Tailoring Elution of Tetraalkylammonium Ions. Ideal Electrostatic

Dec 13, 2006 - Although ion exchange is often depicted as a process driven by electrostatic forces, ionic solvation or hydro- phobic forces contribute...
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Anal. Chem. 2007, 79, 769-772

Tailoring Elution of Tetraalkylammonium Ions. Ideal Electrostatic Selectivity Elution Order on a Polymeric Ion Exchanger Bingcheng Yang, Masaki Takeuchi, and Purnendu K. Dasgupta*

Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061 Tomonari Umemura

Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Yuji Ueki and Kin-Ichi Tsunoda

Department of Chemistry, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan

Although ion exchange is often depicted as a process driven by electrostatic forces, ionic solvation or hydrophobic forces contribute greatly to ion exchange selectivity and is often the dominant factor. On a variety of commercial anion exchange columns, monovalent ClO4elutes after doubly charged SO42- and even triply charged PO43-. For identically charged alkali metal ions, electrostatic charge densities based on crystal radii would suggest Li+ to be the most strongly retained on a cation exchanger. In practice, it is typically the least strongly held cation on most cation exchangers, because of its very high hydration energy and with most eluents its capacity factor approaches zero. Even when the ion is very poorly solvated, as with tetraalkylammonium (NR4+) cations, there has never been a report on a polymeric ion exchanger of an ideal electrostatic selectivity order where NR4+ cations elute in their increasing charge density order: R ) n-butyl first, followed by n-propyl, ethyl, and last, methyl. We show that this selectivity order is easily achieved 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+ (and other alkali cations) elutes after NMe4+. Ion exchange processes are fundamental in nature. In analytical chemistry, ion exchange chromatography (IC) and ion-selective potentiometry are at the forefront of techniques that depend on different selectivities that ion exchangers display to different ions. A large number of ion exchanger stationary phases with different selectivities have been developed over the years. From the nature of the ion exchange group, to variations in the substituents around the functional group, to the nature of the substrate to which the functional group is bonded and even the nature of groups in the * To whom correspondence should be addressed. E-mail: [email protected]. 10.1021/ac061648i CCC: $37.00 Published on Web 12/13/2006

© 2007 American Chemical Society

vicinity of the ion exchange groups can all dramatically affect selectivity. A key to efficient practice of the current art of suppressed conductometric IC with hydroxide-based eluents are “hydroxide-selective” columns in which hydroxide selectivity is brought about by alkanolamine rather than amine substituents in the traditional quaternary ammonium functional groups.1 Many anion exchangers exhibit a high affinity for perchlorate. But increasing that affinity still further by deliberate design2 may play a key role in dealing with trace perchlorate contamination in water. More recently, monolithic columns have been introduced as attractive media for separations;3 polymeric monolithic IC columns in the capillary scale were introduced by some of the present authors.4 Following inner surface treatment with (3-methacryloxypropyl)trimethoxysilane to promote adhesion, glycidyl methacrylate and ethylene dimethacrylate were polymerized in the capillary by radical-initiated polymerization. Cation exchanger sulfonate functionalities were created by in situ ring opening of the epoxide linkages with Na2SO3; the duration of the sulfitolysis controlled ion exchange capacities. Subsequently, others have described similar monolithic columns that were converted to anion exchangers by subsequent passage of anion exchange latex.5 There are several recent models that address ion exchange behavior and selectivity.6-7 Unfortunately, the selectivity behavior of diverse exchangers is still difficult to predict a priori. For example, the retention order of alkali metals on a typical sulfonic acid-bearing cation exchanger follows the order Li+ < Na+ < K+ < Rb+ < Cs+, but there are early reports that this order can be changed, even fully reversed, at very high cross-linking levels.9 (1) Jackson, P. E.; Pohl, C. A. Trends Anal. Chem. 1997, 16, 393-400. (2) Brown, G. M.; Bonnesen, P. V.; Moyer, B. A.; Gu, B.; Alexandratos, S. D.; Patel, V.; Ober, R. Environ. Sci. Res. 2000, 57, 155-164. (3) Svec, F.; Fre´chet, J. M. J. Anal. Chem. 1992, 64, 820-822. (4) Ueki, Y.; Umemura, T.; Li, J. X.; Odake, T; Tsunoda, K. Anal. Chem. 2004, 76, 7007-7012. (5) Zakaria, P.; Hutchinson, J. P.; Avdalovic, N.; Liu, Y.; Haddad, P. R. Anal. Chem. 2005, 77, 417-423. (6) Ståhlberg, J. Anal. Chem. 1994, 66, 440-449. (7) Okada, T. Anal. Chem. 1998, 70, 1692-1700. (8) Ståhlberg, J. J. Chromatogr., A 1999, 855, 3-55.

