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Topic: gas chromatography: eluition order  (Read 1656 times)

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Offline xshadow

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gas chromatography: eluition order
« on: October 05, 2021, 04:55:50 AM »
I'm studying gas-chromatography but I don't undertand one crucial point.

When I'm in liquid chromatography if I use  a polar compound (-SiOH) as stationary phase the most polar solute will eluite first.
When I use some apolar phase, the less polar solte will eluite first.


I wonder if this difference will occour also in GC...or is the eluition order always defined by the relative vapor pressure (volatility) of the different solutes

i.e: the most volatile solute will always elute first

THANKS :)

Offline Babcock_Hall

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Re: gas chromatography: eluition order
« Reply #1 on: October 05, 2021, 09:40:33 AM »
In the case of alkanes, elution order and volatility are closely related for at least some stationary phases.  I don't have other examples handy at the moment.

Offline xshadow

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Re: gas chromatography: eluition order
« Reply #2 on: October 09, 2021, 06:50:30 AM »
In the case of alkanes, elution order and volatility are closely related for at least some stationary phases.  I don't have other examples handy at the moment.

thanks

Offline Babcock_Hall

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Re: gas chromatography: eluition order
« Reply #3 on: October 09, 2021, 01:31:39 PM »
https://thetruthaboutforensicscience.com/the-top-2-problems-seen-with-gas-chromatography-bac-results/

I did happen to see two chromatograms of ethanol and other simple alcohols (and possibly acetone) recently.  The context was the GC separation and quantitation of ethanol for blood alcohol determination.  The elution order was not identical between two stationary phases.  At the link above, I did not see some information that would be of interests to chemists.

Offline xshadow

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Re: gas chromatography: eluition order
« Reply #4 on: October 10, 2021, 03:44:11 PM »
https://thetruthaboutforensicscience.com/the-top-2-problems-seen-with-gas-chromatography-bac-results/

I did happen to see two chromatograms of ethanol and other simple alcohols (and possibly acetone) recently.  The context was the GC separation and quantitation of ethanol for blood alcohol determination.  The elution order was not identical between two stationary phases.  At the link above, I did not see some information that would be of interests to chemists.
I've read the text...interesting :)


I have a last doubt about a phrase that I've read when it speaks about injection system :

"" A splitless vent opened too earlly causes loss in response of heavier analytes ""

I wonder why....perhaps because the heavier analytes have a lower vapor pressure so if I open the purge valve too early I lost many of those ...

Can make it sense ???
Thanks

BUt i'm not sure because I thought  I was getting a flash vaporization of ALL the samples...p

Offline Babcock_Hall

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Re: gas chromatography: eluition order
« Reply #5 on: October 11, 2021, 08:18:16 AM »
I afraid that such a question exceeds my knowledge of gas chromatography.  However, some of the issues raised in the link I gave are worthy of consideration.  Sample carryover as a source of ghost peaks is a genuine issue.

Offline Mravche

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Re: gas chromatography: eluition order
« Reply #6 on: December 06, 2021, 04:13:37 PM »
Hey,

I don't really understand what you wrote about liquid chromatography. I mean if the stationary phase is polar, the most polar compound will eluite last.

I'am missing something here?

Unfortunately, I can't answer your question but I am very confused by this.  ??? ???

Offline Babcock_Hall

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Re: gas chromatography: eluition order
« Reply #7 on: December 06, 2021, 05:25:14 PM »
The stationary phase in gas chromatography is a liquid and the mobile phase is a carrier gas, such as helium.  In liquid chromatography the stationary phase is a solid and mobile phase is a liquid.  Gas chromatography is sometimes called gas liquid chromatography, which might be a little confusing.  Does that help?

Offline rjb

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Re: gas chromatography: eluition order
« Reply #8 on: January 04, 2022, 08:58:03 AM »
I'm studying gas-chromatography but I don't undertand one crucial point.

When I'm in liquid chromatography if I use  a polar compound (-SiOH) as stationary phase the most polar solute will eluite first.
When I use some apolar phase, the less polar solte will eluite first.

I wonder if this difference will occour also in GC...or is the eluition order always defined by the relative vapor pressure (volatility) of the different solutes

i.e: the most volatile solute will always elute first

THANKS :)

I wouldn't normally respond to a thread that is nearly 3 months old, but as the author appears to be a regular/frequent poster, I figured why not as I had some data to hand that might help...
Like Mravche, I think there is something wrong with the author's comments regarding LC...

With normal phase HPLC (i.e. Polar stationary phase and non-polar mobile phase) polar analytes will be retained for longer on the column than less polar analytes and hence elute later.
With reverse phase HPLC (i.e. Non-polar (like C18) stationary phase and polar mobile phase) non-polar analytes will be retained for longer than polar analytes and hence elute later. The classic 'like attracts like' really helps getting to grips with this. 

GC is (in my view) a bit simpler than HPLC in some ways. Volatility does play an important part in elution order but other factors (such as the choice of column) are as, if not more important. The following samples were run on a simple GC-FID using an isothermal temperature programme and most importantly using a highly polar ZB-WAX+ column.

Reference    Sample    RT (mins)   VP (kPa)
TMP           Liquid   1.410   5.1
Octane   Liquid   1.502   1.33
Methanol   Liquid   1.647   12.9
Ethanol   Liquid   1.743   5.8
Propanol   Liquid   2.241   2

You can probably see that Octane (which has the lowest VP) comes off the column fairly rapidly, whereas methanol (which has the highest VP) comes off after Octane. Using a different column (such as a ZB-1) and similar conditions the elution order and retention times would be quite different, although you would expect that VP would play a significant part in defining order of elution of 'similar' compounds like these simple alcohols.

Hope that Helps

R



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