Microscopy Flashcards
What are microscopes?
Devices that produce magnified images of structures too small to see with the unaided eye
What are the 3 primary types of microscopes?
- Compound microscope
- Dissecting microscope
- Electron microscope
What does a compound microscope do?
- produces a magnified image by focusing visible light rays
- uses 2 lenses
- can observe living or non living specimens
- magnification ranges from 2.5 to 1300x
What limits the use of a compound light microscope?
Light must be able to pass through the specimen - s considerable preparation may be required
What is a comparative microscope?
- type of compound microscope
- two separate items can be viewed simultaneously under same lighting conditions and magnification
- allows for meticulous comparison in court
When is a dissecting microscope used?
When viewing large specimens
What does a dissecting microscope do?
- 3-D image is viewed
- light does not pass through object but the surface is magnified and a light source allows adjustment to contrast imaging
What is the main disadvantage of a dissecting microscope?
Lower magnification than compound microscope
What is the main advantage of a dissecting microscope?
Evidence rarely requires pre-treatment
What does an electron microscope do?
- focuses beams of electrons to magnify and image
- magnifies 1000x or more than a compound microscope
- can not be used to study living specimens because they must be dried in a vacuum inside the microscope
What are the different types of electron microscopes?
- scanning electron microscope
- environmental scanning electron microscope
- transmission electron microscope
- scanning tunnelling microscope
What are the advantages of light microscopes? (5)
- magnify details and spatial relationships of objects
- no need for sample pre-treatment
- ability to observe living cells
- flexible sample types
- image capture and storage
What is the disadvantage of light microscopes?
Low resolution
What are the advantages of electron microscopes? (4)
- higher resolutions
- all kinds of samples
- alternative information
- can image 3D external shape
What are some disadvantages of electron microscopes? (2)
- elaborate sample preparation
- dry so cannot view living cells
What are light microscopes used for in forensics generally?
Used for comparison between evidence item and suspected linked object
Examples of things light microscopes are used for in forensics
- striations on bullets
- tool marks
- fingerprints
- type impressions
- manufacture marks
- comparison of structure and colour of hair, clothing or plant fibres, paint fragments, biological specimens and skeletal trauma marks
What is SEM generally used for in forensics?
For characterising surface structure and comparison of surface composition
Examples of SEM uses in forensics
- striations on bullets
- composite soils (rocks, sediments, soils and dust)
- insect taxonomic characteristics
- paint comparisons
- manufacture batch matching (eg glass, paper, tape and metals)
What is the basic principle of SEM?
- uses beam of electrons
- electron beam hits sample and interacts with samples - electrons are scattered
What are secondary electrons?
- ejection of loosely bound outer orbital electrons from specimen atoms by in elastic scattering
- requires little energy for this and resultant secondary electrons have low energy
- only SES produced near the surface are ejected and detected
- reflect structural information
What are back scattered electrons?
- primary beam electrons elastically scattered by atom nuclei in the sample back out of the sample
- have high energy
- come from a larger volume within the sample
- heavier elements = more scattering = more BSEs
- reflect compositional information
Regions of high atomic number will appear ….
Bright
What is the magnification of SEM?
10 - 500,000x
What is the resolution of SEM?
Up to 1nm
What does the quality and resolution of SEM depend on?
- instrument performance
- selection of imaging parameters
- nature of the sample
