Analytical Separations Gas Chromatography

Instrument

Types of Gas chromatography Gas solid chromatography (GSC) Solid stationary has limited application owing to semipermanent retention if active or polar molecules and severe tailing of elution peaks. Gas Liquid chromatography (GLC) A liquid phase immobilizes on surface of an inert solid.

Liquid Phase for GLC Low volatility－the boiling point of the liquid should be 100oC higher than the maximum operating temperature. Thermal stability Chemical inertness Solvent characteristics－k and a values for the solutes to be resolved fall within a suitable range.

Principle of Gas chromatography Retention volume The effect of pressure and temperature Retention volumes rather than the retention times

Retention volume

Carrier Gas The carrier gas must be chemically inert. Commonly used gases include N2, He, Ar. The choice of carrier gas is often dependant upon the type of detector which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities.

Columns－ Packed columns There are two general types of column, packed and capillary (open tubular). Packed columns- contain a finely divided, inert, solid support material (commonly based on diatomaceous earth) coated with liquid stationary phase. Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm.

Columns－ Capillary columns Capillary columns have an internal diameter of a few tenths of a millimeter (0.25mm or 0.42mm). Two types－wall-coated open tubular (WCOT) or support-coated open tubular (SCOT). WCOT －a capillary tube whose walls are coated with liquid stationary phase. SCOT － the inner wall of the capillary is lined with a thin layer of support material such as diatomaceous earth, onto which the stationary phase has been adsorbed. SCOT columns are generally less efficient than WCOT columns.

WCOT column These columns are flexible and can be wound into coils. They have the advantages of physical strength, flexibility and low reactivity.

Instrumental Components Sample injection Two modes－split or splitless. The injector contains a heated chamber containing a glass liner into which the sample is injected through the septum.

Split Mode The carrier gas enters the chamber and can leave by three routes The sample vaporizes to form a mixture of carrier gas, vaporized solvent and vaporized solutes. A proportion of this mixture passes onto the column, but most exits through the split outlet. The septum purge outlet prevents septum bleed components from entering the column.

Injection Method For optimum column efficiency, the sample should not be too large Sampleshould be as a "plug" of vapour - slow injection of large samples causes band broadening and loss of resolution. Microsyringe is used to inject sample The temperature of the sample port is usually about 50°C higher than the boiling point . For packed columns, sample size ranges from tenths of a microliter up to 20 microliters. Capillary columns need much less sample, typically around 10-3 mL.

Column temperature The optimum column temperature is dependant upon the boiling point of the sample. Minimal temperatures give good resolution, but increase elution times. If a sample has a wide boiling range, then temperature programming can be useful.

Flame Ion Detector The effluent from the column is mixed with hydrogen and air, and ignited. A large electrical potential is applied at the burner tip, and a collector electrode is located above the flame. The current resulting from the pyrolysis of any organic compounds is measured.

Advantages of FID Changes in mobile phase flow rate do not affect the detector's response. The FID is a useful general detector for the analysis of organic compounds It has high sensitivity, a large linear response range, and low noise. Unfortunately, it destroys the sample.

Thermal Conductivity Detector (TCD) One pair is placed in the column effluent to detect the separated components as they leave the column One pair is placed in a separate reference column.

TCD The temperature of the sensing element depends on the thermal conductivity of the gas flowing around it. The TCD is not as sensitive as other detectors. Advantages It is non-specific and non-destructive. Simplicity

Electron Capture Detector (ECD) Uses a radioactive e- to ionize some of the carrier gas and produce a current between electrodes. Molecules capture some of the electrons and reduce the current measured between the electrodes.

Advantages of ECD Specific for the organic molecules contain electronegative functional groups, such as halogens, phosphorous, and nitro groups. The ECD is as sensitive as the FID but finds its greatest application in analysis of halogenated compounds.