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Anal. Chem. 1984, 5 6 , 610-614
Liquid Chromatographic Separation of Metal Ions on a Silica Column Ronald L. S m i t h and Donald J. Pietrzyk*
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242
Metal ions were shown to be retained on a silica column from an aqueous mobile phase by cation exchange. An equation, which relates cation retention to the mobile phase variables, the equilibrium constants describing the cation exchange, and the dissociation of the silanol sites, was derived. The major mobile phase varlables, which were evaluated, include moblie phase pH, ionic strength, and solvent composition, type and concentration of countercation, inorganic anaiyte cation concentration, and the use of compiexing agents as mobile phase additives. Chromatographic conditions for the separation of alkali metals, alkaline earths, and several rare earths on sliica are discussed.
Silica has been recognized as an adsorbent for nonionics and ionics for well over 50 years. Even though ita composition as well as its chromatographic properties are complex ( I , 2), silica has been widely used in chromatography. It is not surprising, therefore, that silica was a major stationary phase in the early period of the development of modern high-performance liquid chromatography (HPLC). Although silica columns are very versatile and readily meet the high standards required in HPLC, they have been largely replaced in applications by other stationary phases and consequently their current role in HPLC is a minor one. When silica columns are used in HPLC they are generally used as a normal stationary phase, because of their hydrophilic surface, in combination with nonaqueous, aprotic type mobile phases. Typical analytes separated on silica columns range from nonpolar to medium polarity (2). One of the unique properties of silica, which has been recognized for a long time, is its cation exchange properties (1,2). The ion exchange equilibria between silica and mono-, di-, and trivalent metal ions, particularly monovalent, in aqueous electrolyte solutions appear to be simple ion exchange (1-8) although the exact nature of the acidic silica surface, which is a key factor in its exchange properties, remains somewhat uncertain. For example, ita pK, has been estimated to be about 7.1 (2);however, other values have been reported and it appears that the type of silica and environment used strongly influences the pK,. Several models have been proposed to explain the increased acidity relative to monosilicic acid. Vysotskii and Strazhesko (9) suggested two possible models. In one an electron accepting effect of many undissociated hydroxyl groups is transmitted by (d-p)r interactions along the chain of Si-0 bonds causing an increased acidity of the silanol group. In the second, polarized water molecules are possibly coordinated to unsaturated surface silicon atoms via vacant 3d orbitals providing an exchangeable H30+. Burwell et al. (10)suggests that the greater acidity is due to hydrogen bonding between adjacent surface silanol groups while Unger (2)suggests that all three models contribute to the increased acidity. Like the acidity, the exchange capacity of silica is very dependent on the silica surface and the environment (2). Theoretical capacities approach 3 to 4 mequiv/g for silica with
surface area of about 400-500 m2/g. However, observed capacities a t about pH 9 are more likely to be about 1.2-1.5 mequiv/g. As pH decreases the capacity drops sharply. The degree of hydroxylation of the silica surface, silica surface area, and ionic strength also have a large effect on capacity. Even though perhaps only 10 to 20% of the theoretical capacity or a small amount of the surface hydroxyls are available a t a favorable pH, this low capacity should still be significant enough for silica to be used effectively as an ion exchanger for the separation of cations by HPLC techniques. Several studies, which are reviewed elsewhere (1, 2), using classical column and batch techniques have demonstrated that silica can be used for separating metal ions. The separations obtained this way are often inefficient, provide poorly defined peak shapes, and are time consuming. This report focuses on the utility of silica as a stationary phase for the HPLC separation of metal ions, such as alkali metals and alkaline earths. The major mobile phase variables, which are operator controlled, include pH and type and concentration of added electrolyte, chelating agent, and organic modifier.
