Yeah, it is expensive. Even used they stuff is not cheap. Nothing to do with vacuum systems is.
Yes, you can get a pretty good idea of the mixture in the tube.
You are thinking of a normal optical spectrometer where it will read emission or absorption and compare that to known samples. This works differently. It ionizes gas and the ions are sorted by their mass weight. For example water has a mass of 18 amu (atomic mass units), two hydrogen at 1 amu, and one oxygen at 16amu. So you get a peak at 18 amu on the graph. But it is possible for other things to have a mass of 18 as well. What makes things easier is that when the gasses are ionized in the chamber and accelerated some of the molecules are broken apart and maintain a statistical ratio. So water breaks up into a few other peaks, One at mass 1 for Hydrogen, and also another peak at mass 17 for HO. You may also get a peak at 16 for monoatomic oxygen. In the graph below you can see an analog plot off my 100 AMU Dycor RGA. It interfaces to a PC so I just use an old laptop to control it. You can see the peak at mass 18 an the little peaks below it. At the time I had a pretty large air leak in the system so you see two massive peaks at Mass 28 (Nitrogen, N2) and Oxygen O2 at Mass 32. You also see a peak at 14 from monoatomic nitrogen and a slight bump from carbon at AMU 12. The bump at 40 is Argon. Xenon is a pretty massive atom at 132 AMU and has isotopes up to 138. So I can measure it on my portable unit but not my big system.
IMG_1220 by macona, on Flickr