Anal. Chem. 1964, 56,769-772
769
Ion Chromatographic Separation of Anions with Ion Interaction Reagents and an Annular Helical Suppressor Purnendu K. Dasgupta Department of Chemistry, Box 4260, Texas Tech University, Lubbock, Texas 79409
Capacity factors for more than 30 common anions on a macroporous poiy(styrenedivinylbnzene) statlonary phase with tetraethyl- and tetrapropylammonlum hydroxide as ion Interaction reagents are reported. Conductometrlc detection Is used after eluent suppression wtth a contlnuoudy regenerated filament filled heilcal membrane suppressor. Separations of test solutes are shown by use of lsocratlc elution wlth an NaOH. The feasibiilty of graaqueous solution of NPr,OH dient elution utllizing an increaslng gradient of NaOH:NPr,OH is demonstrated.
+
Ion chromatography (IC) with eluent conductance suppression and conductometric detection (1)has become the technique of choice for a variety of applications. No less than 22 review papers in several languages and two monographs have appeared within the first 5 years since the inception of the technique (2) and numerous papers have been published since. A recent paper by the original author outlines the present status of the field (3). The current scope of IC spans a range of alternative detection methods. However, electrical conductivity, which uniquely characterizes ionic solutes in solution, remains the most widely used method of detection. The utility of nonsuppressed IC with conductometric detection has been demonstrated and a recent book details the scope (4). The relative merits of suppressed vs. nonsuppressed IC have been discussed in the literature (5-7). Molnar et al. (5) and Cassidy and Elchuk (8)have reported the use of conductivity detection in conjunction with a nonpolar column and an ion interaction reagent (IIR), which is adsorbed on the column and effectively generates exchange sites for counterions. The particular system used by Molnar et al. was nonsuppressed, although in parallel studies involving bonded ion exchanger stationary phases, the authors found superior base line stability and thus superior detectability for suppressed, compared to nonsuppressed, systems. Cassidy and Elchuk have rejected the applicability of suppression primarily from a consideration of band dispersion induced by the suppressor. With the introduction of membrane-based continuous suppression systems (9) many of the objections against suppressed IC have been removed. Band broadening in such suppressors has been reduced by packing the membrane tube with inert beads of optimized size (IO),drawing a very narrow bore tubing from commercially available membrane tubing (11,121, and most recently by constructing the membrane suppressor in the form of an annular helix (13). The last configuration involves the insertion of a closely fitting inert filament inside the membrane tube and coiling the filament-filled membrane into a very small diameter, low-pitch, helix to produce a filament-filled helix (FFH). During operation, the solution to be ion exchanged is pumped through the helix, whereupon it flows in the narrow annular space between the filament and the membrane while an appropriate reagenerant solution flows outside the FFH to keep the ion exchange capacity of the membrnae rejuvenated. Detailed practical and theoretical considerations on such systems have
been published (13,14). Concerning band broadening, true microcapillary ion chromatographic systems incorporating hollow fiber suppressors have been shown to produce excellent results (15), thus there remain few compelling arguments against suppressed IC. The separation of anionic solutes on a nonpolar column requires a cationic IIR. For maximum sensitivity with conductometric detection, the IIR counterion (the eluent ion) should be hydroxide, regardless of whether or not suppression is used. This is because in the unsuppressed mode, the hydroxide ion has the highest mobility (or equivalent conductance) of any anion, thus yielding the highest difference signal (16). With such an eluent, Cassidy and Elchuk (8) have suggested the introduction of a weak organic acid a t the column exit to produce a hydrophobic ion pair and thus reduce the background conductance. The present paper shows that the judicious choice of an IIR in the hydroxide form leads to an acceptable compromise in a membrane-based suppressor system