Polymer based sorbent materials for thermal ionization mass spectrometric determination of uranium(vi) and plutonium(iv) ions

Abstract

Thermal ionization mass spectrometry (TIMS) is a widely used method for obtaining information about the isotopic composition of individual isotopes with high resolution and accurate quantification with considerably low detection limits. However, analyses by TIMS require an elaborate sample preparation step to eliminate the matrix and subsequent manual loading of small volumes of purified aqueous samples on a filament surface for thermal ionization. This is cumbersome, particularly for the handling of radioactive solutions. Therefore, in the present work, polymeric material based sorbents were explored for the single step matrix elimination and source preparation for the loading of U(VI) and Pu(IV) ions preconcentrated from a variety of environmental and nuclear fuel reprocessing samples. The solid phase loading offers a number of advantages for handling radioactive materials, and is amenable to matrix elimination and preconcentration of analytes that would further improve the detection limit of TIMS. The polymer based sorbents were prepared by anchoring neutral and acidic phosphate functional groups selective to actinide ions in porous poly(propylene) and poly(ethersulfone) matrices. One of the procedures used for the preparation of polymeric sorbents involved grafting of the monomers phosphoric acid 2-hydroxyethyl methacrylate ester and 2-ethylhexylmethacrylate (EHM) in 1 : 1 molar proportion by UV-initiator induced polymerization in the pores of the host matrix. In another route, the liquid extractants tris(2-ethylhexyl) phosphate (TEHP) along with bis(2-ethylhexyl) phosphoric acid (HDEHP), in different molar proportions, were physically immobilized by capillary force in the pores of poly(propylene) and poly(ethersulfone) membranes and beads. It was observed that the poly(propylene) pyrolyzed easily at a filament temperature close to that used for solution based sample loading, and thus was best suited for the analyses of U(VI) and Pu(IV) by TIMS. The composition of the polymer membrane supported liquid extractant based sorbents was optimized for the preconcentration of U(VI) from ground water and seawater, and also for the preconcentration of Pu(IV) from 3 mol L⁻¹ HNO₃, which is normally encountered in nuclear fuel reprocessing facilities. The parameters affecting the analytical performance of polymer sorbent based TIMS were evaluated, and tested for the quantification of U and Pu in the ppb concentration range in seawater and urine samples using the isotope dilution method.