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Gas chromatography

Gas chromatography is a method for separating components of mixtures of volatile compounds. In most applications the separations are made to identify and determine the quantity of each component of a sample of the mixture, and analytical gas chromatographic apparatus includes additional devices for this purpose. In some applications, separations are made for preparative purposes, but the scale is not generally greater than that required for quantities of the order of 100g.

The central item in the apparatus for gas chromatography is the chromatographic column, a long tube packed permeably with some adsorbent. In the commonest technique of gas chromatography, the elution technique, a stream of inert gas, the carrier gas, passes continuously through the column, and the mixture to be separated is introduced at the beginning of the column as a sample either of a gas or a volatile liquid. Let us suppose that the sample consists of one pure component. After introduction, it is swept by the carrier gas on to the column, first evaporating to form a vapour if it is introduced as a liquid. When it reach the column, it is largely adsorbed, but the equilibrium is set up between the column and the gas in the interstices of the column so that a proportion of the sample always remains in the gas phase. This portion moves a little further along the column in the carrier gas stream, where it again equilibrates with the column. At the same time, material already adsorbed in the column re - enters the gas phase so as to restore equilibrium with the clean carrier gas which follows up the zone of vapour.

The speed at which the zone moves depends on two factors, the rate of flow of the carrier gas and the extent to which the vapour is adsorbed. The faster the flow of carrier gas, the faster the zone moves; and the more strongly the vapour is adsorbed on the column, the more slowly the zone moves. When two or more components are present in the sample, each usually behaves, independently of the others so that for a given carrier gas flow rate, the speed of the zone of each component will depend on the extent to which it is adsorbed. Since different substances differ in their adsorption, they may therefore be separated by making use of their different speeds of progress through the column. If they are eluted to the far end of the column, they will appear one after the other in the gas stream, the fastest first and the lowest last.

Adsorbents such as carbon, alumina, or silicagel are used as the packing material for columns, but in more than 90% of applications, the column material is the liquid held in place on the column by being adsorbed on an inert solid support. Gas chromatography with this kind of column is called Gas Liquid Chromatography (G L G). This method is used for separating solutes from mixed solutions.

Exercises

I. Answer the following questions:

1. What does gas chromatography mean?

2. What is the central item in the apparatus for gas chromatography?

3. What gas passes through the column?

4. How does the process of chromatography pass?

5. What are adsorbents such as carbon, alumina, or silicagel used for?

6. What method is used for separating solutes from mixed solutions?

II. Read and translate the following derivative:

1. analytical, analysis, analyse;

2. scheme, schematic, schematically;

3. compress, compression;

4. identify, identification;

5. include, inclusion, inclusive;

6. prepare, preparation, preparative;

7. adsorb, adsorbent, adsorption;

8. technique, technical, technician;

9. evaporation, vapour;

10. equilibrium, equilibrate, equilibration;

11. behavior, behave;

12. solute, solution, solvent, soluble, dissolve;

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

1. The mixture is introduced as a sample either of a gas or a volatile liquid.

2. The material already adsorbed in the column re - enters the gas phase so as to restore equilibrium with the clean carrier gas.

3. The speed at which the zone moves depends on the rate of flow of the carrier gas and the extent to which the vapour is adsorbed.

4. The faster the flow of the carrier gas, the faster the zone moves.

5. The more strongly the vapour is adsorbed on the column, the more slowly the zone moves.

6. The speed of the zone of each component will depend on the extent to which it is adsorbed.

IV. Translate into Vietnamese paying attention to the function of the infinitive:

1. The mixture to be separated is introduced at the beginning of the column.

2. The electrons have come to be known as valence electrons, because the valence of any given element depends on the number of these electrons.

3. Valence is the tendency of the atoms of any given element to donate or accept a strictly definite number of electrons.

4. Chlorine requires only one electron to form the stable electron layer of the following inert gas.

5. Hydrogen is a combustible gas, burning in air or oxygen to form water.

6. The organic compounds to be discussed are of great importance in the study of plastics.

7. Carbon has a tendency to combine with oxygen from the air to form gaseous carbon dioxide.

8. There is no problem to be associated with exuding, leaching or deterioration with polyethylene plastic.

9. The stability of polyethylene plastic in storage is to be associated with the lack of chemical active functional groups in the molecular structure.

10. To regard the polyethylene as a substitute for stainless steel is not proper design procedure.

V. Translate into English:

1. S¾c kÝ khÝ lµ ph­¬ng ph¸p t¸ch c¸c cÊu tö cña hçn hîp hoÆc t¸ch c¸c chÊt dÔ bay h¬i .

2. ViÖc t¸ch ®­îc tiÕn hµnh ®Ó ®Þnh l­îng mçi cÊu tö trong hçn hîp.

3. Bé phËn chÝnh cña thiÕt bÞ s¾c kÝ khÝ lµ cét s¾c kÝ( lµ mét èng dµi ®­îc nhåi mét chÊt nµo ®ã cã kh¶ n¨ng hÊp phô ).

4. KhÝ mang liªn tôc ®i qua cét ®ã. Hçn hîp cÇn t¸ch ®­îc cho vµo cét hoÆc ë d¹ng khÝ hé¨c ë d¹ng láng dÔ bay h¬i.

VI. Write a short summary of the text