2116
Anal. Chem. 1988, 60, 2116-2119
Micelle Exclusion Chromatography of Heavy-Metal Cations Tetsuo Okada Faculty of Liberal Arts, Shizuoka University, 836 Ohya, Shizuoka 422, Japan
Mlcelle exclusion chromatography, wherein a micellar moblle phase and a size exclusion column are utilized for lonlc separation, was applled to the separation of heavy-metal cations. Usefulness of slmple mlcelle exclusion chromatography was llmlted by the poor resolution of metal catlons, but addition of complexing agents In the sodlum dodecyl sulfate eluent made possible the simultaneous separatlon of many divalent and trlvalent transltlon- and rareearth-metal catlons. Additlonally, the Chromatographic retention behavior was used to calculate partltion coemcients of metal Ions between mkelles and the bulk solution and between the Imbibed solution and the statlonary phase.
High-performance liquid chromatographic (HPLC) determination of metal cations has been investigated by many workers (1-11). Cation-exchange chromatography and reversed-phase chromatography have both been successfully employed. Although ion chromatography (IC) is better known for its usefulness in anion analysis (1,12-15), IC has been also applied to cation analysis (1-3) and has proved to be useful for the determination of alkali- and alkaline-earth-metal ions. However, in simple cation-exchange chromatography, the poor resolution between metal cations, especially between divalent heavy-metal cations, is a serious disadvantage. The use of complexing eluents has therefore been investigated to separate such cations. Sevenich et al. reported the successful singlecolumn ion chromatographic separation and conductometric determination of divalent and trivalent transition- and rareearth-metal ions with ethylenediamine eluents containing auxiliary complexing agents ( 2 ) . In regard to metal ion separation on reversed phase (5-101, the metal cations are not directly retained on such stationary phases. Some studies have focused on the separation (or speciation) of organometallics, e.g., organoarsenic, organotin, and organolead species, with atomic absorption spectrometric detection (5, 6). Other workers have investigated the separation of metal chelates (7,8). For example, metal chelates of the derivatives of diethyldithiocarbamate have been separated on reversed phase. Eluents containing ion-pair or chelating reagents have also been used in the reversed-phase separation of metal ions (4, 9-11). Such methods are called ion-pair chromatography or ion-interaction chromatography (IIC). IIC has been successfully used in anion chromatography with cationic ammonium salts as ion-pair reagents (9,16-18). However, IIC of cationic species, especially metal ions, has been investigated by relatively few researchers. Micelle exclusion chromatography, which utilizes a micellar mobile phase and a size exclusion column for ionic separation, was previously reported by the author (19). In this method, three separate equilibria are involved in the separation: partition between an external solution and micelles, partition between an inner solution and the stationary phase, and partition between the external and inner solutions. The order of partition to a micelle phase or to a stationary phase was similar to that to chemically bonded ion-exchange resins, but the actual elution order is very different from that of conventional separation methods. This is because the separation
selectivity is controlled both by the stationary and the mobile phases in this case, whereas in conventional cases, it is determined primarily by the stationary phase. In this paper, results of fundamental studies on micelle exclusion chromatography of metal ions (Co(II),Cu(II), Fe(II), Fe(III), Pb(II), Mn(II), Ni(II), Zn(I1)) and further applications are described.
