What is the formula for retention factor in chromatography?

• A. Retention factor = distance analyte / solvent distance
• B. Retention factor = solvent distance / analyte distance
• C. Retention time / run time
• D. Distance to solvent front / wait time

Answer: (A)

Retention factor tells you how far a compound moves on a chromatography plate relative to the solvent’s progress. It’s defined as the distance traveled by the analyte from the origin divided by the distance traveled by the solvent front from the origin. This yields a unitless value, typically between 0 and 1 in TLC; a small value means the compound interacts strongly with the stationary phase, while a value near 1 means it moves with the solvent front. So the correct form is the distance the analyte travels divided by the distance the solvent front travels. Using time instead of distance, or reversing the ratio, doesn’t measure this same proportion.