Anal. Chem. 1987, 5 9 , 2237-2241
the sample lie within the optimum range of I T P analysis. Extraction and derivatization losses are avoided. As described, the method has two weaknesses. Firstly it is not preparative, and identification of sample components must be done by standard addition. This necessitates the availability of quite a wide range of standard compounds and still leaves some uncertainty in identification. Secondly, the reduction of selectivity and separation capacity a t higher molecular weights limits the applicability. For compounds with carbon number >6 the increasing number of isomers and the decreasing differences in p K and mobility values make analysis of complex mixtures problematical. However, when the method is used as a quantitative screen for formation waters, these drawbacks do not seriously disturb the analysis. The scope of the I T P analysis is sufficient to give a clear picture of the levels of all the target compounds considered most important in the geochemical applications; the C1 to C6aliphatic acids, benzoic and methylbenzoic acids, and bifunctional compounds of the same carbon numbers. I t is also faster, more easily performed and as accurate as the alternative methods while covering a broader spectrum of compounds.
ACKNOWLEDGMENT I am grateful to Statoil and Norsk Hydro for supplying samples of formation waters for analysis. Registry No. Formic acid, 64-18-6;acetic acid, 64-19-7;propanoic acid, 79-09-4; butanoic acid, 107-92-6;pentanoic acid, 109-52-4;hexanoic acid, 142-62-1; heptanoic acid, 111-14-8;octanoic acid, 124-07-2;benzoic acid, 65-85-0;oxalic acid, 144-62-7; malonic acid, 141-82-2;lactic acid, 50-21-5; 2-methylpropanoic acid, 79-31-2; 2-methylbutanoicacid, 116-53-0;3-methylbutanoic acid, 503-74-2; 2,2-dimethylbutanoic acid, 595-37-9; 2-hydroxyacetic acid, 79-14-1; 2-hydroxypropanoic acid, 50-21-5; 4hydroxybutanoic acid, 591-81-1; succinic acid, 110-15-6; phenylacetic acid, 103-82-2; 2-methylbenzoic acid, 118-90-1; 3-
2237
methylbenzoic acid, 99-04-7; 4-methylbenzoic acid, 99-94-5; 2hydroxybenzoic acid, 69-12-7; 3-hydroxybenzoic acid, 99-06-9; 4-hydroxybenzoic acid, 99-96-7; phthalic acid, 88-99-3; 1,3benzenediacid, 121-91-5; 1,4-benzenediacid, 100-21-0; water, 7732-18-5; cyclohexanoic acid, 98-89-5.
LITERATURE CITED Carothers, William W.; Kharaka, Yousif K. AAPG Bull. 1978, 62, 2441-2453. Kawamura, Kimitaka; Tannenbaum, Eli; Huizinga, Bradley J.; Kapian, Isaac R. Geochem. J . 1985, 20,51-59. Kawamura, Kimitaka; Tannenbaum. Eli; Huizinga, Bradley, J.; Kapian, Isaac R. Org. Geochem. 1988, 70, 1059-1065. Kharaka, Yousif K.; Carothers, Wiiiiam W.; Rosenbauer, Robert J. Geochim. Cosmochim. Acta 1983, 47,397-402. Kharaka, Yousif; Law, Leroy M.; Carothers, William W.; Goerlitz, Donaid F. I n Roles of Organic Matter in Sediment Diagenesis: SEPM Special Publication 38, SEPM: Tulsa, OK, 1988; pp 111-122. Surdam, Ronald C.; Boese, Stephen W.; Crossey, Laura J. In Clastic Diagenesis; McDonald, D. A.; Surdam, R. D., Eds.; AAPG Memoir 37, AAPB: Tulsa, OK, 1984; pp 127-151. Norenkova, I . K.; Arkhangei’skaya, R. A,; Tarasova, T. G. Geokhimliya 1978, 3,408-414. Chem. Abstr. 197& 88, 194026~. Barth, Tanja, Chemolab, in press. Pempowiak, Janusz J . Chromatogr. 1983, 258,93-102. Bocek, Petr; Gebauer, Petr; Dolnik, Viadislav; Foret, Frantisek, J . Chromatogr. 1985, 334, 157-195. Everaerts, Frans M.; Beckers, Jo L.; Verheggen, Theo P. E. M. Isotachophoresis ; Journal of Chromatography Library 6, Eisevier: Amsterdam, 1976. Hirokawa, Takeshi; Nishino, Makoto; Aoki, Nobuyaki; Kiso, YoshiYuko: Sawamoto, Yasnyo; Yagi, Takao; Akiyama, J.-I. J . Chromatogr. 1983, 277,D1-106. Everaertes, Frans M.; Verheggen, Theo P. E. M.; Mikkers, Frans E. P. J . Chromatogr. 1979, 769, 21-38. Perrin, D. D.; Dempsey, Boyd; Serjant, E. P. pK, Prediction for Organic Acids and Bases; Chapman and Hail: London, 1981. Pospichal, Jan; Deml, Mirko; Zemiova, Zdenka; Bocek, Petr J . Chrornatogr. 1985, 320, 139-146.
