103
Anal. Chem. 1984. 56,103-105
for realistic values of the influent concentration, the transfer through the wall is likely to become rate limiting long before this maximum value can be attained.
CONCLUSIONS Although we have focused our attention in this article on the use of the FFH as an efficient continuous cation exchanger, mass transfer efficiency and low dead volume considerations are of paramount importance in many other systems, including dialyzers in artificial kidney machines. Detailed theoretical considerations on the hydrodynamics of helical flow through an annulus are clearly necessary. We believe that such a flow configuration will be of utility in a large number of membrane-based separation systems. The computational approach presented here can be extended to other systems. For example, in designing dialyzers with porous membranes, the density of pore distribution is analogous to exchange site density of an ionomeric membrane, while the selectivity coefficient will be controlled by the pore size distribution and the size of the molecule to be dialyzed. Additionally, in open tubular capillary LC, the generation of active sites in the walls of such small capillaries is a significant task. It is considerably easier to produce such sites on a filament. A thermally shrinkable jacket could then be shrunk down on the filament, yielding very small annular gaps. With appropriate membrane material, an FFH could be used for efficient mixing in of a reagent without large band dispersions. We hope that the considerations outlined in this paper will promote the use of annular and annular helical flow in analytical chemistry.
ACKNOWLEDGMENT Discussions with Daniel P. Y. Chang, Department of Civil Engineering, University of California at Davis, were invaluable during all phases of this work. The author is indebted to Donald Chinn, Lubbock High School, for carrying out the bulk of the experimental work. Wescan Instruments is thanked for the gift of the conductivity detector used in this work. Registry No. K, 7440-09-7;NEt4+,66-40-0;H3P04,7664-38-2; HOAc, 64-19-7; HC104,7601-90-3; H2S04,7664-93-9.
LITERATURE CITED (1) Braman, R . S.; Shelley, T. J.; McClenny, W. A. Anal. Chem. 1982, 5 4 , 358-364. (2) Ferm, M. Atmos. Environ. 1979, 73,1385-1393. (3) Gormley, P. 0.; Kennedy, M. Proc. R . I r . Acad., Sect. A 1949, 52A, 163-169. (4) D'Ottavlo, T.; Garber, R.; Tanner, R. L.; Newman, L. Atmos. Environ. 1981, 75, 197-203. ( 5 ) Snyder, L. R.; Kirkland, J. J. "Introduction to Modern Liquid Chromatography", 2nd ed.; Wiley: New York, 1982; p 87. (6) Stevens, T. S.;Davis, J. C.; Small, H. Anal. Chem. 1981, 53, 1488- 1492. (7) Klrkland, J. J., E.I. du Pont de Nemours and Co., personal communlcatlon, Sept 1981. (8) Dean, W. R. Philos. Mag. 1927, 4 , 207-223. (9) Dean, W. R. Phllos. Mag. 1928, 5 , 673-695. (10) Truesdell, L. C., Jr.; Adler, R. J. AIChE J. 1970, 16, 1010-1015. (11) Austln, L. R.; Seader, J. D. AIChE J. 1973, 79, 85-93. (12) Janssen, L. M. Chem. Eng. Sci. 1976, 37,215-218. (13) Deelder, R. S.; Kroll, M. G. F.; Beeren, A. J. B.; Van den Berg, J. H. M. J . Chromatogr. 