484 (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)
Anal. Chem. 1985, 57,484-489 McKone, H. T. J . Chem. Educ. 1980, 57, 380. Willeford. B. R.; Veening, H. J . Chromatogr. 1982, 257, 61. Schlogl, K.; Widhalm, M. Chem. Ber. 1982, 775,3042. O'Laughiin, J. W. J . L i q . Chromatogr. 1984. 7 ,Suppl. 1, 127. Armstrong, D. W.; DeMond, W. J . Chromatogr. Sci. 1984, 22, 411. Armstrong, D. W.; DeMond, W.; Alak, A,; Hinze, W. L.; Riehl, T. E.; Bui. K. H. Anal. Chem. 1985. 57, 234. Hinze, W. L.; Riehl, T. E.; Armstrong, D. W. DeMond, W.; Alak, A,; Ward, T. Anal. Chem. 1985, 57, 237. Arimoto. F. S.;Haven, A. C., Jr. J . Am. Chem. SOC. 1955, 77,6295. Hill, E. A.; Richards, J. H. J . Am. Chem. Soc. 1961, 83, 3840. Sokolov, V. I.;Petrovskii, P. V.; Reutov, 0. A. J . Organometal. Chem. 1973, 59. C27. Rausch, M.; Vogel, M.; Rosenberg, H. J . Org. Chem. 1957, 22, 903. Perevalova, E. G.; Ustynyuk, Y. A.; Nesmeyanov, A. N. Izv. Akad. Nauk SSSR, Ser. Khim. 1983, 1972. Chem. Abstr. 1983, 59, 75576. Misterkiewicz, B.; Kajdas, C.; Dominiak, M.; Dabrowski, J.; Wasilewski, J. Chem. Ind. (London) 1981, 4 3 3 . Ratajczak. A.; Misterkiewicz, 8. J . Organometal. Chem. 1975, 9 7 , 73. Ratajczak, A.: Misterkiewicz. B.; Czech, B. Bull. Acad. Pol. Sci. Ser. Sci. 1977, 25,27.
(25) Ratajczak, A.; Czech, B.; Misterkiewicz, B. Bull. Acad. Pol. Sci., Ser. Sci. Chim. 1977, 25, 541. (26) Sokolov, V. I.; Troitskaya, L. L.; Reuter, 0.A. Dokl. Adad. Nauk SSSR 1977, 237, 1376. Chem. Abstr. 1978, 88, 1 3 6 7 6 1 ~ . (27) Trainor, G. L.; Breslow, R. J . Am. Chem. SOC.1981, 703,154. (28) Breslow, R.; Trainor, G.; Veno, A. J . Am. Chem. SOC. 1983, 705, 2739. (29) le Noble, W. J.; Srivastava, S.;Breslow, R.; Trainor, G. J . Am. Chem. SOC. 1983, 705,2745. (30) Harda, A.; Takahashi, S. J , Chem. SOC.,Chem. Commun. 1984, 645. (3 1) Bender, M. L.; Komiyarna, M. "Cyclodextrin Chemistry"; Springer Verlag: New York, 1978. (32) Hinze, W. L. Sep. Purif. Methods 1981, 70, 159. (33) Szejtli, J. "Cyclodextrins and Their Inclusions Complexes"; Akademai Kiado: Budapest, 1982.
RECEIVED for review September 7,1984. Accepted November 5, 1984. The support of this work by the Department of Energy, Office of Basic Energy Research (DEAS0584ER13159), is gratefully acknowledged.
Dual Membrane Annular Helical Suppressors in Ion Chromatography Purnendu K. Dasgupta,* R. Quin Bligh, and Marita A. Mercurio Department of Chemistry, Texas Tech University, Lubbock, Texas 79409-4260
A dual membrane annular helical configuration is shown to be a highly efficient low dlsperslon suppressor for ion chromatography. The device contains one filament-fllled membrane tube Inserted inside another closely fitting membrane tube of the same type. The dual membrane assembly is coiled as a small diameter helix, the shape being retalned by the filament. The column effluent fit INS In the annular space between the two membranes; regtnerant flows through two separate channels, Inside the inner membrane and through a Jacket whlch surrounds the entire device. Compared to simple filament-filled membrane helices, these devices exhibit a substantlaliy larger available membrane surface area per unit dispersion of an Injected band.
