118
Anal. Chem. 1989, 6 1 , 118-122
changes occur within 3 h of storage. A microbial intoxicant, m-cresol(29), was added to some stored subsamples 4 h after sampling to test whether microbial activity causes changes in sulfide concentrations during storage. The concentrations of some sulfide species stabilized, but some others decreased with time (particularly MeSH) after addition of the phenol. The data are not necessarily conclusive with regard to m-cresol suppressing microbial effects, but they show that sulfide analysis do change in the sample after collection. Thus concentrations of sulfides may be biased in samples stored for only a few hours.
CONCLUSIONS A simple and reliable method of analysis for volatile sulfides was developed for freshwater samples. The HECD is a key factor for achieving low detectability, multiple component calibration, and reproducibility of the calibration curves. The signal of the detector is linear with the amount of analyte, and the working range is greater than 3 orders of magnitude. Detection limits in the range of picograms of sulfur are generally better than those found in other studies. The column, however, does not decisively separate H a from COS at 40 "C. Replicate analyses of water samples show changes in reduced sulfide concentrations during storage of only a few hours, probably due to microbial activity. An effective preservative agent for these compounds in water is needed. Chemical agents and containers for sample preservation are under investigation at the moment. ACKNOWLEDGMENT Critical comments on the manuscript by Y. K. Chau and P. Takats, technical advice from P. Brassard, and the use of some laboratory installations by H. P. Schwarcz are appreciated. We thank J. 0. Nriagu for sharing with us an unpublished manuscript on DMS in freshwaters (13). &&try NO.DMS, 75-18-3;DES, 352-93-2;DMDS, 624-92-0; HzS, 7783-06-4; COS, 463-58-1;MeSH, 75-18-3;CSz,75-15-0;SOz, 7446-09-5; PrSH, 111-47-7;HzO, 7732-18-5.
LITERATURE CITED (1) Lovelock. J. E.; Maggs, R. J.; Rasmwsen, R. A. Nature 1972, 237, 452-453. (2) Cullis, C. F.; Hirschler, M. M. Afmos. Environ. 1980, 14, 1263-1278. (3) Moller, D. Atmos. Envkon. 1984, 18, 29-39. (4) Berner, E. K.; Berner, R. A. The Global Wafer Cycfe; Prentlce-Hall: EnglewW Cliffs, NJ, 1987. (5) Aneja, V. P.; Aneja, A. P.; Adams, D. F. J . Air Pollut. Control Assoc. 1982, 3 2 , 803-807. (6) Barnard, W. R.; Andreae, M. 0.; Watklns, W. E., Blngemer, H.,Georgli, H.-W. J . Geophvs. Res., C : Ocaens 1982, 87. 8787-8793. (7) Luria, M.; Van Valln, C. C.; WeHman, D. L.; Pueschei, R. F. Envlron. Sei. Technol. 1986, 20, 91-95. (8) Steudler, P. A.; Peterson, B. J. Nature 1984, 311, 455-457. (9) Jorgensen, B. B.; Okholm-Hansen, B. Atmos. Environ. 1985, 19, 1737-1749. (10) Adams, D. F.; Farwell. S. 0.;Robinson, E.; Pack, M. R.; Bamesberger, W. L. Eflvk~fl.SCi. Techno/. 1981, 15, 1493-1498. (11) Andreae, M. 0.; Barnard, W. R.; Ammons, J. M. Envhnmental Biogeochemkrtry. Ecol. Bull. 1983, 35, 167-177. (12) Bechard, M. J.; Rayburn, W. R. J . phycd. 1979, 15, 379-383. (13) Nriagu, J. 0.;Holdway, D. A. Tellus, in press. (14) Andreae, M. 0.; Barnard, W. R. Anal. Chem. 1983, 55, 606-612. (15) Gaudry, A.; Bonsang, B.; Nguyen. B. C.; Nadaud, Ph. Chemosphere 1981, 10, 731-744. (16) Luce, C.;Carller, P.; Glrard, R.; Hannachl, H.; Fresnet, P.;Mouvler, G. A n a l ~ ~ l1984, S 12, 350-357. (17) Holdway, D. A.; Nriagu, J. 0. Int. J . Environ. Anal. Chem. 1987, 3 2 , 177-186. (18) Hall, R. C. J . Chromatogr. Sci. 1974, 12, 152-160. (19) O'Keefe, A. E.; Ortman, G. C. Anal. Chem. 1966, 38, 760-763. (20) Scaringelli, F. P.; O'Keefe, A. E.; Rosenberg, E.; Bell, J. P. Anal. Chem. 1970, 42, 871-876. (21) Farweli, S. 0.; Gluck, S. J. Anal. Chem. 1980, 5 2 , 1968-1971. (22) DeSouza, T. L. C.; Bathia, S. P. Anal. Chem. 1978, 48. 