Agilent 6890 GC with 5975 MS; helium gas as carrier, 30m x 0.25 mm column. I've noticed that if I run the air/water check when in splitless mode (inlet pressure = 10.6 psi, total inlet flow = 4.8 mL/min, column flow = 1.2 mL/min, constant flow), I get high values for the nitrogen and oxygen (569.1, 161.67) but if I do the same again in split mode (50:1, inlet pressure 10.6 psi, total inlet flow = 64.5 mL/min, column flow = 1.2 mL/min, constant flow) then the values are way more reasonable (9.94, 2.21). This would seem to indicate a small leak that is perhaps masked at higher flows?
In fact, if I set the split ratio to 0.1:1 (inlet pressure = 10.6 psi, total inlet flow = 4.7 mL/min), I do observe higher values as well (167.87, 47.57). I leaked-checked the GC before installing the column in the inlet (new septa, new liner, new gold seal) and didn't even see a decimal of a drop over well an hour. I have sprayed both cooling spray (1,1,1,2-tetrafluoroethane) and argon on every single point in the pathway while running in splitless mode, plus I have checked the entire pathway more than once with an electronic leak detector.
What am I missing?