Reduced or mssing alkene and alkyne peaks

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

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Hi all,
I'd like to thank you all in advance for reading the post and posting potential solutions
We recently replaced the FID collector assembly and the jet of the GC, but after this we could not see any alkene or alkyne peak. We have trim ~10 cm of the outlet of the column.
I am not an expert on column, but the phenomena looks like the column is activated due to the contact with air when changing the jet. Should I trim the column outlet even more? such as 20~50 cm?

Please see details of the column, calibration gas used and the operation configuration below

GC agilent 7890B
Column 19091P-S12 (Fused silica, 0.32mm ID, 27 M length, more information here: https://www.agilent.com/store/fr_FR/Pro ... 19091P-S12 )
Operation configuration:
Inlet
Manual Injection Split 1:80, He flow 5 ml/min, Heater 250 C, Pressure 16 psi Septum Purge 3 ml/min
Detector
Heater 250 C Air Flow 400 ml/min, H2 40ml/min, Makeup 27 ml/min
He flow 3.5 ml/min
Oven
60 C
Ramp 1 20 C/min to 80 C
Ramp 2 30 C/min to 190 C

Our inlet doesn't have a liner, and all we do is inject hydrocarbons into the GC.

Calibration Gas composition

The bold font denoted the gas we could not observe

2 METHYL 2 BUTENE 0.0500 % - this one is higher (than previous, "normal" calibration)
1 PENTENE 0.1000 % - none shown
ACETYLENE 1.000 % - much lower
CIS 2 PENTENE 0.1000 % - much lower
METHYL ACETYLENE 1.000 % - much lower
CIS 2 BUTENE 0.3000 % - this one is lower
HEXANE 0.0500 %
ISOPENTANE 0.1000 %
N PENTANE 0.1000 %
TRANS 2 PENTENE 0.1000 %
ARGON 0.2000 %
1 BUTENE 0.3000 %
1,3 BUTADIENE 0.3000 %
ISOBUTANE 0.3000 %
ISOBUTYLENE 0.3000 %
N BUTANE 0.3000 %
TRANS 2 BUTENE 0.3000 %
PROPADIENE 1.000 %
PROPYLENE 1.000 %
CARBON MONOXIDE 1.500 %
ETHYLENE 2.000 %
PROPANE 2.000 %
CARBON DIOXIDE 3.500 %
ETHANE 4.000 %
METHANE 5.000 %
HYDROGEN 12.00 %
If you are using a splitless/split inlet, how can you not have a liner? How do you make the seal at the top of the inlet where the o-ring normally wraps around the liner to make the gas seal that allows column flow? Something amiss...

Why are you doing such a high split? Even though only a 0.32 the biggest peak in your mix is only 5%.

It is possible for alumina to become deactivated with water so a long bake out at a reasonable temperature (150 say) is not an unreasonable starting point.

Have to answer the liner question first, though. That is where I would start.

Best regards,

AICMM
Thanks AICMM.
I need to clarify that I may have made a mistake explaining the liner problem.
Our gas chromatography use a gas bag to inject the sample gas to the system, and I do not know where the liner is. In the chemstation interface, I also noticed that in the inlet part it also shows that: a liner has not been selected. This is why I say that there may not have a liner. But since you point that out, I am not that sure. I will open it up and have a look.

I will try the bake out first, thanks.but just curious where is the alumina at?
Hi all,
Just an quick update. A closer look clarifies that no peak is missing. Rather, the residence time is change by -0.1 min that shift everything, and hence leading us to assume that there are missing peak.

However, I have a hard time understanding why changing the FID jet would change the residence time so much. The agilent currently offer the jet with the wide open inlet as compared to the previous narrow and long inlet. I am wondering if this is what caused the issue
A jet is a very narrow bore hole in the tip of a fitting. It can come in different sizes, depending on the flow that may be passing through it (capillary vs packed for example.) It is fine enough of a hole that it can act as a restrictor. Changing from one jet size to another jet size will change the restrictor and thus the backpressure on the column. This can affect retention times.

Your retention time stability is due in part to a manual injection BTW. There is always some variance in a manual injection of a gas sample that is not present when using a valve injection.

Best regards,

AICMM
Thank you AICMM!
HydrocarbonCrash wrote:
Hi all,
Just an quick update. A closer look clarifies that no peak is missing. Rather, the residence time is change by -0.1 min that shift everything, and hence leading us to assume that there are missing peak.

However, I have a hard time understanding why changing the FID jet would change the residence time so much. The agilent currently offer the jet with the wide open inlet as compared to the previous narrow and long inlet. I am wondering if this is what caused the issue


The alumina would be the stationary phase of the column.

You did remove 0.1m from a 25m column, did you tell the instrument the column length was now 19.9m? If not the flow will be slightly higher since the GC did not reduce the inlet pressure to compensate for the shorter column. In the software where you tell it what column you have, you can also "calibrate" the column and tell it you removed 0.1m from it and it will automatically compensate for that and usually keep the retention times very close to where they were. Another trick I have used if the times shift 0.1 minutes is to add 0.1 minutes to the initial oven temp hold time if using a ramped temperature program. That usually corrects most of the times.
The past is there to guide us into the future, not to dwell in.
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