stationary phase

stationary phase, in analytical chemistry, the phase over which the mobile phase passes in the technique of chromatography. Chromatography is a separation process involving two phases, one stationary and the other mobile. Typically, the stationary phase is a porous solid (e.g., glass, silica, or alumina) that is packed into a glass or metal tube or that constitutes the walls of an open-tube capillary. The mobile phase flows through the packed bed or column. The sample to be separated is injected at the beginning of the column and is transported through the system by the mobile phase. In their travel through the column, the different substances distribute themselves according to their relative affinity for the two phases.

Chromatography usually is divided into two categories depending on the type of mobile phase that is used. If the mobile phase is a liquid, the technique is liquid chromatography; if it is a gas, the technique is gas chromatography. In a simple liquid chromatographic apparatus the stationary phase is held in place either in a column or on a plane (such as a plate of glass, metal, or plastic or a sheet of paper). In the case of a column, the lower end is loosely plugged, often with glass wool or a sintered glass disk. Prior to the separation, the column is filled with the mobile phase to a level that is slightly above the level of the stationary phase. The mixture to be separated is added to the top of the column and is allowed to drain onto the stationary phase.

In the most common form of chromatography, known as elution chromatography, the mobile phase is continuously added to the top of the column as solution flows from the bottom. The stationary phase must be continuously immersed in the mobile phase to prevent air bubbles from entering the column and impeding the mobile-phase flow. As the components of the mixture are flushed through the column, they are partitioned between the two phases depending on their attractions to the stationary phase. Because different mixture components have different attractions for the stationary phase, a separation occurs. The components that are more attracted to the stationary phase remain in the column longer, while those components that are less attracted are flushed more rapidly from the column. The separated components are collected as they exit the column.

A similar process occurs during separations that are performed on a plane. In such a case, however, the separations occur in space after a fixed time period rather than in time at a fixed location as was described for column chromatography. The separated components appear as spots on the plane.

In gas chromatography the stationary phase is contained in a column. The column generally is a coiled metallic or glass tube. An injector near the entrance to the column is used to add the analyte. The mobile phase gas usually is contained in a high pressure gas cylinder that is attached by metallic tubing to the injector and the column. A detector, placed at the exit from the column, responds to the separated components of the analyte.