1503
Anal. Chem. 1980, 52, 1503-1505
LITERATURE CITED (1) Fox, D. L.; Jeffries,H. E . Anal. Chem. 1979, 57, 22R. (2) Chane, L. W.; McClenny,W. A. Environ. Sci. Technoi. 1977. 1 1 , 1186. (3) Polasek, J. C.; Buliin, J. A. Environ. Sci. Technol. 1978, 12, 708. (4) Horning, E . C.; Horning, M. G.; Carroll, D. I.; Dzidic, I.; Stillwell, R . N . Anal. Chern. 1973, 45, 936. (5) Siegel, M. W.; Fite, W . L. J . Phys. Chem. 1978, 80,2871. (6) Reid, N. M.; French, J. B.; Buckky, J. A,; Lane, D. A,; Lovett, A. M. Sciex Inc. Application Note No. 677, 1977. (7) Kambara, H.; Kanomata. I. Anal. Chem. 1977, 49, 270. (8) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J . T. J . Phys. Chem. Ref. Data 1977, 6 , 267.
(9) Payzant, J. D.; Kebarle, P. J . Chem. Phys. 1970, 53, 4723. (10) Meot-Ner, M.; Field, F. H . J . Chern. Phys. 1974, 6 1 , 3742. (11) Ng, C. Y.; Trevor, D. J.; Mahan, B. H.; Lee, Y . T . J . Chem. Phys. 1977. 66, 446. (12) Kambara, H.; Kanomata, I. Int. J . Mass Spectrom. Ion phys. 1977, 25, 129. (13) Kambara, H.; Mitsui, Y . ; Kanomata, I. Anal. Chern. 1979, 5 1 . 1447. (14) Kambara, H.; Misui, Y.; Kanomata. I. Int. J . Mass Spectrorn. Ion phys.,
in press.
RECEIVED for review January 28, 1980. Accepted April 14, 1980.
Tubular Flow Donnan Dialysis James A. Cox" and Zbigniew Twardowski Department
of Chemistry and Biochemistry,
Southern Illinois University at Carbondale, Carbondale, Illinois 6290 1
Pumping the receiver electrolyte through cation-exchange tubing is demonstrated to yield more efficient Donnan dialysis enrichments than the use of static systems. Twofold enrichments are achieved in a 1.4min dngie pass experiment, and 50-fold enrichments are attained in 20 min by circulation of an aliquot of electrolyte through a receiver loop. The dialysis rate is directly proportional to sample concentration and independent of sample matrix factors over a wide range of conditions; hence, quantitation of real samples can be performed with a linear calibration curve. In this method the metals in the dialysate are determined by differential pulse polarography or anodic stripping voitammetry.
The determination of trace quantities of ions in real samples often requires pretreatment steps. The original matrix may not be compatible with the analytical method, matrix variation may not be corrected by an internal standard or standard addition approach, or the sensitivity of the analytical method may be too low t o perform the determinations without preconcentration. Donnan dialysis has been demonstrated to be a useful technique for matrix normalization and ion enrichments (1-3). Typically it has been performed by contacting a 50-200 mL stirred sample to a few milliliters of static receiver electrolyte through a n ion-exchange membrane. The uses of stirred receiver electrolytes ( 4 ) and circulated samples and receivers (5) have also been reported. The latter had a receiver chamber which was a 0.16 cm deep rectangular solid indentation with the ion-exchange membrane covering one face; baffles were used to increase turbulence. The static receiver approach has the disadvantage of yielding rather modest enrichment factors; values of 5-10 were achieved with 1-h dialyses. With the stirred receivers, greater enrichments were attained; values of 100 were possible by allowing the systems to reach Donnan equilibrium. For monovalent ions, 2 h was required whereas about 20 h was needed to reach Donnan equilibrium for divalent species ( 4 ) . The above limitations are a result of an unfavorable ratio of receiver volume to ion-exchange membrane surface area in cases where milliliter-size aliquots are needed for the quantitation step. For example, 10-fold enrichments can be attained in a few minutes if microliter volumes are used for the receiver ( 3 ) . 0003-2700/80/0352-1503$01 .OO/O
