Strange peak plateau/tailing in GC-FID

[hellovorld]

Hello all,

Lately I have been having a very troublesome problem. I have been trying to develop method for analysis of impurities in isoprene. The first image shows a chromatogram of a isoprene sample spiked with impurities that we will want to quantitate. The peak just before the main isoprene is the one that is causing the problems, isopropenyl acetylene (2-Methyl-1-buten-3-yne).



That is what I had been seeing. However, after changing inlet liners, I see the following.



Sometimes the peak will stay at the maximum into the isoprene peak. Other times it will gradually slope down. But I never see it as it is seen in the first picture.

I'm at a loss for what it could be. I've changed columns (same phase). I've changed the inlet liner again, changed septum, performed blank runs, ran other samples, but when I go to run a sample with this compound in it, it behaves like the second picture. This makes me think it may be detector related.

[marek2]

In my opinion the oven temperature is higher than in the first experiment or you have set a higher flow rate. The detector is OK

[chromatographer1]

Your sample size has changed by a factor of 10.

Were you splitting initially.

Are you splitting the sample now?

best wishes,

Rod

[jdezeeuw]

HI
In GC when a component is introduced at HIGH concentration, you always see an impact on the peaks that elute in front. I have seen this amny times. THis is because the big peak will act as "phase" causeing the component that elutes before, to look like that. You can improve this by:
- Injecting less
- use a column with higher capacity (larger ID, thicker film)
- use a columnn with a better selctivity (that these peaks are better resolved.
- do a heart cut

Jaap de zeeuw, Restek corporation

[hellovorld]

Thank you for all of the responses.

I looked at the data and there was a difference in ID and flows between the two chromatograms shown. I chose those two as it illustrated the problem the best. I will get better representative data of the problem.

I may need to find a new phase that is more suitable for this separation. At the moment we're having to start at -15 C and hold for 10 minutes.

I also am having another problem while I am troubleshooting this and I don't want to spam these forums with all of my problems. Is the problem with this baseline common? It has me stumped.

[Steve Reimer]

Do you have liquid condensing on your column (at -15degrees)?
With a 10 minute hold at -15 your column phase would be variable amounts of liquid. If you need that temperature for retention you should look for a different coumn.

[chromatographer1]

Sorry you did not address all my questions. But if you would:

perhaps you can tell me why the peaks are 10 times larger in the later injections than the first injection you posted.

Did you block your splitter vent with polymerized rubber?

best wishes,

Rod

[hellovorld]

Sorry for not answering. Both injections were split. I have noticed a large variance in peak areas between injections.

I will check the splitter vent for blockage. I am also looking into different autosampler syringes. Someone suggested to me that a syringe with PTFE plunger may help for injection of volatiles.

[chromatographer1]

I have never tried to inject isoprene directly using a syringe due to the catalytic nature of hot metal which tends to initiate polymerization.

I have always used a liquid injection valve (like they use for LC) which is heated minimally (like 35C) with a replaceable 1 ft 2mm ID retention gap (fused silica or glass lined steel) to ensure full evaporation of the 2 microliter sample.

I suspect you should be monitoring your split flow immediately before and immediately after injecting your sample, and visually verify the amount of liquid injected if you continue to use a syringe. We used to get partial polymerization at times even after taking the most strenuous precautions.

Your split (but actually variable splitless) injection is causing the broadening and the distortion of peaks (I suspect).

good luck,

Rod

[Peter Apps]

The two chromatograms in the first post show multiple differences after the inlet liner change. There is a general degradation in resolution, which does not affect only the first peak, all the peaks are larger (as Rod noted), the retention times are all shorter, and the peaks are overloaded iin the second chromatogram. This points to multiple inlet problems with contamination and gas flow peturbations.

The resolution on the first chromatogram is optimal - you are not wasting time generating long sections of flat baseline, and all the peaks are baseline resolved, so whatever column you used then, it has the correct stationary phase. You should probably try a thicker film though - which will reduce the need for very low starting temperatures.

As noted, your sample is a difficult one. To give any further advice we would need to know your current operating conditions.

Peter