Scanning electron microscopy (SEM) – What is standard specimen preparation before imaging for conductive coating?

Introduction / Context:
SEM images surfaces by scanning with a focused electron beam and detecting secondary or backscattered electrons. Biological specimens are inherently nonconductive and beam-sensitive. Standard preparation minimizes charging and damage by chemical fixation, dehydration, drying, and coating with a thin conductive metal film (e.g., gold, gold–palladium, platinum).

Given Data / Assumptions:

• We consider conventional high-vacuum SEM of biological samples.
• Conductive coating provides a path for electrons to dissipate, reducing charging artifacts.
• Fixatives (e.g., glutaraldehyde) preserve ultrastructure prior to drying and coating.

Concept / Approach:
Charging distorts SEM images and reduces signal stability. Thin metal coatings (a few nanometers) improve conductivity and secondary electron yield, enhancing surface detail. While many coating metals are also transition metals, the key concept tested is the presence of a thin heavy-metal coating after proper fixation and drying, not the periodic table classification term itself.

Step-by-Step Solution:

Verification / Alternative check:
Comparing uncoated versus coated samples shows dramatic reduction in charging artifacts and improved resolution, validating the necessity of metallic coating in standard protocols.

Why Other Options Are Wrong:

• “Not fixed”: risks collapse and artifacts; fixation is standard.
• “Transition metal” wording is imprecise to the concept; the point is conductive heavy-metal coating after fixation.
• “None of the above” and uncoated room-pressure scanning do not describe conventional SEM workflows (except in variable-pressure or cryo-SEM special cases).

Common Pitfalls:
Confusing SEM with TEM preparation or assuming coating thickness should be large; overly thick coatings obscure fine surface details.

Final Answer:
A specimen is fixed and then coated with a thin layer of a heavy metal