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Basic apparatus of gas chromatography

The instrumentation of gas chromatography is remarkably simple compare to many other recent analytical techniques. The fingure shows a block schematic diagram of a typical gas- liquid chromatography apparatus, such as might be used for analysis.

Carrier gas from the tank of compressed gas first passes to a controller, the usual purpose of which is to maintain a constant flow of gas.. The gas then passes to the beginning of the column, at the inlet to which is a Sample Injector through which the sample to be analysed can be introduced. The carrier gas then elutes the components of the mixture through the column.

At the far end is a device, the Detector, the purpose of which is to detect the separate components of the mixture as they emerge one by one. The detector use some physical or chemical properties of the vapours by which they can be indicated and, if possible, measured. A further piece of apparatus not always incorporated is a Flowmeter to measure the rate of flow of gas.

1. Any gas, which is easily distinguishable in the detector from any components of the mixture, can be chosen as the carrier gas. In most cases a permanents gas such as helium, argon, nitrogen or hydrogen is used.

2. Flow controls can either be true flow controls, which allow a definite rate of flow of gas, or they can be pressure controls, which control the input pressure to the column. Pressure controls are simpler than flow controls and are used more commonly.

3. For analytical purposes, reasonable dimensions for a permeably packed column might be 6 feet long by 1/4 inch diameter. In G. L. C the actual dissolving agent is a liquid which must remain stationary in the column without draining. The supporting material is often called the Support. The liquid solvent is called the Stationary liquid. The two together are called the Stationary Phase. The stationary liquid must dissolve all the components of the mixture and they must have boiling points, which are high in comparison with the boiling points of the components of the mixture. Almost any high-boiling point may be used, for example, silicone oil, high-boiling esters such as phtalates, high-boiling point paraffines, and many others.

4. One of the most commonly used properties for detecting the vapour is the thermal conductivity of the gas, which changes when a vapour is present.

There are different types of detectors such as: 1) Differentiating detectors, which record concentrations or rate flow of mass of the vapour. (Differentiating detectors are used in the great majority of applications); 2) integrating detectors, in which the deflection is proportional to the total quantity rather than to the concentration. When no vapour is passing through, the reorder gives a straight line.

5. Sample injectors, aim to inject a sample of a mixture of interest and not an accurately known weight. The sample injector remains the least satisfactory item of gas chromatographic equipment. Sample injectors must satisfy the general conditions that their volume should be small and that the temperatures of the devices designed to handle liquids or solids be sufficiently high.

The chromatogram

Analytical useful information obtainable from the chromatogram is of three kinds.

The simplest thing a chromatogram can show is whether or not a given sample is pure. If the chromatogram contains more than one peak, the sample contains more than one component. Very often this technique is used as a test of purity of substances.

The second use of chromatogram is to enable one to identify the individual components of a mixture qualitatively.

The third kind of information obtainable from the chromatogram is a quantitative analysis of the mixture, which is provided by the detector rather than column. With most detectors, the deflection of the meter is proportional to the concentration of vapour, and so, if the flow rate is constant, the area of peak will give the total amount of vapour. This may be made the basis for the exact quantitative analysis.

Gas chromatography is mainly used for quantitative analysis, for the method particularly suitable for the routine analysis of industrial sample, the interpretation of the data is simple, and the apparatus does not require skilled personnel.

Exercise

I. Answer the following questions:

1. What is the technique of gas chromatography?

2. Where does carrier gas from the tank of compressed gas first pass to?

3. What is the usual purpose of controller?

4. What does the carrier gas elute?

5. What is the purpose of Detector?

6. What types of Detectors do you know?

7. What can a chromatogram show?

8. What is gas chromatography used for?

II. Read and translate the following derivative:

1. chromatography, chromatographic;

2. separate, separation, separator;

3. volatile, volatility, volatilization, volatilize;

4. detector, detect, detection;

5. concentrate, concentration;

6. solid, solidity, solidify;

7. quality, qualitative, qualitatively;

8. quantity, quantitative, quantitatively;

9. particular, particularly;

III. Read and translate the following sentences paying attention to the functions of the Infinitive:

1. Carrier gas from the tank of compressed gas first passes to a controller, the usual purpose of which is to maintain a constant flow of gas.

2. In the inlet to the column there is a simple injector through which the sample to be analysed can be introduced.

3. The purpose of the detector is to detect the separate components of the mixture as they emerge one by one.

4. A further piece of apparatus not always incorporated is a flowmeter to measure the rate of the flow of gas.

5. Almost any high boiling liquid may be used.

6. Sample injectors aim to inject a temperature control of the column.

7. In the years to come many new synthetic products will appear.

8. It has taken centuries of scientific research and invention to develop the civilization of the modern age.

9. The chemical industry of the Soviet Union began to develop in the pre-war five year - periods.

10. Chemical processing method made it possible rationally to utilize industrial wastes to speed up technological processes and to ensure automation.

11. The second use of the chromatogram is to enable one to identify the individual components of a mixture qualitatively.

12. For the preparation of an aerosol the substance to be dispersed is first evaporated and the vapour is then quickly cooled.

IV. Translate into Vietnamese paying attention to the words in bold type:

1. Not a single property, but rather the combination of the properties has made this substance outstanding.

2. In integrating detectors the deflection is proportional to the total quantity rather than to the concentration.

3. The supernatant fraction appeared to degrade lysolecithin rather than to utilize it for lecithin production.

4. When no vapour is passing through, the recorder gives a straight line.

5. No traces of water have been obtained during the test.

6. A quantitative analysis of the mixture is provided by the detector rather than the column.

7. Gas chromatography is mainly used for quantitative analysis, for the method is suitable for the routine analysis of industrial samples, the interpretation of the data is simple, and the apparatus does not require skilled personnel.

V. Translate into English:

1. ThiÕt bÞ ®­îc sö dông trong s¾c kÝ khÝ rÊt ®¬n gi¶n.

2. NhiÖm vô cña ®ªtect¬ lµ ®Ó ph¸t hiÖn c¸c cÊu tö riªng rÏ cña hçn hîp khi chóng lÇn l­ît ®i ra khái cét .

3. Dªtect¬ sö dông mét sè tÝnh chÊt ho¸ häc vµ vËt lý cña h¬i, mµ nhê chóng c¸c cÊu tö ®­îc ph¸t hiÖn vµ nÕu cã thÓ ®­îc x¸c ®Þnh.

4. Trong s¾c kÝ khÝ- láng th× chÊt láng cè ®Þnh lµ c¸c chÊt cã kh¶ n¨ng hoµ tan c¸c cÊu tö cña mÉu.

5. ChÊt láng cè ®Þnh cÇn ph¶i hoµ tan tÊt c¶ c¸c cÊu tö trong hçn hîp.

VI. Make up an outline of the text and retell the text