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Difficulty: MediumProcedural Steps

To prepare delicate biological specimens, such as insect eyes or fungal spores, for imaging under a Scanning Electron Microscope (SEM), researchers must execute a meticulous multi-step preparation protocol. Because the SEM operates under a high vacuum and utilizes a high-energy electron beam, wet specimens would collapse and accumulate electrical charges. First, researchers stabilize the specimen’s structure using chemical fixatives like glutaraldehyde. Next, they dehydrate the tissue by soaking it in a graded series of ethanol solutions of increasing concentration, ending with absolute ethanol. Once dehydrated, the specimen undergoes critical point drying. In this step, liquid carbon dioxide replaces the ethanol under high pressure, and the temperature is raised until the carbon dioxide transitions directly from liquid to gas, avoiding the destructive surface tension of evaporating liquids. Following drying, the researcher mounts the specimen onto a metal stub using conductive carbon tape. Finally, the specimen is transferred to a sputter coater, where a thin layer of gold or palladium is deposited onto its surface. This metallic coating is essential for conducting electrons away from the specimen during imaging, thereby preventing image distortion caused by electrical charging.

According to the passage, if a researcher attempts to perform critical point drying on a specimen without first soaking it in a graded series of ethanol solutions, what is the most likely consequence for the specimen?

  1. A
    The biological specimen will accumulate electrical charges during imaging, resulting in distorted images.
  2. B
    The specimen will fail to mount securely to the metal stub due to residual moisture interfering with the conductive carbon tape.
  3. The specimen’s structures will be damaged by surface tension because liquid carbon dioxide replaces ethanol, not water, to prevent liquid-to-gas evaporation effects.Answer
  4. D
    The chemical fixatives like glutaraldehyde will remain active, dissolving the specimen's tissue under high vacuum conditions.

Answer

The specimen’s structures will be damaged by surface tension because liquid carbon dioxide replaces ethanol, not water, to prevent liquid-to-gas evaporation effects.
The correct answer is correct because the passage explains that the graded ethanol washes dehydrate the tissue (removing water), and that during critical point drying, liquid carbon dioxide replaces this ethanol. If the ethanol washes are skipped, water remains in the specimen and cannot be replaced by the liquid carbon dioxide. This water then undergoes normal liquid-to-gas evaporation, subjecting the delicate structures to the destructive surface tension that the critical point drying process is specifically designed to avoid.

Step-by-Step Solution

1
Identify the step that is being omitted and its role in the process.
Soaking the specimen in graded ethanol solutions is described as a method to dehydrate the tissue.
Dehydration is necessary because it removes water, replacing it with ethanol.
2
Analyze how this omission affects the subsequent critical point drying step.
During critical point drying, liquid carbon dioxide replaces the ethanol.
If there is no ethanol to replace, the water remains in the specimen.
3
Determine the outcome of having unreplaced water in the specimen during drying.
The water will evaporate, subjecting the specimen to destructive surface tension.
Critical point drying prevents damage by using carbon dioxide to bypass liquid-to-gas evaporation, but this requires the prior replacement of ethanol, not water.

Key Concept

Procedural Steps and Consequences of Omission
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Procedural Steps

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