Chromatography Forum

We have a problem with the chromatographic 7890 A. The peaks of the FID move (delayed) but however not TCD. We have verified that there are no leaks in the injector but do not know why the peaks have been delayed.
If anyone could help me I would be very grateful.
thank you very much

If You really want help why You do not post details about Your set-up????

Are detectors connected simultaneously via splitter or column is re-connected between detectors? What kind of column? Column temperatures, flow? What kind of analytes? Gas sample, liquid sample, split, splitless?

What means delayed? 0.00001 min? 0.1 min? 5 minutes? What about solvent?

If You really want help why You do not post details about Your set-up????

Are detectors connected simultaneously via splitter or column is re-connected between detectors? What kind of column? Column temperatures, flow? What kind of analytes? Gas sample, liquid sample, split, splitless?

What means delayed? 0.00001 min? 0.1 min? 5 minutes? What about solvent?

Thank you very much for your answer.

The gas chromatographic equipment have such as carrier gas helium. We analyze in continuous a gas mixture containing alcohols, hydrocarbons, hydrogen, nitrogen and carbon monoxide predominantly.
The detectors are two FID and one TCD. The FID are connected with a dean swith. The peaks which are delayed are from FID. The peaks delayed about 2 minutes and they are delayed more with the time.
The column are: HPFFAP, GAS-PRO and DB-1.

If you need some more details, please answer me...thank you very much

The GasPro is a PLOT and the FFAP and -1 are capillary. At first glance, you would not expect the retention times to be anywhere near each other with this combo without more details. Which column is connected to which detector then?

Best regards,

AICMM

If I had to guess I'd say his TCD is the gas-pro, his FFAP is for his alcohols, and his DB-1 for hydrocarbons. In fact I'd say this is definitely what's going on because as we speak I've got a similar setup to the OP. I'm having a hard time figuring out your question though, what exactly do you mean by delayed? Your retention time on each dector/column being different from each other? Ex. Ethane retention time on the DB-1 not matching FFAP retention times? Need some more details on what's going on and I can help some more.

If I had to guess I'd say his TCD is the gas-pro, his FFAP is for his alcohols, and his DB-1 for hydrocarbons. In fact I'd say this is definitely what's going on because as we speak I've got a similar setup to the OP. I'm having a hard time figuring out your question though, what exactly do you mean by delayed? Your retention time on each dector/column being different from each other? Ex. Ethane retention time on the DB-1 not matching FFAP retention times? Need some more details on what's going on and I can help some more.

The columns are: HP-FFAP for FID A and GAS PRO for FID B. I have two FID and like i said in a previous post I have a dean switch.
For TCD I have other columns (DB1, PORAPLOT Q AND HP PLOT MOLECULAR SIEVE) but this channel not problem.
Delayed is that in different test, one compounds out to different retention times, namely, a certain compound out increasingly delayed in time in continuous test.
If you need more information, please answer me.

Thank you very much for the help!!

The most obvious problems are a leaking septum or a blocked FID jet, have you tried replacing these?

Are you using the Deans switch? and if so, how?

Your problem probably lies in a change of flow, perhaps caused by your Deans switch. There are of course, other causes. Retention time changes are caused by a change in flow rates, or in film thickness or temperature.

Leaks, blockages, temperature regulation can all be causes. A contaminated Deans Switch looms high on my list from what you have described.

Good luck,

Rod

The most obvious problems are a leaking septum or a blocked FID jet, have you tried replacing these?

The septum have been replacing but the jet not because I have two FID and if the problem was the jet only the peaks of one channel would move.

Thank you for your answer

Are you using the Deans switch? and if so, how?

Your problem probably lies in a change of flow, perhaps caused by your Deans switch. There are of course, other causes. Retention time changes are caused by a change in flow rates, or in film thickness or temperature.

Leaks, blockages, temperature regulation can all be causes. A contaminated Deans Switch looms high on my list from what you have described.

Good luck,

Rod

Yes, I am using a Dean Switch. It is similar to the one shown on page 6 of the following link: http://www.chem.agilent.com/library/bro ... en-web.pdf

How can I clean the deans switch?

Thank you for your answer

Follow the procedure in your documentation to clean the Deans switch. I hope you have documentation.

best wishes,

Rod

Follow the procedure in your documentation to clean the Deans switch. I hope you have documentation.

best wishes,

Rod

In my documentation guide, there's any information about how to clean the dean switch. If you have information, please if you don't mind, give me this information.

Thank you very much.

Contact your vendor (Agilent?).

They will give you the best advice.

best wishes,

Rod

Of course, I trust you have eliminated any chance of leaks causing your problem.

Rod

Of course, I trust you have eliminated any chance of leaks causing your problem.

Rod

Yes I did the injector leak test and no leakage.

I contacted with my vendor (Agilent) and said me:

There isn't any procedure to clean the dean switch. The only thing, cut the ends of the columns and re-install with ferrules suitable internal diameter SilTite your columns but this has been done and has not solved the problem.

Thank you very much