIR variants

A

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

43
Q

transmission FTIR

A

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

44
Q

FTIR-ATR - attenuated total reflectance

A

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

45
Q

advantages of ATR

A

Simple

Cheap

Non destructive

Excellent results for larger samples of fibre material

46
Q

disadvantage of ATR

A

not appropriate for individual fibres

47
Q

FTIR microscopy

A

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

48
Q

FTIR

A

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

49
Q

FTIR - rapid interpretation

A

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

50
Q

advantages if TLC

A

Further confirm any colour result after FTIR

Simple to perform

Inexpensive

Very discriminating

51
Q

disadvantages of TLC

A

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

52
Q

other techniques

A

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

53
Q

SEM-EDX - scanning electron microscopy

A

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

54
Q

PyrGC-MS

A

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

55
Q

burning tests - presumptive testing of bulk samples

A

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

56
Q

interpretation and reporting

A

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

57
Q

analysis should allow the RO to define what 5 things?

A 
Fibre type and variant 
Shape and cross section
Dimensions 
Colour and dye 
Delustrant type and distribution
58
Q

how common is a fibre

A

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

59
Q

give 6 examples of uncommon fibres as a result of their type/damage/fault

A

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

60
Q

databases and scientific papers do exist with some high quality information of value in many cases

A

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

61
Q

give 2 examples of uncommon fibres

A

Red dyed Octalobal Nylon 6 fibres or bilobal polypropylene fibre

62
Q

give an example of a common fibre

A

round blue cotton fibres from jeans

63
Q

number of fibres - could my fibre be there by chance?

A

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

64
Q

location of fibres - could my fibre be there by chance?

A
  • May be more consistent with prosecution hypothesis and not a typical secondary transfer scenario as may be alleged by the defence
65
Q

what can eliminate a fibre by chance

A

two way transfer or cross transfer

66
Q

evaluation

A

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

67
Q

what is optical path difference in birefringence

A

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)

68
Q

how can birefringence be calculated and a negative of this?

A

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

69
Q

what is one of the most common methods of calculating birefringence

A

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

70
Q

how does the quartz wedge method of calculating birefringence work?

A

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

71
Q

what is a simpler method for calculating birefringence?

A

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

72
Q

how does the michel-levy chart work for calculating birefringence

A

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

73
Q

what can be determined by adding a first order retardation plate in PLM - sign of elongation

A

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

74
Q

how can you determine the sign of elongation by orientation in PLM

A

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

75
Q

how do you know if a fibre has a positive or negative sign of elongation

A

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)

76
Q

advantages of SEM-EDX

A

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

77
Q

negative of PyrGC-MS

A

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

78
Q

advantage of PyrGC-MS

A

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

Materials Identification (17 decks)

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