Split inlet -- preferential loss of aromatics

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

6 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.
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