Flashcards in Electron Microscopy 2 Deck (35):
What is the wavelength of a electron?
Picometer wavelength - usually around 2 picometers in EM.
How are the electrons focussed in EM?
Using electromagnetic lenses
What interacts more strongly with matter, X-rays or electrons?
Which parts of the specimen scatter more, thick or thin?
Thick - greater than 100nm makes EM very difficult.
Why are cryo EM images often under exposed?
Because the electron dosage has to be limited.
What is helping improve under-exposure of EM image?
The development of newer cameras.
What are the two advantages of using a holey carbon grid for viewing the sample?
1. It does not scatter electrons - which would generate noise.
2. The holes allow for pockets of solvent to form - keeping proteins in hydrated state.
Why is EM kept under a vacuum?
As electrons scatter very easily.
List 3 types of electron guns in increasing order of quality.
1. Tungsten filament
2. LaB6 (Lanthanum hexaboride crystal)
3. Field Emission Gun (FEG)
Describe the how the tungsten filament and LaB6 electron guns work?
The (blunt) tip is heated and this ejects electrons.
How does the field emission gun work?
This has an extremely sharp tip, which when a current is past through it electrons are emitted.
What is spatial coherence?
Electrons that originate from the same point
What is temporal coherence?
Electrons that all have the same wavelength
Why is the FEG better than the tungsten filament or LaB6 gun?
It produces spatially and temporally coherent electrons due to its sharp tip.
Name five methods for generating a vacuum in EM?
1. Mechanical pump
3. Turbo-molecular pump
4. Oil diffusion pump
5. Ion getter pump
Describe how the mechanical pump generates a vacuum.
A paddle scoops gas out the tube that leads to the microscope column.
Describe how the cryopump generates a vacuum.
Cool a metal slab using liquid nitrogen to ~77K. Gas molecules freeze on surface, immobilising them or retarding them for a long time.
Describe how the turbo molecular pump generates a vacuum.
High energy fan spinning at thousands of hertz that filters out gas molecules.
Describe how the oil diffusion pump generates a vacuum.
There is a layer of super heated oil that evaporates into a water cooled chamber. The oil combines with any gas and then cools and runs down the side of the chamber.
Describe how the ion getter pump generates a vacuum.
Cosmic rays break covalent bonds in the gas. The column contains anodes and cathodes which attract the ions that are generated.
Name the equation for force applied to electrons.
F = -e(v x B)
v = velocity
B = magnetic field
e = electron charge
(think left hand rule)
How is the magnification altered?
By changing the amount of current going through the coil.
What is spherical aberration?
Highly scattered electrons are over-focussed by the lens
How do glass lenses remove spherical aberration?
They are parabolic - thinner at the edges.
Can spherical aberration be corrected for in electromagnetic lenses?
How can spherical aberration be reduce in EM?
Centre the electron beam on the optical axis using a deflector pair and remove highly scattered electrons with an aperture.
What is chromatic aberration?
This is where electrons have varying energies (white electrons), resulting in them being focussed differently.
What is elastic scattering?
When electrons interact with the nucleus of an atom and no energy is lost (monochromatic).
What is inelastic scattering?
This is when electrons interact with the electron cloud of an atom and deposit energy into sample (chromatic aberrations). The energy usually results in a covalent bond breaking.
Do thicker specimens result in more inelastic scattering?
What is constructive interference?
When the wavefronts of two scattering events are coherent.
What is spatial frequency?
The distance between the atoms.
What does kind of scattering does high spatial frequency cause?
High angle scattering.
What is the Fourier Transform in EM?
The scattering of the electrons.