Introduction. The interaction of inorganic cations with charged aqueous interfaces is important for a variety of settings relevant to environmental chemistry, nanotechnology, energy processes, as well as industrial and engineering applications. A major challenge is to quantify the number of adsorbed ions per area under conditions of aqueous flow or as a function of ion concentration, and to assess the extent to which adsorption and desorption are reversible. Of the surface-selective experimental approaches that provide these capabilities, X-ray reflectivity, quartz crystal microbalance mass estimates, and nonlinear optical spectroscopy are particularly powerful. The latter two techniques circumvent the strong absorber problem of water in the ultraviolet and midinfrared regions, which prevents the probing of most inorganic ions at surfaces using conventional electronic or vibrational spectroscopic techniques (notable exceptions are oxyanions like arsenate, chromate, phosphate, or sulfate.) Interface-specific vibrational spectroscopies such as sum frequency generation probe the response of water's O-H oscillators in the interfacial region to the presence of oxy- and non-oxy ions, from which metal cation number densities, or even relative surface coverages, have not yet been quantified due to absorptive-dispersive mixing of the Stern and diffuse layer contributions to the nonlinear optical response. Electronic spectroscopies are challenging as most common inorganic ions do not possess strong enough electronic transitions that are readily accessible at buried aqueous interfaces. A notable exception is what has been achieved at the air:water interface using second harmonic generation (SHG) measurements that access solvent to solute charge transfer resonances in the ultraviolet down to 210 nm. Electronic resonances involving n-p* transitions of ions such as nitrate (lmax, SHG=300 nm) or ligand-to-metal charge transfer processes of some select transition metal ions (CrO4 2-, lmax, SHG=290 nm) or uranyl (lmax, SHG=305-310 nm) at buried aqueous interfaces have also been detected in our laboratory using resonantly enhanced second harmonic generation (SHG).