Chromatographic Separations
Chromatographic separations resolve mixtures into their components by exploiting the differential distribution of analytes between a stationary and a mobile phase.
Definition
Chromatography is a family of analytical separation methods in which the components of a mixture are partitioned between a stationary phase and a moving mobile phase, so that they migrate at different rates and emerge resolved in time or space.
Scope
This area covers the major separation techniques of analytical chemistry: gas chromatography, liquid chromatography in its high-performance form, and capillary electrophoresis, together with the underlying theory of band migration and broadening. It treats column technology, mobile-phase and stationary-phase selection, detection, and the figures of merit—retention, resolution, and plate count—that describe a separation. Coupling of these separations to spectrometric and mass-spectrometric detectors is noted but the detectors themselves are treated in their own areas.
Sub-topics
Core questions
- How does differential partitioning between two phases separate the components of a mixture?
- What determines resolution, and how are efficiency, selectivity, and retention balanced?
- How are gas, liquid, and electrophoretic methods chosen for a given analyte class?
- How is a chromatographic peak related quantitatively to the amount of analyte?
Key theories
- Plate theory and the rate (van Deemter) theory
- A column's efficiency is expressed as a number of theoretical plates; the rate theory of van Deemter relates plate height to flow velocity through contributions from eddy diffusion, longitudinal diffusion, and resistance to mass transfer, predicting an optimum velocity for minimum band broadening.
- Differential partitioning and retention
- Each analyte distributes between mobile and stationary phases according to its distribution constant; the resulting retention factor sets how long it stays on the column, and differences in retention among analytes, combined with column efficiency, determine whether they are resolved.
Mechanisms
A sample is introduced into a stream of mobile phase passing over or through a stationary phase. Analytes that interact more strongly with the stationary phase move more slowly, so components separate as they traverse the column and reach the detector at different times. The detector records a series of peaks; their position identifies analytes against standards and their area quantifies them. Band broadening from diffusion and mass-transfer resistance limits how closely spaced peaks can still be resolved.
Clinical relevance
Chromatographic separations are indispensable across pharmaceutical analysis, clinical and forensic toxicology, environmental contaminant monitoring, food and flavour analysis, and biotechnology, because they can resolve and quantify many analytes from complex matrices in a single run.
History
Chromatography began with Mikhail Tswett's 1906 separation of plant pigments on a packed column. Martin and Synge's partition theory in the 1940s, for which they received the Nobel Prize, provided the conceptual foundation and led to gas–liquid chromatography. The rate theory of van Deemter and the open-tubular capillary column of Golay in the 1950s established the modern quantitative understanding of separations.
Key figures
- Mikhail Tswett
- Archer Martin
- Richard Synge
- Marcel Golay
Related topics
Seminal works
- tswett1906
- vandeemter1956
- skoog2017
Frequently asked questions
- What is the difference between gas and liquid chromatography?
- Both separate by differential partitioning, but gas chromatography uses an inert gas as the mobile phase and suits volatile, thermally stable analytes, while liquid chromatography uses a liquid mobile phase and handles non-volatile, polar, or thermally labile compounds.
- What does resolution mean in chromatography?
- Resolution measures how completely two adjacent peaks are separated; it improves with greater differences in retention, higher column efficiency, and appropriate selectivity, and is needed to quantify each component without overlap.