very strange GCMS baseline

[yyaann]

Hi all,
I am using agilent 7890GC 5975MS for SPME analysis, scan range 28-200. recently I found small and sharp peaks in the chromatogram (sample and also blank) m/z is 28, they appear constantly at intervals through the whole chromatogram, I have no clear idea what's the problem.
Thank you in advance for your suggestions.

[rb6banjo]

What does your tune look like? m/z=28 is nitrogen (N2). Perhaps you have a leak.

I recently put a helium tank on my systems and the tune gave a huge m/z=28. It was actually the base peak. I swapped out the tank and after an hour or 2 all went back to normal.

[Gizmo]

Unless I needed that low end of the scan range for some specific reason I'd raise the range up to avoid interference from N2 and O2

[James_Ball]

It is most likely trace nitrogen in the carrier. Since the lower limit of the scan range is set to 28, then if it reads the nitrogen peak at 27.5 it will reject it but at 28 and above it displays it. The best way to solve the problem is set the scan lower limit to 27 and you should have a smoother baseline just showing the trace nitrogen.

If you do not absolutely need to see 28m/z, then move the lower limit to about 35 for a much cleaner baseline.

[yyaann]

It is most likely trace nitrogen in the carrier. Since the lower limit of the scan range is set to 28, then if it reads the nitrogen peak at 27.5 it will reject it but at 28 and above it displays it. The best way to solve the problem is set the scan lower limit to 27 and you should have a smoother baseline just showing the trace nitrogen.

If you do not absolutely need to see 28m/z, then move the lower limit to about 35 for a much cleaner baseline.

Thanks a lot, peaks disappeared after setting the limit to 27.

[yyaann]

Thanks for all.