How do ion-exchange chromatography techniques function at a basic level?

• A. By binding positive ions to negatively charged stationary phases
• B. By separating molecules based on size differences
• C. By utilizing specific intermolecular attractions
• D. By altering the polarity of molecules

Answer: (A)

Ion-exchange chromatography operates by utilizing the interactions between charged molecules and a charged stationary phase. In this technique, the stationary phase is typically composed of a resin that has either a net negative or net positive charge, allowing it to attract and bind ions of the opposite charge.

When a mixture containing charged molecules is passed through the column, positively charged ions in the mixture will be attracted to and bind to the negatively charged sites on the stationary phase. This binding allows for the separation of molecules based on their charge. Compounds with a stronger affinity for the stationary phase will elute later than those with weaker affinities. By controlling the conditions, such as the salt concentration of the mobile phase, one can selectively elute different molecules based on their charge.

This principle distinguishes ion-exchange chromatography from other techniques. For example, methods based on size-exclusion chromatography separate molecules by size and do not involve charge interactions, while techniques focused on intermolecular attractions may include affinity chromatography, which targets specific interactions (such as enzyme-substrate or antibody-antigen) rather than general charge. Similarly, altering the polarity of molecules pertains to solvent interactions and does not apply to the charge-driven mechanisms at play in ion-exchange chromatography.