RECEIWQfor review January 27,1987. Accepted May 29,1987. This work was supported by the Norwegian Research Council and Statoil, who have kindly permitted the publication of this material.
Enrichment of Enantiomers and Other Isomers with Aqueous Liquid Membranes Containing Cyclodextrin Carriers Daniel W. Armstrong* and Heng L. Jin
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-4260
Water-based liquid membranes utilizing CY-, p-, and y-cyclodextrin carriers are able to selectively transport certaln isomers. Enantlomerlc enrlchments were demonstrated from slx racemic mixtures, two of whlch were drugs. Isomeric enrichments from mixtures of several geometrical and structural Isomers were demonstrated as well. Two separation schemes are ldentlfled and discussed. The factors that affect and control isomeric selectivity are examined. This study Is an important first step in demonstrating the feaslbliity of membranebased Isomer separatlons. These and analogous membrane systems have tremendous potential as a tool for large, continuous, preparatlve-scale isomer separatlons.
The isolation and purification of isomers is one of the more important and interesting areas of separation in science. Recently, considerable attention has been focused on the resolution of enantiomers, particularly by liquid chromatography (LC) methods ( I d ) . It is apparent that much progress
has been made in separating a variety of enantiomers and other isomers on the analytical scale. Much less has been written about preparative-scale separation of isomers although some commercially available chiral stationay phases (CSPs) are available in preparative and semipreparative columns. It is known that scaling sensitive analytical separations generally results in a loss of resolution. However, isomeric separations continue to be done via preparative LC because of a lack of viable alternatives. Many of the problems of routine, preparative-scale LC have been addressed (6). From an industrial point of view, the selectivity and efficiency of preparative LC are attractive while the low-capacity, high-cost, and batchseparation approach are detriments to the technique. A low-cost continuous separation process that has the isomeric selectivity of CSP-LC would be particularly attractive. Unfortunately, there are few such alternatives a t the present time. Membrane mediated separations are attractive from a preparative standpoint, because they can be used in continuous processes and the cost and energy requirements are often reasonable (7).However, they generally lack the selectivity
0003-2700/87/0359-2237$01.50/00 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987
and efficiency of chromatographic systems, which often is necessary to obtain isomeric separations. Liquid membranes that employ mobile carriers are among the most selective of all membrane techniques. The type and degree of selectivity stem from the nature of the carrier molecule employed. Perhaps the best characterized system employs organic liquid membranes between bulk aqueous solutions (8-10). The carrier molecules are usually macrocyclic ligands such as crown ethers or cryptands. Highly selective transport of ions has been demonstrated by a number of groups (11-15). Furthermore, the transport of ions against concentration gradients (coupled-transport) has been demonstrated (8). Newcomb et al. employed a chiral crown ether carrier in an analogous system for the selective transport of enantiomers of phenylglycine esters (16). Analogous studies subsequently were completed by another group ( I 7). Few studies have considered using water-based liquid membranes for the separation of organic molecules. To our knowledge, there are no reports on the use of cyclodextrins as membrane carrier molecules to separate hydrophobic isomers. In this work the first enantiomeric and isomeric enrichments using aqueous cyclodextrin liquid membranes are reported.