Membrane based suppressors (1-4) and postsuppressors (5, 6) have led to significant improvements in the practice of ion chromatography (IC). Membrane-based reactors for postcolumn application (e.g., detection of a transition metal ion with a suitably chelating chromogenic dye) have also been introduced (7). For application as a IC suppressor, several membrane suppressor designs have appeared. The simplest of these is a small diameter hollow membrane tube as introduced by Stevens et al. ( I ) . A single fiber configuration is ideal for minimizing dispersion a t the inlet/outlet connections and has been so used by Japanese investigators (8-10). However, the length necessary for a given application may result in an unacceptably high pressure drop, and multiple fiber configurations are necessary, as in the original design of Stevens et al. ( I ) . The ratio of available membrane surface area to holdup volume for a hollow tubular membrane is 2 / r where r is the inner radius of the membrane tube. A reduction in diameter not only increases the available surface area per unit holdup volume but also does not affect mass transfer efficiency to the wall under laminar flow conditions, according to the Gormley-Kennedy equation (11). Further, band dispersion is 0003-2700/85/0357-0484$0 1.50/0
acutely dependent on diameter and is greatly reduced with a reduction in diameter. There are practical limits to which the diameter can be reduced however. First, the smallest bore commercially available ionomeric membrane tube is approximately 900 pm in diameter (wet dimension, all ionomeric membranes expand significantly upon wetting) and band dispersion with useful lengths ( 2 ) are undesirably large. Second, although it is possible to custom synthesize narrow bore ionomeric membrane tubes (1, 12) or to reduce the diameters of the commercially available tubes (8-10, 12),there are limits to which the bore can be reduced. The current technological limit for the inner diameter of any type (not necessarily ionomeric) of tubular membrane is approximately -20 pm. Third, if the chromatographic process is to be carried out a t conventional flow rates (0.5-5 mL/min), the pressure drop is going to be too large for a single fiber configuration and multiple fiber designs will defeat the objective of a low dispersion device by virtue of dispersion introduced a t the inlet/outlet connections. Working with commercially available membrane tubes, Stevens et al. (2) found that low dispersion and efficient mass transfer to the wall can be achieved by the single bead string reactor design (13),by packing the membrane tube with inert beads of optimized diameter (60% of tube internal diameter). The packed bead suppressor of this type display a few shortcomings (4). With continued use, pressure expands the elastomeric membrane, allowing the mobile beads to pack down densely and less uniformly, leaving occasional large voids in the tube. Thus, dead volume and dispersion characteristics deteriorate with use. Further, the mobility of the beads contribute to the pressure induced rupture of the membrane. In more recent work (14), these problems have been solved by enclosing the membrane within a jacket tube and packing the space between the jacket and the membrane with beads as well. This prevents any movement of the beads inside the membrane and also improves efficiency of regenerant utilization. The same study of Stevens et al(14) also reports that ion exchange resin 1385 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
beads of the appropriate type can be used as packing and leads t o better exchange efficiencies compared t o their inert counterparts. T h e presence of a n ultrasonic field was found t o improve the efficiency of mass transport to the wall, especially for configurations that are not otherwise particularly efficient; t h e dispersion of a n injected band, however, increased unacceptably. Many other configurations, such as the presence of a n inert metallic wire (either in linear or in a waved configuration, with the membrane being intentionally constricted or pinched in places, ref 14),the membrane being constricted at close and regular intervals by applying the heated t i p of a soldering iron ( I @ , and ironing the membrane flat to produce a flat ribon configuration, then winding it tightly on a porous support and introducing regenerant t h o ugh the support (I@, have been attempted. I n related work with porous PTFE membranes, Sunden e t al. (5) have found that t h e mass transport efficiency to the wall increases in the following order: hollow tube, helical hollow tube, inserted fishing line, inserted notched fishing line, inserted twisted pair of stainless steel wires, and an inserted knotted fishing line (with knots as close as possible, the overall device is much like a single bead string reactor). In the past ( 3 , 4 ) ,we have shown that filament filled helical (FFH) membrane suppressors are particularly attractive in terms of their low dispersion characteristics and highly efficient mass transport t o t h e wall. Devices incorporating t h e FFH or t h e dual bead packed (both inside and outside packing) configuration are demonstrably superior to all other designs investigated. In practical IC systems, mass transfer efficiency both to and through t h e membrane are important. If high eluent concentrations or large lipophilic ions are t o be exchanged ( I n , mass transport efficiency through t h e membrane becomes increasingly important. I n this work, we show t h a t a dual membrane annular helical (DMAH) suppressor provides a large available membrane surface area per unit band dispersion induced by t h e device and is therefore particularly attractive for IC applications.
EXPERIMENTAL SECTION R e a g e n t s and Equipment.
