In Reverse Phase chromatography of a mixture of protein and sodium chloride:
A. The salt will elute before the protein
B. The salt and protein will co-elute
C. The salt will elute after the protein
D. The salt will elute but the protein will not

 

Reverse Phase Chromatography: What Elutes First—Protein or Salt?

Reverse phase chromatography (RPC) is a powerful technique used in analytical and preparative chemistry, especially in protein purification. It operates on the principle of hydrophobic interactions between the analyte and the stationary phase.

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The Key Players: Protein and Sodium Chloride

When a mixture of protein and sodium chloride is subjected to RPC:

• The stationary phase is non-polar (hydrophobic)
• The mobile phase is relatively polar (aqueous or organic solvent mixture)

The interaction strength of the analyte with the hydrophobic stationary phase determines its retention time.

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What Happens During Elution?

Let’s consider the properties:

• Sodium chloride (NaCl): Very polar and highly soluble in the aqueous mobile phase. It does not interact with the hydrophobic column.
• Proteins: Many proteins have hydrophobic patches that bind to the hydrophobic stationary phase.

Hence, during the chromatographic run:

✅ The salt will elute before the protein

Because:

• Salt (NaCl) passes through unretained
• Protein binds and is retained until eluted using a more hydrophobic mobile phase (e.g., increasing organic solvent)

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Correct Answer: A. The salt will elute before the protein

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Why Other Options Are Incorrect

• B. Co-elution: Incorrect—salt doesn’t bind, protein does.
• C. Salt after protein: Opposite of what’s true.
• D. Salt only elutes: Protein also elutes eventually with a change in solvent polarity.

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Key Takeaways

• Reverse phase chromatography separates components based on hydrophobicity.
• Polar compounds like NaCl elute quickly, while hydrophobic molecules like proteins are retained.

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Key Phrase: reverse phase chromatography protein salt elution