Characterization of replacement-ion chromatography employing

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Anal. Chem. 1987,5 9 , 2293-2302

that was obtained by using identical experimental parameters. The similarity of these spectra indicates that the m/z 164 ion present in the clove oil sample was indeed the molecular ion of eugenol. Both of these spectra are remarkably similar to the 70-eV electron impact spectrum of pure eugenol as shown in Figure 5C, with many of the same characteristic fragment ions observed over the entire mass range. It appears that two factors are particularly important in the success of these experiments. First, the use of elevated trapping potentials probably enables a larger population of ions to be trapped in the cell at the high pressures used in the source. Therefore, even if a low-efficiency ion transfer occurs, a sufficient number of ions for detection may still migrate into the analyzer. Second, by using a longer collision time in combination with the high pressure of collision gas, there is also an increased probability of collisionally cooling translationally excited ions that otherwise might not pass through the small orifice of the conductance limit. Finally, it is also possible that the sudden reduction in trapping voltage at the time of ion transfer may perturb theions in a way that increases transfer efficiency. CONCLUSION

LITERATURE CITED Tandem Mass SepctroTetry; McLafferty, F. W., Ed.; Wiley: New York, 1983. Sack, T. A.; Lapp, R. L.; Gross, M. L.; Kimble, B. J. Int. J . Mass Spectrom. Ion Processes 1984, 61, 191-213. Cooks, R. G. Spectros.: Int. J . 1984, 3, 129-131. Johnson, J. V.; Yost, R. A. Anal. Chem. 1985, 5 7 , 75SA-788A. McLafferty, F. W.; Bockoff, F. M. Anal. Chem. 1978, 50, 69-76. Kondrat, R. W.; Cooks, R. G. Anal. Chem. 1978, 50, 81A-92A. Yost, R. A.; Enke, C. G. Anal. Cbem. 1979, 5 1 , 1251A-1264A. Glish, Gary L.; McLuckey, Scott A. Anal. Chem. 1986, 5 8 , 1889-1892. Wise, M. B. Ph.D. Thesis, Purdue University, 1984. McLuckey, Scott A.; Glish, Gary L.; Kelley, Paul E. Anal. Cbem. 1987, 59, 1670-1674. Johlman, Carolyn L.; White, Robert L.; Wilkins, Charles L. Mass Spectram. Rev. 1983, 2 , 389-415. Cody, R. B.; Burnier, R. C.; Freiser, B. S. Anal. Chem. 1982, 5 4 , 96-101. Cody, R. B.; Burnier, R. C.; Cassady, C. J.; Freiser, B. S. Anal. Chem. 1982, 5 4 , 2225-2228. Cody, R. 8.; Freiser, B. S.Anal. Cbem. 1982, 5 4 , 1433-1435. White, R. L.; Wilkins, C. L. Anal. Cbem. 1982, 5 4 , 2211-2215. Cody, Robert B.; Amster, I.Jonathan; McLafferty, Fred W. Proc. Natl. Acad. Sci. U . S . A . 1985, 8 2 , 6367-6370. Hunt, Donald F.; Shabanowitz, Jeffrey; Yates, John R.; McIver, R. T., Jr.; Hunter, R. L.; Syka, John E. P.; Amy, Jon Anal. Chem. 1985, 5 7 , 2733-2735. Hettich, R. L.; Freiser, B. S. J . Am. Chem. SOC. 1985, 107, 6222. Carlin, T. J.; Freiser. B. S. Anal. Chern. 1983, 55, 574-578. McIver, R. T.; Hunter, R. L.; Bowers, W. D. Int. J . Mass Spectrom. Ion Processes 1965, 64 67. Cody, Robert 8.; Kinsinger, James A.; Ghaderi, Sahba Anal. Chim. Acta 1985, 178, 43-66. Cody, R. B., Nicolet Instruments, Inc., private communication, 1986. Giancaspro, Carlo; Verndun, R. Francis Anal. Chem. 1986, 58, 2099-2101. Masada, Yoshiro Analysis of Essential Oils by Chromatography and Mass Spectrometry; Wiley: New York, 1976. ~

The results of this work indicate that both parent ions and daughter ions generated under static high-pressure CID conditions in the source cell of the FTMS, can be partitioned between the source and low-pressure analyzer in abundances that are easily detectable. By the use of the technique described in this paper, it is possible to routinely obtain highresolution daughter ion spectra without the use of pulsed valves or serious loss of CID efficiency. Finally, the reproducibility of results over a period of several months has been excellent, further adding to the analytical utility of this high-resolution CID method.

RECEIVED for review December 16, 1986. Accepted June 1, 1987. Research sponsored by the Office of Health and Environmental Research, U.S.Department of Energy under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.

Characterization of Replacement-Ion Chromatography Employing Cation Replacement and Flame-Spectrometric Detection Leonard J. Galante and Gary M. Hieftje*

Department of Chemistry, Indiana University, Bloomington, Indiana 47405 A relatively new detection method for ion chromatography (IC)called replacement-Ion chromatography (RIC)Is critically evaluated. The capabilities and limitations of cationreplacement RIC for cation and anion separatlon and determination were explored by uslng a conventlonal dual-column IC system followed by a continuously regenerating Nafion cation-exchange fiber (the replacement column). This third cdumn converts all lonk solutes into their lithium salts, whlch are detected by an acetylene-air flame-emission spectrometer. Although poor figures of merit were observed for cation chromatography, the method is analytically useful for anion chromatography. Anlon detection llmits are 3-50 ng and depend upon the replacement-Ion background produced by the eluent. This background llmitatlon of RIC and the infiuence of varlous system parameters are dlscussed in detail. The universal calibration capabiltly of RIC Is also evaluated.

Ion chromatography (IC) is becoming an increasingly popular technique for separating, identifying, and detecting

solvated ions. At present, the conductivity detector is the most widely used detector in IC because of its simplicity and generality. Two conductometric-based IC detection methods exist. Suppressed IC was introduced in the mid-1970s by Small and co-workers (1). The technique employs a second column called the “suppressor” to minimize the conductivity of eluent (background) ions and enhance the detectability of solute ions. Later Gjerde et al. developed nonsuppressed IC (2-4), in which low-capacity analytical columns and dilute, low-conductivity eluents are employed. The intrinsic advantages and disadvantages of the two methods have been critically compared in the literature ( 5 , 6 ) . Detection limits for suppressed IC are in the range of 0.1-1.0 pg/mL and about an order of magnitude higher for nonsuppressed IC because of its higher background conductivity. Also, separator columns for nonsuppressed IC necessarily have limited sample capacity. These two effects account for the much larger linear dynamic range observed in suppressed IC.