without further reagent addition. Additionally, this approach permits gradient elution which, in spite of its well-established benefits, is rarely practiced in IC due to large changes in background conductance. In this study, the chromatographic behavior of a large number of sample anions using tetraethyl- and tetrapropylammonium hydroxide as IIR on a macroporous poly(styrene-divinylbenzene) column in conjunction with a FFH suppressor has been examined. EXPERIMENTAL SECTION The chromatographic equipment used in this work consists of a ternary pumping system with three Model llOA pumps controlled by a Model 421 controller, a stirred mixing chamber, and a Model 210 injection valve equipped with a 5-pL loop (Altex, Berkeley, CA). The conductivity detector (Model 213, Wescan Instruments, Santa Clara, CA) was equipped with a flow cell of nominal volume of 2 HL. The flow cell was thermally insulated by a polyurethane foam mold. All experiments were conducted at 22 f 1 "C. The FFH membrane suppressor was fabricated from Nafion 811x cation exchanger membrane tubing (Du Pont Polymer Products Division, Wilmington, DE). The original length of the membrane tubing was 50 cm, the filament diameter was 0.66 mm, and the coil diameter was approximately 2 mm. The construction and operation of the FFH suppressor have been described in more detail earlier (13, 14). Tetramethyl- and tetraethylammonium hydroxide were obtained as the crystalline pentahydrate and in 20% aqueous solution, respectively (Aldrich, Milwaukee, WI). Tetra(npropy1)ammonium hydroxide was made by ion exchanging the corresponding iodide (Aldrich) on hydroxide-form Dowex 1-X8 anion exchange resin. Reasonable care was taken to prevent COz absorption, by employing soda-lime guard tubes. All stock IIR hydroxide solutions were standardized by potentiometric titration with primary standard KHP. Sodium hydroxide solutions were similiarly standardized and were prepared from a carbonate-free stock reagent (50% NaOH). The solute anions were injected as the free acid or the sodium/potassium/ammoniumsalt. The usual concentration injected was 0.1 mM, with occasional changes made as necessary for very fast or very slow eluting species to obtain adequate detector response.
0003-2700/84/0356-0769$01.50/00 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
Table I. Capacity Factors ( k ’ )for Various Sample Anions
solute Io; FNH,SO; CH,COOHCOOc1NO; BrO; Br N3-
NO;
c10; 1c10; SCNsulfanilate
IIR NEt,OH, NEt,OH, NEt,OH, 1mM 5mM 10mM 0.12 0.12 0.14 0.19 0.21 0.16 0.19 0.22 0.22 0.28 0.28 0.33 0.42 1.00 1.19 1.33
0.40 0.40 0.42 0.44 0.46 0.46 0.53 0.56 0.56 0.65 0.66 0.79 1.23 2.67 3.16 3.30
0.44 0.44 0.46 0.49 0.52 0.52 0.60 0.69 0.69 0.84 0.86 1.28 2.56 3.79 4.42 4.12
NPr,OH, 1 mM 3.16 3.19 3.46 3.56 3.60 3.81 4.81 5.20 5.20 6.02 6.76 9.37 13.2 60.2 49.9 30.9
Macroreticular poly(styrene-divinylbenzene) (PRP-1, 10 pm) was obtained from Hamilton Co. (Reno, NV). This resin was slurry packed (in an aqueous medium containing 10% glycerol and 2.5% NaCl according to manufacturer’s recommendation) into a 2.4 mm i.d., 15 or 50 cm long stainless steel tube. We lack appropriate packing equipment and the stainless steel tubing used for the column does not meet the requirements of high-performance work. As a result, the observed column efficiency was poor. With 80:20 acetonitri1e:water flowing at 0.5 mL/min, only 2500-3500 plates/m was observed, with benzene and toluene as test solutes. Commercially packed PRP-1 columns (4.1 mm id.) are reportedly an order of magnitude more efficient. With 4.6 mm i.d. columns of 10-30 cm length, made from HPLC grade column tubing, we have been able to attain as much as 10000 plates/m as tested under the above conditions; but this is still far short of what is attained in the commercial columns containing this packing. Capacity factors for sample anions with tetraethylammonium hydroxide as IIR were determined on a commercially packed 150 X 4.1 mm PRP-1 column at a flow rate of 1.5 mL/min (flow velocity, 1.89 mm/s). For reasons discussed in the next section, smaller flow rates were required in working with tetrapropylammonium hydroxide as IIR and the homepacked 150 X 2.4 mm column was used at a flow rate of 0.5 mL/min (flow velocity, 1.84 mm/s). The reported capacity factors are corrected for the dead volume of the suppressor and associated connections (90 pL + 35 pL).
