ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: naved_s9994 on December 16, 2009, 10:33:01 am
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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
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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.
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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!
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sorry to bring this up again,
GC will have a single peak on the graph from the actual carrier gas correct?
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^yeah
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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 :)
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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?
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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.
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Also which peak on the graph relates to the mobile phase?
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Also which peak on the graph relates to the mobile phase?
All of them. Unless one didnt move.
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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.
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There can be.
It would have to stipulate it though, usually its removed from the chromatography. (AFAIK).
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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?
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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.
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See VCAA 2002 Short Answer Question 1b.
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hm fair enough . VCAA > Insight. although change of study design. Maybe change of answers ? :S
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See VCAA 2002 Short Answer Question 1b.
This is talking about the solvent which the stanozolol was dissolved in. Not the carrier gas (nitrogen).
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.
They do say that.. Does anyone know if this is actually correct?
I would think that it flows at a steady rate. What is there to push it all out? The role of the carrier gas is to push things through the column, so what pushes this through so that it all comes as a peak?
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Shit, you're right. In that case, then there mustn't be a peak.
When I think about it, if the carrier gas produces a peak, then it would always be present on the chormoatgram as one neverending peak. I've never seen this. I'm now leaning towards no peak.
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if the carrier does produce a peak, that indicates its reacted with something...which u dont want happening.
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I would think that if the carrier produced a peak the question would be nice enough to tell us. Thankfully insight did.
(and on that exam i skimmed that and missed it :-\)
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Why does it produce a peak when it reacts with something?
EDIT: wait I got it, it produces a new compound which has different attraction to the stationary phase => different retention time => separate peak.
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Just thinking about GC in general...............
Polarity still has a play in this as that determines how much it will dissolve yes?
yet its more dependent on the molar mass?
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The peak is due to the solvent not beacuse of the carrier gas