Metal absorption lines from the z_abs = 2.1871 system toward the quasar HE0001–2340 (solid-line histograms). Synthetic profiles are plotted by the smooth curves. The signal-to-noise ratio, S/N, per pixel in the co-added spectrum is indicated in each panel. Also shown are the relative velocity offsets ΔV: for the Si ii λ1190 Å line and the Mg ii doublet they are with respect to the Si ii λλ1260,1526 Å lines, and for the Fe ii lines – relative to the Fe ii λ2344 Å line. The zero radial velocity is fixed at z = 2.1871.