Chromatography Forum

Hello anyone. Firstly, thanks for the attention.I would like to enquire regarding the following problem.

Do you have any idea regarding automated thermal desorption instrument attached to GCMS?? Can I request for some insight if you have any idea about this combination. I have an Agilent GCMS combined with a Perkin Elmer ATD instrument via back inlet volatile interface on Agilent GCMS.

I have this issue and its not a typical problem. I have the VI temperature set at 240degC and everything works fine. Then, one day, after venting down the system for some ion source cleaning and PM on the GCMS, it seems that I cant achieved back the back inlet pressure using the pre-set temperature of 240degC. I need to increase the temperature in order to achieve the required pressure. Up till now, I've increased the VI temperature to about 295degC to maintain the required pressure. There seems to be no leakage problem when I check the system using autotune evaluation.

Is there any insights that you can tell me as to why this problem occurs? How can I solve this issue once and for all? Increasing the VI temperature will not help as if this continues, the ferrule max temperature will be breached and the ferrule will decompose by then.

Hope you can give me some advice and thanks a lot for your attention!!

God bless~

My guess is that one of the ferrules on the volatiles interface hase already loosened, and is leaking.

You need to check this using a leak detector, not by doing the air and water check on the MS.

What happens when you change the EPC column flow control from constant pressure to constant flow and vice versa ? Are you using the VI splitless, split, or direct ?.

If you have a problem with gas flow or pressure you need to be adjusting gas controls, not temperature.

Peter

Hi there. I am using VI split mode.

Thats the issue here; I can increase the pressure by increasing the temperature. Its not typical but this seems to be the way to solve the problem; short term.

You mentioned that the ferrule has lossened; which I believed too. Perhaps, it has aged. Anyway, if thats the case, why increasing the temperature can increased the pressure?

I have tried changing the flow to constant flow or pressure but still, I cant get the required pressure.

I have a new issue today. Not sure where does this comes from but seems that when I used the front inlet (FID) for analyses, the pressure for back inlet (VI) also drops even though it doesnt have any problem with the same setting under MS mode. Why is this problem? Is it because there is a serious problem with pressure of the split of the system and not just because of the VI? Do advise me about this problem that is mind boggling.FYI, back inlet is for MSD while front inlet is for FID.
Thanks~

The pressure goes up when the temperature increases because the ferrule that is leaking expands when it gets hot and seals better as a result.

You probably have either a leak or a blockage somewhere in the GC - thermal desorb sampler flow paths. Your first step must be to check the whole system, including all the connections inside the instruments, with a leak seeker.

Ensure that your gas supply to the instrument is able to supply the required pressure and flow.

Check that your total flow (set on the GC) is high enough to supply both the split and the vent flows of the GC, as well as any purge etc flows on the P&T sampler.

Check the linear flow rates through the columns, it could be that the pressure sensor is malfunctioning and that the pressure is higher than it indicates.

Measure the split flows and purge/vent flows with a bubble flow meter, not with the flow monitors of the instruments.

Please post a complete description of your set up (column dimensions, flows, pressures etc).

Peter

It has been several years since I used a VI, but at that time the ferrules supplied did not properly match the taper of the VI. I threw away the ferrules that came with the instrument and replaced them with a ferrule with the proper taper and my problems went away. The VI used to be very sensitive to how much the ferrule was tightened, too tight and the ferrule would split. Try replacing the ferrule and see if the problem goes away.