The role of the N-C bond in the observed cycling was tested by substituting sodium citrate for DTPA. This substitution eliminates the nitrogen atoms from the chelate backbone and reduces the number of carboxyl groups from five to three. Both changes reduce the interaction of citrate with Nd. To compensate for this reduced interaction and create a citrate sample whose behavior can be compared with DPTA samples, the composition of citrate was chosen to be 10 mM at pH 6. Figure shows that this composition yields a low surface density of Nd ions (0.02 ions nm -2 ) when XFNTR is measured at Qz = 0.018 Å -1 , for which X-rays illuminate only the surface. This demonstrates that Nd ions are nearly fully chelated by the citrate in bulk water under these conditions, like the nearly full chelation of Nd ions by DTPA demonstrated by XFNTR measured at Qz = 0.018 Å -1 in Fig. which revealed a similar low surface density of 0.02 ions nm -2 . As shown in Fig. , reducing the pH of the citrate samples to pH 5 increases the Nd surface density to 0.1 ions nm -2 ; therefore, pH 6 at 10 mM provides the condition at which citrate starts to nearly fully chelate Nd ions. Although it is likely that the composition of the DTPA samples (30 µM DTPA and pH 3.0 or 4.5) provides a slightly stronger chelation of Nd (compare Figures and), the conditions chosen for citrate chelation should be at least equivalently susceptible, if not more so, to the release of Nd when X-rays expose the bulk water.