0003-2700/87/0359-2293$01.50/00 1987 American Chemical Society

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ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987

Unfortunately, the conductivity detector used in both IC methods is strongly influenced by temperature. Most ionic solutions exhibit a 2% change in conductivity per degree Celsius. Thus, the sensitivity achievable in both IC techniques depends on the ability to minimize background noise caused by temperature fluctuation. Of course, temperature control is more critical when background levels are high; under such conditions, detector cells must be well-insulated and are sometimes equipped with temperaturecompensation circuitry (5, 6). Some columns and valves are also environmentally isolated (7). The burden of temperature control and the sensitivity limitation imposed by this temperature dependence are serious drawbacks to conductometric methods. Unfortunately, no other truly general detector has been employed successfully for the separation and measurement of both anions and cations. Electrochemical (6), atomic emission (6), ion-selective electrode (8,9), and UV-visible (6,10,11) detectors have been used but enjoy limited application because they respond to only a few ions. A new detection method called replacement-ion chromatography (RIC) was discussed in a previous publication (12). The method employs a suppressed-IC arrangement and a third column that replaces solute ions or their co-ions by another ionic species. The replacing ions are then monitored by an appropriate detector. As in conventional IC, separation, selectivity, and other chromatographic characteristics are governed in RIC by the separator column, eluent, and flow rate; RIC can therefore be viewed as a new approach to IC detection. This analyte-replacement method offers several potential advantages. First, ion replacement allows otherwise selective detectors to be investigated as general detectors for IC. Accordingly, detectors can be chosen that are less sensitive to temperature and which obviate the need for insulated IC systems. Furthermore, detectors that lack a strong temperature dependence or other serious limiting noise source should provide superior detection limits over those obtained with the conductivity detector. Lastly, an RIC detector should provide a universal response; each solute ion is stochiometrically converted into the same detected form. The integrated detector response per equivalent charge should therefore be identical for all ions, and only one calibration curve would serve to quantitate all eluting species. Interestingly, the relative concentration of each component could then be determined without a calibration curve merely by comparing peak areas. That is, peaks could be quantitated without prior knowledge of their identity. The low selectivity coefficient of Li+ makes it an ideal replacement ion. Furthermore, solutes eluting as their lithium salts can be detected sensitively by a flame spectrometer. However, the first Li+-based RIC system (12) suffered from a number of important limitations. A turbulent (noisy), total-consumption hydrogen-air flame was employed, which increased RIC detection limits. Secondly, a conventional packed-bed column containing a cation-exchange resin was used as the replacement column. Like packed-bed suppressor columns, such columns must be periodically regenerated, making IC analysis less convenient. Other investigators have shown that ionomeric fiber devices perform better than their packed-bed counterparts as eluent suppressors in IC (13-19). They operate continuously and generally cause less band broadening. Furthermore, other problems frequently encountered in IC analysis are eliminated or reduced when fiber-suppressor columns are employed (13-19). In this expanded study of Li+-based RIC, a premixed acetylene-air flame-spectrometric detector and a fiber-re-

SEPARATOR SUPPRESSOR

INJECTOR

HROMATO REPLACEMENT COLUMN

LENS

t FIBER/ HOUSING

NEBULIZER/ SPRAY CHAMBER

AMMETER

Schematic diagram of the RIC apparatus including suppressed-ion chromatographic system, fiber-replacement column, and flame-emission spectrometer. Figure 1.

placement column have been employed. These modifications produced a more stable, sensitive, and convenient instrument than that reported earlier (12). This new system is described, and its optimal operating conditions are determined in this report. In addition, the capabilities and limitations of cation-replacement RIC for cation and anion analysis have been more thoroughly evaluated here. The universal calibration feature of RIC has been critically assessed and the high background and problems encountered in cation analysis have been clarified. Overall, it is found that RIC employing cation replacement is more useful for anion than cation determinations. EXPERIMENTAL SECTION Instrumentation. A schematic diagram of the new RIC system appears in Figure 1. Its components are described in the following sections. Chromatographic Equipment. Two different LC systems were used during these experiments. The fmt employed a high-pressure single-piston pump (Model llOA,Beckman, Berkeley, CA) and injection valve (Model 210, Beckman). The other system utilized a high-pressure syringe pump (Model LC-5OO0, Isco, Inc., Lincoln, NE) and injection valve (Model 7010, Rheodyne, Cotati, CA). Injector valves were equipped with either 20- or 1 0 0 - ~ Lloop volumes. A minipump (Model 396, Milton Roy Co., Riviera Beach, FL) equipped with pulse dampener was used to regenerate suppressor columns or convert packed-bed replacement columns into the Li+ form. The commercial columns used in this study were all Dionex brand. Columns used for anion separation included an anion separator (Model 35311) and a large packed-bed anion suppressor (Model 30828). An anion guard column (Model 30986) was used as the separator column in experiments performed to evaluate the relative RIC response for several common anions. Columns used for cation separation included a cation separator (Model 35371) and a large packed-bed cation suppressor (Model 30834). Suppressor columns (large, Model 30828 and small, Model 30829) were converted into the Li+ form and utilized as packed-bed replacement columns in experiments that required performance comparisons between packed columns and the fiber-replacement column. The replacement column consisted of a Ndion cation-exchange fiber housed in a reservoir, which accommodates the continuously flowing salt solution (regenerant) used to maintain the fiber in the desired ionic form. This unit is sold commercially by Dionex (Model 35350) for use as a fiber-eluent suppressor in anion chromatography and can be adapted easily for use as a cationreplacement column. The fiber is approximately 1.5 m in length and packed with inert glass beads. All columns were connected by 1.6 mm 0.d. X 0.3 mm i.d. Teflon tubing with Omnifit (AtlanticBeach, NY)polypropylene tube-end bushings and grippers. Nonretained injections of Li+,performed to evaluate the interface, employed a prepared column consisting of a 27 cm long X 6.35 mm 0.d. X 4.6 mm i.d. stainless steel column packed with Dowex 1-X8, 100-200 mesh, anion-exchange resin (Dow Chemical Co., Midland, MI).