Nafion perfluorosulfonate cation exchanger membrane tubing was obtained in two different sizes from the manufacturer (E. I. du Pont de Nemours and Co., Polymer Products Division, Wilmington, DE). The nominal wet dimensions of these membrane tubes are 920 pm and 620 pm in internal diameter and 170 pm and 130 pm in wall thickness, respectively, for types 815x and 811x. Dodecylbenzenesulfonic acid, DBSA, was obtained as the commercial surfactant Biosoft-S-100 (approximate purity 85%) from Stepan Chemical Co., Surfactants Dept (Northfield, IL). A stock solution, -0.5 M, was prepared and standardized by titration with a standard base. This was diluted to product a 50 mM DBSA solution and used without further treatment (12) as regenerant. Performances of different DMAH devices as IC suppressors were tested with two eluents commonly used in IC. These are 3 mM NaHC03 + 2.4 mM Naz CO, (designated henceforth as El) and 8 mM NaHC0, + 6 mM Na2C03(designated henceforth as E2). Conductivity was measured with a Model 213 conductivity detector (Wescan Instruments, Santa Clara, CA) equipped with a 2-pL cell. The cell constant was determined to be 34.1 by calibration with 0.01 D KC1. The reported conductance data are corrected to 25' C. All experiments were conducted at 22 f 1 "C. Under these conditions, the conductance of El is 20.8 pS and drops to 0.69 pS upon passing through a packed bed H+-form cation exchanger column. The respective values for E2 are 65.2 and 1.05 pS. The eluent was pumped through the suppressor device with an Altex Model llOA pump with its associated pulse dampener and an Altex Model 210 loop injector (Altex Scientific, Berkeley, CA). One regenerant channel was pumped with a Model E-120-S pump (nonmetallic fluid contact parts, Eldex Labs, Inc., Menlo Park, CA); gravity flow was sufficient for the other regenerant channel.
R
R
'
485
IY
JP G
(e1
Flgure 1. (a) Tube assembly prior to filament insertion: I , inner membrane: T, flanged PTFE insert: R, pair of O-rings; N. 1/4-28 threaded male nut: 0,outer membrane. (b) Alternative insert: S, stainless steel tube: G, polypropylene disk. (c) Tube assembly prior to coiling: F, filament: P, 30 AWG PTFE insert. Drawings are not to scale, gaps are exaggerated for clarity.
Construction of DMAH Suppressors. A segment of 811x membrane tube was inserted inside a shorter length of 815x membrane tube. As supplied, the 815x membrane tube is sufficiently larger than the 811x tube, such that insertion in the dry state for lengths upto 1m pose no special difficulties. The length of the 811x tube taken was approximately twice that of the 815x tube. As the indicated facility of insertion would suggest, the annular gap for a 811x tube inserted inside a 815x tube without further modification is too large for efficient mass transfr to the membrane walls. To reduce the annular gap, we used a solvent swelling/streching technique. Hot stretching methods have been used (8-10) to alter the diameter of Nafion membrane tubes; in our experience, the technique described hereunder is much easier to practice. The inner bores of two 1/4-28 threaded polypropylene male nuts, made for use with 1.6 mm (1/16 in.) 0.d. tubes (Rainin Instrument Co., Emeryville, CA) were widened by drilling with a 3/3z in. bit. These nuts were slipped on top of the 815x tube with the threaded ends facing the ends of the tube assembly. Next two O-rings (Buna-N, no. 004) were slipped onto each end of the 815x tube. A short segment (-3 cm) of a PTFE tube (AWG 19, standard wall, Zeus Industrial Products, Raritan, NJ; nominal dimensions: 965 pm in i.d., 406 pm wall) was taken and flanged at both ends using a hot tip flanger and without using washers. This tube was then cut exactly in half, producing two short (- 1 cm) PTFE tubes with one end of each tube terminating in a flange. These flanged PTFE tube segments were slipped over the protruding ends of the 811x tube, flanged end last, one at each end and moved inward until they butted the 815x tube. This protion of the assembly (where the flanged PTFE tubes butt the 815x tube) was briefly immersed (1min) in methanol, one end at a time. The membrane tubes swelled and the 815x tube ends were slipped over the PTFE tube segments, up to the flange. After letting the methanol evaporate completely (if desired, a hot air blower may be used for expediency), the O-rings were forcibly pushed toward the ends of the 815x tube, until they were located next to the flanges in the PTFE inserts. At this point, the tube assembly looks like Figure l a . After ensuring that approximately equal lengths of the 811x tube are protruding from both inserts in the 815x tube, the assembly of Figure l a was coiled to a convenient diameter (ca. - 5 cm) and immersed into a breaker containing 95% ethanol. The ethanol was then boiled under low heat, covered with a watch glass, for 15-20 min. The membrane tubes imbibe considerable amount of the solvent (most polar organic solvents lead to similar results) and become highly swollen and quite pliable. (Note: They also become virtually invisible in the solvent which leads the unini-
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
