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Anal. Chem. 1984, 56, 1572-1577
(the model assumes an infinite column capacity). Thus, the resulting peak tailing, peak bearding, peak splitting, and peak reversal are not caused by overloading the column; rather they are caused by interaction between the sample components. Finally, if in a given chromatographicsystem the true cause of peak tailing, peak bearding, peak splitting, or peak reversal is interaction between components,then iterative least-squares procedures could be used to estimate quantities related to the interaction parameters S A p and S B / A .
LITERATURE CITED (1) Stranahan, J. J.; Demlng, S. N. Anal. Chem. 1982, 5 4 , 1540-1546. (2) Berek, D.; Bleha, T.; Pevna, 2 . J. chfOm8tOgf. Scl. 1976, 14, 560-563. (3) Rellly, C. N.; Hlldebrand, G. P.; Ashley, J. W. Anal. Chem. 1982, 3 4 , 1198-1213. (4) Scott, R. P. W.; Scott, C. G.; Kucera, P. Anal. Chem. 1972, 4 4 , 100-104. (5) Slais, K.; Krejci, M. J. Chromatogr. 1974, 9 7 , 161-166. (6) Hendrix. D. L.;Lee, R. E., Jr.; Baust, J. G. J. Chromatogr. Scl. 1981, 210,45-53.
(7) Keller, R. A.; Glddlngs, J. C. J. Chromatogr. 1960, 3 , 205-220. (8) Tseng, P. K.; Rogers, L. B. J. Chromatogr. Scl. 1978, 16, 436-438. (9) Stranahan, J. J.; Demlng, S. N.; Sachok, 8. J. Chromatogr. 1980, 202, 233-237. (10) Campos, A.; Borque, L.; Flgueruelo, J. E. J. Chromatogr. 1977, 140, 219-227. (11) Katime, I.; Campos, A.; Rivera, J. M. T. fur. Polym. J. 1979, 15, 291-293. (12) Bdlingmeyer, B. A.; Deming, S. N.; Prlce, W. P., Jr.; Sachok, B.; Petrusek, M. J . Chromatogr. 1979, 786, 419-434. (13) McCormlck, R. M.; Karger, B. L. J. Chromatogr. 1980, 199, 259-273. (14) Parris, N. A. "Instrumental Liquid Chromatography": Elsevier: Amsterdam, 1976; Chapter 12. (15) Denkert, M.; Hackzell, L.; Schlll. G.; Sjogren, E. J. Chromatogr. 1981, 218, 31-43. (16) BOX,G. E. P.; Hunter, W. G.; Hunter, J. S. "Statistlcs for Experimenters. An Introduction to Design, Data Analysis, and Model Building"; Wiley: New York, 1978. (17) Helfferich, F. J. Chem. fduc. 1964, 4 1 , 410-413. (18) Helfferlch, F. Adv. Chem. 1988, No. 79, 30-43.
RECEIVED for review January 23,1984. Accepted March 27, 1984.
Retention of Inorganic and Organic Cations on a Poly(styrene-divinylbenzene) Adsorbent in the Presence of AI ky I suIfonate SaIts Ronald L. Smith and Donald J. Pietrzyk*
Department of Chemistry, T h e University of Iowa, Iowa City, Iowa 52242
Two major equiilbrla appear to be responsible for the retention of Inorganic and organlc analyte cations on PRP-1, a poly(styrene-divlnyibenzene) copolymerlc reversephase adsorbent, from a moblle phase containing an alkylsulfonate (RSOC) salt as an additive. One describes the retention of the RS03- salt onto the statlonary phase whlie the second describes an Ion exchange seiectivlty between the RS03- salt co-cation and the anaiyte cation. Mobile phase varlables which can be optimized when separating Inorganic or organlc analyte cation mlxtures are structure and concentration of the RS03- salt, mobile phase solvent composltlon, type and concentration of co-cation accompanying the RS0,- salt or salt used for ionic strength control, and mobile phase pH.
