La microscopie Électronique à Transmission

A TEM uses an electron gun to emit electrons that are focused through electromagnetic condenser lenses onto a sample. Electrons transmitted through the sample are collected and magnified by objective lenses, intermediate lenses, and projector lenses. The final image is captured on a phosphorescent screen or CCD camera, creating a two-dimensional black and white image where darker areas represent dense regions and lighter areas show regions with higher electron transmission.

In transmission electron microscopy images, contrast reflects electron density and transmission. Darker regions indicate dense areas that transmit few or no electrons, blocking the beam from reaching the detector. Lighter regions show areas where more electrons pass through the sample. This contrast pattern reveals the internal structure and composition of the specimen being examined.

Electromagnetic lenses in a TEM focus and direct the electron beam, functioning similarly to how optical lenses direct light in conventional microscopes. Condenser lenses focus the electron beam onto the sample, while objective lenses collect and focus transmitted electrons to form an intermediate image. Additional intermediate and projector lenses further magnify this image for final viewing or capture.

When the electron beam interacts with a sample, it generates characteristic X-rays that can be detected and analyzed. These X-rays are unique to specific elements, allowing researchers to identify and quantify different elements present in the sample. This analytical capability makes TEM valuable for both structural imaging and elemental composition studies.

Biological samples require chemical fixation to stabilize them for ultrathin slicing because TEM samples must be approximately 100 nanometers thick. Standard preparation involving fixation and dehydration can create artifacts. To overcome these limitations, techniques like cryo-electron microscopy have been developed to allow direct sample imaging without damaging preparation stages.

Modern high-resolution TEM models achieve resolving power greater than 0.5 angstroms with magnifications exceeding 50 million times. In comparison, the best resolving power obtained with light microscopy is currently about 97 nanometers. This superior resolution allows TEM to visualize cellular and molecular structures at atomic scales, far beyond the capabilities of conventional optical microscopy.

The entire TEM setup, including the electron gun, lenses, specimen, and fluorescent screen, is enclosed in a vacuum chamber to prevent energy loss from the electron beam. Without a vacuum, electrons would collide with air molecules, scattering and losing energy before reaching the sample. This vacuum environment ensures the electron beam maintains sufficient energy and coherence for high-resolution imaging.