GC headspace 7890 with 7697a flow/pressure

Hi all,

I have the following question about pressure/flow set up. When i used a column with 320 um or 250 um diameter, i cannot get the flow/pressure up without split(5+). Where might the issue be ?

I changed the transfer line, confirmed the column installation is fine, septum and liner are appropriate. When i used 530 um column, there is no issue.

Thank you

  • The smaller diameter columns require higher inlet pressures.  If your inlet total flow is low, it will take longer to reach the pressure setpoint.  With the headspace sampler in the flow path, it will take even longer because of the extra tubing. The GC will shut off the inlet flow if it takes too long because it thinks there is a leak.  For the smaller diameters, what is the column flow, inlet pressure, and inlet total flow?  For the inlet flow, we recommend that it be at least 20 mL/min.

    Another thought: make sure that the inlet has the correct PID settings for the headspace sampler.  This can be changed using the GC Firmware Update program.  Select GC from the main screen and then your GC model.  At the next screen, select PID update.  Connect via IP address, and it will show what EPC modules are installed.  For the SSL/MMI inlet, there is a drop-down menu to select different PID modes.  One is specifically for the 7697 headspace sampler.

  • HI Lance,

    thank you for the response. The inlet flow is 1.5 ml/min, pressure about 10 psi and total flow is 7.5 ml/min. If i set higher split flow, it does reach the pressure and flow values, but once i drop it back, they dont hold anymore. I also have a G1888 set up on another GC and it had no trouble applying those parameters.

  • Hello Alex,

    I think it's having trouble keeping the pressure because the total flow is only 7.5 mL/min.  As I stated, you're better off having inlet total flow at least 20 mL/min.  The simplest thing would be to increase the split ratio to get to the higher flow.  If you're worried about seeing your lower detection limit, there are larger sample loops available for the 7697.  The units from factory have a 1 mL loop installed; 3 mL and 5 mL loops are also available.

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