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Gas Chromatography

Lecture 39

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a. Thermal Conductivity Detector (TCD)

This is a nondestructive detector which is used for the separation and collection of solutes to further perform some other experiments on each purely separated component. The heart of the detector is a heated filament which is cooled by helium carrier gas. Any solute passes across the filament will not cool it as much as helium does because helium has the highest thermal conductivity. This results in an increase in the temperature of the filament which is related to concentration. The detector is simple, nondestructive, and universal but is not very sensitive and is flow rate sensitive.

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Note that gases should always be flowing through the detector including just before, and few minutes after, the operation of the detector. Otherwise, the filament will melt. Also, keep away any oxygen since oxygen will oxidize the filament and results in its destruction.

Remember that TCD characteristics include:

1. Rugged2. Wide dynamic

range (105)3. Nondestructive4. Insensitive (10-8 g/s) 5. Flow rate sensitive

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b. Flame Ionization Detector (FID)

This is one of the most sensitive and reliable destructive detectors. Separate two gas cylinders, one for fuel and the other for O2 or air are used in the ignition of the flame of the FID. The fuel is usually hydrogen gas. The flow rate of air and hydrogen should be carefully adjusted in order to successfully ignite the flame.

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The FID detector is a mass sensitive detector where solutes are ionized in the flame and electrons emitted are attracted by a positive electrode, where a current is obtained.

The FID detector is not responsive to air, water, carbon disulfide. This is an extremely important advantage where volatile solutes present in water matrix can be easily analyzed without any pretreatment.

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Remember that FID characteristics include:• Rugged• Sensitive (10-13 g/s)• Wide dynamic range (107)• Signal depends on number of carbon atoms

in organic analytes which is referred to as mass sensitive rather than concentration sensitive

• Weakly sensitive to carbonyl, amine, alcohol, amine groups

• Not sensitive to non-combustibles – H2O, CO2, SO2, NOx

• Destructive

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Electron Capture Detector (ECD)

This detector exhibits high intensity for halogen containing compounds and thus has found wide applications in the detection of pesticides and polychlorinated biphenyls. The mechanism of sensing relies on the fact that electronegative atoms, like halogens, will capture electrons from a b emitter (usually 63Ni). In absence of halogenated compounds, a high current signal will be recorded due to high ionization of the carrier gas, which is N2, while in presence of halogenated compounds the signal will decrease due to lower ionization.

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Remember the following facts about ECD:1. Electrons from a b-source ionize the carrier

gas (nitrogen)2. Organic molecules containing

electronegative atoms capture electrons and decrease current

3. Simple and reliable4. Sensitive (10-15 g/s) to electronegative groups

(halogens)5. Largely non-destructive6. Insensitive to amines, alcohols and

hydrocarbons7. Limited dynamic range (102)8. Mass sensitive detector

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Gas Chromatographic Columns and Stationary Phases

Packed ColumnsThese 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. The column is packed with finely divided particles (<100-400 m diameter), which is coated with stationary phase. However, glass tubes are also used for large-scale separations.

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Several types of tubing were used ranging from copper, stainless steel, aluminum and glass. Stainless steel is the most widely used because it is most inert and easy to work with. The column diameters currently in use are ordinarily 1/16" to 1/4" 0.D. Columns exceeding 1/8" are usually used for preparative work while the 1/8" or narrower columns have excellent working properties and yield excellent results in the analytical range. These find excellent and wide use because of easy packing and good routine separation characteristics. Column length can be from few feet for packed columns to more than 100 ft for capillary columns.

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Capillary/Open Tubular

Open tubular or capillary columns are finding broad applications. These are mainly of two types:

• Wall-coated open tubular (WCOT) <1 m thick liquid coating on inside of silica tube

• Support-coated open tubular (SCOT) 30 m thick coating of liquid coated support on inside of silica tube

These are used for fast and efficient separations but are good only for small samples. The most frequently used capillary column, nowadays, is the fused silica open tubular column (FSOT), which is a WCOT column.

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The external surface of the fused silica columns is coated with a polyimide film to increase their strength. The most frequently used internal diameters occur in the range from 260-320 micrometer. However, other larger diameters are known where a 530 micrometer fused silica open tubular column was recently made and is called a megapore column, to distinguish it from other capillary columns. Megapore columns tolerate a larger sample size.

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It should be noted that since capillary columns are not packed with any solid support, but rather a very thin film of stationary phase which adheres to the internal surface of the tubing, the A term in the van Deemter equation which stands for multiple path effects is zero and the equation for capillary columns becomes

H = B/V + CV

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Capillary columns advantages compared to packed columns

1. higher resolution2. shorter analysis times3. greater sensitivity

Capillary columns disadvantage compared to packed columns

1. smaller sample capacity2. Need better experience

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Solid Support Materials

The solid support should ideally have the following properties:

1. Large surface area (at least 1 m2/g)

2. Has a good mechanical stability

3. Thermally stable

4. Inert surface in order to simplify retention behavior and prevent solute adsorption

5. Has a particle size in the range from 100-400 m

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Selection of Stationary Phases

General properties of a good liquid stationary phase are easy to guess where inertness towards solutes is essential. Very low volatility liquids that have good absolute and differential solubilities for analytes are required for successful separations. An additional factor that influences the performance of a stationary phase is its thermal stability where a stationary phase should be thermally stable in order to obtain reproducible results. Nonvolatile liquids assure minimum bleeding of the stationary phase

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Weight of liquid stationary phase * 100% % Loading =

Increasing percent loading would allow for increased sample capacity and cover any active sites on the solid support. These two advantages are very important, however increasing the thickness of stationary phase will affect the C term in the van Deemter equation by increasing HS, and therefore Ht.

Weight of stationary phase plus solid support

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Generally, the film thickness primarily affects the retention character and the sample capacity of a column. Thick films are used with highly volatile analytes, because such films retain solutes for a longer time and thus provide a greater time for separation to take place. Thin films are useful for separating species of low volatility in a reasonable time. On the other hand, a thicker film can tolerate a larger sample size. Film thicknesses in the range from 0.1 – 5 m are common.