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Lesson 1: Introduction to basic cryo-EM
• Vacuum• Cold trap (Anti-contaminator)• Electron gun• Lens system• Deflectors• Specimen transfer• Magnification• Recording camera• Dose monitor• Positions of all knobs
Vacuum System
• Rotary pump
• Diffusion pump
• Ion pump
• Turbo molecular pump
• Column bake-out every 6-10 months
Pumps
• Mechanical pump : 100-0.1 Pa• Diffusion pump: 0.1 – 10-4 Pa• Trubo molecular pump: 10-7 - 10-8 Pa• Ion pump 10-4 – 10-7 Pa• Cryo pump 10-4 – 10-7 Pa
Mechanical and diffusion pump forces the molecules out the system
Ion and cryo pump adsorbs the molecules till warm up
Cold Traps
• Where are they? – Between pump and column– Around specimen area
• How to care of the cold trap?– Daily warm up for specimen cold trap– Monthly warm up for the other traps
TEM Con
Types of Electron Sources
• Tungsten filament
• LaB6 filament
• Field emission gun
Electron Emission
J= AT2Exp(-/kT)
J = Emission current densityA= Constant
T = Temperature = Work function
K= Boltzmann constant
Electron Brightness
Current density per unit solid angle
= Number of electrons per unit time per unit area per unit solid angle (Ampere/cm2/strd)
The smaller the solid angle, the brighter the beam can be
Temporal and Spatial Coherence
• Temporal: Monochromatic electrons (i.e. small energy spread)
• Spatial: Parallel electron beam (i.e. small angular spread)
C1, C2 spot sizeC3 brightnessCM alpha angle
OL and OM forDifferent mags.
IS1 image shift by usersIS2 image shift by engineers
Specimen Dose
• Ispecimen /M2 = Iimage
• Idetectpr/M2 = Iimage
How to measure the dose rate?
• Current density on the screen
• Back calculate the dose rate on the specimen by considering the magnification
What magnification to use?
• D0 = Di /M (Optics)
• Dp = ½ - 1/3 D0 (Sampling Theorem)
• If D0 = 10 Å, then Dp = 10/3 Å
• If Di = 15 x 104 Å (as governed by the detector),
then M = 15x104x3/10=45x103
Expected Resolution On The 4kx4k Gatan CCD Camera with 15 m pixels on the JEM2100
