Nevertheless, XFNTR measurements at Qz = 0.025 Å -1 , for which X-rays illuminate both the surface and the bulk water, did not reveal an increase in surface density of Nd (Fig. ) in the presence of citrate, as observed for DTPA samples (shown in Fig. and Fig. ). These results suggest that the N-C bond in the backbone of DTPA is relevant for the mechanism of X-ray exposure-induced release of Nd ions.