Sine wave baseline in GC-MS

[wangx460]

We have a HP5973 MDS GC-MS. After I took off the EI source, cleaned it and reassamble it back. The baseline showed a sine-wave pattern with a period about 5 min. I did not do anything to other parts (column, gas source, etc). What can be the problem?

[tlahren]

How does your Tune look after the cleaning? Are most of the values close to before the source cleaning?

Sometimes alumina (white cleaning powder) can be left behind on the source and can really screw up the voltages applied on the source. My guess it that the source is contaminated with alumina and it's not correctly focusing the ion cloud to the analyzer section. I would need more information to be sure though.

What is the EM voltage from your last tune?

[wangx460]

It could be this reason.
We took out the EI source and cleaned one more time. After that, the baseline was still sinewave-like, but much lower magnitude. After a few runs, the baseline became flat.

[chromatonewbie]

That sounds like an electrical interference pattern - perhaps you could investigate whether there are other instruments/appliances being used that coincide with your pattern.

[Don_Hilton]

A five minute period may be electrical, but for a time period that long, I tend to look at carrier gas. If there is not enough pressure across a regulator, you will get that kind of oscillation and the time period is likely to be minutes.