Chromatography is a separation technique in which components of a mixture distribute differently between a stationary phase and a mobile phase, causing them to travel at different rates and become separated. The stationary phase remains fixed or relatively fixed, while the mobile phase moves through or across it. Components that interact more strongly with the stationary phase generally move more slowly, whereas components that preferentially remain in the mobile phase generally move more quickly.
Chromatography can therefore separate substances that may have very similar physical properties and cannot easily be separated by simple filtration or ordinary distillation. It is used both for qualitative analysis, where the identity or number of components is investigated, and quantitative analysis, where the amount of each component is measured. Major forms include paper chromatography, thin layer chromatography, gas chromatography, high performance liquid chromatography and ion exchange, size exclusion and affinity chromatography.
The history of chromatography began with experiments by Mikhail Tsvet, who used plant pigments and a column containing a stationary material to separate coloured compounds. The name derives from Greek words associated with colour and writing because early demonstrations produced visible coloured bands. Modern chromatography is much broader than the original pigment separation technique and is routinely used for colourless compounds as well. The central concept remains differential partitioning or interaction between stationary and mobile phases. Chromatographic systems can be designed around differences in polarity, volatility, charge, molecular size, binding affinity or other chemical properties. This flexibility makes chromatography one of the most powerful families of analytical and preparative separation methods.
In paper chromatography and thin layer chromatography, the stationary phase is commonly a polar material such as cellulose or silica, while a liquid solvent acts as the mobile phase. A sample is placed near one end of the stationary material and the mobile phase moves through it by capillary action. Different compounds travel different distances because their interactions with the stationary phase and mobile phase differ.
In thin layer chromatography, the distance travelled by a compound can be compared with the distance travelled by the solvent front using a quantity called the retention factor, commonly abbreviated Rf. Under controlled conditions, Rf values can help compare an unknown sample with known standards. Thin layer chromatography is particularly useful for monitoring chemical reactions, checking sample purity and selecting solvent systems before more sophisticated chromatographic analysis.
Column chromatography uses a stationary phase packed into a column through which a mobile phase flows. Different components move through the column at different rates and therefore emerge at different times. The time between injection and detection of a component is commonly described by its retention time in instrumental chromatography. In high performance liquid chromatography, high pressure forces a liquid mobile phase through a tightly packed column containing very small stationary phase particles. This produces efficient separation and high resolution. Different modes of liquid chromatography exploit different mechanisms, including adsorption, partition, ion exchange, size exclusion and affinity interactions. The choice of stationary phase and mobile phase is therefore central to determining which compounds separate and how strongly they are retained.
Gas chromatography is designed for compounds that can be vaporised without unacceptable decomposition. An inert carrier gas acts as the mobile phase and carries analytes through a column containing a stationary phase. Compounds separate according to volatility and their interactions with the stationary phase. The column temperature can be held constant or programmed to increase during the analysis. Detectors such as flame ionisation detectors, thermal conductivity detectors and mass spectrometers can identify or quantify compounds after separation. Gas chromatography is widely used in environmental analysis, forensic science, petrochemicals, flavour analysis and pharmaceutical testing. Compounds that are nonvolatile or thermally unstable may require derivatisation or a different chromatographic method such as liquid chromatography.
Chromatography can separate substances because their movement through the system is not identical. The distribution of a compound between stationary and mobile phases can be described by partition coefficients or related retention parameters. In an idealised repeated equilibrium picture, a compound continually moves between the phases while the mobile phase carries it forward. Small differences in the fraction of time spent in each phase can accumulate over many theoretical equilibrium stages, producing substantial separation.
Column efficiency is often described using theoretical plate concepts, while resolution depends on factors including selectivity, efficiency and retention. Band broadening can reduce separation quality and can arise from diffusion, uneven flow, slow mass transfer and other mechanisms. Optimising chromatography therefore involves balancing retention, selectivity, efficiency, pressure, sample size and analysis time.
Chromatography is also a major quantitative analytical technique. In instrumental chromatography, a detector generates a signal as separated components pass through. The area or height of a chromatographic peak can be related to the amount of analyte after appropriate calibration. Internal standards, external standards and calibration curves can compensate for differences in detector response and sample handling. Chromatography can also be coupled with mass spectrometry, producing highly informative measurements of molecular mass and fragmentation patterns. Such combinations are used to identify drugs, metabolites, pollutants, pesticides, proteins, lipids and countless other substances. Chromatography can therefore function both as a separation method and as part of a complete analytical workflow in which separated compounds are detected, identified and quantified.
Chromatography has important limitations and should not be treated as a single universal technique. Different analytes require different stationary and mobile phases, and a method effective for one mixture may perform poorly for another. Samples may require extraction, dilution, filtration or chemical modification before injection. Overloading a column can broaden peaks and reduce resolution, while inappropriate solvent composition can cause compounds to elute too quickly or remain strongly retained.
Chromatographic methods can also consume significant quantities of solvents and generate chemical waste, although modern systems increasingly use smaller columns, more efficient separations and greener solvent choices. Despite these limitations, chromatography remains indispensable because it can resolve chemically complex mixtures with a level of selectivity and sensitivity that many simpler separation methods cannot achieve.
