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

Un microscopio elettronico a trasmissione o TEM viene utilizzato per studiare la struttura interna e la composizione di un campione.
Lo strumento è costituito da un cannone elettronico che emette un fascio di elettroni. Il fascio viene quindi focalizzato sul campione utilizzando due o più lenti a condensatore elettromagnetico.
Gli elettroni trasmessi attraverso il campione vengono raccolti e focalizzati dalle lenti dell'obiettivo per formare l'immagine intermedia. Questa immagine è ulteriormente ingrandita da ulteriori lenti intermedie e del proiettore.
L'immagine finale può essere acquisita utilizzando dispositivi di imaging sensibili agli elettroni, come uno schermo fosforescente o una telecamera CCD.
La TEM forma un'immagine bidimensionale in bianco e nero di un campione. Le aree più scure rappresentano regioni dense che trasmettono pochi o nessun elettrone. Al contrario, le aree più chiare indicano le regioni attraverso le quali vengono trasmessi più elettroni.
La TEM viene utilizzata anche per analizzare la composizione del campione. L'interazione fascio di elettroni-campione genera raggi X caratteristici che possono essere utilizzati per identificare e quantificare diversi elementi presenti in un campione.

microscopia elettronica a trasmissione

Transmission Electron Microscopy

Nel 1931, il fisico Ernst Ruska, basandosi sull'idea che i campi magnetici possono dirigere un fascio di elettroni proprio come le lenti possono dirigere un raggio di luce in un microscopio ottico, sviluppò il primo prototipo del microscopio elettronico. Questo sviluppo ha portato allo sviluppo del campo della microscopia elettronica. Nel microscopio elettronico a trasmissione (TEM), gli elettroni vengono prodotti da un elemento caldo di tungsteno e accelerati da una differenza di potenziale in un cannone elettronico, che conferisce loro un'energia cinetica fino a 400 keV. Dopo aver lasciato il cannone elettronico, il fascio di elettroni viene focalizzato da lenti elettromagnetiche (un sistema di lenti condensatrici) e trasmesso attraverso un campione da visualizzare. L'immagine del campione viene ricostruita dal fascio di elettroni trasmesso. L'immagine ingrandita può essere visualizzata direttamente su uno schermo fluorescente oppure indirettamente inviandola, ad esempio, ad una fotocamera digitale o al monitor di un computer.

L'intera configurazione composta dal cannone elettronico, dalle lenti, dal campione e dallo schermo fluorescente è racchiusa in una camera a vuoto per prevenire la perdita di energia dal raggio. I moderni modelli ad alta risoluzione di un TEM possono avere un potere risolutivo maggiore di 0,5 Å e ingrandimenti superiori a 50 milioni di volte. Per fare un confronto, il miglior potere risolutivo ottenuto con la microscopia ottica è attualmente di circa 97 nm.

Una limitazione del TEM è che i campioni devono avere uno spessore di circa 100 nm e i campioni biologici richiedono una preparazione speciale che comporta un “fissaggio” chimico per stabilizzarli per l’affettatura ultrasottile. Per superare le limitazioni, sono stati fatti diversi progressi nelle tecniche TEM, come la crio-TEM, per eliminare gli artefatti e consentire l'imaging diretto del campione senza le fasi dannose del campione di fissazione e disidratazione.

Questo testo è adattato da <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>

Microscopia elettronica a trasmissione

Nel 1931, il fisico Ernst Ruska, basandosi sull'idea che i campi magnetici possono dirigere un fascio di elettroni proprio come le lenti possono dirigere un raggio di luce in un ...

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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

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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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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

cell biology

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How TEM Reveals Tiny Structures

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