EXPERIMENTAL SECTION The chromatographic system was composed of the following instruments two computer-controlledpumps (CCPM and CCPD, Tosoh Co., Ltd.), a column oven (CO-8OO0,Tosoh), a Rheodyne injection valve equipped with a 100-pLsample loop, a UV-vis detector (UV-so00,Tosoh) set to 540 nm, and a separation column (Asahipak GS-310H poly(viny1 alcohol) gel, particle size 5 pm, 7.6-mm i.d. X 250 mm). The separation column was maintained at 35 “C in the oven. Metal ions in the column effluent were detected by postcolumn colorimetry with 4-(2-pyridylazo)resorcinol (PAR). The flow rate of both pumps was 1 mL/min. Standard solutions of transition-metal ions were prepared by dissolving analytical grade reagents of the nitrate salts in distilled deionized water and were kept at a pH of 40000, micelles of SDS, the aggregation number of which is 62 (20), are partly excluded by the stationary phase. The region where micelles can permeate is called the “external solution”. No micelles can permeate the inner part of the stationary phase, but monomeric surfactants are permeable here. Ion-pair or ion-interaction chromatographic retention is dominant in this region, called the “inner solution”. The following equation can be derived by taking these equilibria into account:
where V,, V,, Vi, and V, are the retention volume of a solute, volume of the external solution where micelles can permeate, volume of the inner solution imbibed by the stationary phase where micelles cannot permeate, and volume of the stationary phase; 0 is the molal volume of a micelle; C, is the concentration of micelles in an eluent; Khlw is the partition coefficient
0003-2700/88/0360-2116$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
2117
Table I. Slopes and Intercepts Obtained with 1/( V , - V , ) vs c, Plots mobile phase
SDS
SDS + 0.04 M tartrate, pH 4.4
metal cation Cu(I1) Pb(I1) Zn(I1) C~(II)
Pb(I1) Zn(I1)
c0(11)
slope, mol-' 5.4 x 102 1.6 X lo3 1.0 x io3 1.1 x 103 2.0 X lo3 1.3 X lo3
1.0 x 103
intercept, cm" 1.2 x 10-2 -1.6 X -4.9 x
-3.5 x 10-3 0.14
5.7 X lo-* 2.9 x 10-2
I
-I
0
0.04
m
Figure 1. Typical micelle exclusion chromatogram of transition-metal ions: mobile phase, 0.05 M SDS and 0.08 M NaCi (pH 4.0); sample
concentration, 1 X lo-' M. Other conditions are given in the text.
of a solute between micellar and external solution phases; Ksw is the partition coefficient of a solute between stationary and inner solution phases. The plots of l/(Vr - V,) against C, are linear and provide information on the partition coefficient between the micelle and aqueous phases or between the aqueous and stationary phases. As predicted by eq 1,a linear relation was obtained. However, the partition coefficients could not be calculated because the intercepts of the plots for all metal ions except Cu(I1) are negative. This appears to be due to a difference in critical micelle concentration (cmc) between the bulk solution and the chromatographic system. Slopes and intercepts obtained with plots based on eq 1are listed in Table I. Negative intercepts obviously have no physical meaning. It is well-known that the retention in micelle chromatographyis not necessarily changed at cmc (21). The present results also indicate that cmc in the chromatographic system is not the same as in usual solutions. Although accurate values of partition coefficients cannot be specified, the order of the values can be predicted; that is, Pb(I1) > Zn(I1) = Co(I1) > Cu(II) (other metal cations investigated have almost same value as that of Co2+)for both partition coefficients. This order is well-correlated with the order of increasing hydration energy. A typical micelle exclusion chromatogram of metal cations is demonstrated in Figure 1. Pb(II), which is most strongly retained on a conventional chemically bonded ion-exchange resin, is most rapidly eluted in the present method, because the retention of solute ions is mainly controlled by partition to the micelle rather than to the stationary phase. As previously reported, in micelle exclusion chromatography of inorganic anions, the selectivity was different from that in conventional separation methods (19). In this case, it was possible to simultaneously separate several anions because these anions have various molecular shapes and different hydration energies. However, in the present case, the number of metal cations that can be simultaneously separated is limited because the differences in the hydration energies are small. This is the same problem as in a conventional cation-exchange chromatographic separation of metal ions (2,3). Effect of Adding Complexing Agents on Retention of Metal Ions. In order to improve the resolution of micelle
=
conc. of
tartrate,
mFan 1 0.08
M
Figure 2. Variation of retention times with the concentration of tartaric acid in a mobile phase: concentration of SDS, 0.05 M; pH, 4.4.
