Chromatography Forum

Step 5 of the Leak Check procedure says: "Shut off the carrier supply at the source" (see attached Varian GC3900 manual page).
The question is: can I shut it off using gas cylinder valve? Or I must install a dedicated gas valve between the gas pressure regulator and the GC to be able to perform this procedure correctly?

If you've removed the column and plugged it per the instructions, just shut off the cylinder valve. You're just looking for a pressure drop at the GC. Keep in mind that if you have unions in the gas line between the cylinder and the GC, if one is leaking, it could give you a false positive on your inlet check. This procedure also seems to assume that you only have one GC on the tank that supplies the carrier gas. If you have more than one, you will need a valve in the line that isolates the carrier gas for the GC you are troubleshooting.

If you've removed the column and plugged it per the instructions, just shut off the cylinder valve.

Yeah, I did that. And the GC was holding 80 psi for ~10 minutes. After that time the pressure indicated by the gas pressure regulator (mounted on the gas cylinder) began decreasing and the GC pressure also started to decrease by ~2 psi/min.
I'm not sure what does this mean. Is it leaky or not?

Have you verified that your supply line has no leaks up until it gets to the back of the GC? I'd take that line off the back of the GC, plug it, pressurize the line and then turn off the tank. See how that holds.

OK, thanks, I'll try that.