1978, 749, 669-682. (14) Trivedl, R. N.; Vasudeva, K. Chem. Eng. Sci. 1974, 2 9 , 2291-2295. (15) Trivedi, R. N.; Vasudeva, K. Chem. Eng. Sci. 1975, 30, 317-325. (16) Lundberg, R. E.; Reynolds, W. C.; Kays, W. M. NASA Technical Note D-1972, Aug 1963. (17) Possanzlni, M.; Febo, A,; Libertl, A. Atmos. Environ ., in press. (18) Craig, L. C.; Chen, H. C.; Taylor, W. I. J . Macromoi. Sc;., Chem. 1969, A 3 , 133-149. (19) Stevens, T. S.; Jewett, G. L.; Bredeweg, R. A. Anal. Chem. 1982, 54, 1206-1208. (20) Dasgupta, P. K. Anal. Chem., followlng paper in this issue. (21) Esayian, M., E. I . du Pont de Nemours and Co., personal communication, 1982, 1983. (22) Elsenberg, A., Yeager, H. L., Eds. ACS Symp. Ser. 1982, No. 180. (23) Cox, J. A.; Litwinski, G. R. Anal. Chem. 1983, 55, 1840-1642. (24) Yeager, H. L. ACS Symp. Ser. 1982, No. 180, 41-64. (25) Helfferlch, F. "Ion Exchange"; McGraw-Hill: New York, 1962; p 143. (26) Poole, L.; Borchers, M. "Some Common BASIC Programs", 3rd ed.; Osborne/McGraw-Hill: Berkeley, 1979. (27) Yeager, H. L.; Steck, A. Anal. Chem. 1979, 57, 862-885. (28) Stumm, W.; Morgan, J. J. "Aquatlc Chemistry", 2nd ed.; Wiley: New York, 1981; p 212. (29) Eberhart, J. G.; Sweet, T. R. J. Chem. Educ. 1966, 3 7 , 422-425. (30) E. I . du Pont de Nemours and Co., product information literature, Naflon perfluorosulfonic acid products, 1976.
RECEIVED for review September 15, 1983. Accepted October 7, 1983. This work was supported partially by the Water Resources Center, Texas Tech University, and by the NIH Biomedical Research Support Program at TTU. D.C. was supported by a minority high school student research internship program.
Annular Helical Suppressor for Ion Chromatography Purnendu K. Dasgupta Department of Chemistry, Texas Tech University, Box 4260, Lubbock, Texas 79409
A nylon monofilament filled Nafion perfluorosulfonate catlon exchanger membrane tubing functlons as an efflclent suppressor of low dlsperslon and dead volume for anion chromatography.
Since its introduction in 1975 (I),ion chromatography (IC) with eluent conductance suppression and conductometric detection has established itself as a singularly important and powerful tool, especially in anion analysis. The facility of such analysis as demonstrated by commercially available instrumentation is, at least in part, responsible for resurgent interest in ionic analysis by chromatography. The modern practice of ion chromatography includes alternative detection methods 0003-2700/84/0356-0103$01 SO/O
(2-11). Nonsuppressed single column IC with conductometric detection has also been shown to be useful (12-16). The advantages and disadvantages of nonsuppressed vs. suppressed IC have been discussed in the literature (12, 15,17). In essence, the advantages of suppressed IC include wide dynamic range (large column exchange capacity, low probability of column overloading), rapid equilibration, high sensitivity (good base line stability and low drift), and a wide choice of eluents. It suffers from (a) the need to regenerate the suppressor column, (b) variable retention of weak acids by the suppressor column as a function of its degree of exhaustion, and (c) band broadening induced by the suppressor. With the introduction of the continuously regenerate&hollow fiber suppressor (18) which is always in the same state of regeneration, problems (a) and (b) have been effectively solved. The rather large 0 1983 Amerlcan Chemical Society
104
ANALYTICAL CHEMISTRY, VOL. 56, NO. 1, JANUARY 1984
dispersion caused by the original version of the continuous suppressor has since been reduced by a packed bead configuration (19). In view of continuing advances in improving column efficiency (20), the band broadening introduced by this modified version is still significant. In a companion paper (21),we have introduced the concept of helical flow through an annulus as a highly efficient way to conduct mass transport to the walls of a tube. In the present article, the use of a filament-filled helix (FFH) as an IC suppressor of low dead volume and dispersion is described and the performance is compared with a continuous suppressor of packed bead configuration.