EXPERIMENTAL SECTION Materials. a-,@-,or y-cyclodextrins (CD) were obtained from Advanced Separation Technologies, Inc., and the Ensuiko Sugar Refining Co. as was the @-CDbonded phase column (Cyclobond I). The a-CDlmaltosyl copolymer was generously donated by Ensuiko Sugar Refining, Co. The nitrotoluenes and nitroanilines were obtained from Eastman. The cis- and trans-stilbene, bromobenzoic acids, acetyl-0-hydroxybenzoic acid (aspirin) and acetyl-p-hydroxybenzoic acid were obtained from Sigma. The racemic 2,2’-binaphthyldiyl-14-crown-4 and 2,2’-binaphthyldiyl-20-crown-6were made as previously reported (18) as was racemic S-(1-ferrocenylethy1)thiophenol(19). The mephenytoin and disopyramide were the generous gifts of R. D. Armstrong, University of California at San Francisco Medical School. All bulk solvents were obtained from Fisher. Two types of paper supports were used. The thicker paper was Whatman No. 1filter paper (designated W) and the thinner paper was Schleicher & Schuell sharkskin (designated S). Two types of chambers were used to evaluate the liquid membranes (Figure 1). The first type consisted of two identical glass chambers, of 10-mL volume each, which can be sealed together against an O-ring (Figure 1A). These chambers required a paper support for the liquid membrane. The second type of chamber consisted of a capillary tube (75-mm length by 1.1-mm i.d.). The tube was sealed on one end and the isomeric solution, membrane solution, and bulk water were layered into the capillary consecutively with a syringe. The capillary chambers were used when there was a limited amount of isomeric material available for the experiment (as in the case of enantiomeric mixtures). Methods. The amount of each isomer that traversed the membrane was determined by HPLC. Two microliters of solution from the receiving side were periodically analyzed until the concentration was sufficient for accurate quantitation (i.e., i 5 % standard deviation). A Shimadzu LC 6A liquid chromatograph with a variable wavelength detector and a C-R3A Chromatopac data system were used for all analytical work and data analysis. The chromatographicconditions for the separation of all isomers on a P-cyclodextrin bonded phase column have been reported previously (16-19). When the larger chamber was used (Figure lA), 10 mL of diethyl ether containing an equimolar mixture of the isomers (from 0.0036 to 0.46 M) was placed in one side of the chamber and 10 mL of neat ether was placed on the opposite side. Both sides were stirred at 60 rpm with magnetic stirrers. In the case of the capillary chamber, 30 pL of the appropriate ether solution was placed on either side of the membrane (Figure 1B). Capillary chambers were not stirred. Concentrated cyclodextrin solutions, suitable for membrane use, were made as follows. Solutions of a-and 0-CD up to 0.7 M can be obtained by mixing the appropriate amount of cyclodextrin into 20 mL of water containing 7.5 g of urea and 1.5 g of NaOH. The pH of the final
A
Glass Chamber, IO ml
I rnm Liquid Membrane
6.
Isomeric Ether Solution
t
Ether
75 mm
b
Flgure 1. Illustrations of the chambersLsed in the membrane separation experiments. A shows the larger chamber which utilizes a supported liquid membrane. Each side of the glass chamber is stirred. B is the capillary chamber. Chamber B is used when a limited amount of a compound is available for testing. solution is 11.4. Up to 0.5 M 7-CD can be prepared in a solution of 20 mL of H 2 0 + 7.5 g of urea. The a-CD copolymer was used as a saturated solution in pure water. The supported membranes are formed by dipping the appropriate piece of filter paper in the desired cyclodextrin solution. The pure solution was used in capillary chambers. The liquid membrane consisted of 2 pL of aqueous solution. Three blanks were run for every successful separation. The “blank membranes” consisted of identical aqueous solutions minus the cyclodextrin.
