Boliver Shagnast wrote:
Hello everyone. I've been lurking here for a few months, but finally decided to join the conversation. I'm new to mass spectrometry and have learned a lot here.
I'm looking for some help with troubleshooting my 5975C. Over the course of a day while attempting to calibrate, I see my analyte peak areas continually decrease, making it impossible to get a calibration. I had thought it must be a leak, but my air and water checks come out ok and a varian helium mass spec leak detector couldn't sniff out any leaks.
I now believe the problem is within the mass spec. The other day I did 4 low mass auto tunes about half hour apart and the PFTBA abundances decreased with each one, with lower masses appearing to be affected more. I also have my GC configured such that I can flow calibrant continually to the mass spec so I can manually tune to maximize my target ion (34) abundance. When I do that, I'll profile ions 34, 69, and 131 and can observe them all decreasing over 10 or 20 minutes. Yesterday while doing this, however, ion 34 abundance decreased while the others increased.
The EM is fairly new, and I've cleaned the source and replaced filaments. Does anyone have any ideas what the problem could be?
Thanks
After cleaning a source I normally wait 24 hours before I begin to calibrate. Is this happening the next day after a cleaning or just after the initial 2 hour wait that automatically pops up on the screen?
What are your source and quad temperatures?
Also when I begin to tune, I set it in manual tune and let it scan with the PFTBA gas valve open for 15-30 minutes at least to make sure the 69 mass has stabilized, especially if the instrument was open to atmosphere recently. If it has been running samples every day it will stabilize much faster.
It may not hurt to let it scan a few hours since it is a new Electron Multiplier, just to make sure everything is broken in and stable, putting power on the multiplier will warm it slightly and help it degas any traces of atmospheric gasses that might remain. When I do major cleaning on my instruments I try to do it on Friday, and let them pull vacuum and bake out over the weekend so my first tune and calibration will last longer, but almost always have to return and recalibrate about a week later, after that they remain stable for a long time.