Have you ever been changing your Agilent GC inlet liner (you do change that routinely, right?), particularly a single taper with wool type, and pulled it out to see the wool at the top of the liner (Figure 1) when it should be at the bottom of the liner (Figure 2), the proper placement where injected sample can hit it and be transferred to the GC column? How does this occur? Sudden depressurization of the GC inlet can cause the wool to pop to the top of the liner. It probably doesn’t matter if this happens when you’re changing the liner, since you’re going to throw the old liner away, but it can also happen when you’re changing the GC inlet septum if you haven’t properly depressurized the inlet, and then unknowingly you’re left with the liner wool at the top of the inlet, where it’s not effective for its purposes of keeping the sample from hitting the bottom seal (where compounds like DDT can degrade), heat transfer, and preventing nonvolatile “dirt” from collecting on the head of the more expensive GC column.
In the following sets of figures, I’ll show you how I depressurize my Agilent 6890 GC inlet to prevent the inlet liner wool from jumping up. As always, it is much better to completely cool all heated zones before inlet maintenance procedures.
Go to the Agilent GC Keyboard, and press “Col 1” (Figure 3), depending on the Column and Inlet in use. You’ll see the Display showing similarly to Figure 4. If necessary, use the “Up” or “Down” symbol key on the Keyboard to move the Display arrow to “Pressure” (it is on “Flow” in the Figure 4 example). When the Display arrow is on “Pressure”, use “0.1 Enter” (or even “0 Enter”) from the Keyboard (Figure 5), which should cause the Pressure to drop down relatively quickly, but not so quickly that it causes the inlet liner wool to move (the pressure does not drop all the way to zero, typically, Figure 6). [Note: On newer Agilent GCs you may hear the “rattlesnake” as the pressure drops.] Use the “Up” symbol key to move the Display back to “Pressure” and then press “Off” on the Keyboard (Figure 7). Again, the “Pressure” may not completely zero out, but the GC inlet is now ready for a septum and/or liner change. Why don’t we just use “Off” immediately instead of “0.1”. When you use “Off” the pressure bleeds off very, very slowly through the GC column and if you open the inlet too soon the liner wool can move in the liner.
Don’t forget to restore pressure/flow before you heat the GC inlet and column up, and it’s always good to leak check after inlet maintenance.