Injection quality in column chromatography — A sample injection is considered successful when what happens at the column inlet?

Introduction / Context:
Band focusing at the column head determines resolution. Even with perfect transfer, a poorly focused injection produces broad peaks and loss of separation. Proper injection and solvent conditions minimize extra-column dispersion and maximize efficiency.

Given Data / Assumptions:

• Narrow initial bands increase theoretical plates effectively utilized.
• Mismatch between sample solvent and mobile phase can cause fronting/broadening.
• On-column or split/splitless injections in GC and reduced-volume injections in LC help focus.

Concept / Approach:
Define success by chromatographic performance, not merely by sample transfer. A compact, concentrated plug at the start of the column is ideal because dispersion grows as the band travels (longitudinal diffusion, eddy diffusion, mass transfer limits).

Step-by-Step Solution:

Verification / Alternative check:
Compare peak widths: properly focused injections yield sharper peaks and improved resolution at identical flow and temperature.

Why Other Options Are Wrong:

Common Pitfalls:
Injecting large volumes in strong solvent; not using solvent focusing or temperature programming in GC when needed.

Final Answer:
The sample band is tightly focused at the head of the column.