The effect of X-ray exposure on the desorption of Nd from a surface monolayer of DHDP in the absence of DTPA. The monolayer was held at a surface pressure of 10 mN/m on the surface of water containing 10 μM NdCl3. XFNTR measurements of Nd surface density under continuous X-ray exposure with (A) total reflection of X-rays at Qz = 0.018 Å -1 , which exposes the surface region with a penetration depth of 10 nm, and (B) X-ray reflection at Qz = 0.025 Å -1 , which exposes the surface and the bulk aqueous phase with a penetration depth of 2.8 µm. Values of pH are 4.5 (blue), 3.0 (magenta), 2.5 (orange). Two lines in (B) are fits described in the text.