Using a hydrophobic ion as a mobile phase additive, usually with alkyl-modified silica as the stationary phase, has been a versatile, successful approach for the chromatographic separation of closely or poorly retained organic analyte ions of opposite charge (1-4). Manipulation of the mobile phase variables affecting the analyte ion-hydrophobic ion interaction enhances the analyte retention, often affects selectivity, and subsequently improves resolution. Ion pair formation, ion exchange, their combination, dynamic ion exchange, and ion interaction have been proposed to account for the enhanced retention; these retention models and the experimental results supporting them are discussed elsewhere (4-12). Although retention of the hydrophobic ion on the stationary phase and ion exchange between its counterion and the analyte ion were recognized, initial studies focused on the importance of ion pair formation between the analyte ion and hydrophobic ion
(3-5). However, recent studies with alkylsulfonate (RSO,-) salts (4,6-8) and tetraalkylammonium (R4N+)salts (4,9,10) have suggested that ion pair formation is not always a major factor in the enhanced retention. This appears to be the case when inorganic anions are the analyte and R4N+salts are the mobile phase additives since conductance studies of R4N+Xsalts, where X is an inorganic anion, indicate a very high percent dissociation for these salts in solution (9). In contrast, conductance data for solutions derived from the presence of two hydrophobic ions, for example RNH3+and R'SOc, were interpreted to indicate the absence of ion pairs (6) and the presence of ion pairs (11, 12). Recently, experimentaldata in our laboratory has suggested that retention of inorganic analyte anions (10) on to PRP-1, a poly(styrene-divinylbenzene)(PSDVB) nonpolar adsorbent, from a mobile phase containing a R4N+salt as an additive, an aqueous or mixed solvent, and a buffer and/or inert salt to provide pH and/or ionic strength control is largely influenced by two equilibria. One describes the retention of the R4N+salt and its co-anion C-,on the stationary phase while the second describes an ion exchange selectivity that occurs between the analyte anion, X-, and the co-anion, C-, or any other co-anion that may be in the mobile phase. Experiments also appear to indicate that these two equilibria are the major ones when simple organic anions are the analytes (9). Control of these equilibria via adjustment of the mobile phase parameters allows one to separate inorganic and/or organic analyte anions. PRP-1, rather than the alkyl-modifiedsilica, was used because (1)PRP-1 is stable in a very basic mobile phase and thus these conditions can be used to convert weak organic acid analytes into their anionic form, (2) the PRP-1 surface is less complex because of the presence of free silanol
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sites in alkyl-modified silica, (3) recent studies had indicated that charged analytes are retained on PRP-1 and related adsorbents as a double layer (13, 14), and (4) retention of hydrophobic ions is high on PRP-1. These advantages should also be realized when using alkylsulfonate (RS03-) salts as mobile phase additives. Since PRP-1 is also stable in a very acidic mobile phase, these conditions can be used to ensure that weak basic type analytes are in their cation form. RS03- salts have been successfully used as mobile phase additives for the separation of many classes of organic bases (1-4). Recently, RSO, salts were used as additives to achieve separation of several metal ions on an alkyl-modified silica column; however, little was done to quantitatively establish the retention model (15,16) and/or identify the key, operator-controlled mobile phase variables. In this report, data are provided which demonstrate that enhanced retention of inorganic analyte cations on PRP-1 from a RS03- salt mobile phase follows a retention model similar to that observed when using R4N+salts as additives for the separation of analyte anions (9, 10). Furthermore, control of the mobile phase variables provides a general procedure for separating inorganic cationic analytes on reverse stationary phases and represents an alternate procedure to ion chromatography (17).
