Figure shows that the initial measurement in each cycle starts with a low surface density of Nd. For example, the first data point measured for pH 4.5 samples represents a surface density of 0.02 ions nm -2 . It will be shown later that the surface density of Nd remains at this low value for the duration of a long exposure when X-rays illuminate only the surface by reflecting at Qz = 0.018 Å -1 , which is below the angle of total reflection. This suggests that either direct X-ray exposure of DTPA in the bulk water is required to release Nd ions from DTPA, or that radiolysis products from the bulk water are required to release Nd ions from DTPA. Released Nd ions are then free to diffuse to the surface and adsorb to DHDP.