Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

M3D > Electron Microscopy 2 > Flashcards

Electron Microscopy 2 Flashcards

1
Q

What is the wavelength of a electron?

A

Picometer wavelength - usually around 2 picometers in EM.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

How are the electrons focussed in EM?

A

Using electromagnetic lenses

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What interacts more strongly with matter, X-rays or electrons?

A

Electrons

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Which parts of the specimen scatter more, thick or thin?

A

Thick - greater than 100nm makes EM very difficult.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Why are cryo EM images often under exposed?

A

Because the electron dosage has to be limited.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is helping improve under-exposure of EM image?

A

The development of newer cameras.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What are the two advantages of using a holey carbon grid for viewing the sample?

A
  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.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Why is EM kept under a vacuum?

A

As electrons scatter very easily.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

List 3 types of electron guns in increasing order of quality.

A
  1. Tungsten filament
  2. LaB6 (Lanthanum hexaboride crystal)
  3. Field Emission Gun (FEG)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Describe the how the tungsten filament and LaB6 electron guns work?

A

The (blunt) tip is heated and this ejects electrons.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

How does the field emission gun work?

A

This has an extremely sharp tip, which when a current is past through it electrons are emitted.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is spatial coherence?

A

Electrons that originate from the same point

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is temporal coherence?

A

Electrons that all have the same wavelength

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Why is the FEG better than the tungsten filament or LaB6 gun?

A

It produces spatially and temporally coherent electrons due to its sharp tip.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Name five methods for generating a vacuum in EM?

A
  1. Mechanical pump
  2. Cryopump/trap
  3. Turbo-molecular pump
  4. Oil diffusion pump
  5. Ion getter pump
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Describe how the mechanical pump generates a vacuum.

A

A paddle scoops gas out the tube that leads to the microscope column.

17
Q

Describe how the cryopump generates a vacuum.

A

Cool a metal slab using liquid nitrogen to ~77K. Gas molecules freeze on surface, immobilising them or retarding them for a long time.

18
Q

Describe how the turbo molecular pump generates a vacuum.

A

High energy fan spinning at thousands of hertz that filters out gas molecules.

19
Q

Describe how the oil diffusion pump generates a vacuum.

A

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.

20
Q

Describe how the ion getter pump generates a vacuum.

A

Cosmic rays break covalent bonds in the gas. The column contains anodes and cathodes which attract the ions that are generated.

21
Q

Name the equation for force applied to electrons.

A 
F = -e(v x B) 
v = velocity 
B = magnetic field
e = electron charge 
(think left hand rule)
22
Q

How is the magnification altered?

A

By changing the amount of current going through the coil.

23
Q

What is spherical aberration?

A

Highly scattered electrons are over-focussed by the lens

24
Q

How do glass lenses remove spherical aberration?

A

They are parabolic - thinner at the edges.

25
Q

Can spherical aberration be corrected for in electromagnetic lenses?

A

No

26
Q

How can spherical aberration be reduce in EM?

A

Centre the electron beam on the optical axis using a deflector pair and remove highly scattered electrons with an aperture.

27
Q

What is chromatic aberration?

A

This is where electrons have varying energies (white electrons), resulting in them being focussed differently.

28
Q

What is elastic scattering?

A

When electrons interact with the nucleus of an atom and no energy is lost (monochromatic).

29
Q

What is inelastic scattering?

A

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.

30
Q

Do thicker specimens result in more inelastic scattering?

A

Yes

31
Q

What is constructive interference?

A

When the wavefronts of two scattering events are coherent.

32
Q

What is spatial frequency?

A

The distance between the atoms.

33
Q

What does kind of scattering does high spatial frequency cause?

A

High angle scattering.

34
Q

What is the Fourier Transform in EM?

A

The scattering of the electrons.

35
Q

What is the inverse Fourier Transform in EM?

A

The refocussing of the electrons by the lenses.

M3D (7 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions