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In some cases, as you will see below, the column starts off at a low temperature and then is made steadily hotter under computer control as the analysis proceeds. One of three things might happen to a particular molecule in the mixture injected into the column:.
A compound with a boiling point higher than the temperature of the column will obviously tend to condense at the start of the column. The chances are that it will then condense again a little further along the column. Similarly, some molecules may dissolve in the liquid stationary phase. Some compounds will be more soluble in the liquid than others. The more soluble ones will spend more of their time absorbed into the stationary phase; the less soluble ones will spend more of their time in the gas. The process where a substance divides itself between two immiscible solvents because it is more soluble in one than the other is known as partition.
Now, you might reasonably argue that a gas such as helium can't really be described as a "solvent". But the term partition is still used in gas-liquid chromatography. You can say that a substance partitions itself between the liquid stationary phase and the gas. Any molecule in the substance spends some of its time dissolved in the liquid and some of its time carried along with the gas. The time taken for a particular compound to travel through the column to the detector is known as its retention time.
This time is measured from the time at which the sample is injected to the point at which the display shows a maximum peak height for that compound.
Different compounds have different retention times. For a particular compound, the retention time will vary depending on:. A compound which boils at a temperature higher than the column temperature is going to spend nearly all of its time condensed as a liquid at the beginning of the column. So high boiling point means a long retention time. The more soluble a compound is in the liquid phase, the less time it will spend being carried along by the gas.
High solubility in the liquid phase means a high retention time. A higher temperature will tend to excite molecules into the gas phase - either because they evaporate more readily, or because they are so energetic that the attractions of the liquid no longer hold them.
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A high column temperature shortens retention times for everything in the column. For a given sample and column, there isn't much you can do about the boiling points of the compounds or their solubility in the liquid phase - but you do have control over the temperature. The lower the temperature of the column, the better the separation you will get - but it could take a very long time to get the compounds through which are condensing at the beginning of the column!
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On the other hand, using a high temperature, everything will pass through the column much more quickly - but less well separated out. If everything passed through in a very short time, there isn't going to be much space between their peaks on the chromatogram. The answer is to start with the column relatively cool, and then gradually and very regularly increase the temperature.
At the beginning, compounds which spend most of their time in the gas phase will pass quickly through the column and be detected.
Increasing the temperature a bit will encourage the slightly "stickier" compounds through. Increasing the temperature still more will force the very "sticky" molecules off the stationary phase and through the column. There are several different types of detector in use. The flame ionisation detector described below is commonly used and is easier to describe and explain than the alternatives. In terms of reaction mechanisms, the burning of an organic compound is very complicated.
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During the process, small amounts of ions and electrons are produced in the flame. The presence of these can be detected. Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts. The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online.
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Analytical Chemistry , 91 8 , Carlos A. Manzano, Nathan G. Dodder, Eunha Hoh, Raul Morales. ACS Central Science , 4 8 ,