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At least for alkali metals, the retention behavior of typical carboxylic acid exchangers is the same as that for typical sulfonic acid exchangers mentioned above. However, carboxylate resins with a retention order Li+ > Na+ > K+ have been described.10 The ion exchanger matrix has a profound effect. Variation in the aluminum content in aluminosilicate glass can reverse the selectivity order for alkali metal ions.11 Alumina behaves as either a cation or anion exchanger. Diametrically opposite to the anion exchange selectivity of polymeric resins bearing quaternary ammonium groups, the retention order on alumina is ClO4- < I< Br- < Cl- < F-.12 Although methacrylate-based latexes and anion exchangers have found extensive use in commercially available IC stationary phases, acrylate-based sulfonic acid functionality cation exchangers are relatively new4,5 and little is known about their cation exchange selectivities beyond those for alkali and alkaline earth metals. Capillary columns are particularly amenable to the contactless conductivity detection approach;4 we were exploring such columns with nonsuppressed conductivity detection for cation separations. To attain good limits of detection in a nonsuppressed system, the eluent counterion, which behaves largely as a spectator, should have a low ionic mobility and the eluent ion should have an ionic mobility as different as possible from the target analyte ions. Methanesulfonic acid or HClO4 are thus good choices because CH3SO3- or ClO4- has low mobility and H+ has a far greater mobility than any other ion. However, if the ion exchange sites have a weak affinity for H+, eluent concentrations required to perform reasonable chromatography on a column with significant capacity may result in background conductance values that are far too high for the eluent system to be practical. This indeed proved to be the case for our columns under investigation. We therefore sought alternatives. At the other extreme of eluent ion mobility from H+ are bulky tetraalkylammonium cations that have mobilities well below typical target analyte cations (Na+, K+, etc.), and we chose therefore to explore the use of NR4ClO4 salts as eluents in this system. The relative eluent strengths that we observed as R was varied were completely unanticipated. We provide a brief account of the highly unusual selectivity of this stationary phase in this note. EXPERIMENTAL SECTION Apparatus. The capillary IC system was similar to that previously described.13 Briefly, the pumping system is based on a personal computer-controlled, 48 000-step precision dispenser (model V6P, P/N 24520) coupled to a high-pressure three-way valve header (P/N 26324), and a high-pressure, 1-mL syringe (P/N 23994, all from Kloehn Inc., Reno, NV). An internal loop injector (200 nL) was used for sample injection (P/N CI4W.2, VICI). A flow rate of 3 µL/min was used throughout. The capillary column used was a monolithic methacrylate-based sulfonate functionality cation exchanger (15 cm long, 250 µm i.d.; 171 µequiv/mL) prepared as previously described.4 The column was butt-joined (9) Bregman, J. I. Ann. N. Y. Acad. Sci. 1953, 57, 125-143. (10) Reichenberg, D. In Ion Exchange; Marinsky, J. A., Ed.; Marcel Dekker: New York, 1966; Vol. 1, pp 227-276. (11) Eisenmann, G. In Advances in Analytical Chemistry and Instrumentation; Reilley, C. N., Ed.; Wiley-Interscience: New York, 1965; Vol. 4. (12) Schmitt, G. L.; Pietrzyk, D. J. Anal. Chem. 1985, 57, 2247-2253. (13) Boring, C. B.; Dasgupta, P. K.; Sjo ¨gren, A. J. Chromatogr., A 1998, 804, 45-54.

