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Gas Chromatography

Lecture 38

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Syringe

Injector

Detector

Carrier Gas Cylinder

Column

To Waste or Flow Meter

Flow Controller

Two-Stage Regulator

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Three temperature zones can be identified:

1. Injector temperature, TI, where TI should allow flash vaporization of all sample components.

2. Column temperature, Tc, which is adjusted as the average boiling points of sample components.

3. Detector Temperature, TD, which should exclude any possible condensation inside the detector.

Generally, an intuitive equation can be used to adjust all three zones depending on the average boiling point of the sample components. This equation is formulated as:

TI = TD = Tc + 50 oC

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The Carrier Gas

Unlike liquid chromatography where wide varieties of mobile phase compositions are possible, mobile phases in gas chromatography are very limited. Only slight changes between carrier gases can be identified which places real limitations to chromatographic enhancement by change or modification of carrier gases

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A carrier gas should have the following properties:

1. Highly pure (> 99.9%)2. Inert so that no reaction with stationary phase or

instrumental components can take place, especially at high temperatures.

3. A higher density (larger viscosity) carrier gas is preferred.

4. Compatible with the detector since some detectors require the use of a specific carrier gas.

5. A cheap and available carrier gas is an advantage.

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Longitudinal Diffusion Term

This is an important factor contributing to band broadening which is a function of the diffusivity of the solute in the gaseous mobile phase as well as the molecular diffusion of the carrier gas itself.

HL = K DM /VWhere; DM is the diffusion coefficient of solute

in the carrier gas. This term can be minimized when mobile phases of low diffusion, i.e. high density, are used in conjunction with higher flow rates.

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The same van Deemter equation as in LC can be written for GC where:

H = A + B/V + CVThe optimum carrier gas velocity is given

by the derivative of van Deemter equation

Vopt = { B/C }1/2

However, the obtained velocity is much greater than that obtained in LC.

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The carrier gas pressure ranges from 10-50 psi. Higher pressures potentially increase compression possibility while very low pressures result in large band broadening due to diffusion. Depending on the column dimensions, flow rates from 1-150 mL/min are reported. Conventional analytical columns (1/8”) usually use flow rates in the range from 20-50 mL/min while capillary columns use flow rates from 1-5 mL/min depending on the dimensions and nature of column. In most cases, a selection between helium and nitrogen is made as these two gases are the most versatile and common carrier gases in GC.

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Injectors

Septum type injectors are the most common. These are composed of a glass tube where vaporization of the sample takes place. The sample is introduced into the injector through a self-sealing silicone rubber septum. The carrier gas flows through the injector carrying vaporized solutes. The temperature of the injector should be adjusted so that flash vaporization of all solutes occurs. If the temperature of the injector is not high enough (at least 50 degrees above highest boiling component), band broadening will take place.

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Carrier Gas

Syringe

Vaporization Chamber

To Column

Septum

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Column Configurations and Ovens

The column in chromatography is undoubtedly the heart of the technique. A column can either be a packed or open tubular. Traditionally, packed columns were most common but fast developments in open tubular techniques and reported advantages in terms of efficiency and speed may make open tubular columns the best choice in the near future. Packed columns are relatively short (~2meters) while open tubular columns may be as long as 30-100 meters

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Packed columns are made of stainless steel or glass while open tubular columns are usually made of fused silica. The temperature of the column is adjusted so that it is close to the average boiling point of the sample mixture. However, temperature programming is used very often to achieve better separations. The temperature of the column is assumed to be the same as the oven which houses the column. The oven temperature should be stable and easily changed in order to obtain reproducible results.

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Detection Systems

Several detectors are available for use in GC. Each detector has its own characteristics and features as well as drawbacks. Properties of an ideal detector include:

1. High sensitivity2. Minimum drift3. Wide dynamic range4. Operational temperatures

up to 400 oC.5. Fast response time6. Same response factor for

all solutes7. Good reliability (no fooling)8. Nondestructive9. Responds to all solutes

(universal)