Relationship between the time resolution of kinetics measurements and the fractional change in signal magnitude (|ΔI/I|) by changing the interfacial conditions using the flow-through transmission cell. The lines represent the significance of the measurement, Z = |ΔI/I|/(σ/I) = 4, i.e. the reflectivity change is statistically significant at the 99.99% confidence interval. The values were calculated for Φ = 1 × 1012 photons s−1, R = 5 × 10−7, B = 1 × 10−8, corresponding to those from typical reflectivity measurements of the muscovite (001) surface using the transmission cell at q = 0.49 Å−1 near the Rb K-edge (∼15.2 keV). The calculations were conducted for a perfect surface (ζ = 1 shown by a solid dark blue line) and a surface with a broad reflection (e.g. because of the non-zero mosaicity; ζ = 100 shown by a dashed cyan line), where ζ represents the size of the reflected image relative to the intrinsic size of the beam.