= High-performance liquid chromatography
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<High-performance liquid chromatography> is <liquid chromatography> typically using a finely divided stationary packing and a pump supplying the pressure needed to drive the <chromatographic mobile phase> through it. A sample mixture is injected, its components interact differently with the <chromatographic stationary phase>, and a detector records their elution. IUPAC recognizes both high-performance and high-pressure terminology for this form of <liquid chromatography>.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[Liquid chromatography, entryL03578, also defining high-performance liquid chromatography]{ref}
The pressure supplies flow through the column; differential interactions produce the separation. Raising pressure alone does not specify which component will elute first. The solvent composition, stationary material and analyte chemistry all matter. The useful result is a set of peaks separated sufficiently for the intended identification or measurement.
An original example illustrates how to assess a recorded separation. Suppose two peaks have <chromatographic retention times> $t_{R1}=5.0$ minutes and $t_{R2}=6.0$ minutes, with baseline widths $w_1=w_2=0.40$ minutes. Using widths at the base, their <chromatographic resolution> is
$$
R_s=\frac{2(t_{R2}-t_{R1})}{w_1+w_2}=\frac{2(6.0-5.0)}{0.40+0.40}=2.5.
$$
If the widths become $0.80$ minutes each while the peak times stay fixed, $R_s$ falls to $1.25$. Peak timing alone therefore does not establish separation quality. Widths measured at half-height require the corresponding formula rather than substitution into this baseline-width definition.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[Peak resolution, entryP04465, baseline-width definition]{ref}
Matching a <chromatographic retention time> to a standard under the same conditions is useful evidence, but different substances can coelute. Quantifying a peak also requires a calibrated detector response; its area is not automatically an amount independent of molecular identity and instrument settings.
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