Anal. Chen. 1994,66, 824-829
Ion Exchange Using a Scintillating Polymer with a Charged Mlng Ll and Joseph B. Schlenoff’ Department of Chemistry and Center for Materials Research and Technology (MARTECH), The Florida State University, Tallahassee, Florida 32306 The surface of a polystyrene-based plastic scintillator was sulfonated to yield an ultrathin cation-exchanging layer of welldefined geometry. The thickness of the layer (0.2-10 000 nm) is a strong functionof the concentrationof SO3 in thesdfonating reagent. Radioisotopes absorbedinto this layer cause efficient scintillation, permitting in situ measurement of ion content, selectivity coefficients, and exchange kinetics. Self-exchange was performed using radiolabeled calcium (&emitter) as a probe ion, which could be displaced, with differing efficacy, by the addition of other cations. An electrochemistry-based analysis of the hydrodynamics of the system revealed that the rate-limiting process is diffusion through a thin film of solution at the surface of the exchanger rather than mass transport through the polymer. Ion-exchanging materials are extensively used for purification,’ decontamination2 (extraction of radioisotopes), and prec~ncentration.~Exchangers, which can be organic or in~rganic,~ often show high selectivity between ions.5 Ionexchange resins are typically based on cross-linked polystyrene derivative^.^^^ Cation-exchange resins are prepared by sulfonating styrene/divinylbenzene copolymer beads.7 Anion exchangers are made by chloromethylation of beads, followed by conversion to amines or quaternary ammonium Salk8 In analytical applications, a dilute solution of the ion is preconcentrated by percolation through a bed of exchanger. Following preconcentration, the resin can be decomposed and analyzed directly, or more commonly, the entrapped ions can be eluted using a small volume of acid with quantitative analysis on the eluents3 We report a novel combination of ion exchanger and plastic scintillator which allows direct determination of radioisotopes. This composition was generated during the course of work aimed at developing charged surfaces to which ions and polyelectrolytes would adsorb. Techniques based on isotopic labeling have the potential for high sensitivity. Our work was partly inspired by prior radiochemical studies on the adsorption of monolayers of charged and neutral species (e.g., bisulfate9 ~
(1) Arden, T. V. Water Purification by Ion Exchange; Plenum: New York, 1968. (2) Bibler, J. P. InRecent Deuelopments in IonExchange; Williams, P. A,, Hudson, M. A., Eds.; Elsevier: London 1990; pp 121-133. (3) Rieman, W.; Walton, H. F. Ion Exchange in Analytical Chemistry; Permagon: Oxford, UK, 1970. (4) Qureshi. M. Varshney, K. G.Inorganic Ion Exchangers in Chemical Analysis: CRC Press: Boca Faton, FL, 1990. ( 5 ) Hclffcrich, F. Ion Exchange; McGraw-Hill: New York, 1962. (6) Hohcnstein, W. P.; Mark, H. J . Polym. Sci. 1946, I , 127-145. (7) Pepper, K. W. J . Appl. Chem. 1951, I , 124132. (8) Wheaton. R. M.; Bauman, W. C. Ind. Eng. Chem. 1951,43, 1088-1093. (9) Rice-Jackson, L. M.; Horanyi, G.; Wieckowski, A. Electrochim. Acta 1991, 36, 753-757.
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Ana&rlcalChemistry, Vol. 66,No. 6,March 75, 1994
and pyridine’O) at metal surfaces under potentiostatic control. For example, Wieckowski et al.”13 described simultaneous cyclic voltammetry and radiochemical assay at metal electrodes with detection limits in the percent of a monolayer range. Varga et al. have performed extensive studies on ion sorption phenomena on metal films, powders, and protective coatings using potentiostatic/radiometric techniques.14J5 In our system, the substrate is a charged polymer surface formed on top of a scintillator. Radioisotopes absorbed into the exchanger can thus be determined directly through scintillation counting, making it possible to perform kinetic studies and quantitative extraction and analysis in situ.
EXPER IMENTAL SECT1ON The plastic scintillator was of the type used in detection of high-energy particles.16J7 The scintillating material is polystyrene with dissolved fluorescent dyes (“fluors”). Energy is transferred to the polymer and then, via a primary fluor, to a second dye that shifts the light to a longer wavelength in the visible. The scintillator (SCSN81, Kuraray Inc.) was a blue-fluorescing 2%divinylbenzene cross-linked polystyrene containing proprietary primary and secondary fluors. The concentration of these fluors is low (
