Superparamagnetic bi-functional composite bead for the thermal ionization mass spectrometry of plutonium(IV) ions

Single resin bead-based thermal ionization mass spectrometry (TIMS) offers numerous advantages for Pu(IV) determinations in complex aqueous samples. These include removal of the matrix and interfering ions by a one-step process, selective preconcentration of Pu(IV) ions with a high chemical recovery, transportation without heavy shielding, avoiding the possibility of cross-contamination, and acting as a point source. The single-bead TIMS method reported in literature is based on an anion-exchange resin that lacks sorption-selectivity toward Pu(IV) ions, and where the beads are not easily retrievable from a large volume sample. Therefore, silica-coated superparamagnetic Fe3O4 embedded functionalized porous poly(ethersulfone) (PES) beads were developed for Pu(IV) preconcentration and analysis by TIMS. The beads were functionalized with a phosphate bearing monomer along with or without a quaternary ammonium bearing monomer by UV-induced surface grafting. Since the beads were used in a highly acidic solution, the Fe3O4 nanoparticles were protected with a silica coating formed by the hydrolysis and condensation of tetraethoxysilane. The PES porous beads were prepared by a phase inversion method. The monomers used for UV-grafting were 2-hydroxyethyl methacrylate ester and (3-acrylamidopropyl) trimethylammonium chloride. The functionalized beads were characterized by scanning electron microscopy, energy dispersive analysis, and vibrating sample magnetometry. The sorption studies indicated that the bi-functionalized PES beads consisting of phosphate and quaternary ammonium groups not only have a higher distribution coefficient (K-d) for Pu(IV) but also high selectivity toward Pu(IV) ions in the presence of a large excess of U(VI) ions (K-d(Pu(VI))/K-d(U(IV)) = 11.5). The phosphate-functionalized PES beads showed comparable selectivity (K-d(Pu(VI))/K-d(U(IV)) = 9.1), but a lower K-d value for Pu(IV). The quaternary ammonium-functionalized PES beads were found to have lower selectivity and Kd values toward Pu(IV) ions. The analytical performance of single bi-functionalized bead-based TIMS for the determination of Pu(IV) using isotope dilution was compared with the solution-based TIMS, validated using the Pu isotopic standard reference material NIST SRM-947 and applied to real samples such as dissolver solutions and soil leach liquors.