Inlet pressure problem on 7890A

[JonKlar]

I have been having a problem recently where the inlet flow on my GC will shut down during aquisition. This has not happened in a few runs, however the log book is showing the following errors for every sample:

Front inlet pressure not ready run time 3.69
Front inlet flow not ready run time 3.76
Oven not ready run time 5.77
Front inlet septum purge not ready run time 5.77

I have changed the septum and checked for leaks (although still a possibility).

I am running volatiles to an MS and my GC parameters are as follows

Inlet
Inlet Temp 230
Inlet pressure 19psi (kept constant)
Total flow 252.31 mL/min
Septum purge flow 3.0 mL/min

Column is an Agilent HP-5MS 30mm x .25mm x .25um
Column Flow is 2.5 mL/min
Pressure 19 psi
Avg Velocity 56.825 cm/sec
Holdup time .88 min

Oven goes from 35C to 210C in 20min.
hold time 4m
Ramp 10.94 C/min

Any ideas?

[LWC]

The last time mine did that to me either the septa was worn or the inlet nut (on the top of the GC) was a little too loose.

[MX304]

Are you running in split or splitless injection mode? Try manually setting the pressure on the GC key pad. If the pressure won't reach your 19 psi set point, or jumps around without stabilizing check to be sure the split vent filter isn't clogged. Also check the copper tubing that runs from the inlet to the split vent filter for clogged ends. I have had that happen on my 7890s much more frequently than it does on my 6890s especially when running "dirty" sample in split mode.

[AICMM]

JonKlar,

1) Are you running gas saver mode?
2) Why are you running such a high total flow? I have not used the latest and greatest purge and traps but that seems like a lot of flow across the trap?
3) What is your source pressure?
4) What are your purge and trap details (assuming you are running purge and trap)?

Best regards,

AICMM

[JonKlar]

Sorry for the long delay, things have been hectic lately. The problem has been sorted out with the cause being a combination of our labs gas manifold and a communication problem in the GC.

The total flow is based on a 19psi constant pressure for our column and the purge and trap is a tekmar atomx.

Luckily I have a new problem for you guys to help me with. One of my surrogates (toluene-d8) is coming out high intermittently. There is no pattern to it I can identify, just sometimes RSD is 1-5% and other times 20-30%. I am not having this problem with any other standard. It is being added just before injection with the ISTDs.

All other conditions are the same as before, thoughts?

[JonKlar]

On further inspection, the actual problem seems to be the internal standard related to toluene-d8 is coming in low, causing the surragate (as well as other related compounds) to appear high. maybe the standard injector in the autosampler is problematic? There does not appear to be a leak anywhere.

Thanks,
Jon Klar

[MX304]

On further inspection, the actual problem seems to be the internal standard related to toluene-d8 is coming in low, causing the surragate (as well as other related compounds) to appear high. maybe the standard injector in the autosampler is problematic? There does not appear to be a leak anywhere.

Thanks,
Jon Klar

Which auto sampler do you have?

[JonKlar]

teledyne tekmar atomx

[khogue]

Can you explain how you fixed the communication error? We have the same system setup and are getting a septum purge error which is shutting down the system.

[JonKlar]

We fixed the communication error just by shutting everything down and restarting it. For some reason the version of chemstation that came with this model gcms is very problematic

[jvettleson]

On further inspection, the actual problem seems to be the internal standard related to toluene-d8 is coming in low, causing the surragate (as well as other related compounds) to appear high. maybe the standard injector in the autosampler is problematic? There does not appear to be a leak anywhere.

Thanks,
Jon Klar

Prepare a new ISTD solution for your analyzer. Over time, some standards can fluxuate concentration due to temperature and headspace in the autosampler's standard vial.

If it were your injector, all the ISTD recoveries would be problematic.

[DRFournier5]

We fixed the communication error just by shutting everything down and restarting it. For some reason the version of chemstation that came with this model gcms is very problematic

I encountered this exact problem today and the inlet pressure kept causing the system to shut down, but after reading this I tried shutting down the software, rebooting our GC and MS as well as the computer itself and letting everything come back up to their normal parameters and it too seemed to solve our problem. Just ran a blank to get out the material that did not elute on the first run as it shutdown mid-run and all seems absolutely normal now. The only thing I did differently which may have been the cause of the problem was changing to a lower inlet temperature (250C instead of 280C) than normal- either that or it was completely a coincidence, the same material has also been run at an inlet temperature of 280C with no problem.

Update 02/03/12: this problem did happen the second time I tried lowering the inlet temperature, it was not a coincidence afterall. (Replacing the septum fixed this problem)