is the same in the absence or presence of DTPA (Fig. ). Although it might be expected that the radiolysis effects that released ions from DTPA at higher pH will also do so at lower pH because of the expected increase in radiolysis products at lower pH, it appears that X-ray induced release of Nd ions from DTPA does not affect Nd binding to the DHDP monolayer. At this pH, DTPA is expected to chelate a reduced amount, roughly 40%, of the Nd 3+ ions in solution in the absence of other molecules or effects (Fig. ). These results suggest that there are enough free Nd 3+ to satisfy the adsorption needs of the DHDP monolayer without the additional release of Nd ions from DTPA by Xray exposure. Figure shows that the Nd surface density does not recover when X-rays are turned off and an adsorption cycle cannot be established. Figure also shows that the Nd surface density measured at Qz = 0.018 Å -1 is nearly constant with time when X-rays are confined to the surface. As before, the observation of a temporal variation in the Nd surface density when X-rays penetrate the bulk, but not when X-rays are confined to the surface, indicates that aqueous radiolysis products are responsible for releasing Nd ions from the DHDP monolayer. However, the effect is irreversible suggesting that the X-ray exposure at this low pH is damaging the DHDP monolayer. Shifting the Xray beam to a different position on the sample allows the Qz = 0.025 Å -1 data shown in Fig. to be reproduced (not shown), confirming that the change in Nd surface density observed at Qz = 0.025 Å -1