orgo lab technique question

[tank you]

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in gas chromatography, the area under the curve gives the amount of the substance correct?

also, why does something with a higher boiling point have a longer retention time?

 

[ryche22]

in gas chromatography, the area under the curve gives the amount of the substance correct?

also, why does something with a higher boiling point have a longer retention time?

pretty sure that if a substance has a lower bp, it will elute (evaporate) faster from your stationary phase, leading to less theoretical plates, and a lower retention time.

 

[QofQuimica]

in gas chromatography, the area under the curve gives the amount of the substance correct?

It gives the relative proportions. So, for example, you could conclude based on the peak areas that you have 60% of Compound 1 and 40% of Compound 2, but you could not conclude that you have exactly 6 mg or 6 g or 6 kg of Compound 1 from GC (unless you have also injected a known amount of a GC standard, but that is more complex than you need to know for the MCAT). Although, you can rule out the larger amounts I gave just because it would be impossible to inject that much onto the column.

also, why does something with a higher boiling point have a longer retention time?

GC separates compounds based on two main characteristics: boiling point, and to a lesser extent, polarity. The columns used for GC are a bit different than the ones used for liquid chromatography; the stationary phase in GC is actually a viscous liquid, and the columns are typically not packed, but rather are "open tubular". That means there is an open space in the center for the gas (your mobile phase) to go through. The column is very long (30 m....that's METERS....is a common length) and it is coiled and placed into an oven. The temperature is raised over time, typically from RT to some other moderate temperature. (You can't raise it too much, or you'll destroy the column and/or decompose your compound.) After injection, all of the compounds are vaporized before entering the column. Compounds that are low-boiling remain vaporized and travel rapidly through the column. Compounds that are high-boiling adsorb to the stationary phase more readily, and therefore come off later. If the two compounds have the same boiling point, they might still be separated if they have different affinities for the stationary phase (different polarities).

In general, it is best to think of chromatography as an equilibrium. Both the stationary and mobile phases are "competing" for the compounds. Some compounds prefer the stationary phase, and others prefer the mobile phase, but it is not a winner-takes-all scenario. So, in GC, once the compound vaporizes, it is carried a ways by the gas, then it adsorbs to the stationary phase, unsticks and travels a bit further, sticks again, etc., etc., all down the length of the column.

 

[tank you]

It gives the relative proportions. So, for example, you could conclude based on the peak areas that you have 60% of Compound 1 and 40% of Compound 2, but you could not conclude that you have exactly 6 mg or 6 g or 6 kg of Compound 1 from GC (unless you have also injected a known amount of a GC standard, but that is more complex than you need to know for the MCAT). Although, you can rule out the larger amounts I gave just because it would be impossible to inject that much onto the column.



GC separates compounds based on two main characteristics: boiling point, and to a lesser extent, polarity. The columns used for GC are a bit different than the ones used for liquid chromatography; the stationary phase in GC is actually a viscous liquid, and the columns are typically not packed, but rather are "open tubular". That means there is an open space in the center for the gas (your mobile phase) to go through. The column is very long (30 m....that's METERS....is a common length) and it is coiled and placed into an oven. The temperature is raised over time, typically from RT to some other moderate temperature. (You can't raise it too much, or you'll destroy the column and/or decompose your compound.) After injection, all of the compounds are vaporized before entering the column. Compounds that are low-boiling remain vaporized and travel rapidly through the column. Compounds that are high-boiling adsorb to the stationary phase more readily, and therefore come off later. If the two compounds have the same boiling point, they might still be separated if they have different affinities for the stationary phase (different polarities).

In general, it is best to think of chromatography as an equilibrium. Both the stationary and mobile phases are "competing" for the compounds. Some compounds prefer the stationary phase, and others prefer the mobile phase, but it is not a winner-takes-all scenario. So, in GC, once the compound vaporizes, it is carried a ways by the gas, then it adsorbs to the stationary phase, unsticks and travels a bit further, sticks again, etc., etc., all down the length of the column.

Thank you very much. 👍