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Bad filament??
Posted: Thu Jul 28, 2011 4:50 pm
by mjh32586
How do you know when you have a bad filament in your ms?
my is/ss area counts are gradually going down which makes me suspect I have a bad filament but not sure.
Re: Bad filament??
Posted: Thu Jul 28, 2011 6:49 pm
by willnatalie
Are you talking about EI-MS?
If so then I would look at the EM volts you are running. If you know the horn is good and the source is clean and are running in the upper 2500 region it should be replaced. Another way; some instruments like Agilent have two filaments, if by switching one the EM volts decrease back to a good region <2000, the other filament is bad.
It could be going down simply because the source is getting dirty. Ramp the repeller and see if you get more of a bell curve or a line that continuously increases. If the latter is the case then the source is most likely dirty.
Good luck.
Re: Bad filament??
Posted: Thu Jul 28, 2011 7:13 pm
by mjh32586
What do you mean by ramp the repeller and how do you do this? I'm running drinking water samples so is there a typical number before the source begins to get dirty? Sorry for the dumb questions but I'm new to gcms.
Re: Bad filament??
Posted: Thu Jul 28, 2011 8:00 pm
by willnatalie
Questions are good. Your questions are by no means dumb.
In the manual tune there is a "ramp Repeller" test. There should be a point where the repeller will have a max voltage then start to fall, its at that point the instrument gets its best sensitivity. If you ramp the repeller and do not see a drop in the graph it reports at the end, where the line continuously goes up, this can generally indicate that source source is dirty and needs to be cleans so that you can get more efficient numbers.
here is a good overview
http://www.sisweb.com/art/pdf/ms-maint.pdf
good luck
Re: Bad filament??
Posted: Fri Jul 29, 2011 11:15 am
by carras
I'd like to point out that ramping the repeller you can get a response curve with no apex even if the source is clean. That's my case and the MS works fine, tunes fine and has good sensitivity. I have never asked Agilent about it but I guess the age of the source may be involved. My MSDs are pretty old (more than 10 years).
I analyze drinking water and it is not a specially dirty sample, though it is also a matter of how many samples you run before cleaning the source.
I completely agree with willnatalie about switching filaments, if the problem is due to a bad filament, switching to the other one should fix it.
And a final word about source cleaning: when in doubt, clean it. (If your work schedule lets you)