EXPERIMENTAL SECTION Reagents. Organic analytes, pentyl- (C5S03H),heptyl- (C7S03H), and octylsulfonic acids (C8S03H),were obtained from Eastman Kodak Chemical Co., Aldrich Chemical Co., and MCB. Different metal ion forms of the RS03- salts were prepared by titration with metal hydroxide or by cation exchange on Dowex 50W-X8. Inorganic salts used as analytes and buffers and for ionic strength control were analytical reagent grade. Organic solvents were LC quality from MCB while distilled water was purified via passage through a Barnstead water purification unit. Instrumentation. A Waters Model 202 and a Beckman Model 334 LC equipped with either a Waters 254 nm, a Tracor Model 970 variable wavelength, or a Wescan Model 213A conductivity detector were used. The PRP-1 column (4.1 mm i.d. X 150 mm), which is a 10 pm, spherical, macroporous, high surface area, poly(styrene-divinylbenzene)copolymeric, nonpolar, adsorbent, was obtained prepacked from Hamilton Co. or slurry packed with 10-pm bulk form particles. Typical column efficiencies for a benzene-phenol mixture and 1:9 H,0:CH3CH mobile phase are over 18000 plates/m. Procedures. Column conditioning procedures using RS03salts are similar to those described for R4N+salt mobile phases (9,lO). Analyte solutions (about 1-5 mg/mL) were prepared by dissolving weighed quantities in water, stored in sealed vials, and refrigerated when not in use. Sample injections were usually 2-10 pL. Flow rates were usually 1.0 mL/min and column inlet pressures were about 500 to 1000 psi. All mixed solvents are percent by volume. Ionic strength, where noted, was maintained by adding known amounts of electrolyte and/or buffer salts. Temperature was controlled at 25 "C by a water jacket. Breakthrough volumes were determined by the procedure outlined earlier (9, 10). RESULTS AND DISCUSSION The experimental results obtained in this and related studies (9,ZO) suggest that two major equilibria will influence the retention of an analyte cation, X+, on PRP-1 adsorbent, A, from a mobile phase containing a RS03-C+ salt mobile phase additive, ST+,where C+ is an inorganic cation, an added salt and/or buffer for pH control, and an aqueous mobile phase or one containing small additions of organic modifier. The first, shown in eq 1,defines the retention of the RS03salt on PRP-1 while the second, shown in eq 2, defines an ion exchange like selectivity between C+ and X+. The correA
+ S- + C+ + A-+S-C+
(1)
A...S-C+ + X+ A...S-X+ + C+ (2) sponding equilibrium constants for eq 1and 2 are defined in
1573
and K(s:)8are equilibrium constants eq 3 and 4 where for the retention and exchange processes, respectively.
(4) Several other potential equilibria that might also influence retention, and are briefly described in the following, are either not present or appear to be of a minimal effect. Both experimental results and control of the experimental conditions contribute to this conclusion. (1) Retention of the analyte itself onto the PRP-1 is not a contributing factor. This is particularly true for the inorganic analyte cations since these were shown to have zero retention in the absence of the RSOC salt. For organic analyte cations studied their retention in the absence of the RSO, salt depended on the organic modifier concentration in the mobile phase; in these experiments this was adjusted so that organic analyte retention would be as low as possible while still providing a favorable condition for the retention of the RS03- salt. (2) The formation of ion pairs between RSO, and inorganic cations, such as the alkali metal ions, is assumed to be negligible. Mukerjee et al. (18)has shown through conductance and related studies that a monomer-dimer equilibrium occurs between dodecyl sulfate ions and that their Na+ and Li+ salts are completely dissociated. In our conductance studies (9) we have found tetrapentylammonium halide salts to be over 90% dissociated. Conductance measurements have been used in several chromatographic studies to evaluate whether association occurs between hydrophobic ions and organic analyte ions of opposite charge (6,11,12). Although interpretation of these data may differ, it is conclusive that association will become more important as the organic analyte ion becomes more hydrophobic. For example, Mukerjee et al. (19) detected weak association for tetraalkyl(methy1, ethyl, and propy1)ammonium dodecyl sulfate salts. A significant association (ion pair) constant of about 500 M-l was reported for octylammonium octylsulfonate while for