Injection quality in column chromatography — A sample injection is considered successful when what happens at the column inlet?
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AAll of the sample in the injector is transferred to the column regardless of band shape
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BThe sample band is tightly focused (concentrated) at the head of the column
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CThe sample is spread evenly along the entire column length
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DThe sample is homogeneously smeared to avoid high local concentrations
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EThe solvent front overtakes the analyte band to pre-elute it
Answer
Correct Answer: The sample band is tightly focused (concentrated) at the head of the column
Explanation
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:
Aim for a narrow initial band.Match sample solvent strength to mobile phase to avoid premature migration.Use appropriate injection mode and temperature to condense/focus analytes at the head.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:
a) Complete transfer is good but not sufficient if the band is broad.c,d) Spreading or smearing destroys resolution.e) Pre-elution undermines retention and separation.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.