EXPERIMENTAL SECTION Reagents. Inorganic salts used for analytes, buffers, and ionic strength control and as ligands were analytical reagent grade when possible. Organic solvents were LC quality and H20was purified via a Sybron/Barnstead water purification unit. Zorbax SIL columns were obtained from Du Pont as 6-pm spherical silica particles in 150 mm X 4.6 mm prepacked columns. Instrumentation. The LC instrumentation consisted of Altex Model llOA pumps, an Altex Model 421 controller, a Rheodyne 7125 injection valve, a Wescan Model 213A conductivity detector, a Bioanalytical Systems, Inc., Model LC-224 column heater, and a Spectra Physics 4100 computing integrator-recorder. Procedures. Analyte solutions were prepared by dissolving weighed amounts of the metal chloride salts in water to yield 2-10 mg/mL. These were stored in sealed containers and refrigerated when not in use. Sample aliquots were 1-20 pL and were delivered by a 25-pL syringe (Hamilton). Mobile phase solvent composition is percent by volume while electrolyte and buffer salts were added by weight or by appropriate dilution procedures. Acetate, citrate, and tartrate solutions were made by titrating solutions of their corresponding acids to the desired pH with an alkali metal hydroxide solution. The column was preconditioned by passing >lo0 column volumes of the mobile phase through the column. Column temperature was controlled to 30.0 k 0.1 "C and flow rates were 0.5-2.0 mL/min. Inlet pressures ranged from 500 to 3000 psi depending on flow rate and mobile phase composition. Capacity factors were calculated in the usual way; analytes that are not retained were used for the determination of column void volume. RESULTS AND DISCUSSION The inorganic cation analyte-silica stationary phase interactions are likely to be coulombic in nature and the equilibria representing cation exchange of the analyte on silica is given by -SiO-C+ X+ + -SiO-X+ C+ (la)
+ + + X+ + -SiO-X+ + H+
-SiO-H+ Ob) where -530- represents an accessible silica exchange site, C' and H+ are countercations (the cation form of the exchange
0003-2700/84/0356-0610$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
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20
k’
k’
12
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8-
4.5
5.5
6.5
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7
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M Li acetate mobile phase at 1.0 mL/min and Flgure 1. Retention of metal ions on Zorbax SIL as a function of pH: (A) an aqueous 2.0 X M ammonium citrate mobile phase at 1.0 mL/min and 30.0 f 0.1 O C ; (C) an aqueous 8.0 X 30.0 f 0.1 OC;(B) an aqueous 1.0 X M Li tartrate mobile phase at 2.0 mL/min and 30.0 f 0.1 OC.
site will be determined by the pH, the ionic strength, and the ion exchange selectivities for the analyte cations and countercations), and X+ is the analyte cation. If it is assumed that inorganic analyte cation retention on silica is only by ion exchange, as shown in eq 1, then the analyte retention can be described by treating silica as a weak acid cation exchanger since the silica -SiOH sites are weakly acidic. Thus, considering the ion exchange selectivities and the ionization of the -SOH site, retention of the inorganic analyte cation on silica is given by
l/k’x.si
where k is the capacity factor for the retention of X+ on silica, q is the ratio of stationary phase volume to mobile phase volume, KOis the available exchange capacity, m and s are mobile and solid phase, respectively, Kappis the apparent ionization constant for the -SOH site, and K(Si%+),and K(Si$), are the exchange selectivities for eq l b and la, respectively. Equation 2, which is discussed elsewhere (II,12) and ia very useful because it focuses on the controllable mobile phase parameters that influence retention, indicates that analyte cation retention is indirectly proportional to H+, analyte, and countercation concentration and to silanol K, and directly to the exchange selectivities between the analyte cation and other mobile phase cations. If ligands are included in the mobile phase, eq 2 can be modified to take into account the formation constants for the metal ion-ligand complexes. pH. Even though the pKa of silica is estimated to be 7.1, silica will undergo cation exchange as low as pH 2. However, exchange capacity falls off sharply as the pH approaches the more acidic environment. Figure 1demonstrates that cation retention is significantly affected by mobile phase pH. Elution order in Figure 1A remains constant except for NH4+ and follows the order Cs+ > Rb+ > NH4+> K+> Na+ at pH 6.9. In Figure 2B,C the elution order is Ba2+> S$+ > Ca2+> Mg2+ and La3+ > Ce3+> Pr3+> Nd3+, respectively. These orders are consistent with the results found by classical LC column and/or batch experiments on silica (2). The effect of pH on alkali metal ion retention in Figure 1A is consistent with eq 2. When the presence of the ligand in the mobile phase in Figure 1B,C is taken into account, these data are also in agreement with eq 2. The ligand was used in the mobile phase in order to obtain reasonable elution times since di- and
trivalent cations have very high retention on silica. (Selectivities, in general, follow the order trivalent > divalent > monovalent just as found (13) with conventional ion exchangers.) A stronger eluent cation could also be used to reduce retention and analysis time; however, this approach is not as favorable as provided by using a ligand in the mobile phase. Since silica decomposes above pH 8, this study was restricted to mobile phase pHs below 8. Also, the isoelectric point for silica, which leads to a positive silica surface, occurs at pH
