1574
Anal. Chem. 1986, 58,1574-1575
Electromodulated Ion Exchange Chromatography Sir: Electroactive ionomers are polymers that contain both electroactive and ion exchange functionalities (1-3). We recently described the synthesis and the chemical and electrochemical properties of a series of electroactive ionomers composed of styrene, styrenesulfonate (the ion exchange functionality), and vinylferrocene (the electroactive functionality); the electrochemical characteristics were investigated by using electroactive ionomer-based chemically modified electrodes in conjunction with aqueous supporting electrolyte solutions (1). When this polymer is in the reduced (i.e., neutral ferrocene) form, mobile cations are incorporated into the film to charge compensate the covalently attached sulfonate groups. We have shown, however, that when the ferrocene (Fc) groups are electrochemically oxidized, the Fc' sites created can function as the counterion for the -SO3- groups, and the mobile cations are expelled from the polymer film (1-3). (Note that quantitative expulsion will only occur when the mole percent of vinylferrocene is greater than the mole percent of styrenesulfonate.) Thus, electroactive ionomers are electroreleasing ( 4 ) materials. A variety of applications for such polymers seem likely, including use in drug delivery systems and as membranes in electrochemical sensors (2-4). In addition, these polymers may allow for the development of new ion-exchange-based separation techniques in which the capacity factor for a solute counterion is altered via control of the potential of a flowthrough polymer modified electrode column (Figure 1). We have initiated a research program aimed at assessing the feasibility and advantages of using such electroactive ionomer-based columns/electrodes in separations schemes. We have constructed prototype columns and have demonstrated that the retention volume for an eluting counterion can be drastically and reproducibly altered on these columns. We report preliminary results of these studies in this correspondence.
ca. &fold mol % excess of vinylferrocene) was prepared by use of the techniques described previously (2). A pellicular chromatographic column/flow-through working electrode (Figure 1) was prepared using this polymer. An anion exchange polymer tube (3.18 mm o.d., RAI Research Corp.) was inserted into a custom-made polyethylene cap (Figure l),and a coiled Pt wire was inserted down the length of the tube. The polymer tube was then dry packed with ca. 0.5 g of carbon particles (Carbopack C, Supelco Co., 40 pm mean particle diameter). The particles were coated by percolating a 0.05% (w/v) solution of the polymer in benzene through the column. The first volume incremenh of this, initially pale yellow, solution to emerge from the column were almost totdy decolored, indicating that the polymer did, indeed, adhere to the carbon. After evaporation of the benzene, NaOH (in 70:30 HzOCH3CN)was used to convert the anhydride to the free carboxylate. A Pt counter electrode was coiled around the anion exchange tube, and the external glass tube was filled with supporting electrolyte (0.005 M NaC104 in 70:30 H20-CH3CN). A Ag wire quasi-reference electrode was inserted into this outer electrolyte solution. A conventional three-electrode potentiostat was used to oxidize and reduce the column.
RESULTS AND DISCUSSION The column configuration shown in Figure 1was adopted because it minimizes solution resistance problems in the column; since the column material is an anion exchange polymer tube, analyte cations cannot diffuse radially out of the column into the outer electrolyte phase. Note that the carbon particles were first packed into the column and then coated. This order was used to ensure that conductive carbon-carbon contacts were made between the particles; if precoated particles are packed into the column, these particles are insulated from each other by their polymer films and highly resistive columns are obtained. The organic dication methylviologen (MV2+)was used to investigate the ion exchange characteristics of the electroactive ionomer columns. With the polymer in the reduced (Fc) form (and unpotentiostated), 10 p L of M MV2+was deposited
EXPERIMENTAL SECTION A vinylferrocene/maleic anhydride copolymer (containing a
A
It-
Working electrode contact (see Figure 16)
B
Working electrode contact Pt coil extends
Ag wire quasi-referenceelectrode encased in a perforated le f I o n t u be
Glass t u b e
- P I coil counter electrode
the entire length of tube
- Polymer-coated
A
carbon particles
- Anion exchange
Electrolyte solution -
membrane t u b e packed with polymer-coated carbon particles (see Figure 16)
+Teflon
Anion exchange membrane tube (see Figure 1 A )
cap
Porous f r i t Flgure 1. Schematic representations of the electroactive ionomer-based working electrode/llquid chromatographic column: (A) details of contents of outer tube, (B) details of contents of inner, anion exchange membrane tube. 0003-2700/86/0358-1574$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
A
P O L Y M E R :+ C H ~ C H,$H-$H+
I
c
Fc
c
0 0-
0
No*
No'
0-
ELUENT: 0 . 0 0 5 M NoC1O4 ( 7 0 : 3 0 H 2 0 : C H 3 C N )
0.2.
