In chromatography, components of a solution are separated. The sample is applied to a stationary phase which is solid or a coating on a solid. For separation to take place, the stationary phase must interact with the analyte and retain it temporarily. A mobile phase, which can be gas (GC), liquid (HPLC), or a supercritical fluid (SFC), moves over the adsorbent to elute the sample. If the mobile phase and stationary phase (or adsorbent) are carefully chosen, the components of the sample are separated and elute separately. The analyte must partition rapidly between the mobile phase and the stationary phase, or no separation takes place.

The separation is monitored with some sort of detector. Detectors vary from simple UV-Vis detectors using flow cells with small volumes frequently shaped like narrow cylinders with light transmitted through center of the cylinder to elaborate mass spectrometers. Separation of the analytes prior to detection allows analytical measurement of molecules with similar spectroscopic properties. For example, if all of the analytes are green, the intensity of each green species can be measured separately as each sample elutes.

In chromatography, the time required for the sample to elute is called the retention time. These retention times are reproducible if conditions are carefully controlled. By running frequent standards, the retention times may be used for qualitative analysis of compounds in a mixture.

In GC, HPLC, and SFC, the analytes are detected as they elute from the instrument. The detector displays a peak as each component elutes. The areas of these peaks may be compared to the areas of peaks from standards. By comparing areas, the concentrations of unknowns are calculated.

Flow rate of the mobile phase is a major driving force in the elution of the sample. As flow rates increase the retention time decreases and the analysis time decreases. If the interaction of the analyte with the stationary phase is slow, the mobile phase elutes broad peaks which are not well separated from other components.

About chromatography.