Principles of Chromatography

Chromatography is an analytical technique used to separate the parts of a mixture and identify substances in it. It is used in chemistry, biology, environmental science, and pharmaceuticals. The method depends on two phases: the stationary phase, which stays fixed, and the mobile phase, which moves across it and carries the solutes.

As the mobile phase travels, each mixture component interacts with the phases in a different way. That difference causes the components to separate. Chromatography...

Chromatographic techniques are grouped by the phases they use, how those phases interact, and the way they separate sample components. The mobile phase is usually a liquid or a gas. The stationary phase is usually a solid or a liquid layer on a solid surface.

Many chromatography names start with the type of chromatography, then the mobile phase, and then the stationary phase. For example, gas-solid chromatography uses a gas mobile phase and a solid stationary phase. In liquid-liquid...

Chromatography separates solutes by how they move between the mobile phase and the stationary phase. In many systems, this transfer happens through sorption, which usually means adsorption. The separation depends strongly on polarity, or the overall dipole moment of a molecule.

During chromatography, the solute and the solvent compete for adsorption on the stationary phase. Highly polar compounds and polar solvents tend to adsorb strongly on polar surfaces. This affects how the mixture...

Column diffusion causes band broadening in chromatography columns. When an analyte is added as a narrow band at the top of the column, the solutes begin to separate and spread out. As this happens, the band develops a Gaussian profile, which means the concentration is highest near the center and lower at the edges.

Longitudinal diffusion is one reason this spreading occurs. In this process, solute molecules in the mobile phase move from the more concentrated center of the band into the more...

Peak separation in chromatography depends on how far apart two neighboring peaks appear in a chromatogram. As a solute moves through a chromatographic column, it spreads into a band with a Gaussian shape. The longer the solute stays in the column, the broader that band becomes. This broadening can cause peaks to overlap and reduce separation efficiency.

Chromatographic resolution is the measure used to judge that separation. It is the ratio of the difference in retention time or retention...

Theoretical plates help explain how chromatography columns achieve good separation. Column efficiency is measured by band broadening, and that efficiency depends on the number of theoretical plates, or N. In plate theory, a column is treated as a continuous series of imaginary plates where solute equilibration happens between the stationary and mobile phases.

A higher number of theoretical plates means better column efficiency and stronger separation ability. Plate height affects both band...

Chromatography band broadening in columns explains why separated solute zones spread out as they move through a column. The rate theory of chromatography gives a quantitative way to describe the shape and width of elution bands. These bands follow a Gaussian profile, which means they form a smooth bell-shaped curve as the molecules travel.

As a solute moves through the column, it makes many random transitions between the stationary phase and the mobile phase. Its residence time in each phase...

Chromatography resolution improves when bands stay narrow and the two solutes separate clearly in the column. Good separation often depends on band broadening, plate height, capacity factor, and separation factor. These ideas help explain why some mixtures come apart quickly while others take more time.

Band broadening is the spreading of solute bands as they move through the column. When bands spread out, resolution drops. Plate height, or H, is the length needed for one theoretical plate. A...

Silica gel column chromatography separates compounds by polarity using a column packed with silica gel. The silica gel acts as the stationary phase, and the solvent is the mobile phase. As the mixture moves through the column, the compounds separate into distinct bands.

Polar compounds tend to bind more strongly to the silica gel. Because of this stronger interaction, they move more slowly through the column. Nonpolar compounds bind more weakly to the silica gel. They usually travel faster...

Thin-layer chromatography (TLC) separates compounds by polarity. It uses a polar silica gel plate as the stationary phase. Silica gel is a form of silicon dioxide, and its surface has hydroxyl (OH) groups that can form hydrogen bonds with polar compounds. This affects how strongly a compound sticks to the plate.

To start a TLC analysis, a mixture is placed on the starting line with a thin capillary. The bottom of the plate is then set in the mobile phase, which is usually an organic solvent...

Gas chromatography separates and analyzes volatile compounds in a sample. It is used to identify and measure the parts of complex mixtures. This makes GC useful in environmental analysis, pharmaceuticals, and petrochemicals.

GC is also called vapor-phase chromatography (VPC) or gas-liquid partition chromatography (GLPC). In the process, the sample is vaporized first. It is then mixed with an inert carrier gas, which acts as the mobile phase and carries the sample through a column.

The column...

Gas chromatography uses stationary phases to separate and analyze parts of a sample. In GC, the stationary phase can be liquid or solid. Liquid stationary phases are non-volatile, thermally stable, and chemically inert liquids coated on the column. Solid stationary phases are adsorbent particles, such as silica gel or molecular sieves.

A sample component must interact well enough with the stationary phase to stay in the column long enough to separate. This idea follows “like dissolves like,”...

Gas chromatography uses sample injection methods to control how much material enters the column. The sample is first introduced as a vapor plug into the carrier gas stream. This helps produce high efficiency and good resolution.

A microsyringe places the sample solution into a heated sample port. The heat vaporizes the sample, and it then mixes with the carrier gas. This step prepares the sample for analysis. Some thermally sensitive samples can be injected directly into the column. In that...

Gas chromatography detectors turn chemical properties into signals that appear on a chromatogram. These signals help identify and measure the parts of a mixture. In GC, detectors are often grouped as destructive or non-destructive.

