Chromatography-an analytical tool for separating a mixture into its constituent components. These components are separated on the basis of their relative distribution between the two phases. What are these two phases? What is the difference between the two? How each of these two phases performs its particular function in carrying out a successful chromatographic separation? All these questions form the primary focus of this article.
What is stationary phase
Stationary phase is the phase that stays fixed in its place. During chromatography, it provides a surface for the solvent carrying the analyte components to move over it. The solute components strongly interactive with the stationary phase gets retained on it. A chromatographic stationary phase could either be a solid or a liquid coated on a solid support.
What is mobile phase
Mobile phase is the phase that flows over the stationary phase. It carries the analyte components with it on the basis of their solubility in the mobile phase. A chromatographic mobile phase could either be a liquid or a gas.
After reading those, you can come back here and do a quick revision using the table given below. The table specifically highlights the differences between a stationary phase and a mobile phase in chromatography.
Difference between stationary phase and mobile phase
| Property | Stationary Phase | Mobile Phase |
| Appearance | Stays fixed on a plane or in a column | Flows over the stationary phase |
| Physical State | Solid or Liquid coated on solid support | Liquid or Gas |
| Composition | A single composition of the stationary phase is maintained throughout the chromatographic process | The mobile phase composition can vary via gradient elution when performing a chromatographic separation |
| Interaction with the analyte components | Interacts with and retains analyte components on the basis of affinity, size, shape, charge and chemical bonding etc. | Sweeps the analyte components on the basis of solubility |
| Analyte recovery | Analyte components strongly retained onto the stationary phase, move slowly with the mobile phase and elute out later | Analyte components with high solubility in the mobile phase moves quickly thus faster elution |
| Type of chromatography | On the basis of stationary phase, chromatography can be classified into solid-liquid chromatography, gas-solid chromatography and/or liquid-liquid chromatography | On the basis of mobile phase, chromatography can be classified into liquid-liquid chromatography, gas-liquid chromatography or gas-solid chromatography |
| Polarity | A more polar stationary phase than the mobile phase gives chromatography its name: normal phase chromatography | A chromatographic separation in which the mobile phase is more polar than the stationary phase is called reverse phase chromatography |
Having discussed the generally differences between a stationary phase and a mobile phase in chromatography, we can highlight the differences between the two for specific types of chromatography as shown in the table below.
| Type of Chromatography | Stationary Phase | Mobile Phase |
| Paper Chromatography | Water trapped in the cellulose layers of the paper | Organic solvents (ethanol, methanol, n-hexane etc.) |
| Thin Layer Chromatography (TLC) |
Silica gel or alumina coated on a glass plate | Organic solvents |
| High Performance Liquid Chromatography (HPLC) |
Silica packed in a column (Normal phase) C-8 or C-18 modified silica packed in a column (Reverse phase) |
A combination of organic solvents (such as ethanol, methanol, acetonitrile, chloroform and benzene etc.) and their aqueous solutions Single composition throughout (Isocratic elution) Varying solvent composition (Gradient elution) |
| Gas Chromatography (GC) |
Silica packed column or Liquid coated capillary columns |
Inert carrier gas such as helium used as mobile phase |
| Ion Exchange Chromatography | Cation exchanger or Anion exchanger |
Salt solutions |
| Molecular or Size Exclusion Chromatography |
Porous substances such as silica gel | Organic solvents |
| Affinity Chromatography | Silica support with a specific ligand |
A solution containing a complementary ligand that binds more strongly with the stationary phase than the analyte molecules |
The diagrammatic representation below illustrates a detailed column chromatographic separation using both the stationary phase and the mobile phase.
Conclusion
Thus concluding, the stationary phase as well as the mobile phase are the essential separating media of a chromatographic process. Chromatography cannot be performed in the absence of any of the two phases. Both hold their distinction in terms of appearance, properties, nature and function.
We have for now studied the basics of chromatography in a detailed perspective. The upcoming articles of the series will particularly focus on different types of chromatography in further detail with more insightful information, one at a time.
You can read extensively about what is a stationary phase and what is a mobile phase in chromatography from our separate articles on the topics.
References
1. Calderon, L. d. A. (2016). Chromatography: The Most Versatile Method of Chemical Analysis.
2. Danielson, N. D. (2003). Liquid Chromatography. Encyclopedia of Physical Science and Technology (Third Edition). R. A. Meyers. New York, Academic Press: 673-700.