Split inlet -- preferential loss of aromatics

Discussions about GC and other "gas phase" separation techniques.

11 posts Page 1 of 1
Hi Everyone,

I'm having two problems that I believe are related to my split inlet... I'm quantifying the concentrations of mixtures of C1-C9 hydrocarbons using a gas sampling valve and an FID. The problems are:

1) The total peak areas are highly irreproducible, especially when I use an oven program that starts at cryogenic temperatures. I think this is likely due to fluctuations in the amount of analyte sent through the column vs vented. This isn't really a big deal since I can just normalize by the total peak areas, except...

2) The concentration of aromatics (benzene, toluene, xylenes) I observe stays relatively stable, whereas everything else (methane, ethane, ethylene, propylene, etc.) fluctuates with the total peak areas. The aromatics are at very low pressures (< 1 kPa) so they cannot be condensing.

Does anyone have any idea why this might be happening or any suggestions for fixing it?

Thank you!
BerkleyPhD,

Have you tried heating your line between the valve and the inlet? My experience with high C6+ samples (admittedly LPG) says that a heated line between the valve and the inlet eliminates a great deal of problems.

Best regards,

AICMM
If the concentration of the aromatics is stable, and you are normalising to total peak area, and total peak area fluctuates, then the peak areas for the aromatics must also fluctuate, and yet your point 2 implies that it is only the aliphatics peak areas that fluctuate with total peak area. Also, the title says preferential loss, but the post isn't about that. So can you clarify what it is you see.

Some details on instrument and settings, temperatures, gas flows etc would also help.
Peter Apps
AICMM wrote:
BerkleyPhD,

Have you tried heating your line between the valve and the inlet? My experience with high C6+ samples (admittedly LPG) says that a heated line between the valve and the inlet eliminates a great deal of problems.

Best regards,

AICMM




The lines aren't heated but the concentrations of the components are orders of magnitude below their equilibrium vapor pressures so there's no way they are condensing in the lines... Was that the reason you suggested heating?
Peter Apps wrote:
If the concentration of the aromatics is stable, and you are normalising to total peak area, and total peak area fluctuates, then the peak areas for the aromatics must also fluctuate, and yet your point 2 implies that it is only the aliphatics peak areas that fluctuate with total peak area. Also, the title says preferential loss, but the post isn't about that. So can you clarify what it is you see.

Some details on instrument and settings, temperatures, gas flows etc would also help.



What I mean is that each of my injections into the GC should have the same concentration of all components (including aromatics), but looking at the chromatogram it appears that the aromatics concentration is not stable. All the aliphatics vary appropriately with total peak area, but the aromatics response to the total peak area is much more muted. This is only the case with one GC, the other GC works perfectly even though all the GC parameters are exactly the same.
BerkeleyPhD wrote:
Peter Apps wrote:
If the concentration of the aromatics is stable, and you are normalising to total peak area, and total peak area fluctuates, then the peak areas for the aromatics must also fluctuate, and yet your point 2 implies that it is only the aliphatics peak areas that fluctuate with total peak area. Also, the title says preferential loss, but the post isn't about that. So can you clarify what it is you see.

Some details on instrument and settings, temperatures, gas flows etc would also help.



What I mean is that each of my injections into the GC should have the same concentration of all components (including aromatics), but looking at the chromatogram it appears that the aromatics concentration is not stable. All the aliphatics vary appropriately with total peak area, but the aromatics response to the total peak area is much more muted. This is only the case with one GC, the other GC works perfectly even though all the GC parameters are exactly the same.


Could be some type of high boiling point contamination within the lines that is adsorbing the aromatics. This is one reason to heat the lines, so that any contamination in the system won't hold on to the target analytes.
The past is there to guide us into the future, not to dwell in.
Hi!

What set up in your usage?
I suspect that in your description of the symptoms you are using "concentration" (which should be the tesult you get when you have done all your calculations on the peak areas) interchangably with raw peak areas.

Also, even at very low partial pressures you can get adsorption onto surfaces or absorption, whihc is why you need to heat your lines. The tiniest flake of a graphite ferrule or a little bit of carbonised deposit will selectively adsorb aromatics.

Peter
Peter Apps
Something that has caught me before-

At least on my Agilent 7820, there's a carbon filter on the split vent.

When it gets overloaded, it tends to result in loss of reproducibility, but it's only certain types of compounds(and there seems to be no particular pattern to what is affected and what isn't). I don't often catch it in my routine use as I do a lot of splitless work, but the people who do split on the same instrument will complain to me and I change it.

If your instrument has such a filter and you don't know when it was last changed, it might be worth ordering and installing a fresh one.
Thanks for pointing this out. To what temperature would you expect graphite would need to be heated to in order to prevent adsorption of aromatics?

Peter Apps wrote:
I suspect that in your description of the symptoms you are using "concentration" (which should be the tesult you get when you have done all your calculations on the peak areas) interchangably with raw peak areas.

Also, even at very low partial pressures you can get adsorption onto surfaces or absorption, whihc is why you need to heat your lines. The tiniest flake of a graphite ferrule or a little bit of carbonised deposit will selectively adsorb aromatics.

Peter
BerkeleyPhD wrote:
Thanks for pointing this out. To what temperature would you expect graphite would need to be heated to in order to prevent adsorption of aromatics?

Peter Apps wrote:
I suspect that in your description of the symptoms you are using "concentration" (which should be the tesult you get when you have done all your calculations on the peak areas) interchangably with raw peak areas.

Also, even at very low partial pressures you can get adsorption onto surfaces or absorption, whihc is why you need to heat your lines. The tiniest flake of a graphite ferrule or a little bit of carbonised deposit will selectively adsorb aromatics.

Peter


Usually best if above the boiling point of the analyte of interest. 150C will cover most of the lighter aromatics and also make sure there is no water condensing in the lines. At 250c and higher you might start getting some carbonized deposits if there are any heavy boiling contaminates present.
The past is there to guide us into the future, not to dwell in.
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