catecholammonium (much less hydrophobic than octylammonium cation) octylsulfonate salts modest constants of about 18 M-' were reported (see ref 11 for a discussion of these data and a partial review of other literature conductance data). Thus, it can be concluded that an equilibrium describing ion pair formation is perhaps only significant when considering more hydrophobic organic anions as analytes. Also, reducing the alkyl chain length within the RS03- salt (C, to C4 was used in this study) and maintaining modest amounts of organic solvent in the mobile phase will further decrease the effect of ion pair formation. (3) The combined effect of modest alkyl chain lengths in the RSO; salt and low RS03- salt concentration in the mobile phase indicates that the experimental conditions were well below critical micelle formation. Thus, equilibria describing micelle formation are negligible. (4) Retention of buffer components and salts on PRP-1 was shown to be absent. (5) An aqueous mobile phase or one containing a small amount of organic modifier is used which also favors dissociation. A series of chromatographic experiments, which are discussed in detail elsewhere (ZO),demonstrated that the RS03salts are retained on PRP-1. These results, see Figure 1,and the parameters which influence the retention are summarized in the following: (1) When RS03-Na+ salts are used as the analytes and organic modifier-H20 mixtures are used as the mobile phase, retention on PRP-1 decreases as the mobile phase CH3OH or CH3CN concentration increases; only data for CH30H-H20 mixtures are shown in Figure 1A. If CH&N is used, RS03- salt retention is sharply reduced. For example,
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
0.4
0.3
I/ k' 0.2
0.1
0
in m o b k
phasa, M
Figure 1. Retention of alkyl sulfonate salts (A) and the generation of apparent ion exchange sites (6)on PRP-1: (A) A 4.1 mm X 150 mm, 10-pm PRP-1 column using RSO, salts as analytes, a 1.0 mL/min flow rate, and conductivity detection; anaryte retention is shown as a function of MeOH:H,O mobile phase composltion in the absence of added salts and as a function of ionic strength (NaCI) in a 1:9 CH,CN:H,O solvent. (6) Same as A except C7S0-Li+ is varied in 2 5 9 7 . 5 CH,CN:H,O (a) M C7SO3-Li'is constant in varying CH,OH:H,O (b). or 2.5 X
in 15 organic solvent:H20 the k' for CSSO3-Na+retention in MeOH:H,O is almost five times that in CH3CN:H20. (2) Retention of the RS03- salts on PRP-1 is dependent on the hydrophobicity of the alkyl group; the retention at a given organic modifier-H20 ratio in Figure 1A follows the order C8S03- > C7S03-> C5S03-> C4S03-. Furthermore, the data indicate that log k ' for the retention of analytical samples of the RS03-salts increases linearly as the carbon number in the linear alkylsulfonate increases. For example, in 5:95 CH3CNH20the plot of log k'vs. C number yields the line log k' = 0.417 (C number) -2.192 with a correlation of 0.9982. (3) Increasing the ionic strength increases retention of the RS03- salts on PRP-1in a regular manner. According to the Stern-Guoy-Chapman theory of electrical double-layer adsorption a plot of llk'for the retention of the RS03-salt vs. 1/p1I2, where p is the ionic strength and C1and Cz are constants,should yield a linear relationship according to eq 5 (13).
1/12' = 1/c1
+ C2/p1/2
(5)
Figure 1A illustrates that RS03- salt retention on PRP-1 is consistent with this prediction; although several RS03- salts were studied, only data for C7S03-Na+and CsS03-Na+are shown in Figure 1A. In these studies NaCl was added to the mobile phase to vary the ionic strength over the range of 5.0 x to 1.0 X M. Thus, it can be concluded that RSO15 at 5% MeOH to about 5 at 20% MeOH (see Figure 1A). The effect of CH3CN compared to MeOH is much greater. Ionic strength influences ion exchange capacity, and although these data are not shown here (20), it was found that the capacity increased as ionic strength increased. For example, for a mobile phase containing 1.00 x loT3M C7S03-Li+,2.5:97.5 CH3CN:H20,0.001 M LiCl the apparent ion exchange capacity corresponds to about 10.5 pmol of C7S03-Li+sorbed per column which is about a 25% increase over the capacity if LiCl is omitted from the mobile phase. This effect of ionic strength is also consistent with the ionic strength studies shown in Figure 1A. The surface coverage for the adsorption isotherm shown in Figure 1B can be calculated since the PRP-1 surface area (415 m2/g) and PRP-1column weight (0.8 g) are known. Thus, from 0.0005 to 0.01 M C7SO
