Brainscape's Knowledge GenomeTM

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


See full index

MSF1 > Electron Microscopy 1 > Flashcards

Electron Microscopy 1 Flashcards

1
Q

Who invented EM and when?

A

Ernst Ruska in 1933

Note - did not receive the Nobel prize till 50 years later.

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

List the sizes of cells, bacteria, viruses, proteins and atoms.

A 
Cells - 10^-5m
Bacteria - 10^-6m
Viruses - 10^-7/10^-8m 
Proteins - 10^-9m
Atoms - 10^-10m
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

When did the resolution revolution occur?

A

Around 2 years ago people started to use EM as a way of resolving atomic structures due to technological innovations.

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

What is the source of the light microscope?

A

Light

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

What is the source of the electron microscope?

A

Electron gun - made up of a cathode filament and an anode.

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

What is the job of the cathode?

A

This is the electron source.

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

What is the job of the anode?

A

This is the electron accelerator.

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

What are the lens in EM?

A

They are electromagnetic coils.

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

What is the first lens and what is its job?

A

First lens is the condenser lens and it concentrates the beam of electrons.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the second lens and what is its job?

A

The second lens is the objective lens and it focuses the beam onto certain parts of the specimen.

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

Why do we require electrons?

A

If you want to see objects that are smaller than a photon, then it must be imaged with particles smaller than a photon i.e. electrons.

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

What is the Rayleigh criteria and what does it define?

A

d = (0.61 x lambda)/(N.A)

It defines the theoretical resolution limit.

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

Define all the parameters in the Rayleigh criteria.

A

d = distance between two points (this is your ability to resolve them).
lambda - wavelength of the radiation
N.A. the numerical aperture of the lens.

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

What is the only thing that limits the resolution if the rest of the imaging system is perfect?

A

Diffraction

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

If the wavelength of visible light is 400-700nm and there is a perfect lens what is the resolution limited to?

A

~200nm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is the wavelength of electrons at 200kV?

A

0.0025nm

However, not possible to get this resolution for a structure

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Define an electron.

A

Electrons are negatively charged particles that interact with other particles.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What type of wave is an electron beam?

A

A plane wave - the frequency is constant and wavefronts are infinite parallel planes.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What happens when an electron encounters an atom?

A

Electrons interact (are scattered by) biological material much more strongly than X-rays.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What is the ratio of elastically scattered electrons and inelastically scattered electrons by biological molecules?

A

3 times as many inelastically scattered electrons as scattered electrons.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What does this bad ratio of inelastic and elastic scattering of electrons implicate?

A

Produces a bad signal to noise ratio.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

How many electrons are unscattered?

A

80%

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What happens when electrons interact with the nucleus?

A

They are back scattered.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What happens when electrons interact with the electron cloud?

A

They are either elastically or inelastically scattered.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

Which electrons contribute to the image and which ones produce noise in the image?

A

Elastically scattered electrons contribute to image.

Inelastically scattered electrons produce noise.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What is the signal to noise ratio normally like?

A

Tends to be poor.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

What is bad about inelastically scattered electrons, other than creating noise?

A

They also produce radiation that is damaging to biological molecules.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

Why does EM operate under a strict vacuum?

A

To prevent unwanted electron scattering - like scattering from air.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

What are the two types of electron source?

A

Field emission gun and thermal emission gun

30
Q

What does the condenser system do?

A

Controls intensity and convergence of beam.

31
Q

What does the objective lens do?

A

Generates contrast and contributes to the contrast transfer function (CTF)

32
Q

What does the projector system do?

A

Magnifies the image.

33
Q

How are electrons emitted from the source?

A

The cathode filament is heated with energy and then the electrons are pulled off the metal.

34
Q

What is the difference between the FEG and TEG?

A

The FEG has a very small tip so electrons coming off will be very coherent.

35
Q

What does aperture do overall?

A

Remove highly scattered electrons - electrons you do not want to contribute to your image.

36
Q

What does the condenser aperture do?

