Separation of Cationic Species by Ion Chromatography Using Zirconium−Modified Silica Gel as Stationary Phase

Ion chromatography (IC), developed by Small et al. [1], is widely recognized as a simple and powerful analytical technique for the determination of various inorganic and organic anions and cations [2]. Sulfonated and carboxylated polymer resins and silicas are commonly employed as cation−exchange stationary phases in IC for cationic species. The application of unmodified silica gels as cation−exchange stationary phases in IC with conductimetric detection (IC−CD) for various cationic species (alkali, alkaline earth and transition metal cations) is also possible [3−8], because the silanol group on the surface of silica gel behaves as a weak acid with a pKa of 7.1 [9]. The greatest advantage of the use of unmodified silica gels in IC−CD is that the simultaneous separation of major monovalent and divalent cations (Na, NH4, K, Mg and Ca) can be achieved [8]. In the IC−CD, lithium ion (Li) with the lowest limiting equivalent ionic conductance was mainly utilized as the eluent competing ion and the eluent pH was approximately neutral, in order to separate and detect these monovalent and divalent cations. Unfortunately, the IC conditions resulted in very low detection sensitivities of these analyte cations and short lifetime of silica gel column [10]. The use of acidic eluent was expected to be one of the best ways for eliminating these above drawbacks. It is well known that commercially available silica gels, except for pure silica gel synthesized by the hydrolysis of high purity tetraethoxysilane [Si (OC2H5)4], contain various metals as impurities in the silica matrix and several polyvalent metals cause the enhancement of the acidity of the silanol group on the surface of silica gel [11−14]. The cation−exchange properties of various unmodified silica gels under acidic condition (2 mM HNO3 at pH 2.7) were evaluated. Some silica gels (Develosil 30−5, Kaseisorb LC− 60−5 and Zorbax BP−SIL silica gels) showed cation−exchange behavior under the acidic condition. These silica substrates were apFocusing Review