Mass spectra don't really fit into this definition of spectrosopy. Is this a problem?
On the surface, it would seem so. But, according to Einstein's famous identity, E=mc^2, mass and energy are the same thing. one could, in principle, plot a mass spectrum in energy units rather than in mass units. That this is not done in practice does not overshadow the interchangeability of the units. Particle physicists, for instance, do you energy-derived units as a measure of mass--speaking of how many electron-volts a particle masses, for instance.
Mass spectroscopy is significantly different from the other spectrometers on the page. MS measures the deflection, dependant on mass, of ions in a magnetic field. The plots are of discrete ion-mass (the deflection), against how many of the corresponding atoms are present in the sample (intensity). You couldn't legimately ever put MS results in to an energy/frequency graph. I think it should be treated as an exception. -- sodium