Gas Chromatography (GC)

[naved_s9994]

If anyone could explain the reason why in Gas Chromatography (GC), the flow rate of carier gas
effects the Retention time, and that it doesnt matter whatever gas you use,
the Retention time will stay the same?
Makes no sense, because shouldn't some gas be more reactive, and hence alter the Retention Time etc?

Thanks

[NE2000]

The point of the carrier gas is to not react at all. The molecules in a gas chromatography column will travel along the column and adsorb onto the stationary phase (the column itself) with the amount of adsorption generally reliant on the size of the molecule (as that affects the number of instantaneous dipoles that can exist at any one time). They do not react with the carrier gas. There are only a few carrier gases you can use but we generally look at helium and nitrogen, with hydrogen famously used in this year's unit 3 exam under the condition that the molecules passing through the column will not react with hydrogen.

[naved_s9994]

The point of the carrier gas is to not react at all. The molecules in a gas chromatography column will travel along the column and adsorb onto the stationary phase (the column itself) with the amount of adsorption generally reliant on the size of the molecule (as that affects the number of instantaneous dipoles that can exist at any one time). They do not react with the carrier gas. There are only a few carrier gases you can use but we generally look at helium and nitrogen, with hydrogen famously used in this year's unit 3 exam under the condition that the molecules passing through the column will not react with hydrogen.

Hahaha..!!  "under the condition that the molecules passing through the column will not react with hydrogen"

Thanks man, I got it all clear now!

[chansthename]

sorry to bring this up again,

GC will have a single peak on the graph from the actual carrier gas correct?

[stonecold]

^yeah

[stonecold]

So you can use H2 but only for unsaturated compounds. Good exam Q, coz if you put a unsaturated sample through, it'll react with the H2.

You mean saturated

[chansthename]

So you can use H2 but only for unsaturated compounds. Good exam Q, coz if you put a unsaturated sample through, it'll react with the H2.

yes you do mean saturated, and wasn't this a question on 2009 VCAA?

[azn_dj]

This is a question on 2009 VCAA. And well, I dont think you would go out of your way to use Hydrogen.
I believe there is problems at the end as well, in the detector
Because if its a flame ionization detector................ Hydrogen...............
you wont have much of a Gas Chromatography as well.
AND one more thing - it doesnt actually adsorb. In gas chromatography, it actually dissolves.

[chansthename]

Also which peak on the graph relates to the mobile phase?

[happyhappyland]

Also which peak on the graph relates to the mobile phase?

All of them. Unless one didnt move.

[chansthename]

All of them. Unless one didnt move.

isn't there supposed to be one that is specifically only the carrier gas
say if there are 4 peaks on the graph then there are 3 components in the sample.

[Martoman]

There can be.

It would have to stipulate it though, usually its removed from the chromatography. (AFAIK).

[m@tty]

Are you sure there is a peak for the carrier gas? I would think that it comes through at a steady rate, and justs raises the base line of the chromatogram.. If suddenly all of the carrier gas was gone, what would drive the analytes through?

[Studyinghard]

Are you sure there is a peak for the carrier gas? I would think that it comes through at a steady rate, and justs raises the base line of the chromatogram.. If suddenly all of the carrier gas was gone, what would drive the analytes through?

Q9 , insight 2008. Peak for carrier gas.

[stonecold]

See VCAA 2002 Short Answer Question 1b.