[rose9090]

why do i need a high vacuum in the mass-spectrometer?

[Arkcon]

OK, here's one you might be able to figure out for yourself.  What does a mass spec do?  How does it work?  What would happen if there was atmospheric pressure within it?  Here's a hint:  sometimes we allow a small amount of inert gas to be in the system.  If you can find some info on that procedure, and its effects, you can extrapolate.

Or, you know, just check your class notes/textbook and give the exact answer the instructor wants.  That works better for "definition" type questions, such as this one.