  Atom and, of late, molecule interferometers find application in both the
crucible of fundamental research and industrial pursuits. A prevalent
methodology in the construction of atom interferometers involves the
utilisation of gratings fashioned from laser beams. While this approach imparts
commendable precision, it is hampered by its incapacity to attain exceedingly
short wavelengths and its dependence on intricate laser systems for operational
efficacy. All applications require the control of matter waves, particularly
the particle's velocity. In this manuscript, we propose a continuous beam
monochromator scheme reaching enormously high velocity purification with speed
ratios in the order of $10^3$ based on atom-surface diffraction. Beyond these
high purifications, the proposed scheme simplifies the application by reducing
the degree of freedom to a single angle, selecting the wanted particle's
velocity.