2234-2240. (23) Massart, D. L.; Dljkstra, A.; Kaufman, L. Evakretbn and Optbnkatkm of Laboratory Mssthods and Analytlcel Rocedures ; Elsevier: Amsterdam, 1978. (24) Gluck, S. J . Chromatogr. Sci. 1982, 2 0 , 103-108. (25) Ehrlich, B. J.; Hall, R. C.; Anderson, R. J.; Cox, H. G. J . Chromatogr. SCi. 1981, 19, 245-249. (26) Mlshalanie, E. A.; Birks, J. W. Anal. Chem. 1986. 58 918-923. (27) Johnson, J. E.; Lovelock, J. E. Anal. Chem. 1988, 60, 812-816. (28) James, D. E. Culturing Algae; Carollna Biobglcal Supply Co.: Burlhgton, NC, 1978. (29) Liu, D.;Strachan, W. M. J. Arch. Hydrobiol. Beih. 1979. 12, 24-31.
RECEIVED for review April 18, 1988. Accepted October 13, 1988. This research was supported in part by a Natural Science and Engineering Research Council (NSERC) operating grant for J.R.K.
Measurement of Diffusion Coefficients of C,* Unsaturated Fatty Acid Methyl Esters, Naphthalene, and Benzene in Supercritical Carbon Dioxide by a Tracer Response Technique Toshitaka Funazukuri, Sumito Hachisu, and Noriaki Wakao*
Department of Chemical Engineering, Yokohama National University, Yokohama 240, Japan
Blnary dmuslon coeffklents of C,, unsaturated fatty acld methyl esters, naphthalene, and benzene In supercritlcal carbon dloxlde were measured by the Taylor-Ark tracer response technique. The dWfushm coeffldentp of deic, linoleic, and y-tlnolenlc acld methyl esters In SC-C02 at 313 K and 16.0 MPa were found to be inappreciably dmerent from each other. For #nolelc acld methyl ester the dWuslon coeffklents were correlated with temperature In the range from 308 to 328 K, respectively, at constant pressure of 19.0 MPa and at constant denslty of 800 kg/ms.
* Author to whom correspondence should be addressed. 0003-2700/89/0361-0118$01.50/0
INTRODUCTION A considerable attention is being paid to extraction by using supercritical fluid. Especially in the field of food industries, COZ has widely been used as a supercritical fluid. This is due to the fact that COZ is nontoxic and nonflammable, in addition to the low critical temperature and some other advantageous points. One of the physical properties needed for the design of supercritical extractors/separators is the molecular diffusion coefficients under the supercritical conditions. However, measurements of the diffusion coefficients have been limited (1-7). In this work the binary diffusion coefficients for C18 unsaturated fatty acid methyl esters, naphthalene, and 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 2, JANUARY 15, 1989
119
Table I. Comparison of Measured Diffusion Coefficients, D12,of Benzene and Naphthalene in SC-C02 with Those in the Literature compound
T,K
P, MPa
concn, mol %
benzene (ref 2) (ref 5) (this work) naphthalene (ref 3) (ref 5) (this work)
313.2 313.3 313.4 313.2 313.3 313.2
16.0 16.0 16.2 16.0 16.0 16.2
pure pure 30
pure 7.5 1.0
solvent
DeSc1l2
&on
none none
n-hexane none n-hexane n- hexane
1.30 1.07
3.82D12/
Equation 1 may be rewritten in the normalized form as
hence C* = C/(’SmC T O dt)
(2a)
where DeKis the effective axial dispersion coefficient,D12/ is the
meter
C02 c y l i n d e r Figure 1. A
/
Water bath
Capi 1I ary column schematic of the experimental apparatus.
