The X-axis on an RGA represents (molecular mass)/(molecular charge)
example: Oxygen generally has a mass of 16 AMU (isotopes exist, yielding +/- mass, in whole numbers). But, like most elements, oxygen is usually bonded with other atoms in molecules. Oxygen forms a paired gas, O2, with itself, O2 = 32 AMU. This molecule, like most molecules, generally has neutral charge (#protons=#electrons); but the RGA rips off electrons, Ionizing the molecules with charges of -1, -2, etc...
With that in mind let's reexamine the X-axis on an RGA spectra: (molecular mass)/(molecular charge).
The first ionization of O2 will yield a spectra peak at 32/1=32
the second ionization of O2 will yield a spectra peak at 32/2=16
*insert spectra showing this*