  We study the effect of varying the boundary condition on the spectral
function of a finite 1D Hubbard chain, which we compute using direct (Lanczos)
diagonalization of the Hamiltonian. By direct comparison with the two-body
response functions and with the exact solution of the Bethe Ansatz equations,
we can identify both spinon and holon features in the spectra. At half-filling
the spectra have the well-known structure of a low energy holon band and its
shadow---which span the whole Brillouin zone---and a spinon band present for
momenta less than the Fermi momentum. New features related to the twisted
boundary condition are cusps in the spinon band. We show that the spectral
building principle, adapted to account for both the finite system size and the
twisted boundary condition, describes the spectra well in terms of single
spinon and holon excitations. We argue that these finite-size effects are a
signature of spin-charge separation and that their study should help establish
the existence and nature of spin-charge separation in finite-size systems.
