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: Introduction:
The task is to identify the non-fixation method for Transmission Electron Microscopy (TEM). Fixation preserves a biological sample's structure.

Step 2: Analysis:
Consider the options for TEM sample preparation:
\[\begin{array}{rl} \bullet & \text{1. Formaldehyde and 2. Glutaraldehyde: These aldehydes are the primary chemical fixatives for TEM. They stabilize the cell's ultrastructure by cross-linking proteins. } \\ \bullet & \text{3. Osmium tetroxide: Used as a secondary fixative after aldehyde fixation. It stabilizes cell membranes (lipids) and increases image contrast by acting as a heavy metal stain. } \\ \bullet & \text{4. Coating with gold and palladium: This is sputter coating, which deposits a conductive metal layer onto the specimen's surface. This makes the sample electrically conductive and prevents static charge buildup during Scanning Electron Microscopy (SEM). This is exclusive to SEM, which visualizes the surface, not TEM, which uses thin sample sections. } \\ \end{array}\]

Step 3: Conclusion:
Coating with gold and palladium is an SEM technique, not a TEM fixation method.