Chromatographic Interface. An Omnifit bushing and gripper containing a 50 mm long X 1.6 mm 0.d. X 1.0 mm i.d. plastic tube was used to connect the replacement column to the 60 mm long x 1mm 0.d. X 0.6 mm i.d. stainless steel aspirator tube of the nebulizer chamber. The plastic tube was fit snugly over 3 cm of the aspirator tube, so that the bushing fit flush against the outer wall of the burner chamber and was held firmly by a clamp. The approximate detector void volume including aspirator tube and interfacing tube was 33 pL. Acetylene-Air Flame-Spectrometric Detector. The flame spectrometer included a commercial pneumatic nebulizer-premix burner unit (Model 25958, Instrumentation Laboratory, Inc., Wilmington, MA), a 4-cm-diameter lens with a 5-cm focal length (Ealing Optics, South Matick, MA), a monochromator (JY H-20, Instruments SA, Inc., Metuchen, NJ), and a red-sensitive photomultiplier tube (PMT, Model R446, Hamamatsu Corp., Middesex, NJ). The entrance slit was 0.5 mm and produced a spectral band-pass of 2 nm; the slit height was adjusted to -3.0 mm. The output current of the photomultiplier tube (PMT) was measured by a picoammeter (Model 414S, Keithley Instruments, Inc., Cleveland, OH), filtered by a passive RC filter with a 1-s time constant, and received by a strip-chart recorder (Model SR-204, Heath Co., Benton Harbor, MI). A high-voltage power supply (Model EU-42A, Heath Co.) maintained the voltage between 800 and lo00 V across the PMT. The monochromator was set at the Li 670.8 nm emission line and the burner parameters and optics were optimized for the best Li signal-to-base-line noise ratio (S/N). This performance was achieved by a unity magnification of the flame onto the entrance slit of the monochromator. A field-stop aperture was used to reduce the collection angle of the lens in order to exactly fill the grating of the monochromator. The flame, at a distance 2.4 cm above the burner head, was focused onto the center of the slit. The optimal gas mixture was found to be fuel rich and consisted of 1.6 L/min of acetylene and 11.6 L/min of air. Reagents. Salts or acids used to prepare eluents, regenerant solutions,and ion samples were ACS reagent grade supplied from either Mallinckrodt (St. Louis, MO) or MCB (Cincinnati, OH). Sample solutions used in relative-response experiments were prepared from gold label grade salts supplied by Aldrich (Milwaukee, WI). The water used in all experiments was distilled and deionized. Procedures. The anion suppressor was regenerated with approximately 0.4 N HN03 and the cation suppressor was regenerated with approximately 0.3 N filtered Ba(OH)2. Packed-bed replacement columns were regenerated with approximately 1 N LiOH. Columns were regenerated by pumping the appropriate solution through the column for at least 30 min at a rate of 2.0 mL/min. Columns were then rinsed copiously with water. The operating conditions of the fiber replacement column were adopted from those recommended by the manufacturer for its use as a suppressor column. A solution of 0.025 N Lif as Li2S04 was used as the regenerant. Regenerant flow was maintained between 2.0 and 3.0 mL/min countercurrent to column flow and regulated by gravity (20). Replacement efficiency and background studies required quantitating the lithium present in the column effluent. Lithium concentrations were determined from 20-40-mL aliquots of column effluent collected after system equilibration. These samples were then assayed along with lithium carbonate standards by the flame spectrometer. The RIC response of several monovalent anions and phosphate ion was determined by carefully preparing a 4.0 mM solution of each anion diluted in mobile phase (3.0 mM NaHC03/2.4 mM Na2C03). A 2.0 mM SOZ- solution was prepared because of the ion’s double charge. Six to eight 20-pL aliquots of each solution were injected both with and without the separator column in place. Chromatograms were collected, stored, and integrated by use of a MINC-11/23 computer (Digital Equipment Corp., Marlboro, MA). The time constant was reduced from 1.0 to 0.3 s in these experiments for early eluting peaks and for peaks collected without a separator column. RESULTS AND DISCUSSION Flame Characterization. The anal* transport efficiency into the flame for an aspiration rate of 2.0 mL/min was de-

2

1

>

IH v)

I

NEBULIZER

NEBULIZER OVERFED

+ UNDERFED

1.5

I

-

2295

I

2 W

I-

z

-

1

W

>

H I-

Figure 2.

for a

Variation of Li signal with chromatographic pump flow rate M Li’ solution and a nebulizer intrinsic uptake rate of 2.0

mL/min.

termined by the indirect method (21) and with the flame ignited. The value obtained, 5.9%, is similar to those reported by other researchers for typical atomic absorption nebulizer-spray chamber-burner head assemblies (21). The dynamic range of the flame spectrometer was determined by aspirating Li+ standards ranging from to M. As expected, a sigmoidally shaped emission curve typical of an alkali metal (22,23) wm observed. The plateau at high concentrations begins between and M and arises from self-absorption. Curvature also begins between and lo4 M and results from ionization effects. The linear dynamic range extends between and M. Interface Evaluation. The sensitivity of the interfaced flame spectrometer was evaluated by injecting several 1OO-pL aliquots of M Lif onto the prepared column (containing anion-exchange resin). Lithium ions injected in this manner are not retained and elute with distilled water. The average peak signal-to-noise ratio (S/N) produced by this flow-injection procedure was 18, indicating the inherent sensitivity of the flame detector in the absence of significant chromatographic dilution or Li background emission. Base-line stability and sensitivity were found to be superior to those provided by the total-consumption burner and hydrogen-air flame detector utilized previously (12). The S/N reported here is almost an order of magnitude greater than that reported for the previous detection system (12) but similar to those reported for Li in other acetylene-fueled flames (24, 25). Signal-to-noise ratios in RIC did not depend upon whether the pump flow rate was above or below the nebulizer’s intrinsic uptake rate. Typically, both rates were maintained as close as possible and held a t 2.0 mL/min throughout the study. However, the magnitude of the Li signal did depend upon whether the nebulizer was underfed or overfed by the liquid chromatographic pump; the Li signal produced by a M Li+ solution pumped into the nebulizer a t flow rates above and below the nebulizer’s intrinsic uptake rate (2.0 mL/min) is shown in Figure 2. The intensities shown in Figure 2 fall within the linear range of the flame spectrometer, suggesting that the signal variation results from changes in sample throughput (transport efficiency). Because of the dependence illustrated in Figure 2, subsequent RIC experiments that required quantitating Li+ in column effluent at different pump rates were performed by the sample-collection and standardization procedure described in the Experimental Section and not by direct sampling of the column effluent. Optimal Operating Conditions and Exchange Efficiency of Fiber-Replacement Column. The exchange efficiencies of fiber devices employed as eluent suppressors in IC have been investigated before (14,26-28). Most of these studies have been purely empirical, although Dasgupta (27) described theoretically the diffusion processes that govern ion

2296


exchange in hollow and filament-filled fiber suppressors and defined the parameters (i.e., fiber length and regenerant concentration) that affect exchange efficiency. Effect of Regenerant Flow Rate and Concentration. When a fiber column is employed as a suppressor column in conventional IC, it must continuously exchange an eluent cation at a concentration typically between 5 and 10 mequiv/L. Accordingly, high regenerant flow rates and concentrations are required when these or more concentrated eluents are employed (26,27). In contrast, to provide maximum analyte signal and good reproducibility as a replacement column in RIC, the fiber need quantitatively exchange only solute ions, which elute discontinuously and experience considerable dilution during the separation. Therefore, the demands on the fiber are lower when it is employed as a replacement column than when it is used conventionally as a suppressor column. Consequently, regenerant flow rate and concentration have little effect on replacement-column performance and can be varied over a wide range. Here, a regenerant solution of 0.025 M Li+ maintained at a flow rate of 2.0-3.0 mL/min provided satisfactory performance. Effect of Fiber Length. At low influent ion concentrations such as those encountered in RIC, ion-exchange efficiency through an ionomeric-fiber tube is independent of the ion concentration and limited by the diffusion of ions to the interior surface of the fiber. Mass transfer to the walls of a cylindrical hollow fiber from a laminar stream flowing through it can be described by the Gormley-Kennedy equation (27). A three-term approximation of the equation (27) is 1-f= Oa81g1e-3.657"DLIF + 0.0975e-22.3"DLlF

+ oe0325e-57"DL/F (1)

