Flashcards in Lab 4 - The Electron Microscope Deck (25)
Why does the electron microscope need a high vacuum?
Electrons can’t travel far in air, so this allows for them to travel more quickly and further
How is a high vacuum maintained?
A cold trap; a metal insert in the column of the microscope that is cooled by liquid nitrogen
Why is a tungsten wire used for the filament in the electron gun?
It works well as a cathode; emitting electrons from its surface when it’s heated
Compare the lenses of a light microscope and an electron microscope.
Light microscope: uses glass and moving the stage to focus the specimen
Electron microscope: use negative voltage to control electron emission and shape of the beam via strong magnets
How can the brightness of illumination through the specimen be varied?
A condenser lens (same as light microscope)
Describe in general, how the beam of electrons in a transmission electron microscope is affected by the specimen and how this leads to the formation of an image
Electrons pass through specimen, thicker parts of the specimen allow fewer electrons to pass through and vice versa, this means the photographic plate will show that area as darker, and coming together to create an image.
How does the final image in a transmission electron microscope become visualized?
Image is visualized on a photographic plate
Explain the advantages of the high voltage electron microscope
Accelerating the voltage = penetrating power increase = 10 times thickness is still possible for a good image
In general, how is the formation of images produced by a scanning electron microscope different from the formation of images in a transmission electron micrscope
In SEM there is no cathode, the electrons are excited from the coated surface of the specimen.
What is a scintillator and what role does it play in image formation? What are the electrons called that affect the scintillator?
It emits photon of light when excited by electrons, allowing us to visualize it. Ionizing? (Double check)
How is a high vacuum in electron microscopes created?
By the cold trap: a metal column inserted into the column of the microscope that is cooled by liquid nitrogen
What is the purpose of the tungsten wire filament?
To act as a cathode: emits electrons when heated
How can the brightness of illumination through a specimen be modified?
By using the condenser
How is the beam of electrons in a TEM affected by the specimen and how does this lead to the formation of an image?
1. Electrons pass through specimen, as it passes through the more dense areas the image will show up as darker, when it passes through less dense areas the image will show up as lighter, through this variation a contrast image is created
How is the final image in a transmission electron visualized?
Electrons are captured and emitted as photons by a photographic plate
What is the advantage of a higher voltage electron microscope?
The higher the voltage, the higher the penetrating power
How does a scanning electron microscope work?
Specimens surface is scanned by a beam of electrons that are controlled and focussed by beam deflectors, electrons on the surface of the specimen are excited and shoot out secondary electrons, these secondary electrons are captured by a detector
What allows us to visualize an image in the SEM?
Describe how samples are prepared for TEM
Fixed, dehydrated, cast in resin, sliced with an ultramicrotome
What is the name of a common fixative?
What is the name of a common poststaining dye?
Lead and uranium
What is immunoelectron microscopy?
Antibodies are linked to electron dense substances like gold molecules to better visualize immune cells
What stain is used for negative contrast?
What type of microscope is used to visualize freeze fracture?