It's All in the Split Vent Trap

Recently I came across a situation where I wanted to make a series of constant flow GC-FID runs, during which, the inlet pressure would increase from 2 to 6 PSI over the temperature program. I also wanted to use split injection with a split ratio of at least 20:1, in this case that translated to a split vent flow of about 150 mL/min.

I realized that I would be pushing the lower limit of reliable inlet pressure control but those parameters should be within the range of a (relatively) modern EPC controlled HP 6890 GC.  Of course, as I installed the column and got to work it was clear that something wasn't right.

The inlet pressure would rise above the set point and stay there completely stable, like there was some sort of constant offset preventing the GC from becoming ready. I noticed that as I increased the split flow the real-time inlet pressure would rise further and further above the set point.

 

I started going through the standard troubleshooting motions, pressure decay test, leak checking, everything was good! At one point I flipped the inlet into splitless mode and the pressure equilibrated right to the set point. Now we are getting somewhere.

Looking at a flow diagram for the inlet and EPC system I became convinced that there must be some kind of flow restriction between the pressure sensor and the proportional valve on the split vent. This mystery restriction had to be the cause of the  build up of inlet pressure.

 

 

 

I set about replacing the 1/8'' copper split vent line, replaced the split vent trap, cleaned the inlet fitting with methanol, and installed a Low Pressure Drop Precision Split Inlet Liner. None of this worked and I was quickly running out of options.

What I didn't know at the time was that there are two different styles of split vent trap commonly used on the 6890. The classic "pencil trap" (top in figure) was standard on instruments made before 1997 and the newer "cartridge type" (bottom in figure) that is now used on the 7890 series. My instrument was equipped with a pencil trap.

 

 

Ah-ha, this must be the key! The two different styles of trap actually provide different amounts of flow restriction and I had never realized this until I attempted to run the instrument at low inlet pressure, where the slight difference becomes apparent.  Sure enough when I swapped my brand new pencil trap for the cartridge type everything worked fine and my inlet pressure went right to the set point.

 

 

In fact, using the cartridge type trap and the Low Pressure Drop Precision Split Inlet Liner I was able to get split vent flows of up to 500 mL/min at 5 PSI inlet pressure. If you ever run into a problem like this at low inlet pressures make sure to take a look at your split vent trap!