Equation 1 suggests that an exponential relationship exists between ion-exchange efficiency ( f ) and such parameters as fiber length ( L ) ,flow rate through the fiber (F),and the diffusion coefficient of the entering ion (D). The length of fiber required in RIC for efficient replacement can be predicted from eq 1 by using the diffusion coefficient of H+ at 25 "C, 9.31 X m2/s (29). (Anions enter the replacement column as their conjugate acids in anion chromatography.) For example, exchange efficiencies of 96.7 70 and 99.9% are predicted for 1.0- and 2.0-m fibers, respectively, at an influent flow rate of 2.0 mL/min. Shorter fibers would be sufficient if they are packed with inert beads (18) or filaments (27) because ions are then transported to the fiber wall more efficiently. The bead-packed fiber used in this study is about 1.5 m long and has been reported to provide adequate exchange for pump rates up to 4.0 mL/min when it is employed as an eluent suppressor (26). According to eq 1,the fiber should perform more efficiently as a replacement column than as a suppressor column in anion chromatography; in the former case, H+ is exchanged for Li+, while in the latter case, Na+ is exchanged for H+. The latter process is less efficient because the solution diffusion coefficient of Na+ is less than that of H+by almost a factor of 5 a t 25 "C (29). T o verify that anions eluting from the eluent suppressor in their acid form are converted stoichiometrically into their lithium salts by the fiber replacement column, solutions of NaCl between 8 X M and M were pumped continuously at 2.0 mL/min through the eluent suppressor and fiber-replacement columns. The resulting linear calibration equation (Li emission signal produced by column effluent vs. influent NaCl concentration (in moles per liter)) is y = (13419 f 1 4 4 ) ~+ (0.111 f 0.081). For this plot, the correlation coefficient (r) = 1.00 and the standard error of estimate (SEE) is 0.104. The equation produced by Li+ standards, introduced into the flame spectrometer directly and over the same con-

Table I. Background Lithium Concentrations Produced by Various Eluents in RIC Li+ back-

eluent HZO

7.0 mM HN03 10.0 mM NaOH 1.35 mM NaZB,O7 2.4 mM NaZCO3/3.0mM NaHC03b

suppressor product

ground," mM 0.0091

HzO" HzOd

0.013 0.045

5.4 mM H3B0zd 5.4 mM H2C03d

0.072 0.43

Values obtained by utilizing the lithium-fiber replacement column and an eluent flow rate of 2.0 mL/min. *"Standard" eluent for anions. Cation-suppressor column employed. Anion-suppressor column employed. centration range, is y = (13416 f 153)x - (0.0371 f 0.0767). For this latter equation, r = 1.00 and the SEE = 0.129. The excellent agreement between the two slopes indicates that H+ generated in the suppressor is stoichiometrically replaced by Li+ in the fiber within the accuracy of this experiment and under the selected conditions. According to eq 1, the fiber length required in the RIC experiment should be independent of the chosen replacement cation; both the equation and model assume that the fiber acts as a "perfect sink". That is, an influent ion readily exchanges with a replacement ion residing on the ionic sites a t the wall. However, the model would be expected to become less exact as the affinity of the replacement ion for these sites becomes significantly greater than that for H+. Longer fibers would then be required for quantitative replacement (e.g., an infinitely long fiber would be required to replace solute ions if an irreversibly bound replacement ion was employed). Of course, this problem does not exist in a Li+-based RIC method because of the low affinity of Li+ for Nafion. In fact, the selectivity coefficient for the exchange of H+ for Li+ is slightly more favorable (larger) than that for the conventional suppression reaction, exchange of Na+ for H+. Values of 1.73 and 1.22, respectively, have been reported for Nafion-120 a t 25 "C (30). Sources of Background in RIC. In RIC, a significant background signal exists that forms the base line upon which solute peaks are observed. This constant background signal is caused by background replacement ions (in this case, Li+) that are continuously introduced into the column effluent as it passes through the replacement column. The presence of detectable background ions is particularly troublesome if multiplicative noise sources exist within the detection system. In the flame spectrometer, for example, nebulizer drift and flame flutter are multiplicative noise sources that cause fluctuation in an otherwise stable background signal. In such a situation, detection limits are degraded in proportion to both the multiplicative noise and the background level. High background Li+ concentrations degraded detection limits and precision in previous RIC work (12). Studies reported here were undertaken to more fully understand the cause of this background and hopefully to eliminate it or to define operating conditions under which it is minimal. The determination of background concentrations under a variety of conditions makes it possible to estimate the attractiveness of other replacement-ion-detector schemes if the relationship between base-line relative standard deviation and background signal is known. Table I lists the background Li+ concentrations that are produced by several eluents pumped a t 2.0 mL/min through the appropriate three-column RIC system. Water Contaminants. The low background Li+ concentration observed when distilled water is pumped through the three-column system (Table I) can be attributed to a number

ANALYTICAL CHEMISTRY. VOL. 59. NO. 18, SEPTEMBER 15, 1987

2297

Regermant Solution

Fiber lnlsrla

C q + l$O=HIC03=

c 02

HCq-

+ H+

Li'

/ \

1,

W O s

L,'

"

L,+ so2 ; Regenwont Sohllion

Flgure 3. Schematic diagram depicting chemical reawons involving Ionized and mama1 forms of COAaq) in a Iongnudinal aoss secHon of the fiber-repbcement column.

of minor contaminant sources, including impurities (mostly Na*, Caz+, and Mg2+) in our laboratory water that are not completely removed by distillation and deionization. Contaminant ions can also he introduced by metallic chromatographic components that contact the flowing liquid. In addition, the pH of our laboratory water is typically between 5.5 and 6.0 because of dissolved COP. Protons from carbonic acid can exchange with Li' across the ionomeric membrane of the fiber-replacement column and further increase background replacement-ion concentrations. Unfortunately, it is difficult to accurately assess the contribution of dissolved COz to background levels because of the variety of equilibrium processes that me likely involved. The relevant reactions are depicted in Figure 3. To complicate matters, carbon dioxide and aqueous carbonic acid exhibit some permeability through Nafon and can thus pass through the fiber ( 2 6 2 7 ) ; losses of these species would serve to reduce background levels. However, carbon dioxide and carbonic acid exist in dynamic equilibrium with the acid dissociation products. Consequently, as protons are exchanged for Li+ in the fiber, the governing equilibrium should he shifted toward the dissociation of HzC03(aq) in order to replenish the depleted proton concentration. As a result, the amount of lithium background produced hy HZCO3(aq)88 it travels through the fiber might be considerably larger than that predicted simply by the acid dissociation constant and the initial equilibrium concentration of the acid. Leachate from Packed-Bed Columns. Background Lif levels a t an eluent (water) flow rate of 2.0 mL/min were a factor of 2-5 higher when packed-bed replacement columns were substituted for the fiber replacement column. Moreover, the base line did not appear as stable and exhihited both short-term drift and a gradual long-term decrease. Packed columns are known to "bleed" low Concentrations of regenerant species following regeneration even after being rinsed with copious amounts of water (13). Leachate from any packed-bed column employed in an RIC system as an eluent suppressor, replacement column, or both potentially contributes to background replacement-ion concentrations. Regenerant Penetration. Regenerant penetration is an additional background Li+ source from the fiber replacement column. Regenerant salts are not completely excluded from the inner flow stream; the penetration rate depends upon the concentration and identity of the regenerant co-ion and upon fiber characteristics (14,27,31). A regenerant salt and concentration should be & s e n which provides adequate exchange but minimal background from regenerant penetration. Background Li+ concentrations were measured a t distilled-water eluent flow rates between 0.25 and 3.0 mL/min. The resulting hackground was inversely related to pump rate, indicating that regenerant penetration was significant under these conditions. The best fit of these data to the function y = a + b/x (r = 0.995 and SEE = 1.37 X lo4) yields a rate