The availability of tubular cation-exchange membranes introduces a convenient means of decreasing the receiver volume-to-surface ratio while using receiver volumes in the milliliter range. Further, by pumping the receiver through the tubing rather than using a static system, the effect of concentration polarization can be decreased. Only recently (6) has concentration polarization in the receiver been suggested to be important in determining the overall transport rate. In the present paper, it is demonstrated that such a flow system results in significantly greater enrichment factors while maintaining the matrix normalization function of Donnan dialysis. EXPERIMENTAL The cation-exchange tubing used was Nafion 811 (DuPont Polymer Products, Wilmington, Del.) of dimensions 0.025-inch i.d. and 0.035-inch 0.d. Nafion is a sulfonated fluorocarbon polymer of equivalent weight 1100 in the protonated form. Figure 1 shows the dialysis assembly. Generally 245 cm of the Nafion 811 is coiled around a 3-cm diameter perforated polyethylene tube. The ends of the cation-exchange tubing are tied to the support with nylon thread. Connections to the peristaltic pump (Buchler Instrument Company) are made with 0.8-mm i.d. Teflon tubing. The cation-exchange tubing coil is completely immersed in the sample solution. The latter is magnetically stirred. Up to lo00 mL of sample is used. The cation-exchange tubing volume was 0.77 mL, and the total volume of the receiver system was 2.17 mL in the typical experiment. The dialysis experiments were performed in two ways. One approach was to continuously pump receiver electrolyte through the cation-exchange tubing into a collection chamber. In this method whenever the nature of the sample was drastically changed, the dialysate was diverted to a waste vessel for 5 min prior to collection in order to condition the membrane. Alternatively, 5 mL of the receiver electrolyte was circulated through the tubing for a prescribed time, diverted into a 10-mLvolumetric flask, and diluted to volume. After each set of experiments, the ion exchanger was cleaned by dipping in 1 M HCl while the same concentration of the acid was circulated through the system. After 1 h, the HC1 solutions were replaced with 2 M MgS04. The latter solutions were replaced after an additional 2 h with ones of the same composition as the receiver electrolyte. The system was stored in the electrolyte for at least 2 h prior to initiation of a new series of experiments. I t should be noted that for routine work a few rinses with receiver electrolyte in both the sample chamber and the tubing is sufficient. The receiver electrolyte was generally a 0.2 M MgS04,5 X lo4 M A12(S0& mixture (2). The MgS04 was purified by controlled potential electrolysis of a 1 M solution at a stirred Hg pool (%: 1980
American
Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
1504
per
S'UIL
c
53 Jtl
OP
Table I. Noncirculating Tubular Flowa Donnan Dialysis Enrichment of Metals
reservo r 3"
.r-:-,
s
t
concnb M x lo6 1.0 3.0 6.0
diolysnte collector
10.0 30.0
rno3r-e- c
it
rrr'
60.0 80.0
Figure 1. Tubular flow Donnan dialysis system for single-pass ex-
periments
av.
rel.std.dev.
r
:
S
LLJ
1
enrichment factorsC into 0 . 2 M MgSO,, into 0 . 2 M MgSO, 0 . 5 mM Al,(SO,), Cu Zn Cd Cu Zn Cd 2.2 2.1 2.0 2.0 2.0 1.9
2.4 2.3 2.1 2.1 2.0 1.8 1.7
1.5 1.6 1.7 1.8 1.8 1.7 1.6
2.1 2.1 2.1 2.2 2.4 2.3 2.3
2.4 2.4 2.5 2.5 2.5 2.4 2.4
2.3 2.4 2.5 2.4 2.5 2.3 2.2
1.8
2.00 0.13
2.06 0.25
1.67
0.11
2.21 0.12
2.44 0.05
2.37 0.11
a Flow rate, 3.5 mL/min. Samples contain equal concentrations of Cu(II), Zn(II), and Cd(I1). Concentration in undiluted dialysate divided by original sample concentration; sample volume, 1000 mL.
1
I? \
i
Flgure 2. Effect of flow rate on the enrichment efficiency. Receiver electrolyte, 0.2 M Mg(II), 0.5 mM AI(II1); sample, 2 X lo-' M Cu(II), Cd(II), and Zn(I1). EF, concentration of metal in the receiver divided by the initial sample concentration. Cu, 0 ;Cd, Zn, 0
0;
electrode at -1.3 V for at least 5 days. Aliquots were taken and diluted as needed. All of the chemicals were ACS Reagent Grade. The stock solutions of Cu(II),Zn(II), and Cd(I1) were prepared by dissolving the appropriate metals (Matheson, Coleman and Bell) in nitric acid. Distilled water which was further purified with a Cole Parmer Adsorber/Universal/Research cartridge system was used throughout. The metal determinations were made by either differential pulse polarography or anodic stripping voltammetry. The latter was used when the concentrations were below lo4 M. The instrument was a PAR 174A polarograph. Working curve quantitations were periodically verified by the standard additions method.
