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GAS CHROMATOGRAPHY

Gas chromatography requires a certain minimum amount of instrumentation. Some gas chromatographs are very elaborate and expensive; bigger  equipment  allows the user to take advantage of the great versatility that gas chromatography offers, All  gas chromatographs, however, have the same basic components.

A CYLINDER OF CARRIER GAS
The inert gas that is to be used as the moving phase. As what is noted, this is generally helium or nitrogen.  Attached  to the cylinder is a pressure regulating valve and a guage to show the pressure. Close control of carrier gas pressure and flow rate are significant, and there will generally be a second regulating valve and guage in the gas chromatograph itself.

FLOW METER
To measure the rate of flow of the carrier gas. This is usually of the rotameter type, a vertical glass tube with a v-shaped section that is wider at the top than the bottom. Inside this tube is a small metal ball or a cone-shaped piece of metal that is lifted by the gas stream and rises higher in the tube, as a result of the gas pressure. The level which the ball floats shows the speed of the gas.

THE INJECTION PORT
The place where the sample is introduced into the carrier gas stream. Usually the samples are liquids at room temperature and are injected by a special syringe, like the hypodermic syringes used in medicine but smaller. The needle is introduced through a septum of silicone rubber which closes again after the needle is withdrawn. It is imperative for the sample to be vapourized completely as soon as it is introduced. Therefore, the injection port is embedded in heavy block of metal that can be heated electrically. The temperature is adjustable, and a guage on the front of the instrument indicated the temperature. The injection port should be at higher temperature. The injection port should be at a higher temperature than that of the column.

Samples of liquids at room temperature and are injected by a special syringe, like the hypodermic syringes used in medicine but smaller. The needle is introduced through a septum of silicone rubber which closes again after the needle is withdrawn. It is important for the sample to be vapourized completely as soon as it is introduced. The temperature is adjustable and the guage on the front of the instrument indicates the temperature. The injection port should be at higher temperature than that of the column.

THE COLUMN
This is the heart of the chromatograph. It consist of a tube of copper or stainless steel, or sometimes 2 meters or more. The inside diameter is variable, depending on the type of detector used. With thermal conductivity detectors the diameter is about 5 millimeters; for flame ionization detectors it probably be half this. Gas chromatographs used for preparative purposes, have much wider columns.

The column is enclosed in an oven, and to save space the column is bent in the form of a U, or into a coil. The temperature of the oven is controlled by thermostats, and air is circulated inside the oven by a fan, to make the temperature uniform . In most instruments the oven temperature can be adjusted continuously to any desired value.
Some low cost instruments have only certain temperature settings. More expensive instruments have temperature programming, whereby the temperature of the ovum can be continuously raised in a controlled way as the analysis proceeds.

The column are packed with solid granules that carry a film of non- volatile liquid which is the real stationary phase. The nature of the stationary phase is very important; different types of sample need different stationary phases. Users of gas chromatograps should keep two or three different columns on hand, filled with different kind of stationary phase, so that the columns can be changed in the instrument as needed. Once the column has been packed and bent into a coil or U-shape, it is almost impossible to empty or repack it.

THE RECORDER
The recorder traces a curve automatically and makes the analysis visible. It is connected to the detector in the form of a very small electric current that rises or falls when the substrate passes through the detector. The current flows through a high resistance and sets up an electromotive force between one end of the resistance and the other, and it is this electromotive force that controls the movement of the pen of the recorder accross the paper. The line traced by the recorder is therefore a graph of electromotive force against time. It is also interpreted as a graph of substrate concentration against volume of carrier gas.

THE DETECTOR
As the carrier gas passes out of the column, it takes with it the various substrates which emerge at different times, depending on how strongly they were held at different times, depending on how strongly they were held by fixed phase.  The most popular method until recently, has been the measurement of thermal conductivity. The heavier a molecule is, the more slowly it moves, and the more poorly it conducts heat across a gas. There also exist other factors like the way the molecules can rotate and vibrate, but the main factor is the ability of the gas to conduct heat in the speed at which the molecules can diffuse. The carrier gas has small rapidly moving molecules and conducts heat. Helium is the best choice of the carrier gas if thermal conductivity is used. Hydrogen conducts even better than helium, but it is inflammable and dangerous, and also reacts to organic compounds. The molecules of organic compounds on the other hand are relatively heavy , and lowers the heat conductivity if present.

Thermodynamics is concerned with  the study of the interaction of heat and work energies in systems and their surroundings, and the processes which bring about a change of state in such systems.

A SYSTEM
A thermodynamic system is a region in space that is clearly defined and which contains matter that is of interest and under investigation or study. It must be noted that the region of space in the system is not necessarily a constant volume because the system can change position and shape.

THE SYSTEM BOUNDARY
The boundary of a system is the real or imaginary envelope that encloses the system.

THE SURROUNDING
The surrounding to the system is the region of space external or outside of the system, but which shares a common boundary with the region.

TYPES OF SYSTEM

CLOSED SYSTEM
A closed system is a region in which the matter contained in it has constant mass, and where mass or matter can neither go into, or out of the system. However, heat and work energies can go in or out of a closed system.

AN OPEN SYSTEM
A thermodynamic system is said to be an open system when matter or mass can leave or enter the system through at least one opening at boundary. Thus an open system will have at least one opening in its boundary where matter may leave or enter the system, and because of this matter contained in the system is not necessarily constant.

AN ISOLATED SYSTEM
An isolated system in a special case of a closed  system. An isolated system does not interact with its surrounding in any way. Thus an isolated system does not allow matter in or out of the system and in addition, it does not allow heat and work interactions across its boundary.

SYSTEM EQUILIBRIUM
A system is said to be in equilibrium when it is capable of spontaneous change and when it is in complete balance with the surroundings. Another condition that must be satisfied for the system to be in equilibrium is that any property of the system must have a single value at its equilibrium state. On the whole, we can say a system is in equilibrium if no changes can occur in the state of the system without the aid of an external stimulus. Thus we can test if a system is on equilibrium by isolating it and observing whether any change in its state occurs. An equilibrium state of a system is a very important and useful condition because an equilibrium is that any change in its state occurs. An equilibrium state of a system is very important and useful conditions because an equilibrium state is subject to precise mathematical description and as such, single values of properties of the system can be obtained at this state.

AN ISOLATED SYSTEM
An isolated is one that neither allows matter in or out of the system and addition does not allow heat or work energies to enter or leave the system. From the obove description, an isolated system is also  a closed system. In order words, only a closed system may be isolated system.

THERMODYNAMIC PROPERTY
A  thermodynamic property is a characteristic of a system which has value, and is measurable directly or indirectly and more importantly, it is used to define the state of a thermodynamic system.

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