Chromatography Forum

Dear everyone

I analyze ethylene gas with GC14B.

After sample injection (1mL), the baseline peak drop suddenly.

What is the problem? Please, help me..

GC conditions
Packed column Porapak Q
Column INIT temp = 50 oC
DET temp. = 100 oC
Injector temp. = 80 oC
Flow 80 kPa

dasida,

What are you using to inject the gas sample, syringe or valve. If it is a valve, is a 6 port or 10 port. Does the baseline slowly return to the starting value. Are you using a TCD (Thermal Conductivity Detector).

Gasman

Dear, Gasman

I used sampling with 1 mL plastic syringe.

Detector is FID detector.

Drop of baseline occurred 35 samples (all 50 samples).

Thanks a lot.

Can you see my chromatogarm?
If can't you see, how to attach my chromatogarm.
Please, let me know.

Best regards,

Dasida,

I am not able to see your chromatogram. Please see instructions in the Liquid Chromatography section on how to add images.

Gasman

Dear, GasMan

Thanks you.

Dasida,

I am not familiar with the Shimadzu GC, so please excuse some of my questions.

1. When you get this problem, does it happen ALWAYS at the same time in the run.

2. Are you able to see the signal value on the GC. Does this change at the same time as you see the change on the printout.

3. Does this happen if you start the run but do not inject the sample.

At the moment it looks to me as either a problem with the data system or there is a change in the hydrogen supply to the FID.

Gasman

Dear, GasMan

Answer of your request

1. Yes. drop of peak time is same.

2. I can't see signal value.

3. Only after sample injection, baseline drop.

Last year, this machine had not been problem at all.
I use only GC harvest season.

Anyway, I request to Shimadzu company.
I will notice you their answer.
Thanks a lot.

best regards,

check your timed events. You may have an auto zero event in your data system method.

best wishes,

Rod

Dear, Rod

Thanks for your advice.

I am not use time programme, but I'll check for data system.

I found a new problem.
I said that drop of peak hapened after sample injection. It was my mistake.

Today, I test only plotting without sample.
(Atenuation (3), Range 10^0, chart speed 1)

Peak drop showed rountinely about 20 min.

What is problem?



Sincerely yours,

Jin-Gook

I suspect an electronics problem.

Your baseline should not be rising during an isothermal oven.

good luck,

Rod

Dasida,

To check if this is an electronics problem as suggested by chromatographer1, do the same check but with the flame out on the detector. If you still have the problem, then it is electronics.

However,I would suspect the hydrogen supply to the FID. If there is a manual pressure controller for the hydrogen supply, check this. You may find that it is 'sticking', allowing the pressure to slowly rise, and then it 'unsticks', and resets to the original pressure.

Gasman

Dear, GasMan and Rod

I tried to your suggestion.
The baseline was straight with no flame.
It might mean no problem in electronics.

Gasman,
I don't understand your suggestion about hydrogen valve.
You mean that hydrogen supply was irregular.
Is it needed change hydrogen valve (manual controller) ?
I turn the hydrogen valve many times.
But, the baseline was increase and dropped reguraly.

Anyway, thanks a lot.

Best wishes,

dasida

Gasman probably has it figured out.

Review your detector connections and check the flows repeatedly with no flame and see if the flows are reproducible and constant when the detector is capped off from the oven.

Pneumatics of this GC should be checked thoroughly for leaks and hardware failures in pressure and flow control.

best wishes,

Rod

Dear, GasMan and Rod

Today, I solved the problem.
Shimadzu thechnical supporter came to Lab and repaired GC.

The problem was an air. Air was trapped by the silica gel.
Silica gel was old. Then, air was wet condition.
The time of drop of peak was the back pressure of air pump.

We changed to new silica gel. The baseline was straight in now.
But, peak dropped a little when operate pump.
(GC setting-atteuation is very high and also range is narrow; analyze trace amount of ethylene)
He recommened the use of hydrocarbon trap and inter-valve within air pump.

Thank for your sincere advice .

Best regards,

dasida