RESULTS AND DISCUSSION Table I gives the liquid membrane separation conditions, transport rates, and isomeric selectivity for several enantiomers and other isomers. There are a t least two general schemes for transporting isomers through aqueous-cyclodextrin-based liquid membranes. In the first case (Figure 2A) the solute is unable to permeate the aqueous membrane without the cyclodextrin carrier molecule. The isomer must form a CD inclusion complex a t the aqueous-organic interface, then the complex must diffuse across the membrane and the isomer is released a t the opposite interface. Selectivity is assumed to arise from the different binding constants of the isomers to cyclodextrin. The binding constants at both interfaces and the rate of diffusion of the complex and free cyclodextrin are assumed to be equal. This scenario basically describes the behavior of the first six isomeric solutes in Table I. Blank runs (i.e., using aqueous membranes without CD) indicated that very little of any isomeric specie permeated the membrane after several hours. In the second case (Figure 2B) the solute can be transported across the membrane in the complexed or uncomplexed state. This is a more complex system in which a t least five equilibria must be considered (more if a solute is ionizable). The last five isomeric solutes in Table I show this type of behavior and solute permeation is observed in blank runs. Isomers which can slowly permeate the water membrane without the aid of a carrier molecule may or may not show isomeric selectivity. Enantiomers, obviously, would show no selectivity. Some other isomers such a 0- and p nitrotoluene and cis- and trans-stilbene (Table I) show little or no selectivity. Some structural isomers (i.e., the nitroanilines and bromobenzoic acids in Table I) selectively per-
ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987
2239
Table I. Membrane SeDaration Conditions and Competitive Transport Rates for Several Enantiomers and Geometrical and Structural Isomers
initial concn, M
initial compounds (.t)-S-(1-ferrocenylethy1)thiophenol
d,l-1'-benzylnornicotine (.t)-2,2-binaphthyldiyl-14-crown-4 (.t)2,2-binaphthyldiyl-20-crown-6
mephenytoin (racemic) disopyramide (racemic) cis-, trans-stilbene o-, p-nitroaniline
o-, p-nitrotoluene o-. rn-bromobenzoic acid
aspirin, acetyl-p-hydrox: -enzoic aci-
membrane"
temp, "C
D D A B IV, 0-CD IV, 7-CD IV, blank IV, 0-CD B, a-CD polymer IV, a-CD IV, 7-CD IV, blank IV, p-CD IV, blank IV, 7-CD IV, blank IV, 7-CD IV, blank
40 40 40 26 40 40 40 40 26 26 26 26 40 40 26 26 26 26 26 26 26 26
A
0.031 0.031 0.031 0.022 0.012 0.012 0.046 0.029 0.09 0.09 0.09 0.058 0.072 0.23 0.23 0.23 0.23 0.23 0.04 0.04 0.018 0.018
B C A
isomer in excess
+ +
+ S (-14 (-1-S
unknown unknown cis cis trans P P P P P P P 0 0
aspirin aspirin
permeation ratiob 7.0 3.1 2.0 3.0 2.0 1.4 1.5 1.5 26 . 17 0.7 38 15 14 11 4 5.2 1.1 32 15 8.3 4
transport rate (mol/h X 107~
R,,: 17
9.5 29 0.01 6.3
10
5 8
0.08
57
11
"The membrane types are as follows: A = 3 p-CD capillary membranes in series, B = 2 6-CD capillary membranes in series, C = 1 a-CD-maltosyl polymer capillary membrane, D = 1 p-CD capillary membrane, and IV = a 3-layer supported liquid membrane as described in Table 11. Membranes A-D all contained 0.7 M 0-CD. bThe first observed isomeric ratio where peak area quantitation was accurate to within f5% SD (see Experimental Section). Maximum permeation ratios (RmJ are determined by extrapolating plots of permeation ratios vs. time to time = 0, as in Figure 3. dThe transport rate for the isomer in excess is indicated. These were calculated as previously reported ( 11-13). A. Organic Solution Containing Isomers
I
Table 11. Effect of Membrane Configuration on Selectivity Aqueous Liquid Membrane with Cyclodextrin Carrier
Organic Layer
;Isomer
Isomer
B. Organic Solution Containing Isomers
I
Aqueous Liquid Membrane with Cyclodextrin Carrier
&
Organic Layer
Bulk Solution \
Isomers-
I
IC
II
\
*
Isomers
Figure 2. Illustration of two possible separation mechanisms in cyclodextrin-based aqueous membrane systems.
