Chromatography Forum

... Is it possible I got bad Helium gas tank? I don’t recall this ever happened before. Just to be clear I also tried to isolate MSD from GC and no leak whatsoever. I am now tempted to order another He tank.
...

... Is it possible I got bad Helium gas tank? I don’t recall this ever happened before. Just to be clear I also tried to isolate MSD from GC and no leak whatsoever. I am now tempted to order another He tank.
...

Yes, it is possible.

Leaks in pressurised parts of line won't allow air to enter carier gas line.

So contaminated gas in cylinder or leaks in MSD.

... Is it possible I got bad Helium gas tank? I don’t recall this ever happened before. Just to be clear I also tried to isolate MSD from GC and no leak whatsoever. I am now tempted to order another He tank.
...

Yes, it is possible.

Leaks in pressurised parts of line won't allow air to enter carier gas line.Leaks anywhere in the line, whether pressurized or not, allow air to enter by diffusion. Why, otherwise, would it be always be so difficult to get the air levels down, and why do air levels always rise the first time that a septum is punctured ? Peter

So contaminated gas in cylinder or leaks in MSD.

Try connecting the He tank to another gcms if possible to see if you get the same result (trying a secong He tank will accomplish the same thing). If you do it's the tank, if not it may be a leak in your gc system or possibly a problem with your split flow module. I had this happen to me a month or two back and it took forever for me to diagnose.

Hope this helps,

Jon

Try connecting the He tank to another gcms if possible to see if you get the same result (trying a secong He tank will accomplish the same thing). If you do it's the tank, if not it may be a leak in your gc system or possibly a problem with your split flow module. I had this happen to me a month or two back and it took forever for me to diagnose.

Hope this helps,

Jon

Came back after the weekend and the leak has magically disappeared. However, the tune issue with HP5970 still remains. 502 peak is too wide and off by a few amu. I'm getting error: high mass peak is too wide. I will try to use a different source although this source was pretty clean...

Leaks anywhere in the line, whether pressurized or not, allow air to enter by diffusion. Why, otherwise, would it be always be so difficult to get the air levels down, and why do air levels always rise the first time that a septum is punctured ? Peter

If you still can't get the tune to work (502 too wide) reload an old tune file before you try again. Especially if you have a bad leak in the system, the tune parameters may get too far off and the algorithm won't converge. Then the system won't find a proper tune from your current starting point. The quickest fix is to load an old file and rename it to your active tune file.

Leaks anywhere in the line, whether pressurized or not, allow air to enter by diffusion. Why, otherwise, would it be always be so difficult to get the air levels down, and why do air levels always rise the first time that a septum is punctured ? Peter

Citation from initial post: "He peak is only 30% of 28 amu"

Leaving detailed % calculations aside, alleged leak in pressurised part of carrier gas line had to act as effective* ejector in this particular situation.

* - effectively replaced helium with air in carrier gas line - provioded leak in line was the culprit of dominant nitrogen percentage in mass spec chamber

...
If you had written that a leak in a pressurized line would not allow so much air into the line that its concentration was higher than the helium, then I would be happy to a agree.
...