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GAS CHROMATOGRAPHY AND HIGH PERFORMANCE LIQUID CHROMATOGRAPHY TECHNIQUES
Gas chromatography is the most extensively employed technique of all the instrumental separation method for analytical purpose.
It provides quick and easy way of determining the number of components in a mixture, the presence of impurities in a substance and in many instances.
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Table of contents
GAS CHROMATOGRAPHY (G.C) 2
The stationery phase 3
SAMPLE INJECTION 3
GAS CHROMATOGAPHY COLUMNS 3
GAS CHROMATOGRAPHY DETECTOR 4
b) Nitrogen phosphorus detector 6
c) Flame photometric detector 6
Other type of detectors include 6
HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC) 7
PUMPING SYSTEM 8
4.The short-stroke piston pumps 9
SAMPLE INJECTORS 10
Advantages of valve loop injectors 10
3.Syringe – loop injectors 11
COLUMN AND COLUMN FITTINGS 11
LIQUID CHROMATOGRPHY DETECTORS 12
ADVANTAGES OF HPLC OVER OTHER FORMS LIQUID CHROMATOGRAPHY 15
COMPARISON BETWEEN HPLC AND GC 15
Preview of the essay: GAS CHROMATOGRAPHY AND HIGH PERFORMANCE LIQUID CHROMATOGRAPHY TECHNIQUES
Stationary phase selection is critical for achieving selectivity. Several attempts have been made to predict the proper selection of liquid immobile phase without resorting exclusively to trial and error techniques.
Phases are selected based on their polarity, keeping in mind that “like dissolve like”. That is polar stationery phase will interact more with polar compounds and vise versa. A phase should be selected in which the solute has solubility. Non-popular liquid phases are generally non selective because these are few forces between the solute and the solvent, and so separation tend to follow the order of the boiling points of the solute, with the low-boiling ones eluting first. Polar liquid phases exhibit several interactions with solute such as dipole interactions, hydrogen bonds, and induction forces, and there is not necessary the same elute correlation with volatility.
There are three methods of inserting samples;
1. The valve method
It is convenient for sampling gas stream
The diagram consists of pair of identical dual path stopcocks. In the position shown, the carner gas is passed through the column, which is in a standby condition. To take a sample, stopcock no.1 is turned 90ºc. so that the reservoir is filled with sample gas. Then no. 1 is turned to its original position, and no.2 turned through 90ºc ...
... dissolved in a solvent and, a part from cross- linked, high molecule mass substances, all organic and ionic inorganic products satisfy this condition.
There are three main important differences
i. The diffusion coefficient of the sample in the mobile phase is much smaller in HPLC than in GC (this is a drawback because the diffusion coefficient is the most important factors which determine the speed of chromatographic analysis.
ii. The viscosity of the mobile phase is higher in HPLC than inn GC (this is a drawback because high viscosity result in small diffusion coefficient and in high flow resistance of the mobile phase)
iii. The compressibility of the mobile phase under pressure is negligibly small in HPLC but not in GC (this is an advantage because as a result the flow velocity of the mobile phase is constant over the whole length of the column. Therefore optimum chromatographic conditions exists everywhere if the flow velocity is chosen correctly. Moreover, incompressibility means that a liquid under high pressure is not dangerous)
The information given below gives a summary of the comparison between HPLC and GC
Characteristic of both methods
•Only small sample required
•May be non destructive of sample
•Readly adapted to quantitative
Advantages of HPLC
•Simple and inexpensive equipment
•Unparalleled resolution (with capillary)
•Easily interfaced with mass spectrometry.
Safety in the HPLC laboratory
Three health risks are inherent in HPLC there being
a) Toxic solvents
b) Pulmonary irritation from the stationary phase and
c) Danger resulting from the use of high pressures.
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