EXPERIMENTAL SECTION Construction of devices and other experimental details have been described in ref 21. The devices used in this work consist of 0.66 mm diameter nylon monofilament (40 lb strength fishing line, Stren, Du Pont) filled Nafion 811X (Du Pont Polymer Products Division, Wilmington, DE) perfluorosulfonate cation exchanger membrane tubing (-0.7 mm id.) coiled into a helix with a coil diameter of 2 mm. The helix was enclosed in a glass jacket (3 mm i.d.1 provided with inlet/outlet T's for flow of regenerant solution (design b in ref 21). Sulfuric acid solution (12.5 mM) was used as regenerant, the choice being dictated by considerations outlined in ref 21. The regenerant solution was allowed to flow by gravity countercurrent to the flow in the helix. The reported conductance data refer to a cell constant of 34.1. The experiments were carried out at 30 "C; the conductance data are corrected to 25 "C. The packed bead suppressor described in ref 19 was obtained as its commercial version (Dionex Corp., Sunnyvale, CA). Apparently the use of glass rather than poly(styrenedivinylbenzene) beads is the only major difference from the literature description of this device. Two solutions, 3 mM NaHC03 + 2.4 mM Na2C03and 8 mM NaHC03+ 6 mM Na2C03, which are widely used in anion chromatography, were used in this work for testing ion exchange efficiency and are designated as E l and E2, respectively. Dead volume and band dispersion experiments were carried out with water pumped at 2 mL/min through the device, injecting 20 WLof 1 mM KNOBsolution with a loop injector (Altex Model 210, Berkeley, CA), and recording the optical absorbance as determined at 230 nm by a Schoeffel Model 770 detector (Kratos Inc., Westwood, NJ) with a time constant of 0.5 s at a chart rate of 25 cm/min. In all cases, values for connections only were also determined and were significant. The dead volume of the device under study was taken to be F(t - t') less half the injection volume (10 wL) where F is the flow rate and t and t'are the retention times of the KNOBslug for the device (including terminating connections) and connections only. The band broadening introduced by the device was calculated as (W - W'2)1/2where Wand W'are the respective band volumes (twice the peak width at half height) for the device plus terminations and terminations only (22). RESULTS AND DISCUSSION Quantitative exchange is essential for proper operation in IC. Unfortunately, with Nafion membrane based continuous exchangers, the degree of cation exchange observed with carbonate solutions is difficult to evaluate accurately due to the facile loss of C02 through the membrane. Thus, such membrane-based continuous exchangers often lead to suppressed eluent conductances significantly below those obtained with a conventional packed column suppressor. It is therefore important to realize that the attainment of the packed suppressor conductance does not guarantee quantitative exchange. A lack of quantitative exchange manifests itself in serious loss of sensitivity in determining sample ions. The calibration plot (peak conductance vs. concentration of sample ion injected) becomes highly nonlinear a t low sample ion concentrations, the linear portion suggesting a negative intercept on the conductance axis at zero sample ion concentration. With El, we have found that if the suppressed eluent conductance is 95% or below that of the conductance obtained after passage through a conventional packed column suppressor, calibration
Table I. Performance as an IC Suppressor flow rate, mL/ min 0.5 1.0
1.5 2.0 2.5 3.0 3.5 4.0
E l a conductance, p S 50cm 75 cm 100cm 0.48-0.59 0.47-0.56 0.52-1.00 0.54-1.30 0.77-1.80 1.29-2.29
0.56-0.59 0.54-0.66 0.45-0.46 0.50-0.54 0.65-0.80 0.84-1.06
2.20-3.00
1.18-1.60
50 cm,& Dionex
0.56-0.84 0.54-0.66
0.50-0.53 0.80-0.85
0.50-0.51 0.50-0.52 0.45-0.50 0.50-0.55 0.48-0.62
1.20-1.65 2.00-3.40 3.10-5.20
flow
rate, mL/ min 0.5 1.0
1.5 2.0 2.5 3.0 4.0
E2Cconductance, p S 50 cm 75 cm 100 cm 0.47-0.78 0.77-0.96 1.10-1.51 3.70-4.75
0.85-1.08 0.74-0.92 0.91-1.15 1.50-2.31
10.6-12.2 16.3-22.0
3.71-6.23 6.30-11.6
0.83-1.25 0.85-1.12 0.75-0.86 0.84-1.15 1.22-1.69 2.66-4.90
a Direct conductance 20.8 p S . Completely exchanged conductance 0.69 pS. 50 cm length from commercially available packed bead suppressor. Direct conductance 65.2 p S ; completely exchanged conductance 1.05 MS.
