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Inlet pressure drop following headspace injection

aburke

Hi,

We have a PAL RTC autosampler equipped with static headspace syringe tool and connected to an Agilent 7250 GC-qToF-MS. When a headspace sample is injected we see a pressure drop in the inlet at all split levels below 40 and it goes into pneumatic shutdown. When split level is >40, the pressure instead increases and the same pneumatic shutdown occurs. Gas saver is turned off. Diagnostics indicate an inlet leak, but this goes away after some time and doesn't occur when injecting liquid samples. What am I doing wrong here? I have tried changing the liner/septum and get the same result. I feel this may be due to low flow rates and the headspace syringe directly injecting a large volume (1mL) into the inlet, but that doesn't explain the pressure rise above certain split ratio. Ideally we would like to do splitless injections to recover as many analytes as possible. 

Thanks

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    You cannot do splitless injections with gas samples. It's not a viable option. 

    How old is the HS Syringe plunger O ring?  That is an operator replaceable consumable part. It does not last all that long as the syringe is heated.  If that plunger O ring is leaking the inlet pressure may be pushing the sample up out of the syringe.

    The HS syringe has quite a large needle diameter. Make sure and use a new septum in the inlet and that it is not overtightened.  That septum will only last maybe 25 to 50 HS injections...

    What injection cycle are you using? The syringe plunger injection speed is important when injecting 1 ml. -- typical is about 400 µL per second.  If it's too fast and the split flow is too low, the inlet pressure regulation cannot compensate, and you get errors.

    Inlet questions: How long since a PM? - how long since the split vent trap was replaced?  The copper split vent line from the injection port to the split vent trap?  Do you have external traps on the split vent line exhaust?

  • 0 in reply to paulsalverda

    Thanks for your reply. The syringe O rings are quite old so I will change them. I was able to overcome the issue by using a pulsed split mode on the inlet. Your other points are very helpful info and give me things to investigate. 

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