Chromatography Forum

dear all,

I have just begun to work with MS/GC for my project, and unfortunatelly nobody from my lab group have worked before with MS/GC, so I am a little bit lost.
I am starting to work with this MS/GC and I having some problem (maybe there are obvious for you guys).
1. when I run the experiments I always have a background of water in the MS (arround 120000 in absorbance). I have checked for leaks and the result is tha I have a great peack for water (from 70-200 % with respecth amu 69 in the tune mode) but the peak for air remains small (2-10 %). The pressure is arroun 1 10-5 torr ( I think this pressure is somethin high, but it is possible to run the experiment ?? ). So it that a sign of a leak in my system???
2. if I use the scan mode, the cromatoghraph I obtain have several peaks some of then at high amu (199 and bigger). is that a problem with the mechanical pump oil (I have installed a mist trap) or is because my MS is dirty???
3. In the future I will have to work with high concentration of solvent and I don't want to burn out my detector. Could any one give me an estimation fo the maximum signal I can have in the detector and about how much time ???
I have a HP 5971A/5980 series II MS/GC
Thanks to all for any solution to my problems, I really appreciate

Hi,

First - carefuly read the manual, many things are expaned there (unfortunately sometimes on different page than you would expect).

I will try to answer your questions although I don't work on this instrument for some time.
We start with question 3:
In the instrument software you shoud have seting "Solvent delay" or something similar. That means- you start data aquisition with some delay after injection. Usualy value around 3 minutes is suitable. There is also security feature instaled - software automaticaly disables ion source if vacuum value is too high during measurement.
Question 1. If air peak of air is low and water is high most likely there is no leak in the system. If your system was standing long time switched-off accumulated humidity is still remaining on parts inside. You should heat the detector for some time and then check the air and water again. Water shoud decrease after heating. I presume you are using software menu command for air and water check. From the dialog box for that you can access help with the limits for air and water.
Question 2. Your information is not sufficient to give qualified advice. I suppose that pump oil is not the problem - you would have low vacuum rather than particular peaks, but I might be wrong. You should describe the situation more precisely. First: do you see the peaks during chromatogram aquisition (do they depend on temperature/solvent other factors) or do you mean peaks in tune scan (or background scan ?)? What m/z have these peaks?

Good luck,
Rolandas

First, i think you have a contaminated cylinder.Change your gas cylinder first and wait 2 hours.Check if you have water again.It is the first thing to do

Do you have moisture traps on your gas? Have they been replaced recently?