Air/Water Check Leak?

Discussions about GC and other "gas phase" separation techniques.

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Hi,

I am using 7890A GC and 7000 MSD. Yesterday i am realised after the injection, internal peaks does not appear in chromatogram. After that i tried autotune and air/water check. Autotune was ok. But Air and Water Check results was like this
PFTBA (69.00) 495032
Water 9622 1.94% <= 20.00% OK
Oxygen 243908 49.27% <= 2.50% ---
Nitrogen 909045 183.63% <= 10.00% ---

So i tried tighten the backflush,inlet and interface connections of column. It didn't work.And then vented the MSD, cleaned the ion source(it was monthly routine) and then pump down like 16 hours until the morning. But ıt didnt work. And then i read some topics from here and changed the helium cylinder and check the connections also. I dont have any solutions,i am waiting for your help.

By the way sorry about the bad english.
Hello,

Quote from this Agilent document fits here:

Troubleshooting leaks in GC/MS is a process of elimination,
looking at each site where a leak can occur


Blindly tightening connections is not a good practice. Do you have an electronic leak detector?

Also, you shouldn't be cleaning your ion source monthly. Once or twice a year, depending on the sample load and setup should be more than enough.
Thank you for answer.
Unfortunately, i dont have an electronic leak detector. I didnt blindly tighten the column connections actually, i just checked the connections if they are tight enough. Still i cant figure out where leak comes from?
Since you have backflush you can try a couple of things that can hint at the location of the leak.

I would advice to start with cooling down your source and quads to ~ 100°C and running low emission current trough your filament if you need to turn it on (~use 10 µA instead of the standard 35 µA).

I assume your setup is:

Inlet 1 = column 1 = T connection with inlet 2 => column 2 => MS

Put the system in backflush mode by increasing the inlet 2 pressure and decreasing the inlet 1 pressure. Does the air decrease? Then the leak is located before the T connection (1) . If not, after or at the T connection (2).

If 1: check if the inlet is tightened correctly on top, if column 1 is inserted correctly (remake connection). A leak detector is useful in this case ..

If 2: I look at m/z 10-100 and use a dust removal spray (to clean your PC parts) to find the location of the leak. If it's not located in the oven, it can be located around the source itself.

A different possibility is that the leak is located before your GC. Again, a leak detector is not a luxury if you have a GC-MS!

Good luck
Welcome to the forum.

The strongest clue is "internal peaks does not appear in chromatogram" which suggests a massive leak in the inlet - most likely a leaky septum or, far less likely, a broken column or column inlet connection.

Replace the septum and re-connect the column to the inlet (as in completely re-do the connection after cutting 10cm or so off the beginning of the column.

rndirk is right - a leak seeker is a necessity in a GC lab.

Peter
Peter Apps
Hi Peter,

I replaced the septum and inlet, cut the column 10cm and connected the inlet again. But it didnt work. I have 2 days off so i let the instrument. When i came to here in the morning i tried air/water check again. I saw that N2 and O2 decrease. I waited for a while. Then again i tried air/water check and surprise here is the results:


PFTBA (69.00) 587729
Water 1181936 201.10% <= 20.00% ---
Oxygen 10636 1.81% <= 2.50% OK
Nitrogen 39640 6.74% <= 10.00% OK

I dont have any idea why its been like this. I did not intervene.


@Rndirk thank you i understood what you said but i dont have that information about instrument.
Was a helium cylinder changed just before you first noticed the high levels of air ?

Peter
Peter Apps
The same thing happened to me. I did recently change the tank. Usually when I have leaks I either check the connections or condition my column and the issue goes away within minutes. You are saying it took you two days to stabilize your system? why is that. I am always afraid to leave my carrier gas flowing when I have a leak because my He tank gets empty by the next day.
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