Normalized spectra comparing the intrinsic absorption features in Mrk 509 from COS spectra obtained in 2009 (red) and 2012 (black). 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). 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 3).