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to the detector tube (0.101 mm i.d. × 0.368 mm o.d.) on which contactless electrodes consisting of two tubular stainless steel segments (0.41 mm i.d. × 0.66 mm o.d. × 15 mm long) were affixed with epoxy adhesive with a 1-mm interelectrode gap. Rather than measuring conductance, we measured capacitance; under our measurement conditions and range, capacitance was linearly related to conductance. Details of this will be presented elsewhere. All experiments were carried out at laboratory temperature of 22 ( 1 °C. All chemicals used were reagent grade, and all solutions were prepared with 18.0 MΩ‚cm deionized water. RESULTS AND DISCUSSION Mixed Exchange Sites. As previously noted,4 this stationary phase exhibits normal retention order for alkali metals and alkaline earth metals (e.g., Cs+ > K+ > Na+ > Li+). Even though not deliberately incorporated, polymeric columns always contain COOH sites14 and the present column is no exception. With HCl as the injected analyte and 6 mM NaClO4 as the eluent, the retention time for H+ decreased from 34.3 to 27.6 to 21.7 min as the pH of the eluent was adjusted from 6.4 to 3.9 to 3.0. Figure S1 (in Supporting Information) will indicate that relative to a tailing peak arising from mixed-mode retention (as well as local perturbation of pH by the sample) at higher eluent pH, the analyte response becomes Gaussian at pH 3 when COOH groups are no longer ionized. The relative retention times also indicate that overall the more numerous SO3H groups are the primary contributors to retention. Retention Order of Tetraalkylammonium Cations. Ionic retention is a function of both electrostatic and hydrophobic factors. The latter is controlled by two opposing forces: the preference of the cation to stay solvated in the eluent phase (this is directly related to the hydration/solvation energy of the ion) and true attractive interactions with the stationary-phase substrate (this would include, for example, π-π interactions between an aromatic cation and a poly(styrene-divinylbenzene) (PSDVB) stationary phase). Tetraalkylammonium cations are very poorly hydrated, and their effective size in solution increases with their crystallographic size.15 They present to the water structure a hydrocarbon-like, hydrophobic exterior and so tend to be pushed by the water structures into the less-structured resin phase, the more so the larger the ion. The hydrophobic interaction is thus essentially the attractive interaction with the stationary-phase matrix or the repulsive action by an aqueous eluent, depending on one’s point of view. If such hydrophobic interactions were absent, based on electrostatic considerations alone, retention should increase with decreasing size of the alkyl group moiety as the surface charge density increases. The best chance of achieving such an ideal electrostatic selectivity is obviously when hydrophobic interactions with the substrate can be reduced to a minimum. Larson and Pfeiffer16 were able to show that, on a silicabased cation exchanger, the retention order NMe4+ > NEt4+ > NPr4+ > NBu4+ is attained at high hydroorganic modifier (14) DeBorba, B. M.; Kinchin, C. M.; Sherman, D.; Cook, T. K.; Dasgupta, P. K.; Srinivasan, K.; Pohl, C. A. Anal. Chem. 2000, 72, 96-100. (15) Diamond, R. M.; Whitney, D. C. Resin selectivity in Dilute to Concentrated Aqueous solutions. In Ion Exchange; Marinsky, J. A., Ed.; Marcel Dekker: New York, 1966; Vol. 1, pp 277-351. (16) Larson, J. R.; Pfeiffer, C. D. Anal. Chem. 1983, 55, 393-396.

concentrations (70% acetonitrile (ACN) or methanol). Even on silica, a very polar substrate, this order of ideal electrostatic selectivity was lost by the time the hydroorganic modifier content was reduced to 50%. By the time the ACN content was reduced to 30%, the elution order became NPr4+ ∼ NEt4+ > NBu4+ > NMe4+,17 indicating the strong presence of mixed-mode retention. In any case, because of pH limitations, silica-based exchangers have no practical use in present day IC. On a polymeric PSDVBbased cation exchanger, n-π electronic interactions would be expected to contribute to increased retention for NR4+ cations of increasing size. It has been reported that at 60-70% ACN the retention on a sulfonated PSDVB column follows the order NMe4+ > NEt4+ > NBu4+ > NPr4+.18 (Elsewhere, the lead author has reported a different retention order of the same ions on the same column with the same eluent,17 so the system is likely susceptible to minor perturbations). Notably, all of these previous studies have been conducted with significant concentrations of other large quaternary ammonium cations (typically benzyltrimethylammonium) as the eluent ion. The use of such ions as the eluent leads to strong possibilities of interactions of the eluent ion bound on the stationary phase with the analyte, rather than interactions being limited to that with the stationary phase itself. In other words, the behavior reported in these studies may not be reflective of just the analyte-stationary-phase interactions. In the present work, we studied retention behavior with only alkali metals as eluent ions, with and without added ACN. Increasing [ACN] decreases any hydrophobic interactions. Similarly, one way to look at the effect of increasing the concentration of the eluent ion (when it is a hard cation like an alkali metal ion) is that overall retention from electrostatic interaction decreases because, on the basis of a dynamic equilibrium, less ion exchange sites are now available to interact with the analyte ion. The organic modifier concentration and the eluent ion concentration can be independently varied. They can also be varied in tandem in a binary eluent mixture mode: while one is decreased, the other is increased. Figure 1a shows a log-log plot of the adjusted retention time (tR′, the void time was 1.2-1.3 min in all experiments) versus the eluent ion (K+) concentration at a constant ACN content of 30% v/v. In all cases, an excellent linear relationship is exhibited (r2 ranged from 0.9919 to 0.9992; one aberrant point for NBu4+ is excluded) with a slope that is indistinguishable from unity; this linear behavior with unity slope is expected from classical ion exchange theory where the analyte and the eluent ion both have the same charge magnitude.19 Moreover, these plots have nearly the same slope, suggesting that the retention order will remain the same (NMe4+ > NEt4+ > NPr4+ > NBu4+) within a reasonable range of eluent concentration around the range studied. In comparison, Figure 1b shows a plot of log tR′ (the eluent containing a constant concentration of 6 mM KClO4) with volume percent ACN plotted on the abscissa in a log scale on a similar second column. Interestingly, the log tR′ - log [ACN] plots are also highly linear at high [ACN] values, but predictably the larger the cation, the lower limit of linearity prevails at increasingly higher [ACN] concentrations. Whereas linearity (r2 >0.95) of the log tR′ - log (17) Walker, T. A.; Akbari, N.; Ho, T. V. J. Liq. Chromatogr. 1991, 14, 619641. (18) Walker, T. A. J. Liq. Chromatogr. 1988, 11, 1513-1530. (19) Small, H. Ion Chromatography; Plenum Press: New York, 1989.