apparent diffusion coefficient, and 0 is t / T in which is L/u. - t curve measured is compared in the time domain The C*, with the C*,d - t curves calculated from eq 2 with various assumed values of 012/.If the two curves agree well, the D1i value used for the prediction may be regarded as correct. RESULTS AND DISCUSSION In all cases, the D12/ data were obtained when the two curves, C*e,ptl- t and C*pred- t , agreed within about 2% of the root-mean-square error
Measurements of Diffusion Coefficients for Naphthalene a n d Benzene i n Supercritical C02. Naphthalene dissolved in n-hexane was injected as a shot into the diffusion column. The response curve was measured at 275 nm. Figure 2 shows that the data of D12/first decrease with decreasing u and then level off when u becomes lower than about 0.009 m s-’. The effect of secondary flow due to the column being coiled is found to become insignificant at this low flow rate. Therefore, the limiting value, 1.16 X lo4 m2 s-l, is considered to be the diffusion coefficient, D12,for naphthalene at 313 K and 16.2 MPa. Similarly, the diffusion coefficient, D12,for benzene at 313 K and 16.2 MPa is found to be 1.28 X lo4 m2 s-l. Note that the response concentration of benzene was measured a t 255 nm. These D12data for naphthalene and benzene are listed in Table I together with the values available in the literature (2, 3 , 5 ) . As shown, our D12data are in good agreement with those determined by the previous investigators. There is obviously little solvent effect for naphthalene and benzene. Lauer et al. ( 5 ) also have found no effect on n-hexane on the determination of diffusion coefficients of benzene. Figure 3 shows the Arrhenius plot of D12vs 1/T for naphthalene at 800 kg m-3 together with the lines of Lauer et al. ( 5 ) . The D12 data determined in the present work are fully consistent with the corresponding line of Lauer et al. while
120
ANALYTICAL CHEMISTRY, VOL. 61, NO. 2, JANUARY 15, 1989
Naphthalene
Linoleic acld methyl ester
313 K 16.2MPa
t
I
L,0.005 I
0
1
0,9
'
I
"
"
"
'
0,01
"
0,015
-
-.
0.8
y1
800 ks/m3
N
'
u (WS) Apparent diffusion coefficients, D of naphthalene in supercritical CO, measured at different veloclties of the CO, at 313 K and 16.2 MPa.
,,',
Figure 2.
1 ,E
nc.
I
Y * >
2,9
NaDhthalene
I
3,0
3,l
I
3.2
3.3
3,4
I T ( 10-3 x K - )~ of linoleic acid methyl ester in supercritical COPas a function of temperature at the CO, density of 800 kg/m3; comparison with Wilke-Chang equations.
Flgure 5. Diffusion coefficients, D,,,
1
0.9
1
Linoleic acid methyl ester 19,OMPa
0.8 2 9
3.0
3.1
3 2'
3.3
34
x K-I )
1IT
Figure 3. Diffusion coefficients, D ,, of naphthalene in supercritical
COz as a function of temperature at the CO, density of 800 kg/m3; comparison with Lauer et al. (5).
1. 0
A
\
*-
C18 4YE i n n-hexane Solution peat ClS - AME
G,5
N
i T
(
10-3 x ~ - )1
,,
Figure 8. Diffusion coefficients, D of linoleic acid methyl ester in supercritical CO, as a function of temperature at the CO, pressure of 19.0 MPa; comparison with Wilke-Chang equations.
i 3
0,006
0,008
u
0.01
(rnIS)
Figure 5 shows the Arrhenius plot of D12 vs 1 / T for Cla2-Ah4Eat a constant density of 800 kg m-3. Although some scatter is seen among the data a t high temperatures, the straight solid line representing the data is
Figure 4. Apparent diffusion coefficients, D12),of linoleic acid methyl
ester (Claz-AME) in supercritical COPmeasured at different velocities of the CO, at 3 t 3 K and 16.0 MPa: (0)C,&.AME in n-hexane solution; (A)neat C,*,-AME.