._

I

I

I

I

PUMP RATE

M u r e 4. Influence of eluent flow rate on background lnhlum wncentration In R I G distilled-water eluent (*); standard eluent. 2.4 mM Na,C0,13.0 mM NaHC03(0). The vertical scab on the curve for the distilled-water eluent has been expanded by IOX and is therefore in units of M

v)

coefficient, b, of 1.0 X mol of LizS04/s and a value of 9.7 X lo4 M for the intercept, a. The data points and curve appear in Figure 4 (lower curve). The intercept of this plot, a, deviates significnntlyfrom zero, suggesting that a signifimt background concentration exists from contaminant sources even under conditions of negligible regenerant Penetration (i.e., a t infinite pump rate). Background Li+ caused by regenerant penetration through the ionomeric fiber can be reduced by decreasing the regenerant concentration, particularly for operation a t low pump flow rates. However, alternative regenerant conditions were not explored further because hackgronnd cawed by regenerant penetration was found to be minor compared to other background sources discussed below. CarbonatelBicarbonate Eluent. Carbonate/bicarbonate eluents are commonly used in suppressed IC because such mixtures can separate both monovalent and divalent anions in a single experiment (32). The background level obtained by using the "standard" eluent, 2.4 mM Na2CO3/3.0 mM NaHCO, (6),in Table I is far above impurity levels for the salts employed and an order of magnitude greater than that predicted by the apparent first dissociation constant of carbonic acid, 4.3 X 10.' a t 25 "C (33). This result can again be explained by the dynamic equilibrium effect described previously for dissolved COz and HzC03(aq)in contact with a fiber column in the Li+ form. As illustrated in Figure 3, the eluent suppressor-column products (COz and H2C03),which enter the replacement column, exist in equilibrium with HCOj and H+. In the replacement column, the equilibria are dynamically shifted toward the dissociation of H&O3 as protons are selectively removed (replaced by Li') by the cation-exchange process. In effect, HZCO3is partially converted into LiHCO, as if it had been progressively titrated with LiOH. Less than 10% of the COz or H2C03 generated in the suppressor column is converted to LiHC03 in the fiher-replacement column under the conditions given in Table I. Incomplete conversion of H2C03to LiHC03 appears to be a result primarily of the limited (apparent) dissociation of this weak acid. The concentration of Hf in the diffusion layer above the fiber surface is much lower when a weak acid is introduced than when an equivalent concentration of a strong acid is introduced. In the latter case, the high available H+ concentration (low pH) drives the ion-exchange reaction and results in stoichiometricreplacement of Hf with Li+. In the former case, the buffer formed by the partial conversion of a weak acid to its lithium salt resists changes in pH and presumably limits the availability of H+ a t the fiber surface. Such a buffering effect would explain why only a fraction of the carbonic acid is converted to its lithium salt as it travels through the fiber.

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ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987 10

I

I

1

I

ficients have been assumed and the second deprotonation step of H2C03has been ignored. Equation 3 can be simplified and rearranged into the form of eq 4 because [HCOY] = [Li+]after the replacement process:

I

4 I 2 HCO; PLUS C0:-

I 4

I 6

I 8

I 10

1

12

CONCENTRATION OF ELUENT CM x 1 0 3

Flgure 5. Background liihlum concentration in column effluent as a function of total influent carbonate-plus-bicarbonateconcentration of eluent, determined using small packed-bed replacement column (0) or fiber-replacement column ("). Flow rate was 2.0 mL/min.

Effect of Carbonate/Bicarbonate Eluent Flow Rate on RIC Background. Background increases substantially as the pump rate is decreased when the 2.4 mM Na2CO3/3.0 mM NaHC03 eluent is employed (Figure 4, upper curve), suggesting that background generation is rate-limited. The hydration reaction that exists between dissolved COz and HzC03(aq)(cf. Figure 3) favors the formation of COz. The long half-life of this hydration reaction, about 30 s at low pH (34),might explain the strong dependence of Li+ background on pump rate. Changes in COz concentration or fiber characteristics caused by pressure changes could also be partly responsible, however. Effect of Carbonate/Bicarbonate Eluent Concentration on RIC Background. Commonly, the concentration or ratio of bicarbonate and carbonate is adjusted in order to separate groups of anions with greatly varying resin affinity (35). Accordingly, it is important to characterize how the replacement-ion (Li+) background level varies with eluent strength. Several eluent solutions were prepared with the same carbonate/bicarbonate ion ratio as the "standard eluent" but at different absolute concentrations. These solutions were pumped at 2.0 mL/min through an eluent suppressor followed by a replacement column in the Li+ form. The amount of LiHC03 generated in the column effluent vs. the total influent carbonate and bicarbonate ion concentration is displayed in Figure 5 for both a small packed-bed (0) or fiber (*) replacement column. Background Li+ concentration increases nonlinearly with eluent concentration for both data sets in Figure 5 . This nonlinear response indicates that a contaminant source is not responsible for the high background that is observed when these eluents are employed. Assuming that the high background is caused by the weak dissociation of HzC03(as discussed previously), the relationship between Lit background signal and initial carbonate concentration in the eluent might be explained by considering the equilibria that couple them. The expression that describes a cation-exchange resin (Li+ form) in equilibrium with carbonic acid can be derived by addition of the pertinent individual chemical reactions: H,CO,(aq) + R-Li + R-H + Li+ + HC03- (2)

Keq = KaKLi+H+ =

xH+[Li+][HC03-]

(3) ~~i+[H&031 In eq 3, K , is the dissociation constant for the first deprotonation of HzCO3, KLi+H+ is the selectivity coefficient that describes the relative affinities of H+ and Li+ ion for the resin, K,, is the combined equilibrium constant, and x represents the fraction of ionic sites occupied by the respective ion in the resin (R-Li or R-H). For simplicity, unit activity coef-

Equation 4 suggests that the background lithium concentration introduced in the replacement column should be proportional to the square root of the equilibrium carbonic acid concentration or approximately proportional to the initial acid concentration if the fraction of acid converted to LiHC03 is small. Moreover, because the amount of available acid affects the ratio x L i + / x H + , the proportionality factor, (K,,xL~+/xH+)'/', should decrease as the initial concentration of HzC03is raised. Of course, the degree of this influence will depend on the amount of Li+ that is released during the replacement process. If [R-Li] is large compared to the amount of Li+ released, the proportionality factor should remain effectively constant. The data points in Figure 5 obtained with the fiber column (*) and in units of molarity were analyzed by a least-squares curve-fitting program. The data provide a good fit ( r = 0.997 and SEE = 1.76 X to the function y = AxB where A = (7.3 f 1.3) x and B = 0.53 f 0.03; this curve is fitted to the data in Figure 5 . These data were also recast as background Li+ concentration (moles per liter) vs. the remaining carbonic acid concentration (moles per liter), so the correspondence with eq 4 could be more accurately tested. The concentration of the carbonic acid remaining after the replacement process was calculated as the influent carbonate plus bicarbonate ion concentration minus the resulting Li+ concentration. When the data were formatted in this manner, their statistical fit to the square-root function improved slightly and the coefficients A and B decreased by about 8% and 4%,respectively. The value of B'determined from these experiments agrees satisfactorily with eq 4, even though the equation is only an approximation and should most accurately apply to a static experiment and one involving an ion-exchange resin and not an ionomeric membrane. Conceivably, the equilibria established in the fiber me much more complex because the solution is flowing. Moreover, appreciable amounts of COPor HzC03 could diffuse out of the fiber and therefore change the influent acid concentration in a way not described by eq 4. Background [Li'] increases with eluent concentration with the packed-bed replacement column (top curve, Figure 5 ) just as with the fiber system, but the trend deviates considerably from the theoretical model described by eq 4. In addition, background magnitude and drift were noticeably worse with the packed-bed replacement column for reasons that are not clearly understood. During experiments with the packed column, photocurrents drifted (usually downward) an average of 17% within a 15-20-min period, making it difficult to reliably track the influence of eluent concentration on background. This drift might be caused by the continual depletion of Li+ and concomitant increase of H+ on the resin. In contrast, background Li+ photocurrents drifted only about 5 % (usually upward) for the fiber-replacement column over the same 15-20-min period. The longer equilibration times, base-line instability, and greater background associated with these packed-bed replacement columns are additional reasons for employing fiber columns in future RIC work. Effect of Acidity of Suppressor Product on RIC Background. Qualitatively, Lit background appears to increase as the K, of the suppressor product of the eluent (36) becomes larger, as predicted by eq 3 and 4. However, the background produced by NaOH (cf. Table I) is significantly


