Electron Microscope Learning Objectives Know the difference between

Electron Microscope

Learning Objectives • Know the difference between a transmission and a scanning electron microscope • Explain the benefits of electron microscopes • Compare and contrast EM & LM

Starter – light microscopes • Magnification X 1500 • Resolution 200 nm • Light passes through so need thin specimens • Can be alive or dead • Stains can be used to enhance contrast

Electron Microscope • EM generates beam of electrons • Focused using magnets • Electron beam passes through very thin prepared sample (TEM) or bounces off sample (SEM) • Stain with heavy metals • Vacuum – sample is dead • Resolution is 0. 2 nm (1000 x more than light microscope)

Light vs EM Which is which? What are the main differences?

Preparing Slide of EM • • Fix specimen in gluteraldehyde Dehydrate with ethanol Embed in resin Slice thinly Stain (salts of heavy metals – eg lead, uranium) Mount on copper grid Place in vacuum (allows electrons to travel towards specimen)

SEM vs TEM • A seed viewed with a scanning EM

Transmission Electron Microscope • Electrons pass through denser part of sample less easily so create contrast • 2 D • Magnification up to x 500 000

Scanning Electron Microscope • Electrons ‘bounce’ off metal-salt-stained sample • 3 D • Magnification x 100 000

Task • Construct a table/diagram/revision resource which compares and contrast Light and Electron microscopes. • Include: resolution, magnification, stains, types of samples viewed, level of detail seen.

Light Electron Resolution 200 nm 0. 2 nm Magnification X 1500 TEM x 500 000 SEM x 100000 Stains Dyes Heavy metals Types of sample Living and dead Only dead samples 2 D colour Black and white TEM 2 D SEM 3 D Level of detail False colour can be added by computers

Advantages and disadvantages of EM Advantages of EM Disadvantages of EM Resolution is x 1000 more than LM Samples have to be placed in a vacuum Produces detailed images Very expensive SEM produces 3 D images Need to be highly skilled to create samples

Essay Questions • Using your textbook and your notes answer the essay question: • Compare and contrast the advantages and limitations of EM and LM [12]

Mark Scheme • • • Resolution (marks for actual figures) [2] Detail in image: SEM, TEM, LM [3] Preparing samples [2] How they work: SEM, TEM, LM [3] EM expensive [2] QWC [1]