Chromatography is a broad family of separation techniques used to separate, identify, characterise, purify, or quantify components of a mixture. The fundamental principle is differential distribution of substances between a stationary phase and a mobile phase. The stationary phase remains fixed within the chromatographic system, while the mobile phase moves through or across it. Individual components interact with the two phases to different extents, causing them to migrate at different rates and therefore become spatially or temporally separated.
Depending on the method, separation can be based on differences in adsorption, partition, charge, molecular size, affinity, volatility, or combinations of these properties. Chromatography can therefore be adapted to a very wide range of substances, from small inorganic ions and volatile organic compounds to proteins, nucleic acids, pharmaceuticals, metabolites, lipids, and complex biological mixtures. Analytical chromatography commonly produces a chromatogram in which detector response is plotted against time or another measure of elution.
Preparative chromatography instead focuses on collecting separated material in useful quantities. Chromatography is central to chemistry, biochemistry, pharmaceutical science, environmental analysis, food science, forensic science, life science, clinical research, and industrial manufacturing because complex mixtures frequently cannot be characterised reliably without first resolving their components.
Chromatographic terminology describes both the physical system and the observed separation. The stationary phase may be a solid surface, a porous material, a chemically bonded phase, or a liquid immobilised on a support. The mobile phase may be a gas, a liquid, or, in specialised systems, another fluid. A sample is introduced into the chromatographic system and its components undergo repeated interactions with the stationary and mobile phases.
A compound that spends proportionally more time associated with the stationary phase generally migrates more slowly than one that preferentially remains in the mobile phase. The resulting migration difference creates separation. In column chromatography, compounds typically emerge from the column at different retention times. In planar chromatography, such as thin layer chromatography, compounds migrate different distances across a plate and their positions can be described using retention factors.
Retention time itself is not an intrinsic universal property because it depends on temperature, pressure, flow rate, stationary phase chemistry, mobile phase composition, column dimensions, and instrument configuration. Reliable identification therefore normally requires comparison with standards, spectral detection, retention indices, or multiple analytical characteristics rather than relying on retention time alone.
Major chromatographic modes include adsorption chromatography, partition chromatography, ion exchange chromatography, size exclusion chromatography, and affinity chromatography. Adsorption chromatography separates substances according to interactions with a solid surface, while partition chromatography depends on differential distribution between phases. Ion exchange chromatography uses charged stationary phase groups to retain analytes according to ionic interactions and is widely used for proteins, peptides, nucleic acids, and inorganic ions. Size exclusion chromatography separates molecules according to effective hydrodynamic size as they pass through porous particles.
Large molecules are excluded from some pores and consequently can elute earlier than smaller molecules that enter a greater proportion of the pore volume. Affinity chromatography uses highly specific biological or chemical interactions, such as antigen antibody recognition or ligand binding to a protein, to isolate a target from a complex mixture. Other specialised forms include hydrophobic interaction chromatography, chiral chromatography, supercritical fluid chromatography, and multidimensional chromatography. The appropriate mode depends on analyte properties, matrix complexity, required resolution, sample quantity, detection method, and whether the objective is analytical measurement or purification.
Chromatographic performance is evaluated using concepts such as retention, selectivity, efficiency, resolution, peak width, and capacity. Resolution describes how effectively neighbouring components are separated and depends on differences in retention as well as peak broadening. Selectivity reflects how differently the system retains two compounds, while efficiency describes how narrow chromatographic bands remain during migration.
The theoretical plate concept is commonly used as an empirical measure of column efficiency, and the number of theoretical plates and plate height provide ways to compare column performance under defined conditions. Peak broadening can arise from multiple processes, including multiple flow paths through packed beds, longitudinal diffusion, and finite rates of mass transfer between phases. Flow rate therefore involves tradeoffs between analysis time and efficiency.
Temperature, solvent composition, particle size, column dimensions, stationary phase chemistry, and injection conditions can all affect separation. In practical method development, changing selectivity can be particularly powerful because simply increasing efficiency cannot always resolve compounds with very similar retention behaviour. Chromatographers therefore optimise both the chemical interactions responsible for separation and the physical conditions that control band broadening.