2299

Table 11. Relative RIC Detector Responsen analyte

PK,b

%

ionizationc

HCl HNOB HBr HI03 HZSO,

HF

peak heightd

peak aread

peak areae

100.0 f 99.4 f 100.4 f 92.3 f 93.9 f

1.3 1.2 1.6 1.0 1.1

100.0 f 1.6 99.6 f 1.5 100.4 f 2.2 97.5 f 1.4 97.6 f 0.8

100.0 f 1.7 101.4 f 1.4 91.7 f 2.7 97.7 f 2.1 89.9 f 3.9

0.77

99.8

1.92 2.12 7.21 12.7 3.45

96.8 95.2 0.02

79.4 f 1.4

99.6 f 2.3

99.2 f 3.7

59.7

47.9 f 0.73

95.9 f 2.0

97.1 f 3.9

a Mean response and standard deviations determined from six to eight 20-pL injections of a 4.0 mM solution for all anions except sulfate, which was injected at a concentration of 2.0 mM. Relative responses were computed by comparing each anion’s mean response to that of C1ion. bValuesobtained from ref 36. cCalculated at 25 O C and an acid concentration of 4 X lo-’ N for HI03, H2S04,and HF. A concentration of 12 X lo-’ N was used for HaPOd.dDetermined in the absence of a separator column. eDetermined in the presence of a separator column.

higher than that caused by distilled water, even though the suppressor product (HzO) is nominally the same. Increased absorption of atmospheric COz and the presence of other contaminants in the NaOH solution are probably responsible. The background level produced by 5.4 mM H3B03,the suppressor-column product of 1.35 mM NazB407,lies between those produced by 5.4 mM HzC03and the suppressor-column product of the NaOH eluent. Also, the Li+ background concentration increased by a factor of 1.3 when the NazB407 eluent concentration was doubled, similar to what eq 4 would predict. It is difficult to verify eq 4 by quantitatively comparing the background produced by 5.4 mM HzC03and 5.4 mM H3B03. Equations 3 and 4 suggest that the Li+ background concentration is proportional to both K,’/2 and (XLi+/XH+)1’2, of which K, is the only known variable. The first dissociation constant of H,B03 is smaller than that of H&03 by a factor of approximately 740 (36). Background levels for the two acids should therefore differ by a factor of about 27, yet the values in Table I differ by a factor of only 6. The discrepancy might be caused by an unsurprising increase in the effective XLit/XHt ratio for the weaker acid (H3B03). Anion Analysis. Reproducibility. Synthetic samples containing 0.1 to 10.0 mM F- and C1- were prepared. Four 100-pL aliquots of each solution were injected into the RIC instrument, and the ions were separated by the standard eluent at 2.0 mL/min. Peak-height relative standard deviations (RSDs) were below 2% for concentrations between 0.5 and 10.0 mM and 7% a t 0.1 mM. The relative standard deviation (RSD) values for peak area were between 2 and 6%. A sample chromatogram illustrating the detection of 0.1 mM F- and C1- (1.9 ppm and 3.5 ppm, respectively) appears in Figure 6. Dynamic Range. The peak-area calibration curves for Fand C1- are linear within this concentration range (0.1-10.0 mM). The corresponding linear equations are peak-area response = (3060 f 60)[F-] + (0.425 f 0.319) and peak-area response = (3300 f 4O)[Cl-] + (0.372 f 0.205). For the Pline, r = 0.999 and the SEE = 0.535. For the C1- line, r = 1.00 and the SEE = 0.343. Area determinations at concentrations above 5.0 mM were difficult because peaks became poorly separated because of column overloading. In addition, the detector response became nonlinear at concentrations above 10.0 mM because of self-absorption of the analyte-emission signal from the flame. In general, detector nonlinearity becomes appreciable when peak plus background lithium concentrations exceed 2.0 mM. As described earlier, the flame spectrometer becomes nonlinear also at absolute lithium concentrations below M. This low-end nonlinearity is not a problem in RIC because background lithium concentrations generally exceed lov5M.

t

.J

-

0

1

2

3

TIME (min)

Figure 6. RIC chromatogram demonstrating the separation and detection of 1.9 ppm F- and 3.5 ppm CI- (1 X lo4 M each): eluent, 2.4 mM Na,C03/3.0 mM NaHCO,; flow rate, 2.0 mL/min; injection volume,

100 pL.

Detector Response and Universal Calibration. Although the detector responses for C1- and F- are similar, the slope for F ion derived from linear-regressionanalysis is lower than that for C1- by about 7%. This result indicates that C1- ion is more sensitively detected than F-. Other data verified a consistently higher peak-area response for Cl- than for F and several other anions. To determine which anions exhibit a dissimilar RIC response, the relative sensitivities for F-, C1-, NO3-, Br-, IO3-, and HzPO, were evaluated at an injected concentration of 4.0 mM. (Sulfate was determined at a concentration of 2.0 mM because this doubly charged ion generates an RIC response twice that of a singly charged ion.) For comparison, the peak-area response of each ion was evaluated in the presence and absence of a separator column. Peak heights also were evaluated in the absence of a separator column. The relative responses (normalized with respect to the Cl- response) appear in the last three columns of Table 11. In the absence of a separator column or other discriminatory sources of band broadening, all anions in Table I1 should produce the same peak-height response. However, Table I1 suggests that peak-height response is the same only for anions of the strongest acids, HC1, HN03, and HBr. Anions which form weaker acids produce a lower response. (The pK, values and percent ionization of each acid are compiled in Table I1 for comparison.) For example, the dissociation of HIO, and the second deprotonation of H2S04are not as complete as that of a strong acid and peak heights from those analyte species correspondingly were lower than those of HCl, “OB, or HBr. Peak heights were lowest for the weakest acids, H,P04 and HF. The peaks of these two acids were also noticeably broader than those of the other acids. The H3P04peaks were symmetrical, but the H F peaks were asymmetric and exhibited a fronting peak profile. It is not surprisixig that the weak acids, HI03, H3P04,and HF, exhibit broader peak profiles and lower peak heights than those observed for the strong acids. Because weak acids are only partially ionized and exist to some extent as neutral species, they are not completely excluded from the negatively