RESULTS AND DISCUSSION T h e initial experiment which was performed to verify the predicted advantages of the flow system was to determine the effect of flow rate on the enrichment factor (EF). The enrichment factor was defined as the ratio of the test ion concentration in the receiver electrolyte after dialysis to ita initial concentration in the sample. The noncirculating mode was used for this series. As shown in Figure 2, low flow rates yield the highest E F because of longer residence time of the receiver electrolyte in the cation-exchange tubing. When the results are interpreted as E F per unit time, greater efficiency is found at the higher flow rates. The greater flow rate decreases the diffusion layer thickness a t the membrane/receiver interphase and, thus, diminishes concentration polarization. Above 3 mL/min,
the enrichment efficiency becomes independent of flow rate. In this range concentration polarization in the receiver electrolyte is a negligible factor in determining the rate of diffusion across the cation-exchange membrane. The Reynolds number, Re = ud/u, was calculated from the receiver flow rate, u,the internal diameter, d , of the ion exchange tubing, and the kinematic viscosity of the receiver electrolyte, v. The values ranged from 10 to 270 which are well within the laminar flow region, Re less than 2000. The analytical utility of tubular flow Donnan dialysis (TFDD) was tested on samples containing combinations of equimolar quantities of Cu(II), Zn(II), and Cd(I1). The samples were adjusted to about pH 5 with dilute H N 0 3 in order to prevent the formation of hydroxy complexes from lowering the rate of transport. Both 0.2 M MgSO, and a mixed 0.2 M MgS04,5 X lo4 M A12(S04)3 electrolyte were used as receivers. Results for the noncirculating mode are summarized in Table I. From the tabulated results, it is apparent that the final receiver concentration is directly proportional to the sample concentration, so a working curve approach is feasible for quantitation. With Al(II1) in the receiver, the standard deviations are lower and the enrichment factors are higher. The former is a result of the elimination of a trend toward a slight decrease in EF with increasing sample concentration which occurs when the receiver electrolyte is MgS0, alone. T h e increase in EF with addition of Al(II1) is consistent with results obtained with grafted polymer ion-exchange membranes (2). The Al(II1) has the greatest affinity among the tested ions for the fixed ion-exchange sites in the membrane (7); hence, its presence minimizes interaction between those sites and the analyte ions which would slow transport, as in the case for Cd in Table I, and potentially cause rounding of working curves a t low concentrations. Further, with the Mg/A1 electrolyte, the EF's are independent of the natures of the metal ions as well as the sample compositions even when determined well before steady state (2). The twofold enrichments reported in Table I are achieved in 1.4 min. The increased sensitivity relative to static receiver Donnan dialysis is significant. When it is also considered that matrix effects are eliminated for a wide variety of samples (I, 8), TFDD becomes an interesting means of sample pretreatment. The procedure is rapid and is adaptable to continuous on-stream determinations by feeding the dialysate to an electrochemical, spectrochemical, or other type of detector. If greater enrichment factors are needed, a longer cation-exchange tube or a slower flow rate (Figure 2) can be used a t the expense of response time.
