Isocratic elution in HPLC: During an isocratic method, how does the solvent composition behave over the course of the run?

Introduction / Context:
Chromatographic methods are broadly categorized as isocratic or gradient. Understanding the distinction is essential for method development, robustness studies, and system suitability testing.

Given Data / Assumptions:

• Isocratic elution: fixed % of solvent components throughout the run.
• Gradient elution: changing composition with time (continuous or stepwise).
• Detector and column conditions remain comparable between runs.

Concept / Approach:
In an isocratic method, the mobile phase composition is constant. This yields steady-state retention factors and often simpler validation. It suits samples with a narrow range of polarities; complex mixtures may require gradients for acceptable analysis time and peak shape.

Step-by-Step Solution:
Define isocratic: constant composition (%B and %A do not change with time).Contrast with gradient: composition changes over time to elute late-retaining analytes.Select the option stating that composition remains constant.

Verification / Alternative check:
Instrument method tables for isocratic runs list single fixed percentages for mobile-phase components rather than time-programmed changes.

Why Other Options Are Wrong:

• Changing continuously or stepwise are definitions of gradient elution.
• “None of these” is incorrect because (a) is correct.

Common Pitfalls:
Forgetting that temperature shifts can mimic gradient-like effects by altering solvent strength; nonetheless, composition is still constant by definition.

Final Answer:
remains constant