meate a bulk aqueous membrane largely because of the differential solubilities and pKs of these isomers in water. In these selected cases, cyclodextrin carrier molecules can enhance, inhibit, or reverse the bulk membrane selectivity. In the case of 0- and p-nitroaniline and 0- and m-bromobenzoic acid, the cyclodextrin carrier greatly enhances the transport of the para and ortho isomer, respectively (Table I). In the case of cis- and trans-stilbene, the trans isomer is slightly favored in the blank. However, when a cyclodextrin carrier
membrane configuration" isomer ratiob I I1 I11 IV V VI
4.1 4.6 7.1 9.6 12.3 17.1
time, he 4 5 20 18 21 22
'Membrane I consisted of a single layer of 0.8 M 6-CD impregnated thin cellulose (designated S). Membrane I1 consisted of a single layer of 0.8 M p-CD impregnated thick cellulose (designated W). See Experimental Section for details on the cellulose. Membrane I11 = 3S,membrane IV = 3 W ,membrane V = 3s + 2 W ,and membrane VI = 5W. In membranes I11 and IV, the middle membrane contained ethyl acetate and not cyclodextrin. Likewise, membranes V and VI contained two ethyl acetate layers and 3 CD layers in alternating fashion. bThe numbers indicate the molar ratio of p-nitroaniline to o-nitroaniline that penetrated the membrane. The concentration of each isomer at the beginning of the M. cThis indicates the time required experiments was 3.6 X for a sufficient amount of the nitroanilines to penetrate the membrane and be quantitated to within *5% error (SD). molecule is added to the membrane, the transport of the cis isomer is greatly enhanced (Table I). Enantiomeric selectivity was much more sensitive to the operative transport mechanism (Figure 2). In general, no significant membrane-based enantioselectivity was observed unless the transport was controlled by the cyclodextrin carrier (Figure 2A). Mathematical descriptions of the complex equilibria and possible mechanisms (Figure 2) will be given elsewhere. There are several variables that affect isomeric selectivity in this study. First and foremost is the type of cyclodextrin carrier employed (Table I). Most of the enantiomers contained at least two aromatic rings. These types of compounds showed considerable enantioselectivity with a p-CD carrier but not with cy or y carriers. It was shown previously that these types
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987
Table 111. Effect of Carrier Concentration on Isomer Permeation Ratio
60
isomer
concn of cyclodextrin in liquid membrane, Ma
permeation ratiob
50
c>-CD
2.2 5.3 6.8 6.8
0
0.1 0.2
0.4 0.7 0.9
40
10.6
10.7
30
6-CD 0 0.1
0.2 0.4 0.7 0.9
3.8 4.7 5.1 12.5 11.0 13.0
" A type V membrane was used for the a-CD and a type IV membrane was used for the 0-CD (see Table 11). bNumber represents the molar ratio of p - to o-nitroaniline that first could be quantitated to within f5% standard deviation. The initial concentration was 0.0036 M for each isomer.
of molecules form strong inclusion complexes with P-CD (18-21 ). The membrane configuration significantly affected the transport rate and isomer permeation ratio (Table 11). It is well-known that transport rate is inversely related to membrane thickness (7). However, it is apparent that isomeric discrimination is proportional to membrane thickness (Table 11). This was true for all the isomers in this study regardless of the chamber type used. The concentration of the cyclodextrin carrier in the liquid membrane is very important. This is indicated in Table 111. In general, the higher the carrier concentration, the greater is the isomer permeation ratio for all compounds in this study. Since all cyclodextrins have a limited solubility in water (particularly 8-CD), it is necessary to add urea and alter pH in order to effectively increase their concentration (see Experimental section). I t is known that competitive ratios (through liquid membranes) are time-dependent (10-14). This is illustrated in Figure 3. The highest ratios are observed a t the beginning of each experiment. The time lag between the beginning of the experiment and the point a t which the first accurate measurement of the permeating isomers can be made is dependent on the sensitivity of the analytical technique employed (HPLC in this study) and the permeation ratio of the isomers. For example, if a pair of isomers has a large permeation ratio, one isomer can be detected and quantitated several hours before the other. However, one must wait until both isomers can be accurately quantitated in order to obtain an accurate ratio. As in most membrane-based systems the selectivity decreases with time and eventually disappears as the system approaches equilibrium (Figure 3). The maximum possible isomer permeation ratios (R,,, in Table I) are obtained by extrapolating plots, such as those of Figure 3, to time = 0. Table IV shows the effect of the initial solute concentration on the first detectable isomer permeation ratio. The highest ratios were almost always observed in experiments which utilized high initial solute concentrations. There may be a t least two possible reasons for this. First, the higher starting concentration allows faster quantitation of the isomers (i.e., closer to time = 0 in Figure 3). Also, nonlinear equilibrium behavior may be involved.
20
10
0
Time,
hrs.
Figure 3. Plot of the molar ratio of isomers penetrating a 0.7 M 0-CD liquid membrane vs. time. The solid line represents the ratio of p - to o-nitroaniline. The dashed line represents the ratio of (+)- to (-)S-(ferrocenylethy1)thiophenol. At the start of each competition experiment the isomers were of equal concentration (0.05 M and 0.035 M, respectively).