Figure 1. (a) Plot of log tR′ vs log [KClO4] for four tetraalkylammonium ion analytes. Eluent contains 30% by volume acetonitrile. (b) Plot of log tR′ vs log [acetonitrile] for four tetraalkylammonium ion analytes. Eluent contains 6 mM KClO4. See text for details. All figures: 200-nL injection, Eluent flow rate 3 µL/min, contactless capacitance detection.

[ACN] relationship is maintained at [ACN] g 26% for NMe4+ and NEt4+, [ACN] g 30% is required for NPr4+ and NBu4+. At lower [ACN], the role of the ionic eluent, KClO4, becomes dominant and the retention is much lower than what would be predicted based on the ACN concentration alone by extrapolating the data at high [ACN]. Although the retention data at zero [ACN] cannot of course be plotted on a logarithmic abscissa, these are not remarkably different from the data for the lowest [ACN]; tR′ with a purely aqueous eluent for NMe4+, NEt4+, NPr4+, and NBu4+ are 14.0, 13.5, 11.9, and 11.1 min, respectively. It is also notable that, unlike the case for varying [K+] in the eluent (Figure 1a), the slopes of the lines are not parallel; these consistently increase from Analytical Chemistry, Vol. 79, No. 2, January 15, 2007

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Figure 2. Change in adjusted retention time as a function of eluent composition for four tetraalkylammonium ions. A gradient is run between aqueous 6 mM KCl and pure acetonitrile. Logarithmic axes, the abscissa is log-scaled for [KClO4].

NMe4+ to NBu4+, predictably indicating the increasing importance of hydrophobic interactions along the series. At the specific eluent ion content of 6 mM KClO4, the ideal electrostatic selectivity order is maintained at [ACN] g ∼26%, but below that [ACN], hydrophobic interactions become large enough to affect the selectivity order. This “critical” [ACN] value will depend on the [KClO4]. We can predict that the lower the [KClO4], the lower will be the critical [ACN] above which the ideal selectivity order will be maintained. Very similar behavior was observed with NaClO4, and CsCl as eluents instead of KClO4, establishing that K+ does not play any unique role as the eluent ion. Based on the observations and interpretations above, it should be possible to balance the [ACN] and cationic eluent concentration such that if a gradient is run between the two eluent components they nearly exactly balance each other out for the opposing electrostatic and hydrophobic forces acting on a particular eluent ion. Indeed, Figure 2 shows that any binary eluent composition consisting of ACN and 6 mM KClO4 as the pure eluent components results in a constant retention of NMe4+ while the retention of all the other analyte ions become eluent composition dependent. Figure 3 shows a chromatogram with 6 mM KClO4 and 38% ACN as eluent; NBu4+ virtually elutes in the void volume, and the other ions are well separated in the ideal selectivity order. What is quite remarkable is that, under these same conditions, Li+ elutes well after NMe4+ and Na+ and other alkali metals in

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Figure 3. Chromatogram of four tetraalkylammonium ions using 6 mM KClO4/ 38.1% ACN eluent. 4 nmol each of NMe4+ and NPr4+, and 2 nmol each of NEt4+ and NBu4+.

the usual order after that. To our knowledge, this is an extraordinarily high capacity factor for Li+ relative to what has been reported for other stationary phases in the literature. The unusual retention selectivity of the present stationary phase suggests that the phase is very polar and significant amounts of water likely partition into the phase making it thermodynamically favorable for ions like Li+, with very high hydration energies, to partition into the ion exchanger phase. It is interesting that this polymeric phase promotes ideal electrostatic selectivity even more than silica-based ion exchangers. However, the behavior of silica-based exchangers may be influenced by the hydrocarbon chains used to attach the sulfonic acid groups. ACKNOWLEDGMENT This research was supported by National Science Foundation grant CHE-0518652. We thank Christopher A. Pohl, Dionex Corporation, for much insight. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review November 6, 2006. AC061648I

September

1,

2006.

Accepted