tiated to believe that the membrane has been dissolved.) The tube assembly was then taken out of the solvent, the ends of the 815x tube containing the PTFE inserts were grasped between the thumb and forefingers of each hand, and the 815x membrane tube was stretched to approximately twice its original length. The stretching must be carried out as soon as possible after removing the assembly from the hot solvent, while the tubes are still thoroughly wet. Further, every effort should be made to selectively stretch the outer membrane tube. This selective stretching of the outer tube is important for two reasons. First, the thicker, outer, 815x membrane tube is capable of -250% elongation without rupture under these conditions while the thinner 811x membrane tube typically breaks a t -175% elongation. Second, selective stretching of the outer membrane reduces the annular gap to a greater extent than if the inner tube was simultaneously elongated (and consequently reduced in diameter). Some stretching of the inner tube is unavoidable but most of the protruding lengths of the inner tube should move in smoothly as the outer tube is stretched. Should the innder tube move in asymmetrically (i.e., preferntially from one end), the appropriate end of the inner membrane tube may be pulled outward to correct for the asymmetry. If the smooth movement of the protruding lengths of the inner membrane tube is hindered by the constricting bore of the PTFE inserts, the latter may initially be widened by a suitable tool. (The dry 811x tube passes without any difficulty through a AWG 19 PTFE tube but free passage becomes more difficult as the membrane expands upon solvent swelling.) Optimal dimensions for the PTFE inserts are given here; difficulties may however be occasionally encountered because of significant manufacturing tolerance of the membrane tubes. It is convenient in such cases to use two different sets of PTFE inserts; of the same outer diameter but of different inner diameters. The larger inside diameter tubes need only be short unflanged segments to be used during stretching. After the tubes were stretched and dried, the smoothness of the PTFE surface permits removal of the inserts, to be replaced by smaller inside diameter flanged inserts. This replacement is desirable to avoid unnecessary band dispersion induced by the larger inside diameter inserts. Inserts of material other than PTFE are also useful and may have advantages if higher operating pressures are required. A short segment of a type 316 stainless steel tube (1/16in. o.d., 0.05 in. id.; Alltech Associates, Houston, TX) is forcibly inserted into a PTFE faced gripper fitting (made for 1.6 mm 0.d. tubing, Rainin Instrument Co., Emeryville, CA) or into a 3/16 in. diameter plastic disk with a center hole (e.g., adapter for back pressure microtube, withthe stem removed, P / N 24330, Dionex Corp., Sunnyvale, CA). Such metal inserts (Figure l b ) allow higher operating pressures compared to PTFE inserts before leakage occurs (- 200 psi with PTFE inserts). However, once inserted and the membrane allowed to shrink back, they cannot generally be removed without tearing the membrane, even if the membrane is reswelled with solvent. When the outer membrane tube is stretched approximately to twice its original length, the assembly is dried in the stretched state (tension is maintained) with the aid of a hot air blower. At this stage, a minimum of 3 cm (preferably 2 5 cm) of the inner membrane tube should protrude out of the inserts a t each end. Approximately 15 min of hot air drying is required to completely dry the assembly. Use of high power heat guns is not recommended since the membrane undergoes irreversible decomposition if the air temperature is excessively high. The length of the assembly will decrease by 5 1 0 % once the tension is removed. The next step in the construction of the DMAH suppressors is to insert the filament. Unlike the case of single membrane filament filled helical (FFH) devices (3, 4 ) , the filament is not chosen to be extremely close fitting since the primary purpose of the filament in the present device is to provide structural support for the eventual helical configuration. However, too thin a filament does not provide adequate structural support. Also, substantial deformation and collapsing of the inner membrane may be caused by the pressure of the fluid flowing between the two membranes if the filament is too thin, regardless of its structural strength (i.e., a thin metal wire will hold the helical shape but will not prevent deformation of the inner membrane). A 560 pm diameter nylon monofilament (30 lb. strength fishing line, Stren, Du Pont) was judged to be the optimum choice. A length of this filament, -3 cm shorter than the distance between
the PTFE flanges (after drying and tension release) was taken. Methanol was injected with a syringe (20 gauge needle) from one end of the inner membrane tube to completely fill it and allowed to remain for 1 min. (While methanol and ethanol both swell the membranes, at rmm temperature methanol will swell Ndion more than ethanol. The degree of swelling increases with temperature. Greater swelling occurs with ethanol a t boiling solvent temperature, presumably due to the higher boiling point of ethanol.) The syringe was then removed and the filament pushed in from one end until the free end of it was flush with the insertion end of the inner membrane. If the passage of the filament becomes difficult a t any point, more methanol may be injected from the opposite end. Slightly rotating the filament as it is being pushed in is also helpful. Next, with the aid of a metal wire of suitable size, the flush end of the filament was pushed in, until it was well into the PTFE flange. Excessive use of metahnol during filament insertion will allow some of the solvent to seep through the inner membrane and soften the outer membrane. The assembly will then tend to shrink back toward its original length in the absence of any tension. While it is possible to restretch the tube assembly again to the desired length, it is a good practice to lightly stretch the tube assembly between two rigid supports by ties atop the PTFE inserts before beginning the filament insertion procedure. Once the filament was completely inside the dual membrane assembly (both sides of the filament in or further inward of the PTFE flanges), the protruding free ends of the inner membrane were cut at a distance of a t least 3 and preferably 5 cm from the insert ends. The free ends of the inner membrane were swelled again with methanol and a length of a microbore PTFE tube (30 AWG, standard wall) was inserted a t each end until it butted the filament. The assembly was next allowed to dry completely and any excess lengths of the PTFE microbore insert was cut off flush with the membrane tube. At this point, the assembly looks like Figure