Chromatography involves two phases: the mobile phase and the stationary phase. The mobile phase contains and carries the sample to be analyzed through or across the stationary phase. The stationary phase maintains conditions necessary for separating various substances (i.e., the analytes) within the sample being studied. The mobile phase may be gas or liquid. The stationary phase may also be liquid or solid and is usually housed within a column of some design. Specific chromatographic methods are named according to the phase used. For example, gas-liquid chromatography (GLC) refers to any chromatographic method that uses gas as the mobile phase and liquid as the stationary phase.

Regardless of the phases used, chromatographic principles are generally the same. A sample is mixed by injection with the mobile phase, which carries the sample through a column containing the stationary phase. Depending on the nature of the solid phase, different analytes within the sample will be retained within the column based on size, ionic, charge, or analyte solubility in the mobile phase. As the mobile phase continues to move through the column, analytes with different characteristics will have different retention times and thus will elute (come out) from the column at different times. Eluted analytes pass through a detector that generates a signal. The signal is then translated into an electronic signal that is recorded and used to produce a chromatogram.

The chromatogram is the plot of elution peaks produced by each analyte in the sample. With the inclusion of reliable controls and standards, retention times are used to definitively identify specific analytes. The size of each peak can be used to determine the amount of analyte present in each sample.

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