Name: how tem reveals tiny structures
Uploaded: 2023-04-30T12:01:08.000Z
Duration: 1 min 15 s
Channel: JoVE

Ein Transmissionselektronenmikroskop (TEM) wird verwendet, um die innere Struktur und Zusammensetzung einer Probe zu untersuchen.
Das Instrument besteht aus einer Elektronenkanone, die einen Elektronenstrahl aussendet. Der Strahl wird dann mit Hilfe von zwei oder mehr elektromagnetischen Kondensorlinsen auf die Probe fokussiert.
Die durch die Probe übertragenen Elektronen werden von Objektivlinsen gesammelt und fokussiert, um das Zwischenbild zu erzeugen. Dieses Bild wird durch zusätzliche Zwischen- und Projektorlinsen weiter vergrößert.
Das endgültige Bild kann mit elektronenempfindlichen Bildgebungsgeräten, wie z. B. einem phosphoreszierenden Bildschirm oder einer CCD-Kamera, aufgenommen werden.
TEM erstellt ein zweidimensionales Schwarz-Weiß-Bild einer Probe. Dunklere Bereiche stellen dichte Bereiche dar, die nur wenige oder keine Elektronen übertragen. Hellere Bereiche zeigen dagegen die Bereiche an, durch die mehr Elektronen übertragen werden.
TEM wird auch zur Analyse der Probenzusammensetzung verwendet. Die Elektronenstrahl-Proben-Wechselwirkung erzeugt charakteristische Röntgenstrahlen, die zur Identifizierung und Quantifizierung verschiedener in einer Probe vorhandener Elemente verwendet werden können.

transmissionselektronenmikroskopie

Transmission Electron Microscopy

Im Jahr 1931 entwickelte der Physiker Ernst Ruska – aufbauend auf der Idee, dass Magnetfelder einen Elektronenstrahl genauso lenken können wie Linsen einen Lichtstrahl in einem optischen Mikroskop lenken können – den ersten Prototypen des Elektronenmikroskops. Diese Entwicklung führte zur Entwicklung des Gebiets der Elektronenmikroskopie. Im Transmissionselektronenmikroskop (TEM) werden Elektronen von einem heißen Wolframelement erzeugt und durch eine Potentialdifferenz in einer Elektronenkanone beschleunigt, wodurch sie eine kinetische Energie von bis zu 400 keV erhalten. Nach dem Verlassen der Elektronenkanone wird der Elektronenstrahl durch elektromagnetische Linsen (ein System aus Sammellinsen) fokussiert und durch eine zu betrachtende Probe geleitet. Aus dem durchgelassenen Elektronenstrahl wird das Bild der Probe rekonstruiert. Das vergrößerte Bild kann entweder direkt auf einem fluoreszierenden Bildschirm oder indirekt durch Senden, beispielsweise an eine Digitalkamera oder einen Computermonitor, betrachtet werden.

Der gesamte Aufbau, bestehend aus der Elektronenkanone, den Linsen, der Probe und dem Fluoreszenzschirm, ist in einer Vakuumkammer eingeschlossen, um Energieverluste durch den Strahl zu verhindern. Moderne hochauflösende Modelle eines TEM können ein Auflösungsvermögen von mehr als 0,5 Å und Vergrößerungen von mehr als 50 Millionen Mal haben. Zum Vergleich: Das beste Auflösungsvermögen, das mit der Lichtmikroskopie erreicht wird, liegt derzeit bei etwa 97 nm.

Eine Einschränkung des TEM besteht darin, dass die Proben etwa 100 nm dick sein müssen und biologische Proben eine spezielle Vorbereitung mit chemischer „Fixierung“ erfordern, um sie für das ultradünne Schneiden zu stabilisieren. Um diese Einschränkungen zu überwinden, wurden mehrere Fortschritte bei TEM-Techniken wie Kryo-TEM gemacht, um Artefakte zu beseitigen und eine direkte Probenabbildung ohne die probenschädigenden Schritte der Probenfixierung und -dehydrierung zu ermöglichen.

Dieser Text ist adaptiert von <a href="https://openstax.org/books/university-physics-volume-3/pages/6-6-wave-particle-duality?query=transmission%20electron&target=%7B%22type%22%3A%22search%22%2C%22index%22%3A0%7D#fs-id1163711954948">Openstax, University Physics Volume 3, Chapter 6 Photons and Matter Waves, Section 6.6: Wave-Particle Duality.</a>

Transmissionselektronenmikroskopie

Im Jahr 1931 baute der Physiker Ernst Ruska auf der Idee auf, dass Magnetfelder einen Elektronenstrahl genauso lenken können wie Linsen einen Lichtstrahl in einem ...

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how tem reveals tiny structures

Transmission electron microscopy uses electrons to reveal very small structures inside a specimen. The technique grew from the first electron microscope prototype developed in 1931 by physicist Ernst Ruska. He built on the idea that magnetic fields can direct an electron beam, just as lenses direct light in an optical microscope.

In a transmission electron microscope, a hot tungsten element produces electrons. An electron gun then accelerates them with a potential difference, giving the beam up to 400 keV of kinetic energy. Electromagnetic lenses, arranged as condensing lenses, focus the beam before it passes through the sample.

The image is formed from the electrons that move through the specimen. That reconstructed image can be viewed on a fluorescent screen or sent to a digital camera or computer monitor. The full system, including the electron gun, lenses, specimen, and screen, sits inside a vacuum chamber to reduce energy loss from the beam.

Modern high-resolution TEM models can resolve details smaller than 0.5 Å and reach magnifications above 50 million times. By comparison, the best resolving power of light microscopy is about 97 nm. TEM does have limits, because samples must be about 100 nm thick, and biological samples need chemical fixing before ultrathin slicing.

To reduce these limits, researchers have developed newer TEM methods such as cryo-TEM. These approaches help remove artifacts and can allow direct imaging without the sample-damaging steps of fixation and dehydration.

How TEM Reveals Tiny Structures

imaging biological samples with optical microscopy

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Optical microscopy uses light and lenses to make tiny biological samples visible. Antonie van Leeuwenhoek built the first compound optical microscope in the 17th century to view blood cells, bacteria, and yeast cells. In 1830, Joseph Jackson Lister developed an essentially modern light microscope. During the 20th century, microscopes gained better magnification and resolution. A sample is placed on a glass slide and clipped onto the stage, which is the flat platform of the microscope. The...

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transmission electron microscopy

Transmission electron microscopy uses electrons to reveal very small structures inside a specimen. The technique grew from the first electron microscope prototype developed in 1931 by physicist Ernst Ruska. He built on the idea that magnetic fields can direct an electron beam, just as lenses direct light in an optical microscope. In a transmission electron microscope, a hot tungsten element produces electrons. An electron gun then accelerates them with a potential difference, giving the beam...

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Electron Microscopy Sample Prep Methods

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3D Imaging with Electron Microscopy

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key terms and concepts

Learn how transmission electron microscopy uses electrons, magnetic lenses, and a vacuum to image ultra-thin samples at extreme magnification.

How TEM Reveals Tiny Structures | JoVE K12

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How TEM Reveals Tiny Structures

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