IC. I t is possible to change the order of different operations described here. The filament for example could first be inserted in the inner membrane tube which could then be inserted in the outer membrane tube followed by solvent swelling, stretching, etc. It is worthwhile to note in passing that inserting a filament while the membrane is solvent swelled is considerably easier than the procedure described in our earlier papers (3, 4 ) even when filaments, which would become very tightly fitting once the solvent is evaporated, are inserted for making single membrane FFH devices. A combination of the solvent swelling technique with the fluid pressurized insertion procedure ( 3 ) makes it possible actually to inset a small diameter (340 pm) filament filled prestretched 811x membrane tube inside an unstretched 811x tube. Construction details for such devices are omitted since these devices, although significantly superior in performance compared to the 811x inside 815x devices, are substantially more difficult to construct. The order of operation given here for constructing the 811x inside 815x devices is least likely to present difficulties. The next step in the construction of the DMAH suppressor involves making the helix. The assembly of Figure ICis coiled into a small diameter helix following the procedure described earlier ( 3 ) . Briefly, one end of the tube assembly is securely tied on a 775 pm diameter nichrome wire support (the tie must be made at least 1 cm inward from the end of the filament, not on the microbore tube or the insert), coiled tightly, and the other end tied securely in a similar fashion and then the filament is thermoset by boiling the entire assembly and support in water for 30 min. The ties and support wire are removed following the thermosetting procedure. The liquid inlet/outlet connections were made with 1/4-28 female threaded tees a t each end. While such tees are available commercially, the connecting passages are generally too large for low dispersion applications. Important dimensions of our homemade polypropylene tees are indicated in Figure 2b. As the tees are connected to each end of the tube assembly, the nuts on the tube assembly compress down on the O-rings to seal the outer membrane against the PTFE insert. The inner membrane tube, its microbore PTFE tube filled ends protruding from the tees, are sealed with a flanged end AWG 19 or 20 PTFE tube followed by a washer, two O-rings, and a male nut. The beginning unthreaded portion of the nut should be cut off to obtain enough
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
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Table I. Parameters for Tubular Membrane Permselective Transport Systems bead packed tube inner filament filled tube radius r , bead inner radius r2, radius 0.6r filament radius rl
hollow tube inner radius r
effective hydraulic radius, rh surface area per unit length, S surface area per unit pressure drop, Srh4K holdup volume per unit length, V surface area per unit hold up volume, S / V mean diffusion distance to membrane, d mean diffusion distance per unit residence time, d / t
r 2rr 2ar5K rr2 2/r r/ 2 (1/2rr)(F/L)
0.82r 2rr
a 2.13r2 2.95/r
a a
( r z 2- r12)1/2 2rr2 2rr2(r,2 - r 1 2 P K r ( r z 2- r12) 2 r 2 / ( r 2 - r12) ( r 2- r 1 ) P [ 1 / 2 r ( r 2+ r l ) l ( F / L )
tube within tube inner tube outer radius r l , outer tube inner radius r2 ( r Z 2- r12)1/2 2 d r l + r2) 2a(r1 + r z ) ( r 2- rI2PK a ( r 2 - r12) 2 I h - rl) ( r 2- r J I 4 [ 1 / 4 r ( r 2+ r l ) l ( F / L )
"Flow is turbulent, these parameters are not calculable or meaningful for this design. *The constant K is r/8FqL where F is the volumetric flow rate, q is the viscosity, and L is the length. threaded length to secure a good seal. The liquid line through the short arm of the tee leads to the annular channel of chromatographic interest. Connection to this port is made with a 300 pm bore, 1.5 mm 0.d. PTFE tube, and a gripper fitting to avoid dispersion. After both tees are connected, the flow in the annular channel is tested with water with a 1mL capacity Luer-tip syringe with a Luer adapter. If no flow is obtained with moderate manual pressure, the entire assembly is immersed in methanol for a few minutes and then most of the solvent allowed to evaporate. There are normally no difficulties in obtaining discernible flow with moderate manual pressure a t this stage. I t is important not t o use excessive pressure while the initial test is being conducted; membrane rupture is very likely to occur. A sufficient amount (ca. -25 mL) of water is now pushed through the annular channel to remove any residual methanol. The jacket for the outer channel regenerant flow is most conveniently made from a Plexiglas tube, of appropriate length, with holes on opposite sides at each end for liquid inlet/outlet. The tube inside diameter should be just larger than the dimensions of the heads of the nuts used. The tube is sawed in half and reassembled enclosing the helix and cemented back. The space behind the head of each nut is f i e d with silicone rubber adhesive. Flow in this channel is gravity induced, and PVC tubes of desired size may be cemented to the inlet/outlet holes. A 3/8 in. i.d. Tygon tube may be used also as a jacket, with a slit on the jacket to slip it over the helix. The slit is subsequently covered with silicone rubber adhesive, as is the space behind the heads of the nuts. I t is preferable to use nuts with hexagonal, rather than square, heads, since the former permit a more secure assembly through applying wire crimps to fasten the Tygon tube to the nut head. The complete device is shown in Figure 2a. For performance testing, 50 mM DBSA was pumped a t -3 mL/min through the inner channel, countercurrent to the annular flow, and gravity flow of the same regenerant, also countercurrent to the annular flow, was maintained in the outer channel at 515 mL/min. For long term storage, the device should be stored full of liquid and should not be allowed to dry out.