2300


charged potential barrier of the fiber ionomer or suppressor-column resin. The low response of H2S04,however, is unexpected and cannot be explained by an incomplete-exclusion hypothesis. Although incompletely deprotonated, HS04- is negatively charged and therefore should not pass into the fiber ionomer or suppressor-column resin. The peak areas obtained in the absence of a separator column (Table 11) were also equivalent for strong acids but slightly lower for weaker acids. The responses for HI03 and H2S04were low by only 2-3%. However, this difference is significant at the 95% confidence level. The weakest acid, HF, again produced the lowest response. The increased band-broadening and tailing experienced by the weaker acids might explain these small area losses. Alternatively, the lower areas might reflect fractional losses of solute molecules in the suppressor and replacement columns. These losses could occur by several mechanisms, such as the absorption of neutral analyte molecules by the suppressor-column resin or the diffusion of molecules through the wall of the fiber (replacement column). Weaker acids are more likely to penetrate Nafion-based fibers than strong acids, again presumably because of their partially neutral character (27). Although H F is only partially ionized (Table 11), its integrated RIC response is nearly equivalent to that of HCl. These data suggest that the replacement reaction promotes the dissociation of weakly acidic analytes just as it promotes the dissociation of weak acids (e.g., H2C03and H3B03)formed during the suppression of eluent ions. This behavior was confirmed for acetic acid in a more quantitative manner by continuously introducing 0.5 mM sodium acetate into the RIC instrument (no separator column). In this case, the acetic acid formed in the suppressor is only about 17% dissociated based on its pK, a t 25 "C (36). Yet, the LiAc concentration in the column effluent was equivalent to the influent NaAc concentration within experimental error. The extent to which weakly acidic species are converted into their lithium salts appears to be pK,-dependent. This conclusion is supported by the peak-area response of H3P04 in Table 11, which is similar to that of a monoprotic acid. Evidently, the second and third protons of are not acidic enough to be exchanged in the replacement column. This conclusion is also supported by the limited replacement-ion concentration (background) generated by even weaker acids such as H,C03 and H3B03. Peak areas obtained in the presence and absence of a separator column (Table 11)were similar for all ions except SO4'and Br-. The normalized response for these two ions was unusually low (by 10% and 8%, respectively) when the separator column was inserted into the instrument, for reasons which are not understood. Other data have confirmed a consistently lower RIC response for these ions than for C1-. Possibly, they are being adsorbed onto the resin in the separator column. We have not yet determined whether this loss occurs with other separator columns or under other experimental conditions. The RIC peak-area responses for C1-, NO3-, IO3-, F-, and PO4$ agree within a few percent. Quantitation of these species from a single calibration curve should be acceptable in many applications. Replacement of the large packed-bed suppressor with a fiber suppressor might further improve these results. Comparisons between packed-bed and fiber suppressors indicate that incomplete ion exclusion is more pronounced for packed-bed suppressors and depends dramatically on parameters such as column size and the degree of column exhaustion (5, 13). Detection Limits. Detection limits ( S I N = 2) determined with the standard carbonate and bicarbonate eluent at a flow rate of 2.0 mL/min were 0.26 and 0.43 ppm or 26 and 43 ng

A

TIME ( m i d

B

TIME ( m i d

Sample chromatograms demonstrating the separation and detection of monovalent cations by the tricolumn technique, RIC (A), and conventional dual-column IC (B): eluent, 0.007 N "0,; flow rate, 2.0 mL/min; injection volume, 20 ML. The relative magnification of each chromatogram is shown above each trace. Figure 7.

absolute concentration for F- and C1-, respectively. These values are about an order of magnitude greater than those reported for the conductivity detector under similar conditions (35). RIC detection limits determined with a 0.01 N NaOH eluent were 32 and 67 ppb or 3.2 and 6.7 ng absolute concentration for F- and C1-, respectively. These detection limits are controlled principally by the magnitude of the Li+ background and by the existence of multiplicative noise sources in the flame spectrometer. These noise sources, such as flame flicker and nebulizer instability (37, 38), are multiplicative, so the noise magnitude and therefore S I N will be strongly dependent upon background Li+ concentration. In a flicker-noise-limited emission measurement, eq 5 applies (37). Here, is is the signal current,

(5) -

ib is the average background current, and 12 is the flicker factor (a measure of the amount of multiplicative noise inherent in the background signal). In the RIC experiment, i b is the photocurrent produced by background Li" and i, is the net signal produced by an eluting peak (ipeak - i b = is). The equation predicts an inverse relationship between S I N and background level. This relationship explains in part why RIC detection limits are much poorer than those obtained for unretained lithium injections eluting in the presence of only flame background. Detection limits are degraded also because an analyte experiences chromatographic dilution in the RIC experiment. Equation 5 explains also why RIC detection limits improve by almost a factor of 10 when a NaOH eluent is employed. Background Li+ concentrations decrease by about an order of magnitude with this eluent (cf. Table I). Consequently, detection limits are improved correspondingly. Cation Analysis. The detection of alkali metals by RIC was explored by using a traditional dual-column IC system followed by the Li+ fiber-replacement column. In this mode, cations leave the suppressor column as their hydroxide salts and are exchanged for lithium in the replacement column, to form lithium hydroxide. The selected eluent, HN03, produced a very low lithium background (Table I), so very good detection limits were expected. However, results were somewhat disappointing. Sensitivity and Peak Shape. Solutions containing mixtures of cations between 1.0 and 7.5 mM were injected via a 20-pL sample loop to produce the chromatograms in Figure 7A. Good S I N was obtained at high concentrations, but peak heights decreased disproportionately as lower analyte ion concentrations were injected. In fact, signals disappeared into the base line at cation concentrations below 1.0 mM. The