RESULTS AND DISCUSSION Capacity factors for a number of solute anions using 1,5, and 10 mM NEt40H and 1 mM NPr40H are shown in Table I. Capacity factors were too low with NMeOH as IIR, even a t concentrations up to 50 mM, to be of much interest and are therefore omitted from the table. All capacity factors increase with increasing IIR concentration; the effect of 1mM NPr40H is very significantly larger than 10 mM NEt40H. It is also apparent that many of the solute anions studied are sufficiently lipophilic to interact significantly with the stationary phase itself. Picrate is an extreme example, in that it is so strongly adsorbed on PRP-1, that even in the absence of any IIR, methanol does not completely desorb it. Trichloroacetate and, to a lesser extent, sulfanilate also interact with PRP-1 in the absence of any IIR. Trichloroacetate may indeed be useful as an optically transparent IIR itself; we are not aware of any such reported application in the literature. Table I is arranged according to the ionic charge of the solute anion expected at the operating pH and in order of increasing capacity factors within each group. Phosphate is classified as HP042-,although a significant amount of PO:- is present at the operating pH and the fraction of Po43-increases with increasing pH accompanying the increase in IIR concentration. Consequently, the degree of increase of the capacity factor
solute
c1,coos0,ztartrate so,2picrate
c20,2-
CrO, 2S,O, 2 maleate HPO, 2 -
s,o,zphthalate s,o,*-
citrate Fe(CN), 3 Fe(CN), 4 -
NEt,OH, 1mM 10.1
I1R NEt,OH, NEt,OH, NPr,OH, 5mM 10mM 1mM
28.8
m
m
0.19 0.19 0.23 0.23 0.19
0.79 0.84 0.88 0.88 0.88 0.93 0.91 1.02 1.14 2.46 2.77
0.16
0.23 0.16 0.21 0.37 0.33 0.23 0.30 0.26
1.81
4.88 4.63
1.07 2.16 1.12 1.14 1.19 1.23 1.23 1.67 1.60 3.74 4.81 2.67 8.91 8.44
28.8 37.0 30.9 37.8 41.3
with increasing IIR concentration is high for phosphate compared to most of the other anions. Borate, cyanide, carbonate, and sulfide are common anions conspicuously absent from this study because these very weakly conducting acids are best determined with “resistivity detection” using an eluent with high “suppressed” conductivity ( I 7). Periodate caused very broad tailing peaks; possible oxidative degradation of the stationary phase by this powerful oxidizing agent was suspected and no further studies were conducted with this ion. Maleic acid always produced a second peak with a lower capacity factor. This is presumably due to malic acid, which is generally present as an impurity in commercial maleic acid. Results for some anions with NPr40H as IIR are not reported because of the very high capacity factors. Inasmuch as the retention order to be expected from Table I closely parallels the retention order on conventional ion exchange resins, the dynamic ion exchange concept, as a first approximation, may be used for this type of chromatography. From this viewpoint, the crossover of capacity factors of pairs such as C1- and CH,COO-, thiosulfate and maleate as a function of increasing IIR concentration may be explained in terms of the gradual dominance of electrostatic interaction over lipophilic interaction. However, some situations may involve unique chemical interactions; for example, we are unable to explain the crossover of the capacity factors of SCN- and C104- as the IIR is changed from 10 mM NEt40H to NPr40H. In general, the capacity factors are too low for most anions with 1mM N E 4 0 H as IIR to be of much chromatographic utility. With 5 and 10 mM NEt40H, increasingly larger numbers of anions display capacity factors convenient for chromatographic separation. NPr40H, even a t 1 mM concentration, produces capacity factors that are mostly too large to be useful. It is clear that for the ambitious ion chromatographer, the separation of IO3- and F- and that of Br- and Br03- (the latter pair may