RESULTS A N D D I S C U S S I O N Considerations on Different Design Geometries. First, we wish t o point out that a frequently cited maxim, that very small diameter hollow fibers provide t h e highest surface area/holdup volume ratio, is essentially untrue. Table I shows the pertinent parameters of permselective tubular membrane transport systems. Four designs are considered: hollow tube, bead packed tube (bead radius assumed t o be 0.6 times t h e tube radius), filament filled, and membrane tube in membrane tube. T h e consequences on dispersion and efficiency of transport due to centrifugal or turbulent flow have not been included in this consideration. T h e effective hydraulic radius, rh, is important since t h e pressure drop for any given device length and flow rate is inversely proportional to the fourth power of r,. Consideration of available membrane surface area then leads t o a practical design parameter, available surface area per unit pressure drop. With realistic values for the dimensional bounds, t h e
PT
(b)
Flgure 2. (a) Complete dual membrane annular helical suppressor: Q, PTFE inlet/outlet tube for inner channel regenerant; E, PTFE tube for
column effluent outlet/inlet: M, PVC inlet/outlet tube for outer channel regenerant; K, wire crimp; J, Tygon jacket tube: PT, polypropylene tee. Inset shows cross section of filament filled dual membrane assembly. (b) Details of polypropylene tee and connections: W, washer; G, gripper fitting; other legends as above. Dimension A is 1 cm: dimension B is exaggerated, should be as small as possible: hole H is 0.5 mm in diameter. Drawings are not to scale, gaps are exaggerated for clarity. membrane within membrane design will be clearly superior among the three designs for which the calculation is meaningful. Similarly, consideration of the holdup volume per unit length leads to t h e conclusion that the membrane within membrane design provides the maximum available surface area per unit holdup volume. The values for the bead packed device have been calculated from simple geometric considerations assuming closest possible packing density. The mean diffusion distance to the membrane is the average of distances a molecule/ion must travel to reach the wall when it is located (a) as close to the wall as possible (Le., right at the wall, this is zero for all cases) and (b) when it is located as far as possible from a wall (this is respectively r, rz - rl, and (r2- r1)/2 for
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Table 11. Parameters for DMAHS Devices
device no. 1 2
outer tube initial length, cm
inner tube initial length, cm
filament length, cm
overall device length," cm
hydraulic radius,* pm
20
40
20
50 70 75
35 40 60 55
40 45 65
210 195 200 265
3
35
4
40
60
estimated annular gap, pm
induced band dispersion,d PL
50 45
65 60
47 80
100 145
After stretching, drying, and filament insertion; before coiling. * Calculated from dead volume (residence time) measurements. e Outer diameter of 811x membrane tube in device is estimated to be 800 pm. 20 pL injection, flow rate 0.5 mL/min, see text. Band volume for connections without device is 102 ILL. the hollow, filament filled, and membrane within membrane cases. The residence time t is calculated as V L I F where VL is the total holdup volume and F is the fluid flow rate. Although exact analytical equations are not presently available, as long as the hydrodynamic transport efficiency (nature of the velocity field of the fluid) is the same, mass transport efficiency to the wall will increase as the mean diffusion distance per unit residence time decreases. Once again the DMAH design is superior with respect to this parameter, d / t . Note that the theoretical and experimental superiority of annular diffusion denuders (where both surfaces defining the annulus act as sinks) compared to hollow tube denuders have been demonstrated by Possanzini e t al. (18,19)for the sampling or removal of atmospheric trace gases. While we are not able, in these considerations, to take into account the effects of turbulent (as occurs with the packed bead device) or helical flow, we wish to note that while both results in increased radial mass transport, only helical flow leads to a decrease in axial dispersion. Performance of DMAH Devices as IC Suppressors. Band Dispersion. Dimensional characteristics and band dispersion values for four DMAH suppressor devices are reported in Table 11. There is some disparity in the literature in reporting band dispersion values for postcolumn membrane devices. Stevens et al. ( I , 2, 14) report this value as the band volume (base width of triangulated peak or twice the peak width a t half height) less the injection volume (50 pL) when the device under test is connected directly between the injector and the detector. We have used a computational approach that is more common in the chromatographic literature. Band dispersion induced by the device is defined as the square root of the difference between the squares of the band volumes (twice peak width at half height) obtained with (a) the detector and injector being directly connected by as short a