In HPLC system plumbing, how is dwell volume best defined for gradient methods?

Difficulty: Medium

Correct Answer: the volume of the column between the point at which solvents are mixed and the beginning of the column

Explanation:


Introduction / Context:
Dwell volume (also called gradient delay volume) is critical when transferring gradient HPLC methods between instruments. It explains why the same programmed gradient can produce different chromatograms on two systems.



Given Data / Assumptions:

  • Solvents are mixed either at a low-pressure proportioning valve (pre-mix) or at a high-pressure mixer.
  • The mixed solvent travels through tubing, mixer, pump heads, and injector before entering the column.
  • The “distance” is measured as a volume because flow rate converts volume to time.



Concept / Approach:
Dwell volume is defined as the volume from the mixing point to the head of the column. At a given flow, this volume corresponds to a delay (dwell time) before a programmed change in composition reaches the column inlet. Minimizing or matching dwell volumes is key to method transfer.



Step-by-Step Solution:
Locate the solvent mixing point in the system diagram.Trace the flow path up to the column inlet; integrate all volumes in between.Recognize that the measured physical quantity is volume; the related time is derived from flow rate.



Verification / Alternative check:
A practical test injects a tracer under a programmed step change in %B and measures the response at the detector to compute dwell volume and corresponding dwell time.



Why Other Options Are Wrong:

  • (a) Refers to column void volume, not dwell volume.
  • (b) Is the dwell time (a consequence), not the definition; the question asks for volume.
  • (d) Incorrect because (c) states the accepted definition.



Common Pitfalls:
Confusing dwell volume with extra-column volume downstream of the column or with the column dead volume (void volume).



Final Answer:
the volume of the column between the point at which solvents are mixed and the beginning of the column

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