coexist only in alkaline solution) should be regarded as adequate challenge. A convenient eluent for the separation of most anions could presumably be made from a combination of NEt4OH and NPr40H. However, we find it more convenient to use a combination of NaOH and NPr40H; the former contributes only to the eluting ability while the latter contributes primarily to the observed retaining ability. It is particularly convenient to use a dual pumping system with 1mM NPr40H delivered by one pump while the other pump delivers 10 mM NaOH. By varying the individual pumping rates, a large range of capacity factors can be easily generated. Figure 1 shows a separation of nine anions under isocratic conditions by use of a 500 X 2.1 mm home-packed column. As mentioned in
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4,APRIL 1984
771
so4
F
I
0
I
I
I
I
I
10
I
I
I
minuter
1 1 -
2 0 30
70
Figure 1. Separation of nine anions on a PRP-1 column with 0.87 mM NPr,OH 4- 1.3 mM NaOH as eluent. First peak is system peak; the identities of other peaks are as Indicated. Flow rate was 0.5 mL/min.
the Experimental Section, the efficiency of this particular column was poor; the efficiency measured for the nitrate peak is approximately 2600 plates/m, which is approximately the same as that observed for toluene as solute on this column as described in the Experimental Section. However, the resolution of F- and C1- and SCN- and C10, on this column under isocratic conditions is noteworthy. Perchlorate and thiocyanate are usually so strongly retained by conventional ion-exchanger stationary phases that special low capacity columns are marketed for their separation. This clearly demonstrates the utility of a system of this type to generate easily variable “ion exchange capacities” as noted by Cassidy and Elchuk (8). Gradient elution is rarely practiced in IC because of large changes in background conductance that accompanies such elution. Apparently the only reported application thus far is due to Sunden et al. (18) who utilized a gradient between bicarbonate and carbonate eluents of the same molar concentration such that essentially the same molar concentration of H2C03was observed at the detector. As these authors note, to successfully carry out gradient elution with a NaHC0,Na2C03system, precise matching of the eluent concentrations is necessary. While we find that this may be circumvented by using a ternary gradient system utilizing NaHC03water-NaOH and substituting NaOH for water as the run progresses, the NPr40H-NaOH combination utilized in this work appears to have a greater potential for more facile application of gradient elution. Ideally, a gradient between NPr40H and NaOH with increasing concentrationof the latter during the run should produce no base line change as observed by the detector as long as cation exchange by the suppressor is quantitative. In practice, the stability of the base line signal during gradient elution is directly dependent on the degree of purity of the reagents employed. The presence of traces of strongly retained anions is particularly deleterious, since these tend to accumulate on the stationary phase during the first portion of the run and are eluted later causing a broad hump in the base line. This is not only true for the NPr40H-NaOH system but even more so for the bicarbonate-carbonate system where change in charge magnitude causes additional complications. Preparing the IIR in hydroxide form by ion exchange is particularly recommended
m
*
13-
01
~
0
’
, 6
lb
18 2h minutes
30
36
42
Flgure 3. Peak identitles as in Figure 1: eluent A, 1 mM NPr,OH; eluent B, 10 mM NaOH. Run commenced wlth 87% A and 13% B with linear gradient to 100% B over 4 min, beginning at 6 min. Flow rate was 0.5 mL/min. Solid line indicates gradient profile and dashed line indicates base line without sample injection.