plots for the chloride ion at the 0.1-1 ppm level is linear. In terms of molarities, 0.1 ppm C1- is 2.8 KMwhile E l is 7.8 mM Na+. These results therefore suggest that under these conditions, the eluent is a t least 99.97% exchanged. This is suggested as an arbitrary level of acceptable completeness of exchange. The effluent conductances for E l and E2 a t various flow rates for annular helical devices of different lengths are shown in Table I. These data represent the observed range of results for a number of devices made in this laboratory. As has been noted in ref 21, annular helical exchangers excel in their efficiencies of mass transport to the wall. However, mass transport to the wall is only one of the factors in the observed overall efficiency for solutions of such concentrations ([Na+] = 7.8 and 20 mM for E l and E2, respectively). We believe that the variation of the wall thickness of the membrane tube among different devices (as well as among different regions of the same device) is the primary reason for the variance of the data reported in Table I. Even with a given device, merely changing the direction of flow (while maintaining countercurrent regenerant flow) often produces a significant change in the effluent conductance. This is not observed when a more dilute solution (e.g., 1mM KNOJ is ion exchanged under the same conditions. With such dilute solutions mass transport through the wall is a less important factor (21). The general conclusion that can be reached from Table I is that with E l , 50-cm devices are adequate for flow rates up to 1 mL/min; many perform well up to 2 mL/min. Devices 75 cm in length are adequate for flow rates up to 2 mL/min; many perform well up to 3 mL/min. Devices 1 m long will exchange E l up to 4 mL/min and, more often than not, up to 5 mL/min (data not shown). We have not carried out detailed studies on minimum regenerant flow requirements since it is so dependent on the exact jacket design and dimensions. However, for the dimensions of the device given in the Experimental Section, 12.5 mM H2S04flowing a t 5 mL/min is sufficient to completely exchange E l at 4 mL/min through the 100-cm device. The commercial packed bead suppressor is 150-155 cm long. As have been pointed out by Stevens et al. (19),the diameter
ANALYTICAL CHEMISTRY, VOL. 56, NO. 1, JANUARY 1984
Table 11. Dispersion D a t a device
connections only 50 cm annular helix 75 cm annular helix 100 cm annular helix 150 cm packed bead supressor
dead vol, pL 125 90 135 180 935
band
disper-
vol, pL sion, ,ILL
180
200 220 234 421
90 130 150 380
of the packing beads has been optimized in their design and it is indeed an efficient configuration. Since the 1.5 m length results in complete exchange even a t 4 mL/min, the performance of this device could not be compared with the FFH devices without altering the length. Fifty-centimeter sections of the packed bead device were utilized for comparison and the results appear in Table I. The FFH is more efficient compared to the packed bead device of the same length. It is however, important to recognize that, over a significant portion of the device, the rate of exchange is controlled by mass transport through the wall, which is not dependent on the nature of the flow field. As such, the differences in mass transfer efficiencies (to the wall) between the two configurations are not reflected in these data. The differences decrease even further with the higher concentration eluent E2. Conversely much larger differences are observed with eluents of lower concentration. One of the disadvantages that we have found with the packed bead suppressor is the deterioration of performance, especially with respect to band broadening and dead volume, with use. The beads are reportedly packed