change retention time with Gas Saver Time

Dear Colleagues,

I have GC-FID 8890 and i'm using Gas saver feature but I'm observing when I change time of gas saver , the retention time is changed.

for instance, I set gas saver at 20 ml/min after 2 min then inject methanol and the RT is 4.74 but when I change time to 15 min, the RT of methanol become 7.9 min!.

I'm very confused what's the relation between gas saver and retention time ??.

The column flow 2ml/min.

  • Hi, 

    Are you running split or splitless inlet conditions?  What is your total inlet flow at the time of injection?   Have you tried to run the split vent trap diagnostic to see if you have some restriction in the line?

  • Hi Dear,
    my mode is split and its flow 200ml/min.
    total flow is 205 ml/min.
    how i can run this diagnosis ?

  • Hi AhmedIsmail, 

    On the front screen of your GC, click the house icon to go to the Home view.  Under the Diagnostics tab, go to the Diagnostic Tests section, and locate the inlet you are using.  Split vent restriction test should be an optional test.   See picture below from one of my systems (this is a remote capture, so yours may not look identical).  

    In your case, Gas Saver should decrease the split vent flow from 200 to about 15mL/min after the injection is complete, keeping the septum purge and column flows constant.  There isn't a good reason to continue flushing the liner with 200mL/min of gas after the sample contents are on the column, so 2 minutes is usually plenty of time - especially for something as light as methanol.  Gas Saver should not impact the retention time of your compound significantly unless there is a leak or restriction in your inlet.   Do you notice any alerts or not ready conditions while the run is in progress?   Are you using an autosampler?

  • No Sir I didn't observe any alerts or not ready whether before or after processing.

    I utilise manual injection and syringe in good status and clean.

  • Are you using Prep Run before you make your injection?  Auto-samplers enable this feature as part of a pre-injection readiness, which creates a more consistent inlet environment before the injection.  If there is no auto-sampler, you will need to enable the feature for best performance.  Your software should prompt you by saying 'Waiting for Prep Run'.  On the 8890 touch screen, the icon looks like a vial with a loop.  Its located in the lower right of the screen.  

    We want the conditions of the inlet to be the same for consistent results.  However, if you are not using Prep Run with manual injections, your Gas Saver conditions are still being applied during your next injection, and held until the Gas Saver time is reached.   Gas Saver conditions will be maintained until the GC is triggered to prepare for the next run to save gas while idle.  

    I would encourage you to collect a diagnostic signal for inlet total flow within your method (if you aren't sure how to do this, respond with the type of software you are using, and I'll do my best to assist).  

    For the conditions you have provided thus far, it should look like:

    Prep Run = split flow is closed off, so you should see a small amount of flow (maybe 5mL/min based on the conditions you have provided)--Correction:  This will happen in splitless mode.  Split mode behavior is dependent on if you had Gas Saver enabled on the previous run or not.  Sorry for the confusion! 

    Start of run = 205mL/min

    Gas Saver Start time (2min) should see a very quick drop to 20mL/min for the remainder of the run

    After the temperature program has completed and your run is complete, you should still see a flow of 20mL/min until you hit Prep Run again to prepare for your next injection.  Try this a few times and see if your retention time for methanol is more consistent.   Let me know what happens!


  • Dear Mr. Abbey,

    I'm using Pre run before infection .I was collecting diagnostic signal for my total flow inlet like you recommended and i observed after completing the condition of column, the gas saver value persist until i active the Pre run, the total flow is increased from value of gas saver 20 ml/min to total flow value 205 ml/min ( not like you state as below )

    (maybe 5mL/min based on the conditions you have provided)

    i attached some screen shots during my process.
    1- column parameter 

    2- before injection, Pre run and after conditioning the column 

    3- after using Pre-run

    4- after elapsing 2 min and active gas saver 

    Kindly, let me know your feedback.

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