since this removes the strongly retained anions especially well. Commercially available tetraalkylammonium hydroxides are not adequately pure in our experience for gradient elution. Gandrud and Lazrus (19) reported 100-1700 ppm of total halides in commercial NBbOH and were able to reduce it very significantly by ion exchange (1-5 ppm C1-, 0.01-0.05 ppm Br-, and 0.005-0.01 ppm I-). Another related problem arises from the absorption of COz by the alkaline eluents resulting in a variable carbonic acid background. Note also that the capacity factors reported in this paper were obtained with relatively C02-free eluents. All capacity factors decrease with significant COZ absorption by the eluents, as may be expected. Gradient elution with a relatively small step gradient is shown in Figure 2 for the same column and test solutes as in Figure 1. A minor rise in base line is noticeable. Perchlorate and thiocyanate elutes quicker (note that the time axis is differently scaled for different figures) and with slightly better resolution. A much more drastic example of gradient elution is shown in Figure 3 for the same test solutes. The base line obtained under the same conditions without any injected solute is indicated in the figure in dotted lines and the rise in base line is significant. However, the resolution of the two late eluting ions is significantly improved accompanying a noticeable reduction in retention time. The rise in base line was higher than expected and has since been found to originate primarily from impurities in the NaOH solution used. While
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
few problems are expected for separation and quantitation of test ions in the 0.1-1 mM concentration range (as used in these examples) and the background appears to be very reproducible from run to run for digital subtraction of the base line stored in memory to be easily applicable; further improvement is warranted. Our preliminary experiments with KOH instead of NaOH, the former being available in significantly better purity, shows a very substantial reduction in base line rise. Experiments are also being conducted with an in-line high capacity anion exchanger in OH- form installed upstream of the injection valve. The permeation of C 0 2 through PTFE tubing commonly used on the low-pressure side of the pump appears to be a significant factor in obtaining the minimum possible base line change. The reason for the switch in column dimensions in going from N E 4 0 H to NPr40H deserves comment. T o prevent excessive band broadening, it was arbitrarily decided to limit the length of the membrane tubing in the FFH suppressor to 50 cm (this introduces an approximately 90-pL dispersion as described in ref 14, which is deemed acceptable for most purposes). This suppressor quantitatively exchanged 10 mM NEt40H at flow rates up to 3 mL/min but performance was not acceptable with 1 mM NPr40H even at 1 mL/min. While quantitative exchange was observed at the beginning of a run, a gradual rise in background conductance was observed, indicating that the overall exchange rate was being limited by mass transfer through the wall. This is not surprising in view of the large size of the NPr4+ ion, which leads to a slow diffusive transfer through the membrane matrix. Thus, a further reduction in flow rate was necessary. At a flow rate of 0.5 mL/min, quantitative exchange was observed and did not deteriorate with time. To reduce analysis time, smaller inside diameter columns were chosen. Unfortunately the poor efficiency of these home-packed columns largely offset the desired goal of the shorter analysis time by requiring much longer column lengths for adequate separation. The application of