Will too much high vacuum grease will give high water peak?

[blue]

When I do the autotune in GC-MS, there is too much water ( a big peak at 18, almost 100%). But the vacuum is pretty low (57). The only thing I can think is I put more high vacuum grease than before. Will this be the reason why the result of autotune is abnormal? By the way, there are whole bunch of peaks before 69, I checked for the leaks elsewhere, I can not find it. Any suggestion will be helpful.

[Answer Man]

I do not have an answer to your question, but I do have a few comments on how to troubleshoot and fix your situation. First, cool and vent your MS. Then try cleaning the excess grease using a non-polar solvent such as hexane. Then 'cap-off' the MS from the GC using a blank ferrule. Pump down and heat the MS. Let the system stabilize for at least four hours, then scan m/z 10-100. What do you see? Hopefully this will allow the contamination to be pulled out of the MS.

What exactly did you put grease on?

[jaj]

I am assuming that they put vacuum grease on the o-ring that connects the source to the manifold. I too use Apiezon-L grease on my o-ring, but very lightly. I also have a slight water peak <10% of m/z 69. I have always thought the leak was from where the diffusion pump connects to the manifold. I have sprayed a light gas around all fittings and have found no leaks. The light gas would not show up on a scan if it was beyond the manifold.

In response to wiping the o-ring with hexane, I would not do this. It could damage the o-ring and who would want hexane contamination bleeding from the o-ring into your mass spec? We all need to be careful in giving advice when we do not know the instrument , the o-ring material, and how it would affect the person's application.