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Total Flow

camelia

I am faced with a bit of a puzzle I hope someone can help with.

I am using a 6890GC with EZChrom CDS to run 2 methods. Both methods are operated in split mode with a split flow of 192 mL/min, the column configured in each method is the same and both methods are operated in constant pressure mode with a nominal flow of 2.4 mL/min.

The Total Flow on one method is 197.1 mL/min but it is 196.6 mL/min on the 2nd method.

Why is there a difference in the Total Flow between the 2 methods?

  • 0

    Hi,

     exactly what you said and like what  pointed out,the septum purge flow cannot set on GC 6890 but we can control the septum flow by reduce split vent value or split ratio ,so you can reduce split vent value in method 1 from 197.1 mL/min to 196.3 mL/min and for method 2 196.6 mL/min to 194.7 mL/ min to get 3 mL/min septum purge flow for two methods.

    you can now measure the septum flow by electronic flow meter you should got 3mL/min

    after Carefully checking you can see in method 1 air flow FID detector 350 mL/min and for method 2 air flow FID detector 400 mL/min and I think this difference led to difference in septum flow ,you can change it and see what will you get.

    Dheya

  • 0 in reply to dheyaaldeen

    Hi Dheya,

    the calculated septum purge value for method 1 with a total flow of 196.6 is 2.2 (196.6-194.4) and for method 2 with a total flow of 197.1 is 2.4 (197.1-194.4). The split flow (192) and column flow (2.4) is the same for both methods. I do not want to change the split vent flow in either method as I need to maintain the same split ratio for both methods (192/2.4=80)

    Using your thoughts however, If the total flow for method 1 is increased from 196.6 to 197.4 (0.8 increase) the calculated septum purge would then be 3. Likewise if the total flow for method 2 is increased from 197.1 to 197.4 (0.3) the calculated septum purge would then be 3. I would like to try this and see what the software does. As long as it doesn't affect either the split flow or column flow this may be a way to control the septum purge even though it is not settable in the method. 

    Maybe somehow the methods have been corrupted and writing again from scratch using the same split flow (192), column flow (2.4) and the same adjusted total flow (197.4) for both methods would give the recommended 3 ml septum purge and the expected consistent total flow between methods. 

    Thank you and kind regards,

    Camelia

  • 0 in reply to camelia

    Hi All,

    I changed the total flow but as this affected the split ratio and the split flow it is not a viable option.

    I also created the 2 methods from scratch using the total flow value of 196.6 for both methods whilst maintaining the same split flow and split ratio. 

    I can only think that there was some "glitch" in the software in the original method so i have marked the response from James as the correct answer to close this post off.

    Many thanks

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