Chromatography Forum

Hey guys,

Im setting up a 6890/5973 GCMS for semivolatile analysis.

I am trying to tune the instrument and the tune has a huge nitrogen peak and barely anything else. The PFTBA is full and when I close it, the small amount of peak 69/219/131 that I have disappears but the nitrogen stays high.

The foreline pressure is great and all the other air components are missing which has me scratching my head as we use helium as the carrier. I tried resetting the autotune file by going to reset to default --> autotune but that didn't seem to do anything.

For reference,
Nitrogen is showing a response of about 750,000
Ions 69/219/131 are all in the 10,000 to 20,000 range

Let me know if there's any other information you need
Thanks for your help!

First, I presume it did not behave like that from the day the instrument was installed.

So - did You make any changes/repairs in the system just before the problem appeared? Was instrument not used for a longer period of time? Did somebody else use it - maybe as non-MS instrument?

When You say "as we use helium as the carrier" - did You install the helium bottle and are sure it contains He - mistakes sometimes do happen.... Are You sure nobody messed with the GC tubings and connected wrong tubing to GC gas inlet?

It's possile that there is nitrogen impurity in helium. To test it, disconnect the column from the interface (after venting the MS), cap it off with a no hole ferrule and check the tune.

The instrument was originally used in a different department and is being repurposed. Apparently this problem popped up after a source cleaning.

We have the same helium tanks going to about 7 instruments and this is the only one exibiting this problem.

It was used as a volatiles instrument originally but the concentrator broke down

Has it worked properly in its current configuration? If it was used for volatiles, how was the inlet set up? Was the transfer line spliced into a gas line and is that correctly fixed? If you are only seeing N2 on this instrument does it have its own line scrubber (with a leak?)

It has worked before in this configuration. It does have its own scrubber, I suppose I should replace them as I dont know how old they are. The problem would have to be that combined with a leak somewhere before the inlet correct?

If it has its own scrubber an air leak upstream will only give you nitrogen since the oxygen and water will be removed, at least until the scrubber is exhausted. It sounds like it isn't getting to other instruments so it must be confined to this one.
Capping the interface and checking to MS with no GC input is still a valid check to confirm that you aren't missing something. Check upstream first though, it's a much quicker fix if you find something.

Sounds to me you have a leak, especially after a cleaning. Is the AUX ( transfer line) up to temp? Every time I clean this nut needs checking. If you have any canned air you can set the mass in autotune for the major compound in the "air". Set the MSD to scan and spray ( short bursts) at suspected spots. I would try the transfer line not in the oven, and the O-ring that seals the analyzer ( there is a small cut out on the lower right bottom of the door that floods the O-ring chanel).

Okay, so I tried capping the MS and it makes the problem look a lot worse. It does seem to be an air leak but my foreline pressure is still very good and I tried spraying canned air everywhere I could think of (vent, o-ring, inlet inside and outside the gc) with no change in the tune spectra. Although it seems that the problem is definitely in the MS, I am not really sure what to try next.

Your guys help is always very appreciated

Jon

Is this instrument CI capable? We had one spring a leak in the transfer line cal gas fitting ( it's between the GC and MS towords the back) . Try spraying there.
Some times the O-ring for the high voltage lead ( big white wire with red boots on each end) can leak. You can put a drop of MEOH there and scan .

Good luck. BTW it's difficult to detect a leak using foreline pressure.

New update.

I double checked and the leak is infact NOT in the MS. The ferrule I used to cap in the first time was bad and the next time I capped it the tune looked perfect. I replaced the column and the leak reappeared. Maybe my column is cracked? I will try changing it today. I will also be replacing the scrubbers. I have to say, this one is driving me crazy hah. Appreciate your support!

Before replacing the column, stick the inlet side of the column to a septum and see if there is a leak. If no leak, the column is fine.