Analytical chemistry — In normal-phase HPLC (high-performance liquid chromatography), which pairing best describes the mobile phase and the stationary phase (column) used?
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Anon-polar solvent / polar column
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Bpolar solvent / non-polar column
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Cnon-polar solvent / non-polar column
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Dany of the above
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Epolar solvent / polar column
Answer
Correct Answer: non-polar solvent / polar column
Explanation
Introduction / Context:Normal-phase HPLC is a classic chromatographic mode that separates analytes primarily by polarity using a polar stationary phase and a relatively non-polar mobile phase. Knowing how phases are paired is essential for choosing solvents, predicting elution order, and troubleshooting retention problems.
Given Data / Assumptions:
- The question asks about normal-phase HPLC configuration.
- Stationary phase: typically polar (e.g., bare silica with surface silanols).
- Mobile phase: typically non-polar or weakly polar organic solvents (e.g., hexane with small percentages of isopropanol or ethyl acetate modifiers).
Concept / Approach:In normal-phase mode, polar analytes interact strongly with the polar stationary phase and elute later. Less polar analytes spend more time in the non-polar mobile phase and elute earlier. This is the inverse of reversed-phase HPLC, where a non-polar stationary phase is paired with a polar (water-rich) mobile phase.
Step-by-Step Solution:
Identify normal-phase hallmark: polar stationary phase (silica, alumina).Match the mobile phase: non-polar solvent (e.g., hexane) sometimes with polar modifiers to tune retention.Predict retention: more polar analytes retained longer; less polar elute sooner.Select the configuration that states non-polar solvent / polar column.Verification / Alternative check:Method development guides differentiate normal-phase (polar stationary, non-polar mobile) from reversed-phase (non-polar stationary, polar mobile). Typical solvent systems for normal phase confirm the non-polar mobile phase choice.
Why Other Options Are Wrong:
- Polar solvent / non-polar column describes reversed-phase, not normal-phase.
- Non-polar solvent / non-polar column does not create useful polarity-based separation.
- Any of the above is incorrect because normal-phase has a specific pairing.
- Polar solvent / polar column would lead to very strong retention and is not the standard definition.
Common Pitfalls:Confusing normal-phase with reversed-phase; assuming “normal” means water-based. In chromatography, “normal” historically refers to the early silica-based method, not to aqueous systems.
Final Answer:non-polar solvent / polar column