(a) Crystal truncation rod data from the muscovite-(001)–solution interface measured using a transmission flow-through cell (cyan circles) in comparison with that using a thin-film cell (red squares). The background intensity of these two data are also plotted for comparison. Both data were measured at beamline 6-ID-B with the same experimental setting but with different solution conditions. The transmission-cell data were measured in deionized water at 353 K (labeled as DIW) and the thin-film cell data were in a 1 × 10−4 M MgCl2 solution at 298 K (labeled as MgCl2). (b) Ratio of the fractional uncertainty (i.e. = uncertainty/signal) of the transmission-cell data (σtrans) to that of the thin-film cell data (σthin-film), as a function of momentum transfer, q. The q-dependent increase of the ratio at q ≤ 2.7 Å−1, indicated by the red arrow, is mainly due to the increased background intensity for the transmission-cell data. The ratio decreases at higher q and this is mostly related to smaller background intensity [mostly diffuse scattering from water; see the background intensity variation in (a)] and also larger reflectivity for the transmission-cell data compared with the thin-film cell data (note that these data were measured under different solution conditions).