Electronic isotope shift factors for the Cu $4s \; ^2S_{1/2} - 4p \; ^2P^o_{3/2}$ line

State-of-the-art relativistic multiconfiguration Dirac-Hartree-Fock calculations have been performed to evaluate the electronic field and mass isotope shift factors of the Cu~I resonance line at $\lambda = 324.8$~nm. A linear correlation between the mass factors and the transition energy is found for elaborate correlation models, allowing extrapolation to the observed frequency limit. The relativistic corrections to the recoil operator reduces the transition mass factor by 5~\%.