Tailoring Elution of Tetraalkylammonium Ions. Ideal Electrostatic Selectivity Elution Order on a Polymeric Ion Exchanger

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• 作者：Bingcheng Yang ; Masaki Takeuchi ; Purnendu K. Dasgupta ; Tomonari Umemura ; Yuji Ueki ; Kin-Ichi Tsunoda
• 刊名：Analytical Chemistry
• 出版年：2007
• 出版时间：January 15, 2007
• 年：2007
• 卷：79
• 期：2
• 页码：769 - 772
• 全文大小：131K
• 年卷期：v.79,no.2(January 15, 2007)
• ISSN：1520-6882

文摘

Although ion exchange is often depicted as a processdriven by electrostatic forces, ionic solvation or hydrophobic forces contribute greatly to ion exchange selectivityand is often the dominant factor. On a variety of commercial anion exchange columns, monovalent ClO₄^-elutes after doubly charged SO₄^2- and even triply chargedPO₄^3-. For identically charged alkali metal ions, electrostatic charge densities based on crystal radii wouldsuggest Li⁺ to be the most strongly retained on a cationexchanger. In practice, it is typically the least strongly heldcation on most cation exchangers, because of its very highhydration energy and with most eluents its capacity factorapproaches zero. Even when the ion is very poorlysolvated, as with tetraalkylammonium (NR₄⁺) cations,there has never been a report on a polymeric ion exchanger of an ideal electrostatic selectivity order whereNR₄⁺ cations elute in their increasing charge densityorder: R = n-butyl first, followed by n-propyl, ethyl, andlast, methyl. We show that this selectivity order is easilyachieved on recently described methracrylate-based monolithic capillary cation exchange columns (Ueki, Y.; Umemura, T.; Li, J. X.; Odake, T; Tsunoda, K. Anal. Chem.2004, 76, 7007-7012) with minor amounts of hydroorganic modifiers. Indeed, under such conditions, Li⁺ (andother alkali cations) elutes after NMe₄⁺.