Table IV. Effect of Initial Concentration on Isomer Permeation Ratioo initial concn of each isomer, M
isomer permeation ratiob
0.0036 0.0072 0.014 0.03 0.06 0.115 0.23 0.46
12.1
13.0 11.8
12.6 16.9 17.9 23.6 30.4
A type IV, /3-cyclodextrinmembrane (0.7 M) was used to separate 0- and p-nitroaniline (See Table 11). bNumberrepresents the molar ratio of p - to o-nitroaniline, that first could be quantitated to within f 5 % Standard Deviation. Blanks (Le., Type IV membranes without cyclodextrin) were run at all concentrations. The blank isomeric ratio showed no concentration dependence and averaged 3.8.
The magnitude of the isomeric enrichments found in this study is encouraging and indicates that the use of polar, water-based membranes should be explored further. Cyclodexrin carriers are particularly useful as they are relatively inexpensive, nontoxic, and non-UV absorbing and selectively transport a wide variety of enantiomers, diastereomers, and other isomers. It is likely that cyclodextrin-based membrane systems will play an increasingly important role in future separation processes.
LITERATURE CITED (1) Davankov, V. A.; Kurganov, A. A,; Bocklov, A. S. Advances in Chromatography; Giddings, J. C., Grushka, E., Cazes, J,, Brown, P. R., Eds.; Marcel Dekker: New York, 1983; Vol. 22, p 71. (2) Armstrong, D. W. J. Liq. Chromatogr. 1984, 7(S-2). 353-376. (3) Armstrong, D. W. Anal. Chem. 1987, 59, 84A-91A. (4) Lepage, J.; Lindner, W.; Davies, G.; Karger, B. Anal. Chem. 1979, 5 7 , 433-435.
Anal.
Chem. 1987, 5 9 , 2241-2245
(5) Hare, P. E.; Gil-Av, D. Science 1979, 204, 1226-1228. (6) Guiochon, G.; Colin, H. Chromafogr. Forum 1988, 7 , 21-28. (7) Lonsdale, H. K. J. Membr. Scl. 1982, 10, 81-181. (8) Reusch, C. F.; Cussler, E. L. Am. Inst. Chem. Eng. J. 1973, 19, 736-746. (9) Wong, K. H.; Yagi, K.; Smid, J. J. Membr. B o / . 1974, 18, 379-397. (10) Cussler, E. L.; Evans, D.F. J. Membr. Sci. 1980, 6,113-121. (11) Christensen, J. J.; Lamb, J. D.; Izatt, S. R.; Starr, S. E.; Weed, G. C.; Astin. M. S.;Stitt, 9. D.; Izatt, R. M. J . Am. Chem. SOC. 1978, 100. 3219-3220. (12) Lamb, J. D.;Christensen, J. J.; Oscarson, J. L.; Nielsen, 9. L.; Asay, B. W.; Izatt, R. M. J. Am. Chem. SOC.1980, 102. 6820-6824. (13) Izatt, R. M.; Dearden, D. V.; Brown, P. R.; Bradshaw. J. S.; Lamb, J. D.; Christensen, J. J. J. Am. Chern. SOC. 1983, 105, 1785-1790. (14) Charewlcz, W. A,; Bartsch, R. A. J. Membr. Sci. 1983, 12, 323-333. (15) Oi, 2.: Cussler, E. L. J. Membr. S d . 1984, 19, 259-272. (16) Newcomb, M.; Toner, J. L.; Helgeson, R. C.; Cram, D. J. J. Am. Chem. SOC.1979, 101, 4941-4947.
2241
(17) Yamaguchi, T.; Nishimura, K.; Shinbo, T.; Sugiura, M. Chem. Lett. 1985, 1549-1552. (18) Armstrong, D. W.; Ward, T. J.; Czech, A.; Czech, B. P.; Bartsch, R. A. J. Org. Chem. 1985, 50, 5556-5559. (19) Armstrong, D. W.; DeMond, W.; Czech, 9. P. Anal. Chem. 1985, 5 7 , 481-484. (20) Armstrong, D. W.; DeMond, W.; Aiak, A,; Hinze, W. L.; Riehl, T. E.; Bui, K. H. Anal. Chem. 1985, 5 7 , 234-237. (21) Armstrong, D. W.; Ward, T.J.; Armstrong, R. D.;Beesley, T. E. Science 1986, 232, 1132-1135.
RECEIVED for review March 5, 1987. Accepted June 11, 1987. Support of this work by the National Institute of General Medical Sciences (BMT 1 ROI GM 36292-01) is gratefully acknowledged.