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
L
Y
i
//
l
1
1 15:0
I
/P ,#
*
sample conc. 8x,oi%l
Flgure 3. Calibration curve for TFDD with a circulated receiver electrolyte. Receiver, 5 mL of Mg/AI mixture circulated at 8.0 mL min-' for 20 min. Subsequent metal determinations by anodic stripping voltammetry. Cd, Zn, 0
0;
For the analysis of grab samples or for other types of individual measurements, a greater E F can be achieved by circulation of an aliquot of the receiver electrolyte. When a 5-mL aliquot of a receiver of 0.2 M Mg(II), 0.5 mM Al(II1) is circulated a t 8.0 mL/min for 10, 20, 30, and 40 min, EF's of 17,38,48,and 49, respectively, are obtained for Cu(I1) from a 1000-mL sample of a 2.0 x 10" M Cu(II), Cd(II), and Zn(I1) mixture. The EF's for Cd(I1) and Zn(I1) follow the same trend and are in the same proportion as in Table I. With longer circulation times, the EF's begin to decrease even though the theoretical Donnan equilibrium limit is not approached (calculated equilibrium EF's are about 200). Co-ion transport due to less than ideal permselectivity may be occurring a t a sufficient rate to diminish the EF's. Nevertheless, the 50-fold enrichments which are achieved in 20 min represent a significant improvement over previous reports (including those with turbulent receivers) on Donnan dialysis with receiver electrolytes in the milliliter range and compare favorably with such established preconcentration methods as solvent extraction. As shown in Figure 3, if the dialysis is performed for a prescribed time, the E F is proportional to the sample concentration. The rounding which occurs for Cd below 1 X lo-@ M is probably due to a significant interaction with sulfonate sites on the ion-exchange membrane. If Al(II1) is deleted from the receiver, the rounding becomes more pronounced. The
1505
linear range extends for 2 orders of magnitude. Considering the ability of Donnan dialysis to eliminate the interference of such matrix components as surfactants, ions of charge sign opposite to that of the analyte, and complexing agents (I, 8 , 9 ) together with the linearity of working curves with TFDD, applications should not require standard additions, internal standards, or lengthy chemical treatment of the samples. A grab sample of sewage effluent was used to test this prediction. The sample was filtered through Whatman No. 42 paper and acidified to pH 4 with dilute "OB. With a 0.2 M Mg(II), 0.5 mM Al(II1) supporting electrolyte, a differential pulse polarogram showed only a faradaic current for Zn(I1) reduction prior to discharge of the supporting electrolyte. By standard addition, the concentration was found to be 1.7 X lo4 M. When the filtered sample was spiked with 2 X lo+ M Cu(II), Cd(II), and Zn(II), the subsequently measured pulse polarographic currents were 31%, 4% and 35% lower, respectively, than those for laboratory standards. The sample was then studied by TFDD with a circulated Mg/A1 receiver. With a 10-m section of Nafion 811 tubing, 7 mL of receiver electrolyte, a 6.0 mL/min circulation rate, and a 15-min dialysis time, a Zn(I1) concentration of 1.67 X lo4 M was determined in the original sample with use of a working curve. The excellent agreement of TFDD with the standard addition quantitation attests to the veracity of the TFDD approach. Further, the sensitivity of TFDD was much greater because of the enrichment (EF = 27) and the elimination of the matrix suppression which was observed in the direct determination. Recoveries of Cu, Cd, and Zn from the spiked sample agreed well with recoveries from laboratory standards. From the spiked sample the Cu, Cd, and Zn EF's were 26,27, and 25, respectively, whereas comparable respective values of 28, 29, and 25 were obtained from laboratory standards. For multicomponent determinations, the ability to use a working curve approach to quantitation decreases the total analysis time and generally simplifies the analytical procedure.
LITERATURE CITED (1) (2) (3) (4) (5) (8) (7)
(8) (9)
Cox, J. A,; Twardowski, 2 . Anal. Chim. Acta, in press. Cox, J. A.; DiNunzlo, J. E. Anal. Chem. 1977, 49, 1272-1275. Blaedel, W. J.; Kissel, T. R. Anal. Chem. 1972, 4 4 . 2109-2111. Biaedel, W. J.; Haupert, T. J. Anal. Chem. 1966, 38. 1305-1308. Blaedel, W. J.; Haupert, T. J.; Evenson. M. A. Anal. Chem. 1989, 4 1 , 583-590. Lake, M. A,; Melshelmer, S. S. AIChE J. 1978, 24, 130-137. DiNunzio, J. E. Ph.D. Dissertation, Southern Illinois University at Carbondale, Carbondale, Ill.. 1977. Cox, J. A,; Cheng, K. H. Anal. Lett. 1978, A 1 1 , 653-660. Lundquist. G. L.; Washinger, G.; Cox, J. A. Anal. Chem. 1975, 47, 3 19-322.
RECEIVED for review March 17,1980. Accepted May 12,1980. This work was supported by the National Science Foundation under grant CHE-7908660. Partial support for Z.T. was provided by the Eastern European Universities Exchange grant from the U.S. State Department to SIU-C.