OD4
0 conc. 01
citrate,
0.08 M
Flgure 3. Variation of retention times with the concentratin of citric acid in a mobile phase: concentration of SDS, 0.05 M; pH, 4.3.
exclusion chromatography of metal cations, the addition of complexing agents was studied in cation-exchange chromatography or IIC of metal cations and resulted in successful separation and detection (2, 9). In the present research, tartaric and citric acids, which form labile complexes with metal cations, are employed in mobile phases of SDS. Other strong complexing agents such as ethylenediaminetetraacetic acid (EDTA) are not suitable because resulting complexes are
2118
ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
too stable to detect with PAR postcolumn reagent. Figures 2 and 3 show the effect of adding tartrate and citrate in mobile phases on the retention of metal ions. Both elution orders can be essentially explained by the complex formation. For example, Mn(II), which forms the weakest complex with either tartaric or citric acid, becomes the most strongly retained on the stationary phase upon addition of these complexing agents. On the other hand, Cu(II), which is most strongly retained on the stationary phase without any complexing agent, is most rapidly eluted in the presence of complexing agents in the mobile phase. Cu(I1) and Fe(II1) are the most likely to form negatively charged complexes with both complexing agents. With adsorbed anionic surfactants, the stationary phase excludes negatively charged ions which are excluded by the micelle phase as well. For example, either tartarate or citrate is eluted more rapidly than SDS micelles and is resolved with such a stationary phase. Therefore, negatively charged complexes resulting from the complexation are eluted rapidly relative to other neutral complexes. Retention equilibria of metal ions that form neutral complexes can be described as follows:
Table 11. Partition Coefficients Calculated from Eq 1 and
K M W=~Mm(1 - P)hAe/hAePMe = Mm(1 - P)/PMe
0.1 M SDS
(2)
(3) Ksw = M,hAi/hA,a&li = M,Ai/a&liA, Kd = 1 (4) where A,, Ai, and A, are cross sectional areas of the external solution, the inner solution, and the stationary phase; M,, Me, Mi,and M, are the fractional masses in micelles, in the external solution, in the inner solution, and in the stationary phase; h is the height equivalent of one theoretical plate; 0equals BC,; a,,is the ratio of the concentration of a free metal cation to the total metal concentration; KmT is the ratio of the concentration of metal species including free metal ion and neutral complex in the micelle phase to that in the external solution. It was assumed that a neutral complex as well as free metal ions was partitioned into the micelle phase but not into the stationary phase. Therefore, K m Tis dependent on the concentration of the complexing agent and pH of a mobile phase but will be constant when these conditions are not varied. Partition of a negatively charged complex into either the micelle or the stationary phase can be neglected because of the electrostatic repulsion from the negatively charged phases. The following equation can be derived by using these partition coefficients in the same manner as previously described (19, 22, 23): l / ( V r - VJ = ijCm(KMWT- l)/(Vi + VsKswao) + l/(Vi + VSKSW~O) (5) Although partition coefficients cannot be calculated with eq 1 for reasons mentioned above, it is possible to obtain Ksw values from eq 5 because a. can be calculated from the dissociation constants of the ligands and formation constants of the complexes. Since q is less than 1, the intercept obtained with the plots based on eq 5 is larger than that obtained with eq 1. Therefore, the indistinction in cmc does not induce a serious error in this case. Intercepts obtained from the plots based on eq 5 are listed in Table I. These intercepts are much larger than those obtained with eluents without complexing agents. KSWvalues calculated with those intercepts are listed in Table 11. K m values listed in Table I1 were calculated by using the plot based not on eq 5 but on eq 1,because K m T values contain two equilibria and are dependent on the exact mobile-phase composition. It should be noted, however, that indistinction of cmc in the chromatographic system does not induce error in the slope calculated from eq 1. It is predictable that the partition of a neutral complex into the micelle phase is much lower than that of a free metal ion,
5"
metal cation
KMW
Ksw
Cu(I1)b Pb(I1) Zn(I1) Co(I1)
190 4500 720 480
23 200e 47 31