Effect of (Ethylenediaminetetraacetato)copper(II)and Bis(ethylenediamine)copper(I I)Eluents on Nonsuppressed Ion Chromatography with Indirect Photometric Detection Kazuichi Hayakawa,* Takehiko Sawada, Kazue Shimbo, and Motoichi Miyazaki Faculty of Pharmaceutical Sciences, Kanazawa Uniuersity, 13-1, Takara-machi, Kanazawa, 920 J a p a n
Two copper( I I ) chelates, disodium (ethylenedlaminetetraacetato)copper( I I ) , Na,[Cu(edta)], and bis(ethylened1amlne)copper( I I ) sulfate, [Cu(en),]SO,, were used as eluents In nonsuppressed Ion chromatography wlth lndlrect photometric detectlon. NaJCu(edta)] eluent was able to separately determine lnorganlc and carboxyilc anions on an anion exchange column. The eluent has a slightly lower anion exchange ablllty and a broader ultraviolet absorption avallabie for detectlon than the disodium phthalate eluent which has been commonly used. Wlth addltlon of excess Na,H,edta to the NaJCu(edta)] eluent, not only anions but ako metal ions were determined as metal-edta anion chelates. [Cu( en),]SO, eluent was able to separately determlne lnorganlc catlons on a catlon exchange column. This eluent has a significantly stronger cation exchange ablllty and a broader (ultraviolet and vlslble) absorption than the CuSO, eluent that has been commonly used. The detection llmlts for both anions and cations under the condltlons described above are below 10-ng levels of injected analytes.
Indirect photometric chromatography (IPC) (1) based on the difference of absorbances of sample and eluent ions (2) has attracted much attention in the recent development of ion chromatography (IC). IPC extends the application of IC to any high-performance liquid chromatography (HPLC) system capable of ultraviolet (UV) absorbance detection. The optimization of IPC conditions and applications of the technique have been reported (3-14). The authors have also reported the determination of inorganic cations and anions by similar approaches (15, 16). The choice of eluent for IPC has been limited to several species having large UV absorptions and ion exchange abilities. In the reports cited above, aromatic carboxylate solutions such as phthalate and trimesate have been mainly used as eluents for the determination of anions. On the other hand, copper(I1)
solution such as CuS04 and Cu(NOJ2 have been the main eluents used for inorganic cations. When only these eluents are used, there are some problems with IPC. For example, (1) simultaneous determination of both anions and cations is difficult, (2) a visible absorbance detector is not available, and (3) a column of low ion exchange capacity is necessary. These characteristics have restricted the use of IPC. From these facts, the authors have investigated metal complexes as eluents for IPC and have found some useful ionic metal complexes. There has been only one report on the use of a metal-complex eluent in IPC. Iron(I1) 1,lOphenanthroline was used as an ion interaction reagent for the determination of inorganic anions on a reversed-phase column (17). In this paper the authors describe the properties and the effective use of disodium (ethy1enediaminetetraacetato)copper(II), Naz[Cu(edta)],and bis(ethylenediamine)copper(II) sulfate, [ C ~ ( e n ) ~ ] as S 0eluents ~ for ion exchange columns in nonsuppressed IC with indirect photometric detection.
EXPERIMENTAL SECTION Apparatus. HPLC used in the experiments consisted of the following apparatus: a Shimadzu LC-6A pump with a Rheodyne 7120 sample injector, a Shimadzu CTO-2A column oven, a Shimadzu SPD-6AV variable-wavelength UV-visible absorbance detector, and a Nippon Denshi U-125 recorder. The polarity of the recorder was reversed. An anion separating column (150 X 3.0 mm id., stainless) was packed with Mitsubishi Chemical MCI SCA-01 (polyacrylatebased, anion exchange capacity 0.01 mequiv/g, particle size 20 pm). A cation separating column (250 X 4.6 mm id., stainless), was packed with Mitsubishi Chemical MCI CPK-08 (styrenedivinylbenzene copolymer, cation exchange capacity 1.7 mequiv/g, particle size 20 pm). HPLC Conditions. Typical conditions for inorganic and carboxylic anions were as follows: eluent, 1.5 X M Na2[Cu(edta)];flow rate, 0.5 mL/min; column temperature, 40 "C; detection wavelength, 325 nm (0.02 AUFS); injection volume, 50 pL. Typical conditions for simultaneous determination of both anions and metal ions were as follows: eluent, 5.0 X loT4M
0003-2700/87/0359-2241$01.50/0 0 1987 American Chemical Society
