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An Introduction to Chromatography

Maxum Edition II GC

An Introduction to ChromatographyWhat IS chromatography?

The separation of a mixture by distribution of its components between a mobile and stationary phase over timemobile phase = solventstationary phase = column packing materialThe basis of gas chromatography is the distribution of a sample between two phases namely stationary phase & gas phase ( mobile phase )

The sample being vaporized & injected into the head of chromatographic column

An Introduction to ChromatographyA gas chromatograph uses a flow through a narrow tube known as column through which different chemicals constituents of the sample pass in a gas stream known as carrier gas or mobile phase at different rates .Depending on their various chemical and physical prosperities Their interaction with specific column filling called stationary phase

The sample is transported into the column by the flow of the inert gaseous mobile phase

Components of a Chromatographic SystemSource of Carrier Flow (mobile phase)Cylinder of carrier gas or solvent bottlessample inletColumn with stationary phaseDetector(s)Signal Transducers & Data AnalyzersRecorders, integratorsComputers for library matchingControllersTemperature controls for injectors, columns and detectorFlow controllers and pressure regulators

System block diagram

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Basic ChromatographyA basic Gas Chromatograph (GC) consists of the following parts:Carrier Gas

CarrierGasCarrierRegulatorSample InSample outSVColumnSenseRef.

AtmosVents

Carries the sample through the columnand detector to an atmospheric vent.

Basic ChromatographyA basic Gas Chromatograph (GC) consists of the following parts:Carrier Regulator


AtmosVents

Maintains a constant pressure of carriergas which results in a constant carrierflow rate.

Basic ChromatographyA basic Gas Chromatograph (GC) consists of the following parts:Sample Valve


AtmosVents

Injects a measured amount of sample.

Basic ChromatographyA basic Gas Chromatograph (GC) consists of the following parts:Column


AtmosVents

Separates the sample into individualcomponents.

Basic ChromatographyA basic Gas Chromatograph (GC) consists of the following parts:Detector


AtmosVents

Senses the individual components as they elute off the column.

Carrier Gas-SupplyCarrier gases, which must be chemically inert, include helium, nitrogen, and hydrogen. Associated with the gas supply are pressure regulators, gauges, and flow meters. In addition, the carrier gas system often contains a molecular sieve to remove water or other impurities.

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A carrier gas should have the following properties:

Highly pure (> 99.9%)Inert so that no reaction with stationary phase or instrumental components can take place, especially at high temperatures.When analyzing gas sample , the carrier is sometimes selected based on the sample matrix. For example: When analyzing a mix. In argon an argon carrier is preferred because the argon in the sample doesnt show up on the chromatogram.Compatible with the detector since some detectors require the use of a specific carrier gas.A cheap and available carrier gas is an advantage.

Sample Injection SystemFor quantitative work, more reproducible sample sizes for both liquids and gases are obtained by means of a rotary sample valve. Errors due to sample size can be reduced to 0.5% to 2% relative. The sampling loop is filled by injection of an excess of sample.

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Chromatographic separation involves the use of a stationary phase and a mobile phase. Components of a mixture carried in the mobile phase are differentially attracted to the stationary phase and thus move through the stationary phase at different rates.Columns

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As the carrier gas sweeps the analyte molecules through the column this motion is inhibited by the adsorption of the analyte molecules either into the column wall or into packing materials in the column

the rate at which molecules progress along the column depends on the strength of adsorption which is depend on : The type of molecules The stationary phase material

Since each type of molecules has a different rates of progression , the various components of the analyte mixture are separated as they progress along the column and reach the end of the column at different times ( retention time )

Retention timeRetention time of an analyte is defined as the time it takes after sample injection for the analyte to elute and reach the detector. Columns

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T=0T=10T=20

InjectorDetector

Most Interaction with Stationary Phase LeastFlow of Mobile Phase

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Velocity of a compound through the column depends upon affinity for the stationary phaseArea under curve is ______ of compound adsorbed to stationary phase

Gas phase concentration

Carrier gasmass

Column Configurations

Two general types of columns are encountered in gas chromatography, packed and open tubular, or capillary. Chromatographic columns vary in length from less than 2 m to 50 m or more. They are constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into an oven for thermostating, they are usually formed as coils having diameters of 10 to 30 cm.

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Packed Columns

These columns are fabricated from glass, stainless steel, copper, or other suitable tubes.

Stainless steel is the most common tubing used with internal diameters from 1-4 mm because it is most inert and easy to work with.

The column is packed with finely divided particles (

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