connection as possible and (b) when the suppressor device is installed between the detector and the injector and including the connections used in (a) ( 4 ) . This approach yields a useful measure since dispersions of individual components of the system can be more accurately evaluated. The square of the overall dispersion defined this way (which is mathematically equivalent to the variance of the location of the sample band) is equal to the sum of the squares of the dispersion values originating from different components of the system. As a n example, a device with a measured dispersion of 195 pL was connected in series with another device with a measured dispersion of 110 pL. T h e overall dispersion caused by the combined devices was then measured to be 225 pL, in excellent agreement with the computed value ((19@ + 1102)1/2= 223.9). Unfortunately, there is no simple way to directly compare the dispersion results as reported here with the values reported by Stevens et al. The only device t h a t has been studied by both methods is the packed bead suppressor of Stevens et al. ( 2 ) for which a value of 200 pL was reported by the authors and we obtained a value of 380 pL by our procedure for the commercially marketed version of this device (4). This how-
ever does not imply that the dispersion values computed by the method of Stevens e t al. are always going to be smaller than the values obtained by the method described here. Band dispersion values as low as 50 pL have been reported by Stevens et al for the suppressor devices that are bead packed on both sides of the membrane, as computed by their method (14). Actual band volume obtained with a direct connection between the injector and detector must contribute significantly to this value for such low dispersion devices. Further, reported dispersion values are a function of the injected sample volume and the flow rate used. As may be intuitively apparent, computed dispersion will decrease with increasing sample size. A given device was found to induce apparent band dispersions of 195 pL, 180 pL, and 100 pL for injected volumes of 20 pL, 40 pL, and 100 pL, respectively, as computed by our approach. Increasing flow rate will also affect (increase) reported dispersion values. While this is true for any component of a chromatographic system contributing to overall dispersion, the case for elastomeric membrane based devices can be particularly flow rate dependent. This is because a change in flow rate means a change in pressure drop across the device which in turn results in slight changes in the actual dimensions of the membrane tubes. Clearly, some degree of uniformity is necessary in carrying out dispersion measurements for postcolumn devices if such data are to be compared. In this paper, as well as in our previous paper (4), all reported dispersion values pertain to a 20 pL injected sample a t a flow rate of 0.5 mL/min. Hydraulic Radius and Annular Gap. The hydraulic radius, rh,reported in Table I1 is calculated from the measured dead volume of the device. On the basis of dissection of devices assembled according to the experimental procedure, we estimate that the outside diameters of the inner membrane tube in the actual devices are, on the average, 10% less than their original diameter. Taking the estimated value of rl (Table I) as 400 pm, the annular gap rz - r1 is then computed from the measured value of rh. For devices 1-3, the estimated annular gap is less than the smallest annular gap (64 pm) previously attained for single membrane F F H evices ( 3 ) . When the differences in length of the devices in Table I1 are taken into account, the observed overall dispersion appears to be strongly dependent on the annular gap. The computations for rh and r2 - rl do not take into account the finite contributions made by the end fittings toward the overall dead volume. As such, the reported values should be regarded as upper limits. Exchange Efficiency. Conductance values obtained by ion exchanging E l and E2 through different DMAH suppressors are reported in Table 111. Note that with the high regenerant concentration employed, contribution of regenerant penetration to observed conductance (due primarily to H2S0, impurity in DBSA, ref 12) is significant, especially a t low flow rates. Another factor that affects the observed conductance is variable COz loss, which is likely dependent on the pressure drop as well as the completeness of exchange. I t should be noted, however, that increased background conductance due
ANALYTICAL CHEMISTRY, VOL. 57, NO.
Table 111. Ion Exchange Efficiency of DMAHS Devices conductance, p S eluent E l n
flow rate, mL/min
0.5 1.0
1.5 2.0 2.5 3.0
3.5 4.0
device 1 device 2 0.560 0.550 0.540 0.680 nec
0.565 0.545 0.520 0.660 0.920 nec
eluent E2*
device 3
device 2
device 3
0.690 0.569 0.528 0.508 0.400 0.499 0.505 0.512
0.770
0.940 0.814 0.810 0.986 1.01
0.840 1.04 nec
nec
Conductance 20.8 p S , completely exchanged conductance 0.690 *Conductance 65.2 p S , completely exchanged conductance 1.05 p S . CTheconductance is erratic, at this flow rate and higher, the eluent is not completely exchanged.
pS.
