The spatial variation in Rb Kα fluorescence yield (FY) from a sample mounted in an X-ray thin-film cell. The FY is plotted as a function of distance from the sample center transverse to the vertical scattering plane. The cell contained a 0.1 mM RbOH solution several micrometers in thickness. The data (`before flushing') were collected three hours after solution injection and subsequent reduction of the solution thickness to several micrometers. The large enhancement in FY near the center portion of the sample indicates an increase in Rb concentration. An additional FY enhancement is observed at the beam spot location (−0.7 mm) used for data acquisition for 3 h. This enhancement is due to X-ray-induced damage of the Kapton film leading to Rb incorporation (Lee et al., 2012). Expanding the thin water layer and flushing several milliliters of the same solution (`after flushing') effectively reduced the ion-accumulation near the sample center (this did not fully remove the Rb accumulated under the beam spot). The solid sample was a 10 Å-thick TiO2 layer coated on a Si/Mo multilayer grown by atomic layer deposition (Kohli et al., 2010), with the exposed surface in dimensions of 12 mm × 37 mm (transverse and parallel to the scattering plane, respectively). The data were collected at beamline 6-ID-B (Advanced Photon Source). The incident beam was collimated by slits to a size of 0.05 mm (v) × 1.0 mm (h) with an incident angle of 0.275° (with a 10.5 mm beam footprint on the sample) at a photon energy of 17 keV.