by suction, without pressure. Under use, pressure expands the elastomeric membrane allowing the mobile beads to pack down densely and less uniformly, leaving large voids in the tube. Additionally, the mobility of the beads contributes to pressureinduced rupture of the membrane at lower pressures than that observed with the FFH, presumably by creating local pressure points. From standard ASTM procedures, Esayian (23) estimates the burst pressure of the Nafion 811X tubing to be 400 psi when wet. We suspect that the actual burst pressure is probably somewhat higher since our 1-m devices are in continued operation at 350-400 psi (4 mL/min) for extended periods of time. Concerning pressure drops, the increase in pressure drop with flow rate is less than linear. When the FFH is first used, the flow rate must be brought up slowly, allowing the fluid to force open its passageway. It is also good practice not to allow the FFH to become completely dry. If these precautions are taken, the FFH suppressor provides a long trouble-free life. Occasionally, after
105
continued use, a small gap develops between the microbore insert and the filament (see Figure 1in ref 21). If dead volume and dispersion considerations are important, the inlet needs to be disassembled, the microbore insert is pushed in, excess membrane is cut off, and the device is reassembled. It is important to emphasize that devices much longer than necessary for a given application are undesirable, especially if weak acid anions are to be analyzed. Analogous to the C02 loss, loss of the weak acid occurs through the membrane, resulting in diminished analyte signals and increased nonlinearity in calibration plots. Dispersion and dead volume data are reported in Table 11; here the superiority of the FFH is clearly evident. The extent of the increase in dispersion with device length suggests that a significant portion of the observed broadening is due to the terminations integral to the device and not the FFH itself.
ACKNOWLEDGMENT This research was supported partially by the Water Resources Center at Texas Tech University. Registry No. Nafion 811X, 65666-67-3.
LITERATURE CITED Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 47, 1801-1 809. Girard, J. E. Anal. Chem. 1979, 51, 836-839. Rlcci, G. R.; Shepard, L. S.; Coiovos, G.; Hester, N. E. Anal. Chem. W81, 5 3 , 610-813. Zenki, M. Anal. Chem. 1981, 53, 968-971. Small, H.; Miller, T. E., Jr. Anal. Chem. 1982, 54, 462-469. Denkert, M.; Hackzell, L.; Schill, G.; Sjogren, E. J. Chromatogr. 1981, 218, 31-43. Reeve, R. N. J. Chromatogr. 1979, 177, 393-397. Buytenhuys, F. A. J. Chromatogr. 1981, 218, 57-64. Cortes, H. J. J . Chromatogr. 1982, 234, 517-520. Bond, A. M.; Heritage I . D.; Wallace, G. G.; McCormlck, M. J. Anal. Chem. 1982, 54, 582-585. Wllllams, R. J. Anal. Chem. 1983, 55, 851-854. Molnar, I.; Knauer, H.; Wilk, D. J . Chromatogr. 1980, 201, 223-240. Gjerde, D. T.; Frltz, J. S.; Schmuckler, G. J. Chromatogr. 1979, 186, 509-519. Gjerde, D. T.; Schmuckler, G.; Fritz, J. S. J. Chromatogr. 1980, 187, 35-45. Gjerde, D. T.; Fritz, J. S. Anal. Chem. 1981, 53, 2324-2327. Okada, T.; Kuwamoto, T. Anal. Chem. 1980, 55, 1001-1004. Pohl, C. A.; Johnson, E. L. J . Chromatogr. Sci. 1980, 18, 442-452. Stevens, T. S.; Davis, J. C.; Small, H. Anal. Chem. 1981, 53, 1488-1492. Stevens, T. S.; Jewett, G. L.; Bredeweg, R. A. Anal. Chem. 1982, 54, 1206-1 208. Stevens, T. S.; Langhorst, M. A. Anal. Chem. 1982, 54, 950-953. Dasgupta, P. K. Anal. Chem., precedlng paper in this issue. Snyder, L. R.; Kirkland, J. J. “Introduction to Modern Liquid Chromatography”, 2nd ed.; Wiley: New York, 1982. Esayian, M., E. I . Du Pont de Nemours Co., personal communication, 1982.
RECEIVED for review September 15,1983. Accepted October 7 , 1983.
