I have been experiencing back pressure shutdown issues when the oven temperature reaches about 230'C on our 7890B GC. I have been using the front port, but still keeping pressure in the back column; however, analyses stop because of the back inlet pressure shutdown. In order to be able to analyse samples, I have temporarily put a blank nut at the back inlet and put the pressure off in the method. However I would like to understand what the problem is so that I can fix it.
I have checked for leaks at the inlet, I have tighten the nuts at the inlet and detector, I have changed inlet septum and liner o-ring, with no improvements. The pressure is not kept at the back when the oven temperature is 230'C or more even when using a blank nut. Also, it takes a long time for the back inlet pressure to reach the wanted value, compared to the front inlet.
This happenned suddenly, as I have been running the method for months with no problems. I used the back inlet only a few times (it is pretty new) and we use our GC just for headspace analysis. In addition, we have moisture and O2 filters for the carrier gas (He).
I'm using gas saver mode on, constant pressure mode, 8 psi carrier pressure and a 2:1 split ratio for both ports.
Also, the cylinder pressure for the carrier gas has not been changed.
I hope the details I gave are sufficient for some advice, but please let me know if you need any further information on the method or the GC.
Looking forward to receiving some suggestions on how to tackle this problem.
Thank you in advance.