peaks (Figure 7A) are also unusually asymmetric and exhibit considerable tailing. Sensitivity is obviously degraded because much of the sample is concentrated in the tail of the peak profile. To clarify this behavior, the same solutions were injected into the same system but with the replacement column removed. Without the replacement column, the system is identical with that employed in conventional dual-column IC, and only lithium ion can be observed with the monochromator set a t the analytical wavelength for lithium. Representative peaks for Li+ appear in Figure 7B. These peaks are noticeably greater in height and better shaped than the corresponding RIC peaks at each concentration. The peak heights also change linearly with concentration. Reproducibility. Reproducibility of the RIC cation peak areas was generally poor as well. Occasionally, peak heights decreased or increased by more than 50% between injections. Also, relative heights among peaks within the same chromatogram sometimes varied unpredictably. Frequently, initial injections produced the lowest response and peak heights increased gradually with repeated injections. Peak-height RSDs were calculated for Li+, Na+, and K+; four consecutive injections at 7.5 mM provided typical RSD values of 12.3%, 3.6%, and 8.0%, respectively. These values are unusually high considering the high signal-to-noise ratio of each peak at this concentration. In contrast, the average peak-height RSD for Li+ between 1.0 and 8.0 mM was only 0.84%, using the dual-column system (Figure 7B). Universal Calibration. Conceptually, a universal response should exist for cation as for anion RIC. The area responses for 7.5 mM Li+, Na+, and K+ in Figure 7A appear similar, but a quantitative comparison was not rigorously pursued because of poor reproducibility. The severe peak tailing also made it difficult to define peak base lines accurately. Interactions in the Replacement Column. Similar results were obtained when the packed-bed replacement column was used, suggesting that the unusual effects observed with the fiber column are not caused by the fiber per se. Instead, the cation hydroxides formed in the suppressor column appear to interact physicochemically with the cation-exchange material of the replacement column regardless of whether a fiber or packed-bed column is employed. Extensive penetration of the cation hydroxides into the cation-exchange material of the replacement column could explain the unusual results observed in cation RIC. Most ionic compounds do not appreciably penetrate the cation-exchange medium because the anion associated with the exchangeable cation is effectively excluded from the material. However, the cation-exchange medium becomes more permeable to ionic solutes as the size and charge of the anion decrease (31,39). Unfortunately, in cation separations, cations enter the replacement column as hydroxides and therefore can permeate the replacement medium more easily than would other salts. The more facile passage of LiOH than Li2S04through the Nafion-based fiber-replacement column was verified by measuring the concentration of Li+ in the inner flow stream of the fiber while a LiOH or Li#04 regenerant solution bathed the outside of the fiber. A 0.025 N LiOH regenerant produced approximately an order of magnitude greater Li+ background in distilled water (at a flow of 2.0 mL/min) than either 0.025 or 0.050 N LizS04. The high permeability of other perfluorosulfonic acid based membranes to OH- has been attributed to their high water content (40). Membranes such as those based on perfluorocarboxylic acid contain less water than Nafion and are much less permeable to OH- (40).Perhaps cation analysis would be better accomplished by employing a fiber-replacement column fabricated from such a material.

2301

The unusually low peak-height response observed in cation separations might be caused partially by another mechanism that involves neutralization of solute hydroxides following ion exchange. The negatively charged (ion exchange) sites in the fiber pores exist primarily in the Li+ form when the LizS04 regenerant solution is employed. However, a minute fraction of sites should exist in the H+ form because of the small concentration of H+ ions that exists in the surrounding solvent (HzO). These H+ sites would have no perceptible effect on the exchange efficiency of the fiber when acids or neutral salts are exchanged. In contrast, a cation introduced into the fiber as its hydroxide could be selectively removed from the column effluent upon contact with an H+ site by the following reaction mechanism. First, the cation would exchange with H+ion and deposit on the resin. The H+ ion, then in solution, would react with the co-ion, OH-, which had earlier been associated with the cation solute, to form a molecule of water. Such a mechanism could effectively remove a portion of the injected cations as they travel down the fiber-replacement column and explain the apparent disappearance of cation peaks below an injected concentration of 1.0 mM (for a 20-pL injection volume). Limitations of RIC. There are a number of practical drawbacks in attempting to exploit the universal calibration feature of RIC. Peak-area measurements are needed and require an integrator or computer and adequate chromatographic resolution for maximum accuracy and reproducibility. Moreover, the relative standard deviations of peak areas can be larger than those of peak heights, particularly for broad peaks or peaks that exhibit tailing. RIC possesses a number of other inherent shortcomings. The addition of a third column clearly increases the complexity of IC analysis. However, this disadvantage is minimized by employing a fiber-replacement column. The third column also increases retention times. Fortunately, the fiber void volume is small, measured here to be only 440 ILL. Accordingly, peak retention times are increased by only about 13 s a t a flow rate of 2.0 mL/min. The third column also causes additional band broadening which, in turn, decreases chromatographic resolution and detector sensitivity. The ratio of the band volumes observed in RIC, VRIC, to those observed in dual-column IC, VDCIC, depends upon the dispersion introduced by the replacement column, VRC,as shown in eq 6 (41).

Equation 6 can be used to estimate the relative increase in peak volumes ( V R I c / V D c I c ) caused by any type of replacement column, as long as the column’s band volume contribution, VRC, is known. Fortuitously, Dasgupta (26,28) has measured and compared the band broadening introduced by various types (and lengths) of cation-exchange-fiber columns, each of which could be employed as a replacement column in RIC. The value he reported for a Dionex fiber, similar to the one employed in the present study, is compared in Table I11 to values for two other fiber devices with similar ion-exchange capability. Each of these values was substituted for VRc in eq 6 to determine the relative increase in band volume caused by each fiber device. These calculated ratios appear in the last column of Table 111. One milliliter was used as the assumed band volume eluting from the dual-column IC system, VDcIc,in these calculations because early eluting peaks typically have band volumes of such magnitude. The 7% relative increase in band volume caused by the Dionex replacement column (Table 111) is significant but acceptable. The increases caused by the other devices are minor. These results are not surprising in view of the low dead

2302

ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER

Table 111. Relative Increase in Band Volume Caused by the Fiber-Replacement Column in RIC

dispersion, 1L

re1 increase of a 1.0-mL peak," mL

380b 150b 100'

1.07 1.01 1.005

induced band

length, type of fiber column

cm

Dionex bead-packed filament-filled helix dual membrane annular helix

150 100

65

a Calculated from eq 6 for a 1.0-mL band volume eluting from a dual-column IC system. bData from ref 26; pertains to a 20-1L injected sample at a flow rate of 0.5 mL/min. cData from ref 28; pertains to a 20-bL injected sample at a flow rate of 0.5 mL/min.

volumes of these devices ( 2 6 , B )and the relatively large band volumes that typically elute in dual-column IC; moreover, eq 6 indicates that the relative increase in band volume becomes less severe as the value of V,,,, increases. CONCLUSIONS The present RIC system, employing lithium as the replacement ion and a flame-emission detector, is more stable, sensitive, and convenient than an initial system (12). A Nafion cation-exchange fiber performs better than a packed-bed replacement column. The anion detection mode provides satisfactory figures of merit. However, detection limits are seriously degraded when traditional carbonate/ bicarbonate eluents are employed because high background replacement-ion concentrations result. This high background couples with intrinsic flame noise to generate an unstable base line, which makes it difficult to measure low ion concentrations. A more stable detection system would be less troubled by high background levels and would provide better detection limits. An equivalent detector response exists for several anions. The lower RIC responses produced by other ions have been linked to a number of causes that might be eliminated or reduced by operating under different conditions. Further investigations appear justified. Cation detection by cation replacement does not appear here to be an analytically useful RIC mode. Cations can be detected but only a t high concentrations and with poor reproducbility. Moreover, universal calibration cannot be utilized effectively with this detection mode because of severe peak tailing. However, in a nearly completed study, we have shown that NH4+and alkali metal cations can be successfully quantitated by RIC utilizing an anion-replacement method. ACKNOWLEDGMENT We are grateful to Dionex Corp. for providing the chromatographic columns. Registry No. Li, 7439-93-2;F-,16984-48-8;C1-, 16887-00-6; Na, 7440-23-5;K, 7440-09-7; NH4+,14798-03-9;HC1, 7647-01-0; "OB, 7697-37-2; HBr, 10035-10-6; HI03, 7782-68-5; H,SO,, 7664-93-9; HSP04, 7664-38-2; HF, 7664-39-3.

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RECEIVED for review February 26, 1987. Accepted June 1, 1987. Supported by the National Science Foundation through Grant CHE 83-20053, by the Office of Naval Research, by American Cyanamid, and by Monsanto.

Characterization of replacement-ion chromatography employing

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