Microscopic analysis of fibres Flashcards
How can we get information from synthetic fibres?
When performing a gross examination you can determine the modification ratio - a way of numerically designating tri-lobal fibres, identifying and comparing them.
Dye penetration - understanding how far the dye has penetrated into a fibre
**Crystalline regions **- if something has a crystalline structure, it will potentially interact with light much more - helping in identification
Gas voids
How do we visualise surface properties of fibres?
Scanning electron microscope
Transmission electron microscope
Atomic force
What are the benefits of an electron microscope?
Higher resolutions are achievable using electrons instead of light
Non destructive analysis of very small quantities of material
Allows for rapid accumulation of results in high resolution
Can even give elemental composition of a material
Which microscope is the most suitable for analysis of trace evidence?
SEM has a good resolution of 10nm, best depth of focus and a high field of view (can scan a large area), it is also easy to prepare the specimen however is very expensive.
Light microscopy is cheap, and also a good field of view and easy specimen preparation. However, it has a limited depth of focus as well as a low resolution 200nm.
TEM has the best resolution of 1nm, however it has a lengthy sample preparation and is very expensive.
What is the use of the refractive index?
This is one of the most useful ways you can identify any material.
How is SEM useful for fibres?
Reveals surface features unseen in optical microscopy e.g scale like features
fibre-end fracture morphology of a polyester
SEM cross section of a nylon carpet
elemental analysis of fibres
How is high contrast achieved?
In the event that these two refractive indices are equal, the light passing through the particle does NOT deviate at all, and the particle remains invisible
When the refractive indices are far apart, the light passing through will change direction substantially
if refracted sufficiently, they miss the objective lens and these areas of the particle becomes dark, resulting in contrast.
How does polarised light microscopy useful?
It is useful when applied to anisotropic substances
What is pleochroism?
the property in anisotropic materials that causes it to show different absorption colours when exposed to polarised light coming from different directions.
