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Figure 3. Solution 31P NMR spectra are shown of the amorphous calcium polyphosphate (A), and five-hour-gelled disks with cefuroxime incorporated before (B) and after the elution study (D), and five-hour-gelled disks without cefuroxime incorporated before (C) and after the elution study (E). No significant peak shifts can be seen between the amorphous calcium polyphosphate (A) and the five-hour-gelled disks before the elution study, either with (B) or without (D) cefuroxime incorporation. A chemical phase shift at ~ 1–2 ppm corresponds to an orthophosphate group having no bridging oxygen atoms (Q0), whereas the peak at ~ –9.5 ppm describes a terminal phosphate group with only 1 oxygen atom bridging to a neighboring tetrahedral (Q1). An internal phosphate tetrahedral bond involving 2 bridging oxygen atoms (Q2) is representative of either linear polyphosphate chains (~ –20 ppm) or of metaphosphate ringed structures (~ –22 ppm). Dramatic spectral changes are observed in the post-elution calcium polyphosphate matrices (C&E), showing very few internal phosphate bonds. Spectra of non-gelled and 24-hour-gelled disks did not significantly differ from those of the five-hour-gelled disks and, thus, are not shown (n = 1).
