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We have examined the effect of the mixing ratio of methyl benzoate (MB) and bis(2ethylhexyl)phthalate (DEHP) in the synthetic process on the Li isotope fractionation in the cation exchange reaction. MB / DEHP volume mixing ratios were adjusted to 1 / 9 and 9 / 1. For the purpose, the two kinds of porous-type cation exchange resins with the cross-linkage of 50 wt% degree were synthesized by suspension polymerization. Both the particle size distribution was found to be similar in the synthetic processes. On the other hand, it was found that the surface structure from the condition; MB : DEHP = 1 : 9 is much rougher than that from the condition; MB : DEHP = 9 : 1, judging from the SEM images. Based on this result, the Li isotope fractionation experiments using two kinds of poroustype sulfonated styrene-divinylbenzene resins were performed in the weak basic solution at 298 K. We have confirmed that the Li isotope separation phenomena by confirming a sharp boundary on the chromatographic curve at each band-end where lighter Li isotope are enriched in the resin phase and heavier Li isotope is enriched in the aqueous phase. Two kinds of Li isotope separation coefficients (ε) per unit mass (ε/ Mass) values were respectively 3.3 × 10 -3 for MB : DEHP = 1 : 9 and 1.7 × 10 -3 for MB : DEHP = 9 : 1 in spite of the same cross-linkage degree. Therefore, we have concluded that the much rougher surface structure of the resins is very important parameter to improve ε/ Mass values in the Li isotope cation exchange reactions.

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