The chemical composition of distant objects can be determined by analysing their light spectra.
Atoms do not emit light in a continuous spectrum from red to blue. There are specific energies for which atoms absorb and emit light. The pattern is distinct and unique to each atom in the periodic table. It is the shape of this spectrum that reveals the chemical composition of a distant object.
If the light spectrum is shifted towards the red, we have what we call red-shift. If it is shifted towards the blue end, we have blue-shift.
An interesting thing to note is that the shape is determined by chemical properties only. The various isotopes of an atom all have the same shape. What distinguishes one isotope from another is its blue-shift. Heavier versions of an atom emit bluer light.
In the case of hydrogen, deuterium has a neutron attached to its nucleus. It is therefore twice as massive as regular hydrogen, which has a sole proton in its nucleus. Chemically, the two isotopes are identical. However, their light spectra can be used to tell them apart.
Deuterium can be identified by its blue-shifted spectrum. It is shifted towards the blue end of the spectrum by about 2 angstrom. This is how we know that the water in the tails of comets are rich in deuterium, and therefore not the source of water on our planet, which contains little deuterium.
All of this is easy to confirm in a laboratory. The heavier the atomic nucleus is for a given element, the bluer is its light spectrum.
Halton Arp’s premise about light spectra and the mass of atoms is therefore correct.