HPLC

Meaning: HPLC stands for high-performance (often called high-pressure) liquid chromatography. A pump forces a liquid mobile phase through a column packed with very fine stationary-phase particles. The fine particles give a large surface area and sharp separations, which is why high pressure is needed. Compounds separate by relative affinity: those attracted more strongly to the stationary phase move more slowly and elute later. A detector, commonly UV absorbance, records peaks against time (a chromatogram).

Equation: k = (tR − t0)/t0, where tR is the retention time of a compound (s), t0 is the time an unretained solvent takes to pass through the column (s), and k is the retention factor (unitless). A larger k means stronger retention.

On the MCAT: A passage describes a mixture run on a normal-phase column (polar silica, nonpolar solvent) or a reverse-phase column (nonpolar C18 chains, polar solvent such as water/methanol). You predict elution order from polarity, match peaks to compounds, or read a chromatogram, where retention time helps identify a compound and peak area tracks its amount.

Common confusion: Students memorize that polar compounds elute first, or the opposite. It depends on the column. Like attracts like: in normal-phase, polar compounds are retained and elute later; in reverse-phase, nonpolar compounds are retained and elute later.