A non-destructive detector analyzes a sample without changing or using it up. That means the sample can still be collected after detection for more testing. Thermal conductivity detectors and electron capture detectors are examples of this type.

A destructive...

Gas chromatography uses detectors that respond to different kinds of analytes. The main detectors covered here are the thermal conductivity detector (TCD), flame ionization detector (FID), and electron capture detector (ECD). Each one has a different way of turning a separated compound into a signal.

The thermal conductivity detector, or TCD, is the earliest and one of the most widely used GC detectors. It works by measuring changes in the thermal conductivity of the carrier gas. When a sample...

Gas chromatography uses specialized detectors to identify compounds that general-purpose detectors may miss. These detectors are chosen for specific analytical needs. They can target certain elements, functional groups, or trace analytes with greater precision.

Thermionic detectors are selective for organic compounds that contain phosphorus and nitrogen. They are especially sensitive to phosphorus compounds compared with the flame ionization detector, or FID. That makes them useful for...

High-performance liquid chromatography (HPLC) separates, identifies, and quantifies the parts of a complex mixture. It uses high pressure to push a liquid mobile phase through a tightly packed column. The result is a precise separation process that is useful for analyzing many samples.

In HPLC, the mobile phase is the liquid solvent that carries the sample through the system. The solvent is chosen for its polarity, or how it interacts with other substances. The polarity index helps guide the...

High-performance liquid chromatography (HPLC) uses solvent flow and elution to separate analytes in a column. Elution is the way compounds move through the column and separate because of how they interact with the mobile phase and the stationary phase. These interactions affect retention times, peak shape, and resolution in the chromatogram. Knowing how elution works helps students read chromatographic data and understand how mixtures are separated.

HPLC relies on highly pure HPLC-grade...

High-performance liquid chromatography, or HPLC, uses a set of connected parts to separate liquid samples under high pressure. The system begins with mobile phase reservoirs, which are glass bottles that hold the solvents. HPLC-grade solvents are used to keep the mobile phase as pure as possible. A degasser removes dissolved gases from the solvents before they move through the instrument.

The solvent then reaches the pump, which sends it into the analytical column. Some systems use a...

High-performance liquid chromatography (HPLC) uses detectors to analyze solutes as they leave the chromatographic column. The detector recognizes a property of the solute and turns it into an electrical signal. A computer then converts that signal into a graph called a chromatogram, which shows detector response over elution time.

HPLC includes several detector types, and each one has strengths and limits based on the analyte and the sensitivity needed. No single HPLC detection method can...

Ion-exchange chromatography separates ions by how strongly they bind to a charged stationary phase. The stationary phase is a cross-linked polymer resin with covalently attached ionic groups. These groups may be positively charged or negatively charged, which creates cation exchangers and anion exchangers.

The choice of resin depends on the analyte and the separation needed. A cation exchanger contains a polymeric anion with active cations, while an anion exchanger contains a polymeric cation...

Size-exclusion chromatography separates molecules by size, and it is often used for large molecules such as polymers and biomolecules. It is also called molecular-exclusion chromatography or gel-permeation chromatography. In this method, the stationary phase is made of micron-sized particles or resin beads that contain pores.

The two main stationary phases are silica particles and cross-linked polymer resin beads. Both are porous, but their pore sizes can differ a lot. Silica particles are...

Supercritical fluid chromatography (SFC) is a separation method used for compounds that can be hard to analyze with gas chromatography (GC) or liquid chromatography (LC). It is useful for nonvolatile or thermally unstable samples that do not work well in GC and for compounds that do not have the right functional groups for HPLC analysis. SFC combines useful features of both GC and LC while keeping the analysis efficient.

SFC uses a supercritical fluid as the mobile phase, most often carbon...

Affinity chromatography separates and purifies specific biomolecules from a complex mixture. It depends on a highly selective match between an analyte and a binding partner, such as an antibody and an antigen. The binding partner is fixed on the stationary phase, which forms the affinity column.

The stationary phase is usually a solid support such as agarose or porous glass beads. The mobile phase helps the analyte bind to the ligand and later helps release it. When a sample passes through the...

Electrophoresis separates charged molecules by making them move in an electric field. It is widely used in biochemistry, molecular biology, and analytical chemistry. The technique can separate complex mixtures of amino acids, nucleotides, carbohydrates, and proteins with strong resolution.

Two main formats are used for electrophoretic separations: slab electrophoresis and capillary electrophoresis. Slab electrophoresis uses a gel matrix, such as agarose or polyacrylamide, as a sieve. The pores...

Capillary electrophoresis uses a high-voltage setup, a capillary tube, a sample vial, and a detector to separate charged molecules. The electric field comes from a power supply connected to an anode and a cathode in buffer reservoirs at each end of the capillary. The capillary is made of fused silica and coated with polyimide for added strength.

To start the separation, the sample must be placed inside the capillary. One end of the capillary and its electrode are removed from the buffer...

Capillary electrophoresis uses several separation modes to analyze charged molecules, size-based samples, and even neutral compounds. Each mode works a little differently and is used for different kinds of lab problems. Together, they show why capillary electrophoresis is a flexible tool in analytical chemistry.

Capillary zone electrophoresis (CZE) separates ionic components by their electrophoretic mobility. It can separate proteins, amino acids, and carbohydrates in a short time, so it is an...