Normalized spectra comparing the intrinsic absorption features in Mrk 509 from the COS G140L spectrum obtained in 2012 (red, with 1σ error bars) to FUSE spectra from 1999 (black) and 2000 (green). Relative fluxes are plotted as a function of velocity relative to the host galaxy redshift of z = 0.034397 (Fisher et al. 1995). The blue dotted vertical lines are labeled with the components identified by Kriss et al. (2011) at higher spectral resolution. As a guide to the expected widths of unresolved spectral features, the COS LSF is shown as the thin black line in the middle panel. The boundaries of the selected spectral regions used to evaluate variability in the absorption troughs are indicated along the top horizontal axis (see also Table 5).