our data show some scatter particularly a t high temperatures. Determination of D12of Unsaturated Fatty Acid Methyl Esters. (i) Linoleic Acid Methyl Ester (Cis:.AME). The response concentration signals measured at different wavelengths in the range from 200 to 220 nm gave almost the same D12' values. Figure 4 illustrates D12/ vs u relationship for Cle,.-AME at 313 K and 16.0 MPa. The circles indicate n-hexane solution of C1&.-AME being injected, and the triangles correspond to neat CIa2-AME. No difference is seen between the D l i data when Cla2-AMEis injected with or without n-hexane. The D12/ data approach the limiting value, D12of 6.50 X m2 s-l, a t low flow rate.
where E d is 4.52 kJ mol-' and (D12)dis determined in units of square meters per second. Figure 6 shows the Arrhenius plot of D12 vs 1/T for Cla2-AME a t a constant pressure of 19.0 MPa. The D12data at 19.0 MPa are correlated with temperature, as the solid line in Figure 6 shows, through
(D12),= (8.44
X
lo-')
[4
exp --
(a
where E, is 12.8 kJ mol-' and (D& is determined in units of square meters per second.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 2, JANUARY 15, 1989
Table 11. Experimental Data of L / ( 2 u ) and r2/(3.8'D12) compound benzene naphthalene
linoleic acid methyl ester oleic acid methyl ester y-linolenic acid methyl ester
J~/@u),
9/(3.8'42),
1300-1500 2000-2200
0.4-1.0
2000-2200 2000-2200 2000-2200
1.5-1.8 1.7 1.7
8
0.8-0.9
It is found that the diffusion coefficient (D12)pdepends on temperature more than (D12)d. Swaid and Schneider (2) also observed a similar tendency in the diffusion coefficients for benzene and alkylbenzenes in SC-C02. (ii) Oleic Acid Methyl Ester (Clal-AME) and y-Linolenic Acid Methyl Ester (y-Cla3-AME).From the curves similar to that of Figure 4, the diffusion coefficients of Clal-AME and r-Cla3-AME, respectively, at 313 K and 16.0 MPa are determined to be 6.55 X lo4 and 6.60 X lo4 m2 s-'. Sources of Errors. Table I1 lists the experimental data of the terms of L/(2u) and 9/(3.S2Dl2). Note that the values of r2/(3.82D12')are equal to or even lower than those of r2/ (3.82D12). Equation l b is then found to be fully sound. Some (2, 3, 7) of the previous investigators have pointed out that errors may result from (i) initial peak dispersion, (ii) secondary flow due to the column being coiled, and (iii) adsorption of the substances onto the column walls. However, a preliminary measurement made by installing the UV detector right after the injector and another measurement with the apparatus outlined in Figure 1 showed that the variance of the response curve without the column was lower than about 3% of that with the column. Therefore, it can be assumed that the tracer species has been injected as a delta function. Lauer et al. (5)has pointed out that there will be no secondary flow in the coiled tubing if the following condition is fulfilled: De Sc112 < 10 (6) where Sc is Schmidt number ( = q / p D 1 2 )and De is the Dean number
(7) In the present work it was observed that the D1i data became independent of u at DeSc'12 less than about 13 (for naphthalene and benzene, for instance, refer to Table I). In any case the D12values were determined at sufficiently low flow rates where they did not depend on the flow rate any more. Lauer et al. (5)have pointed out that adsorption of tracer samples can be ignored if an asymmetry factor defined as the ratio of half-peak widths measured at 10% of the peak height is about unity. In the present measurements the asymmetry factors were always in the narrow range from 1.0 to 1.1 (see Table I for Slo for naphthalene and benzene), irrespective of the neat samples or their n-hexane solutions being injected. Therefore, it was considered that little adsorption existed in the diffusion column. Also, the pressure drop (between the inlet and the outlet) was always less than 0.1 MPa. According to Sassiat et al. (7), the pressure drop of 0.1 MPa is too small to affect the change in mobile phase density in the column. Figures 3 and 5 show that the experimental data of D12)s, respectively, for naphthalene and Clk2-AME a t constant density show some scatter, especially at high temperatures. This is considered to result from some experimental errors involved in the pressure measurements: The phase diagram of COz (plotted as pressure on y axis vs density on x axis) shows that in the present supercritical range the pressuredensity isotherm becomes steeper at lower temperature. In other words, a small pressure variation causes a large density
121
variation when temperature is high. Comparison of Binary Diffusion Coefficients Measured and Those Predicted by Wilke-Chang Equations. Wilke-Chang equation (12) has been widely used for the prediction of liquid-phase diffusion coefficients
where q is the Viscosity of the liquid in Pa s, v b is the molecular volume of the solute at its normal boiling point in m3 mol-', M is the molecular weight of C02, $ is called an association factor of the fluid, and D12is in m2 s-l. Sassiat et al. (7) modified the Wilke-Chang equation for the diffusion coefficients of aromatic compounds in SC-C02 in particular
The data of D12for Cla2-AME predicted from the modified Wilke-Chang equation and those obtained from the original Wilke-Chang equation are compared in Figures 5 and 6. Note that v b is estimated by the method of Le Bas (13) to be 4.25 X m3 mol-' for Cla2-AMq, and that $ is assumed to be unity. Also, the viscosity of SC-C02 is estimated from the literature (5,14,15). Although some uncertainty remains in the prediction of $, v b , and I), the experimental data at 19.0 MPa agree with the modified Wilke-Chang line (see Figure 6). However, at 800 kg m-3 (Figure 5) the experimental data differ somewhat from the modified Wilke-Chang line.
