How does the LHC separate the protium isotope to have only protons for the collision? I am preparing a presentation for my physics class about the LHC and the following question arose:
Every text about the LHC says that it collides protons from a gas of hydrogen whose electrons were previously taken away. 
Can collisions be achieved with any hydrogen isotope or is it only protium that is being used?
If so, how is protium separated from the other isotopes?
 A: Given their charge and mass, as soon as you start accelerating particles around a loop with a given magnetic field to deflect them, only particles with the correct mass/charge ratio survive. In effect you have built a giant mass spectrometer - other isotopes of hydrogen are too heavy, and the Lorentz forces are insufficient to deflect them down the tunnel.
As was pointed out by @DMcKee, the process of extracting the protons includes bends in the injector - any particles with the wrong Q/m ratio will be eliminated there, before making it into the main accelerator. You can see that in this diagram (from https://lhc-machine-outreach.web.cern.ch/lhc-machine-outreach/images/complex/Cern-complex.gif)

An of course the first part of the acceleration happens in a LINAC. Although it is a straight line, it selects for the right particles as the RF frequency (and the spacing of the acceleration stages) is tuned for a specific Q/m ratio. So anything that is not a proton will almost certainly never make it to the first bend - anything that did, would "skid out" at that point.