to incomplete exchange is easily distinguished from that due to regenerant penetration; the former leads to an unstable and erratic conductance reading. Comparison of the data in Tables I1 and I11 with those in Tables I and I1 of ref 4 clearly indicates the superior performance of the DMAH suppressors. Other Designs. Sunden e t al. ( 5 ) reports their favorable experience with knotted monofilaments as inserts for postsuppressors; however, no band disperson values were presented. An ample collection of monofilament lines in this laboratory prompted us to evaluate the merit of this design as a Nafion membrane based suppressor. Knots were made on a 200 km diameter filament as close as possible, according to the described procedure (ref 5; we agree that this is a labor intensive procedure, 1800 knots per meter of knotted filament wee required), inserted inside a 811x tube which was then solvent swelled and stretched to obtain a tight fit. The results were disappointing. A 1.5 m long device could not exchange E l a t flow rates greater than 1 mL/min and induced a dispersion of 195 FL. Attempts t o knit very small diameter PTFE hollow fibers with toy knitting machines have been made to fabricate postcolumn reactors (20). We believe that the DMAH design is superior to these approaches. Within the domain of membrane based annular helical postcolumn devices, there are two basic designs. In one design, the outermost (jacket) tube is concentric with the membrane helix (design b, ref 3) as are the DMAH devices presented in this paper. In the other configuration (design a, ref 3), the jacket tube is concentric with the membrane tubes and the filament. T h e limitations of this design encountered for the single membrane F F H devices ( 3 ) d o not apply to dual membrane devices. Implementation of this design with a flexible jacket tube which is able to withstand reasonably high pressures will sllow much higher operating pressures, because membrane rupture is prevented by a totally enclosed closely fitting concentric system. Further, the annular gap can be tailored by connecting the regenerant channels in series and
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489
intentionally applying backpressure. Pressure in the innermost channel expands the inner membrane and pressure in the outermost channel compresses the outer membrane, thus reducing the annular gap. Necessarily, the implementation of such a design is more complex. One disadvantage of the DMAH suppressors described here is the need for a regenerant pump. A regenerant pump is also, however, necessary for other efficient low dispersion designs, such as the dual bead packed design of Stevens e t al. ( 1 4 ) . Although we have demonstrated the superior performance of the DMAH devices only as IC suppressors in this work, it is clear that the concept will be applicable in many other systems of chemical interest. As an example, we were able to produce high resistivity water from local tap water (>lo00 ppm dissolved solids) by inserting a custom synthesized P T F E anion exchanger membrane tube (12) inside a Nafion membrane tube and using NPr,OH as the anion regenerant in the inner channel and poly(styrenesu1fonic acid) as the cation regenerant in the outer channel while the tap water was made to flow in the annular channel.
LITERATURE CITED (1) Stevens, T. S.; Davis, J. C.; Small, H. Anal. Chem. 1981, 5 3 , 1488- 1492. 12) Stevens, T. S.;Jewett. G. L.; Bredewea. R. A. Anal. Chem. 1982. 5 4 , 1206-1208. (3) Dasgupta, P. K. Anal. Chem. 1984, 5 6 , 96-103. (4) Dasgupta, P. K. Anal. Chem. 1984, 5 4 , 103-105. (5) Sunden, T.; Cedergren, A.; Siemer, D. D. Anal. Chem. 1984, 5 6 , 1085- 1089. (6) Siemer, D. D.; Johnson, V. J. Anal. Chem. 1984, 5 6 , 1033-1034. (7) Davis, J. C.; Peterson, D. P., submitted for publication in Anal. Chem. (8) Hanaoka, Y.; Murayama, T.; Muramoto, S.;Matsura, T.; Nanba, A. J. Chromatogr. 1982, 239, 537-548. (9) Rokushika, S.;Sun, 2. L.; Hatano, H. J. Chromatogr. 1982. 2 5 3 , 87-94. (10) Rokushika, S.;Qiu, 2. Y.; Hatano, H. J. Chromatogr. 1983, 2 6 0 , 81-87. (11) Gormley, P. G.; Kennedy, M. Proc. R . I r . Acad., Sect. A . 1949, 52A, 163-169. (12) Dasgupta, P. K.; Bligh, R. Q.; Lee, J.; D'Agostino, V. Anal. Chem. 1985, 5 7 , 253-257. (13) Reign, J. M.; Van der Linden, W. E.; Poppe. H. Anal. Chim. Acta 1981, 123, 229-237. (14) Stevens, T. S.;Jewett, G. L.; Bredeweg, R. A,; Westover, L. 8 . ; Small, H. European Patent Application Publication No. 007537 1, Filed September 17, 1982. (15) Stevens, T. S.,personal communication, May 1964. (16) Dasgupta, P. K., unpublished studies, 1982. ( 1 7 ) Dasgupta, P. K. Anal. Chem. 1984, 5 6 , 769-772. (16) Possanzini, M.; Febo, A.; Liberti, A. Atmos. Environ. 1983, 17, 2605-26 10. (19) Possanzini. M.; Febo, A.; Cecchini, F. Anal. Len. 1984, 17 (A10). 887-896. (20) Berry, V.V.; Shansky, R. E.; Goldberg, A. P. LC, Liq. Chromatogr. HPLC Mag. 1984, 2 , 623.
RECEIVED for review September 21,1984. Accepted October 24, 1984. This work was partially supported by U.S. Department of Energy Grant No. DE-FG05-84ER13281;however, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE.