NOMENCLATURE C = tracer response concentration, mol m-3 C* = normalized tracer response concentration D12= binary molecular diffusion coefficient, m2 s-' D12/ = apparent diffusion coefficient, m2 s-l De = Dean number defined by eq 7 D,ff = effective axial dispersion coefficient, m2 s-' dcoil= coil diameter, m = diameter of tubing, m E = activation energy, kJ mol-' L = column length, m M = molecular weight of C 0 2 R = gas constant, kJ mol-' K-' r = radius of tubing, m , number Sc = q / ( p D 1 2 )Schmidt T = temperature, K t = time, s u = flow rate of supercritical carbon dioxide, m s-' v b = molecular volume of solute at its normal boiling point, m3 mol-' x = column length, m Greek Symbols t = error defined by eq 3 q = viscosity of SC-C02, Pa s 8 = t/r p = density of SC-C02, kg m-3 T = L / u , mean residence time, s $ = association factor of SC-C02 Subscripts d = constant density p = constant pressure Registry No. Benzene, 71-43-2;naphthalene, 91-20-3;linoleic acid methyl ester, 112-63-0; oleic acid methyl ester, 112-62-9; y-linoleic acid methyl ester, 16326-32-2.
LITERATURE CITED (1) Saad, H.; Gulari, E. Ber Bunsen-Ges. Phys. Chem. 1984, 8 8 ,
034-637. (2) Swaid, I.; Schnelder, 0. M. Ber. Bunsen-Ges. f h y s . Chem. 1979, 8 3 , 969-974.
Anal. Chem. 1989, 61, 122-125
122
(3) Feist, R.; Schneider, G. M. Sep. Sci. Techno/. 1082, 17, 261-270. (4) Wilsch, A.; Feist, R.; Schnelder, G. M. F/uidPhase Equilib. 1083, 10, 299-306. (5) Lauer, H. H.: McManlgill. D.: Board, R. D. Anal. Chem. 1083, 55, 1370-1375. (6) Springston, S. R.; Novotny, M. Anal. Chem. 1084, 5 6 , 1762-1766. (7) Sass&, P. R.: Mourier, P.: Caude, M. H.; Rosset, R. H. Anal. Chem. 1087, 5 9 , 1164-1170. (8) Taylor, G. Proc. R. SOC.London, Ser. A 1053, 219, 186-203. (9) Taylor, G. Proc. R. SOC.London, Ser. A 1054, 223, 446-468.
Taylor, G. Proc. R . Soc. London, Ser. A 1054. 225, 473-477. Ark, R. Proc. R . SOC.London, Ser. A . 1856, 235, 67-77. Wilke, C. R.; Chang, P: AIChEJ. 1055, 1, 264-270. Reid, R. C.; Prausnitz, J. M.;Sherwocd, T. K. Le Bas, The Properties of Gases and Liquids, 3rd 4.;McGraw-Hill: New York, 1977; p 58. (14) Iwasaki, H.; Takahashl, M. J. Chem. phvs. 1081, 7 4 , 1930-1943. (15) Diller, D. E.; Ball, M. J. Int. J . Thermophys. 1085, 6 , 619-629.
(10) (11) (12) (13)
RECEIVED for review July 28,1988. Accepted October 24,1988.
