Question

Which of the fixatives is NOT used for sample preparation in TEM?

• A. Formaldehyde
• B. Glutaraldehyde
• C. Osmium tetroxide
• D. Coating with gold and palladium

Hint:

Remember the key difference: TEM (Transmission) looks *through* a very thin slice of the sample. SEM (Scanning) looks *at the surface* of the sample. Therefore, coating the surface with metal is an SEM technique.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The question asks to identify a technique that is not a method of fixation for Transmission Electron Microscopy (TEM). Fixation is the process of preserving a biological sample's structure as closely as possible to its living state.

Step 2: Detailed Explanation:
Let's examine the options in the context of TEM sample preparation:
\[\begin{array}{rl} \bullet & \text{1. Formaldehyde and 2. Glutaraldehyde: These are aldehydes and are the most common primary chemical fixatives for TEM. They work by cross-linking proteins, which stabilizes the cell's ultrastructure. } \\ \bullet & \text{3. Osmium tetroxide: This is typically used as a secondary fixative after aldehyde fixation. It cross-links and stabilizes cell membranes (lipids) and also acts as a heavy metal stain, increasing the contrast of the image. } \\ \bullet & \text{4. Coating with gold and palladium: This is a technique called sputter coating. It involves depositing a thin layer of conductive metal onto the surface of a specimen. This is done to make the sample electrically conductive and to prevent the buildup of static charge when it is scanned by the electron beam. This technique is exclusively used for Scanning Electron Microscopy (SEM), which visualizes the surface of objects, not for TEM, which passes electrons through a thin section of the sample. } \\ \end{array}\]

Step 3: Final Answer:
Coating with gold and palladium is a technique used for SEM, not a fixation method for TEM.