Chromatography Forum

Greetings,
I noticed that I had some spetum fragment peaks for my Agilent 6890/5975 system. I read some posts and learned that I should try to decrease the inlet temperature or increase the septum purge flow. I am using split injection. I have good baseline when I use a really high split ratio (~50-100). But I have very bad baseline when I just use split ratio smaller than 10. My column flow is 1ml/min.

Please tell me how to set purge flow for split injection. Thanks a lot.

The septum purge flow cannot be modified. But you might check to see if it is flowing at all - should be 2-3 ml/min.

Thanks for your reply. How to check if I have purge flow and how to measure the flow? by the way, I do not have flow meter. Thanks a lot.

Hi Zimanli

On the 6890 the septum purge flow comes out of a 1/8 inch Swagelok connection at the top, back, left of the instrument. The split outlet is there also so check the labels to make sure you have the right one. This is in the Operator's Manual.

You cannot measure flow rates without a flow meter, but you do not need a fancy expensive electronic one, you can use a bubble flow meter. You cannot run a GC without a flow meter !

The septum purge does not stop particles of septum falling into the inlet, it is designed to stop volatiles from the septum diffusing into the inlet.

At a 10:1 split the EPC modules on a 6890 can have trouble maintaining stable flow. This seems to vary from instrument to instrument - some people have no problems at 10:1, others report poor performance. You could try doubling both your injection volume and your split ratio.

Peter

Thank you peter. I located the purge vent.
Do I need to buy a flow meter or does the GC come with a flow meter? Excuse me for my stupid questions. I am new in GC/MS.

Agilents come with simple glass flow meters. Look for a 250 to 300cm long box with a strange looking glass tube inside. The idea is to blow bubbles into the flow and time their transit trough the tube.
Electronic ones are easier to use and more accurate for the occasional user but cost $$.

The Agilent part number is 0101-0113, $58 in the 2005-2006 Agilent catalogue. If you have the catalogue you can see the picture (p. 85).
You will also need some bubble solution called Snoop, 8 oz. for $12. The HP gas chromatographs have a built-in stopwatch/flow calculator. You will find instructions in the manual.

Guys, thank you one hundred times!