Background Suppression by Chelation in the Ion-Exchange Chromatographic Separation of Anions Hisakuni Sato* and Akiyoshi Miyanaga
Laboratory of Analytical Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku, Yokohama-shi, Japan 241, and Tokyo Research Center, Tosoh Corporation, Hayakawa 2743-1, Ayase-shi, Japan 252
By the use of chelate formatlon reactions, three new background suppresslon methods are developed for conductlvlty detection In anion chromatography. (1) By postcolumn homogeneous reactions, including chelate formatlon and acldbase reactlons, background conductlvlty and water dlp can be considerably reduced. (2) By use of an elutlng anlon that forms a neutral chelate with a metal Ion (M"'), background conductlvlty can be reduced to a very low level with a M"+-form cation-exchange column as the suppressor. (3) Protonated amlne, whlch Is the countercatlon of the eluting anlon, is made to dissociate in a Cu2+-form cation-exchange column. The dlscharged proton nerdralizesthe anlonlc charge of the eluant Ion by an acld-base reaction. Background conductlvlty depends on the eluting anion. Although these new methods wlll not completely surpass the usual suppression method utilirlng only acid-base reactbns, they have their own dlfferent features, are easler to use, and are expected to expand the posslbliltles of analytlcal ion-exchange chromatography.
To permit conductivity detection in ion-exchange chromatography, Small et al. (1)found a method to suppress the background conductivity of the eluant by using a second ion-exchange column called a "stripper column". For the separation of anions, this stripper column (frequently called a "suppressor column") is packed with cation exchanger in the H+ form, cations in the eluant are exchanged with the H+ ions, and then eluting anions are protonated. Later, in place of the cation-exchange column, fiber suppressors using cation-exchange membrane tubes were used (2, 3). These background supression methods are based on the combination of an ion-exchange reaction and an acid-base reaction. However, since the alkaline eluant is changed to a weak acid solution in the stripper (or suppressor) portion, problems arise in the detection of NOz- and in the detection of conjugate anions of weak acids (4). We have found three new background suppression methods by using chelate formation reactions in place of the acid-base reactions of Small et al. (1). This paper present$ the principles *Author t o whom correspondence should b e addressed a t Yokohama N a t i o n a l University.
of these methods and some experimental results.
PRINCIPLES Method of Postcolumn Homogeneous Reactions. Let the cation and the anion in the eluant be ME and LE, respectively. Similarly, let the cation and the anion in the reacting reagent solution to be added to the effluent from the separation column be Ms and Ls, respectively. Here, the charge on the ion is omitted for the purpose of simplicity. The desirable postcolumn reactions for the reduction of the background conductivity can be expressed as follows:
ME + Ls Ms + LE
+
ME-Ls Ms-LE
(1)
(2) where ME-Ls and Ms-LE are complexes or ion associations. As these reactions progress toward the right, positive and negative charges cancel each other and the conductivity is thereby reduced. Ideally, it is preferable that the charges on the all products are zero. If one of the following acid-base reactions progresses in place of (1)or (2),the background conductivity is also reduced:
ME + Ls Ms + L E
+ nH,O + nH,O
-+
+
+
ME(OH), + H,Ls Ms(OH), + H,LE
(3) (4)
where ME(OH),, Ms(OH),, H,Ls, and H,LE are water-soluble chemical species having smaller charges than ME, Ms, LE, respectively. Thus, it is expected theoretically that the background conductivity of the eluant can be reduced by the combinations of (1)and (2), (1)and (3),or (2) and (4) given above. ME, LE, Ms, and Ls should be suitable for the above reactions, and ME and LE should further be suitable as the eluant components for anion separation. If both ME and MS are limited to metal cations, two kinds of specific chelate formation reactions, as represented in reactions 1 and 2, must be combined. Of course, ME (Ms) must not associate with LE (Ls). If either ME or MS is an amine cation, it is considered that combinations of (1)and (3),or (2) and (4),may be possible. At present, no ideal combinations of ME, LE, Ms, and Ls have been found. A few examples will be given later, where the principles of this method are partially realized. Method of Changing the Eluting Anion to an Uncharged Chelate. Assume that the eluant components are cation ME and anion LE. The effluent from an anion-exchange
0003-2700/89/0361-0122$01.50/00 1989 American Chemical Society