A

Reduces spherical abnormalities - reducing spot size.

37
Q

What does the objective aperture do?

A

Increases contrast, by removing the scattered electrons that reduce contrast.

38
Q

How do the condenser and object lens magnets control the amount of electrons that are allowed through?

A

Through varying hole sizes in the lens magnet.

39
Q

What are the three electromagnetic lenses in EM?

A

Condenser, objective and projector lenses.

40
Q

What are the lens made of and how to they interact with the electrons?

A

They are lots of copper coils going round that generate a magnetic field that bends the electron plane wave as it passes through the centre of the coil.

41
Q

What happens to the electrons coming from the condenser system?

A

They are scattered by the sample situated in the object plan of the objective lens.

42
Q

What does the objective lens do to the electrons scattered in the same direction by the sample?

A

Focuses these scattered electrons in the back focal plane producing the diffraction pattern.

43
Q

What does the projector lens do?

A

Responsible for image formation - magnifies the image.

44
Q

What is the objective lens also used for?

A

Defocussing.

45
Q

What does defocussing do?

A

Generates contrast in the image.

46
Q

What is phase contrast a result of and why is it required?

A

Result of constructive and destructive interference patterns of electron waves.
The structure of a molecule is imprinted in the small variations in phase of the electron waves.

47
Q

Why in EM do you not want the image to be taken in focus?

A

Because all the electron waves are destructively interfering with each other - because all the waves of the positive and negative troughs of the waves cancel each other out.
(remember electrons are plane waves - they are in phase and have same frequency).

48
Q

What does defocussing allow for?

A

Allows for contrast to be introduced.

49
Q

Why does contrast need to be introduced?

A

Because of poor ratio of elastic to inelastic scattering.

50
Q

How is contrast introduced?

A

By introducing a shift in the plane wave

51
Q

Why does defocussing result in a clearer image?

A

Electrons are further away from each other when less focussed so there is less interference.

52
Q

What causes the additional phase shifts?

A

The objective lens

53
Q

What is the contrast transfer function?

A

A mathematical expression describing the phase shifts resulting from defocussing and imperfections inherent in the objective lens.

54
Q

What is the effect of the CTF on the mage?

A

Causes oscillations of positive and negative contrast at different resolution (frequency) ranges.

55
Q

What happens at higher resolution to the contrast?

A

There are more frequent oscillations between positive and negative contrast.

56
Q

What does this oscillation cause and how is it resolved?

A

The oscillation between positive and negative contrast causes huge distortion of the image - to resolve this lenses do Fourier Transforms.

57
Q

How did the images used to be detected?

A

after magnification by the projector lens - the image was developed on film in a dark room - very laborious

58
Q

What was first digital version to be created?

A

Charged coupled devices (CCD).

59
Q

What was advantageous about using CCDs?

A

Much faster and gave instant feedback.

60
Q

What were the disadvantages of CCDs?

A

They were less sensitive than using film - the electrons reacted with the silver on the film making it more sensitive.

61
Q

What is the general process of the CCDs?

A

Electrons are converted to light and then this is converted to a pixel.

62
Q

What converts the electrons to light?

A

A scintillator.

63
Q

What transfers the light image to the image sensor?

A

Transferred via fibre optic.

64
Q

What does the image sensor do?

A

Converts light image to an electronic image.

65
Q

Why is CCDs limited in resolution?

A

Because of scattering of electrons in the scintillator and fibre optic.

66
Q

What is CCDs an example of?

A

Indirect detection.

67
Q

What is direct detection and what does it do to the signal?

A

Directly converts electrons to pixels - produces a more coherent signal.

68
Q

Why is it better to use direct detection?

A

Because if you can detect a single electron interaction with the material then you you do not need as a high of an electron dosage.

69
Q

What does a lower electron dosage result in?

A

Less radiation damge.

70
Q

What can these direct detectors do?

A

Can align all the images to produce the clearest model - more like recording a movie than taking a photograph.

MSF1 (12 decks)

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