conventional packed bed suppressors for cation exchanging large IIRs may be fraught with difficulty. Most conventional ion exchangers are capable of exchanging only a fraction of their rated capacity when the ion to be exchanged is large. For example, the exchange capacity of a sulfonated PSDVB cation exchange resin (10% DVB) for cetyltrimethylammonium ion is only one-tenth of its rated capacity and the efficiency is reduced further with higher cross linking (20). Thus, the use of packed bed exchangers, which would permit a reasonably long operational time, is likely to be accompanied by unacceptably large band dispersion. Very small diameter hollow fiber suppressors constitute a simpler and efficient low dispersion alternative to the annular helical
configuration (11-13). Since narrower inside diameter tubing permits a smaller wall thickness without sacrificing structural strength, the problems associated with the mass transfer limitation through the wall should be a t least partially alleviated with such tubing. Commercial availability of such tubing will facilitate applications analogous to those described in this paper. The applicability of lipophilic cationic IIR's in the hydroxide form for anionic analysis with membranebased suppressors and conductivity detection is clear however and the potential for facile gradient elution seems excellent. Registry No. NEt40H, 77-98-5; NPr40H, 4499-86-9; IO3-, 15454-31-6; F-, 16984-48-8; NH2S03-, 15853-39-1; CH&OO-, 7150-1; HCOO-, 71-47-6; C1-, 16887-00-6; NOT, 14797-65-0; Br03-, 15541-45-4; Br-, 24959-67-9; NL, 14343-69-2; NO3-, 14797-55-8; C103-, 14866-68-3; I-, 20461-54-5; C104-, 14797-73-0; SCN-, 30204-5; CI,COO-, 14357-05-2; SO3", 14265-45-3; S042-,14808-79-8; C2042-,338-70-5; CrO?-, 13907-45-4; S2032-,14383-50-7;HP042-, 14066-19-4; S202-, 14781-81-8; S20g2-,15092-81-6; Fe(CN),3-, 13408-62-3; Fe(CN)6", 13408-63-4; sulfanilate, 2906-34-5; picrate, 14798-26-6; tartrate, 3715-17-1; maleate, 142-44-9; phthalate, 3198-29-6; citrate, 126-44-3.
LITERATURE CITED (1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 47, 1801-1809. (2) Small, H. I n "Trace Analysis"; Lawrence, J. F., Ed.; Academic Press: New York, 1981;Vol. 1, pp 267-322. (3) Small, H. Anal. Chem. 1983, 5 5 , 235A-242A. (4) Fritz, J. S.;Gjerde, D. T.; Pohlandt, C. "Ion Chromatography"; Huthlg Heidelberg, 1b82. Molnar, I.; Knauer, H.; Wllk, D. J. Chromatogr. 1980, 201, 225-240. Gjerde, D. T.; Fritz, J. S.Anal. Chem. 1981, 53, 2324-2327. Pohl, C. A.; Johnson, E. L. J. Chromatogr. Sci. 1980, 18, 442-452. Cassldy, R. M.; Elchuk, S.Anal. Chem. 1982, 54, 1558-1563. Stevens, T. S.;Davis, J. C.; Small, H. Anal. Chem. 1981, 53,
1488-1492. (10) Stevens, T. S.;Jewett, G. L.; Bredeweg, R. A. Anal. Chem. 1982, 54, 1206-1208. (11) Hanaoka, Y.; Murayama, T.; Muramoto, S.; Matsura, T.; Nanba, A. J . Chromatogr. 1982, 239, 537-548. (12) Rokushika, S.;Sun, 2. L.; Hatano, H. J . Chromatogr. 1982, 253, 87-94. (13) Dasgupta, P. K. Anal. Chem. 1984, 56, 96-103. (14) Dasgupta, P. K. Anal. Chem. 1984, 56, 103-105. (15) Rokushika, S.;Qlu, Z. Y.; Hatano, J. J . Chromatogr. 1983, 260, 81-87. (16) Okada, T.; Kuwamoto, T. Anal. Chem. 1983, 55, 1001-1004. (17) Pinschmldt, R. K. In "Ion Chromatographic Analysis of Environmental
Pollutants"; Mullk, J. D., Sawiciki, E., Eds.; Ann Arbor Science: Ann Arbor, MI, 1979;Vol. 2, pp 41-50. (18) . . Sunden. T.: Llndaren, - M.; Cederaren, A.; Siemer, D. D. Anal. Chem. 1983, 55, 2-4. (19) Gandrud, E. W.; Lazrus. A. L. Anal. Chem. 1983, 55, 988-989. (20) Helfferich, F. "Ion Exchange"; McGraw-Hill: New York, 1962;p 160.
RECEIVED for review October 31, 1983. Accepted January 12, 1984. This research was supported by the Water Resources Center, Texas Tech University.