PI
C 0
n
g
0.1-
U
1575
polymer carboxylate sites, a large volume of electrolyte solution is required to elute this cation from the column. The spectrophotometric analysis showed that all of the MV2+added to the column was eluted in the 9th through 13th volume increments. The potential of the working electrode column was then held for 30 min at +0.7 V, where the Fc sites are oxidized to Fc+. With the column potentiostated at +0.7 V, 10 pL of 0.01 M MV2+was deposited at the head of the column and eluted with supporting electrolyte. Because the ion exchange sites are now charge compensated by the Fc+ groups, the affinity of the column for MV2+should be drastically reduced. The chromatogram (Figure 2B) shows that this is, indeed, the case in that M V + is now eluted in the first (1mL) volume fraction. A comparison of the chromatograms in parts A and B of Figure 2 clearly shows the radical difference in retention characteristics of the reduced and oxidized forms of the column. Furthermore, because the electrochemical step is reversible (I),the column can be reversibly switched between the cation-retaining (Figure 2A) and cation-repelling (Figure 2B) states.
CONCLUSIONS
4
Volume E l u e n t , m L P O L Y M E R : f-CH2-$H,YH-$H+
E L U E N T : 0 . 0 0 5 M NaC104 ( 7 0 : 3 0 H 2 0 : C H 3 C N i
These studies show that the ion exchange characteristics of an electroactive ionomer-based liquid chromatographic column can be predictably and reversibly modulated via electrochemical control of the potential of this column. Thus, the feasibility of a new separation technique, which we call electromodulated ion exchange chromatography, has been demonstrated. Clearly, further fundamental investigation will be required before the practical utility of this new technique can be assessed. We are currently attempting to optimize the electrochemical response time of this column so that fast switching between the cation-retaining and cation-expelling states can be achieved. We are also exploring the effect of partial and total oxidation of the column on the separation factors (5) for various pairs of eluting cations. Finally, an attempt to use the potential of a flow-through electrode/ column to change the extent of adsorption on the column has been described previously (6). Because our modified electrode concept allows for more direct and unequivocal control over the retention process, we believe that this modified electrode approach is superior.
ACKNOWLEDGMENT We thank M. W. Espenscheid for assistance with the polymer synthese!.
Registry No. MV2+,4685-14-7. LITERATURE CITED
a
4 Volume
Eluent, mL
Figure 2. Chromatograms for methylvlologen dication on the electroactive lonomer-based column: (A) reduced form of the column, (B) oxidized form of the column.
at the head of the column and eluted with supporting electrolyte solution. The eluent was collected in 1-mL fractions, and each fraction was assayed spectrophotometrically for MV2+(A, = 257 nm). (The 1-mL fractions were diluted with 3 mL of supporting electrolyte prior to UV analysis.) The chromatogram obtained is shown in Figure 2A. Because of the electrostatic interaction between the MV2+ and the
(1) Espenscheid, M. W.; Martin, C. R. J . Elecfroanal. Chem. 1985, 188, 73. (2) Espenscheid, M. W., unpublished results, Texas A&M Unlverslty, 1965. (3) Espenscheid, M. W.; Ghatak-Roy, A. R.; Moore, R. E., 111; Penner, R. M.; Szentlrmay, M. N.; Martin, C. R. J . Chem. Soc., Faraday Trans. 1 , In press. (4) Mliier, L. L.; Lau, A. N. K.; Miller, E. K. J . Am. Chem. SOC. 1982, 104, 5242. (5) Karger, E. L.; Snyder, L. R.; Horvath, C. S. An Introduction to Separation Science; Wiley: New York, 1973; pp 31-33. (6) Strohl, J. H.; Dunlap, K. L. Anal. Chem. 1972, 4 4 , 2166.
Amiya R. Ghatak-Roy Charles R. Martin* Department of Chemistry Texas A&M University College Station, Texas 77843
RECEIVED for review November 25,1985. Accepted February 10,1986. Support for this research was provided by the Robert A. Welch Foundation and the Office of Naval Research.