Parameters: cmc of SDS, 8.6 X M (2.5); V, = 6.45 mL, Vi = 1.30 mL, V, = 3.59 mL; D = 246 mL/mol (27). bMobile phases containing no complexing agents. Calculated by using the results obtained with mobile phases containing 0.04 M tartrate (pH 4.4). Table 111. Comparison of Calculated Retention Times with Experimental Values retention time, min
co
mobile phase 0.125 M SDS
+ 0.04 M
Zn Pb exptl calcd exptl calcd exptl calcd 28.6
27.9
19.4
20.1
12.6
11.7
14
14.6
12.1
13.3
9.6
9.7
14.5
13.9
10.0
10.2
8.0
8.0
17.5
16.0
11.9
11.1
9.0
8.2
tartrate, pH 4.4
+ 0.04 M
tartrate, pH 4.4 0.025 M SDS + 0.08 M tartrate, pH 4.4 0.025 M SDS + 0.08 M tartrate. pH 4.05
because the former is due only to the solubilization of the complex into the hydrophobic micelle core, as in solvent extraction, whereas the latter is due mainly to the electrostatic affinity. Solubility of a hydrophilic complex such as the tartrate complex is especially low. If the partition of a neutral complex can be neglected compared to that of a free metal ion, KwT in eq 5 can be substituted by a&-,. On the basis of this consideration, retention times for Co(II), Zn(II), and Pb(I1) were calculated and compared with the experimental values, as shown in Table 111. Good correlation shows the validity of this approximation. Citrate complexes of metal ions investigated are mixtures of neutral and negatively charged species. Equation 5 is invalid in such cases, and a more complicated retention model should be considered where ion exclusion from both stationary and micelle phases toward negatively charged ions is evaluated. In contrast, tartrate complexes of a l l metal ions except Cu(I1) can be regarded as neutral complexes. Partition coefficients for Co(II), Pb(II), and Zn(I1) can be calculated from the retention data obtained with mobile phases containing 0.04 M tartaric acid. The addition of electrolytes in surfactant solutions brings many changes in the system. For example, the adsorbed amount of a surfactant on the stationary-phase surface increases with increasing the ionic strength (19,24, 25). This effect, called "salting out", often can be seen in chromatography where hydrophobic ion-interaction agents are utilized. The same phenomena can occur in the present system; the adsorbed amount of SDS is changed by the concentration of a complexing agent or a change in pH. An increase in the concentration of counterions reduces the retention of metal ions by the mass effect (19,26). These two effects in micelle exclusion chromatography were previously reported for the retention of inorganic anions. The addition of electrolytes also affects the micelle formation. cmc is reduced by an increase in salinity; cmc of SDS is 1.4 X M in 0.1 M NaCl solution (27). The size of the micelle also increases with increasing salt concentration. The values listed in Table I1 are subject to the above sources of uncertainty. The elution orders of metal ions obtained with micelle exclusion chromatography using mobile phases containing complexing agents agree with those observed in conventional
iNALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER
0.032AU
k
v
con
1, 1988
2119
with eluents containing either citric or tartaric acid as a complexing agent. These ions have been analyzed chromatographically with eluents containing a-hydroxyisobutyricacid (2). In the present study, the separation of six rare-earth-metal ions was investigated with a-hydroxyisobutyric acid as a complexing agent. The elution order is Yb(II1) < Ho(II1) < Dy(II1) < Tb(II1) < Pr(II1) < La(II1). Transition-metal ions investigated above, other than Cu(II), are eluted after La(II1) and are insufficiently separated under this condition. Figure 4 shows simultaneousseparation of rare-earth-metal and other heavy-metal cations with linear gradient elution. Eleven metal cations can be simultaneouslyseparated and detected. In this case, since the condition of the surface of the stationary phase is hardly changed during the gradient, the column can be quickly reconditioned by the starting eluent. In conclusion, micelle exclusion chromatography is applicable to the separation of heavy-metal ions. The addition of complexing agents improves the resolution and permits the estimation of the relevant partition coefficients. Further investigations on the applicability of this method should be carried out with different surfactants and analyte species.
ACKNOWLEDGMENT I thank Asahi Chemical Inc. Co., Ltd., for providing a separation column. Registry No. Cu, 7440-50-8;Fe, 7439-89-6;Mn, 7439-96-5; Co, 7440-48-4; Zn, 7440-66-6; Ni, 7440-02-0; Pb, 7439-92-1; Yb, 7440-64-4; Ho, 7440-60-0; Dy, 7429-91-6; Tb, 7440-27-9; Pr, 7440-10-0.