Chromatography is closely connected to instrumental detection because separation alone does not necessarily establish chemical identity. Detectors can measure properties such as ultraviolet absorption, fluorescence, refractive index, conductivity, flame ionisation, electron capture, or mass to charge ratio. Chromatography coupled with mass spectrometry is particularly powerful because chromatographic separation reduces mixture complexity while mass spectrometry provides molecular mass and structural information.
Gas chromatography commonly couples with mass spectrometry for volatile compounds, while liquid chromatography coupled with mass spectrometry is widely used for pharmaceuticals, metabolites, peptides, environmental contaminants, and other compounds that are unsuitable for direct gas phase analysis. Chromatography can also be combined with nuclear magnetic resonance spectroscopy and other spectroscopic techniques. Detector choice affects sensitivity, selectivity, dynamic range, quantitative performance, and compatibility with the mobile phase. Quantitative chromatography normally requires calibration using standards, internal standards, external standards, or other validated approaches. Matrix effects, detector response differences, analyte recovery, injection variability, and instrument drift must be considered when analytical accuracy is important.
Chromatography has major biological and medical applications because biological systems contain thousands of chemically different substances in complex mixtures. Protein purification frequently uses several chromatographic stages based on different properties, for example ion exchange followed by affinity and size exclusion chromatography. Nucleic acids can be separated according to charge, size, sequence dependent interactions, or affinity for specific ligands. Metabolomics uses chromatographic separation to resolve metabolites before detection, while lipidomics similarly separates diverse lipid species.
Pharmaceutical manufacturing uses chromatography for purification of active pharmaceutical ingredients, removal of impurities, monitoring of reactions, and characterisation of biological medicines. Clinical and toxicological laboratories use chromatographic methods to measure drugs, metabolites, hormones, steroids, therapeutic compounds, and environmental exposures. Food analysis can use chromatography to determine additives, contaminants, flavours, vitamins, pesticides, and other constituents. Environmental laboratories analyse water, soil, air, and biological samples for organic pollutants and other chemical substances. Because chromatographic methods can be highly selective and quantitative, they are often incorporated into validated analytical workflows where reproducibility and traceable measurement are essential.
The history of Chromatography includes the early twentieth century work of Mikhail Tsvet, who used adsorption based separation to separate plant pigments and helped establish the conceptual foundation of the field. The name is derived from Greek terms associated with colour and writing or recording, reflecting the early visual separation of coloured plant compounds, although modern chromatography is frequently applied to colourless substances. Development of paper chromatography and thin layer chromatography made planar separations inexpensive and accessible.
Gas chromatography subsequently enabled high resolution separation of volatile compounds, while high performance liquid chromatography transformed liquid phase analytical separations through high pressure pumping, efficient packed columns, and sophisticated detectors. More recent developments include ultra high performance liquid chromatography, two dimensional chromatography, comprehensive multidimensional systems, miniaturised microfluidic chromatography, and increasingly automated preparative platforms. Stationary phase technology has evolved from simple adsorbents to highly engineered porous particles and chemically bonded materials with controlled surface properties. Modern chromatography is therefore both a classical separation science and a highly sophisticated instrumental discipline in which chemistry, fluid dynamics, materials science, electronics, and data analysis interact to produce reproducible measurements.
Chromatography has limitations that depend strongly on the selected technique. Separation can require substantial method development, expensive equipment, high purity solvents or gases, specialist columns, regular maintenance, and trained operators. Some analytes degrade during analysis, interact irreversibly with stationary phases, or are poorly detected by available detectors. Coelution occurs when two or more components are insufficiently separated and produce overlapping peaks, potentially causing inaccurate identification or quantification.
Carryover, contamination, column ageing, leaks, injector problems, detector drift, and changes in mobile phase composition can also affect results. In preparative chromatography, solvent consumption and scale related performance can become significant practical considerations. Nevertheless, chromatography remains indispensable because its separation capability can convert an analytically intractable mixture into a set of manageable fractions or peaks. Correct interpretation requires understanding that chromatography primarily separates substances according to physicochemical behaviour. Identification and quantification require appropriate detection and validation, and a chromatographic peak should not automatically be interpreted as proof of a particular compound without sufficient supporting evidence.