UlI
lh
F w e 4. Micelle exclusion chromatography of metal ions with gradient elution: mobile phase A, 0.025 M SDS and 0.08 M a4-tydroxyisobutyric acid (pH 4.0); B, 0.025 M SDS and 0.08 M tartaric acid (pH 4.3). Sample concentration is as follows: 5 X M for Pr(II1); 2.5 X lo4 M for the other rare-eartbmetal ions;2 X lo4 M for Mn(I1) and Zn(I1); 1 X lo4 M for the other metal ions. Other conditions are given in the text.
lo4
separation methods (2, 9). In micelle exclusion chromatography of inorganic anions, the addition of salts reversed the elution order and resulted in the same elution order as in conventional ion exchange. This is because the retention of solute ions becomes controlled by the partition to the stationary phase rather than by the partition t~ the micelle phase. In this case, the same mechanism can be considered. The addition of complexing agents results in an increase in the amount of SDS adsorbed on the stationary phase. Moreover, the concentration of free metal ions decreases, and this diminishes the importance of interactions between the micelles and the metal ions. The retention of the metal species thus becomes mainly controlled by the partition to the stationary phase. The addition of complexing agents reduces or eliminates the unusual selectivity otherwise observed in micelle exclusion chromatography but markedly improves the resolution. Further Application. Many metal cations other than those mentioned above also can be separated with micelle exclusion chromatography using eluents containing complexing agents. Rare-earth-metal ions cannot be separated
LITERATURE CITED (1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 4 7 , 180 1-1808. (2) Sevenich, G. J.; Fritz, J. S. Anal. Chem. 1983,55, 12-16. (3) Nordmeyer, F. R.; Hansen, L. D.; Eatough, D.J.; Rollins, D. K.; Lamb, J. D. Anal. Chem. 1980,52, 852-856. (4) DaSgUDta, P. K. Ion ChromatcgraDhy;Tarter, J. G.. Ed.; Dekker: New York, '1987; Chapter 6. (5) Ricci, G. R.; Shepard, L. S.; Colovos, G.; Hester, N. E. Anal. Chem. 1981,53,610-613. (6) Messman, J. D.; Rains, T. C. Anal. Chem. 1981, 53, 1632-1636. (7) Muelier, B. J.; Lovett, R. J. Anal. Chem. 1987,59, 1405-1409. (8) King, J. N.; Fritz, J. S. Anal. Chem. 1987,59, 703-708. (9) CassMy, R. M.; Elchuk, S. Anal. Chem. 1982,54, 1558-1563. (10) Casskly, R. M.; Elchuk, S. J . Chromatogr. Sci. 1981. 19, 503-507. (11) Wada, H.: Nezu, S.; Ozawa, T.; Nakaaawa, G. J . Chromatoar. 1984, 295, 413-421. Okada, T.; Kuwamoto, T. Anal. Chem. 1983,55, 1001-1004. Okada, T.; Kuwamoto, T. Anal. Chem. 1985,57. 829-833. Okada, T. J . Chromatogr. 1987,403. 27-33. Gjerde, D. T.; Fritz, J. S. Anal. Chem. 1981,53, 2324-2327. Iskandarani, 2.; Pietrzyk, D. J. Anal. Chem. 1982,5 4 , 2427-2431. Cassidy, R. M.; Elchuk, S. J . Chromatogr. Scl. 1983,2 7 , 454-459. Mullins. F. G. P.: Kirkbrlaht. G. F. Analyst (London) 1984. 709. 1217-1221. Okada, T. Anal. Chem. 1988v60. 1511-1516. Love, L. J. C.; Habata, J. G.; Dorsey, J. G. Anal. Chem. 1984,56, 1132A-1148A. Landy, J. S.; Dorsey, J. G. Anal. Chlm. Acta 1985, 778, 179-188. Armstrong, D. W.; Nome, F. Anal. Chem. 1981,53, 1662-1666. Herries, D. G.; Bishop, W.; Richards, F. M. J . fhys. Chem. 1984,68, 1842-1 853. Iskandarani, Z.;Pietrzyk, D. J. Anal. Chem. 1982,54, 1065-1071. Rosen. M. J. Surfactants and Interfacial Phenomena .~ : Wilev: New York, 1978. (26) Rigas, P. G.; Pietrzyk, D. J. Anal. Chem. 1985,58, 2226-2233. (27) Berthod, A.; Girard, I.; Gonnet, C. Anal. Chem. 1988, 5 8 , 1362-1367. I
I
.
RECEIVED for review February 16, 1988. Accepted June 3, 1988. This work was partly supported by a Grant-in-Aid for Scientific Research (Grant No. 6240321) from the Ministry of Education Science and Culture, Japan.
