Anal. Chem. 1987, 59, 761-766
761
Voltammetric-Amperometric Dual-Electrode Detection for Flow Injection Analysis and Liquid Chromatography Craig E. Lunte;Thomas H. Ridgway, and William R. Heineman* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221
This artkie describes the appllcatlon of a series conflguration dual-electrode flow-through thln-layer electrochemical cell to vottammetrlc studies. Voltammetric detection Is achieved by scanning the potential at the upstream electrode and following the reaction amperometrically at the downstream electrode. Two detection modes are possible, collection and shielding. I n the collectlon mode the downstream electrode Is used to detect the product of the reaction at the upstream electrode. I n the shleiding mode the downstream electrode is used to determine the depletlon of the anaryte due to reaction at the upstream electrode. These two modes are complementary because the collection mode is more selective but only appllcable to chemically reverslble redox species while the depletion mode is applicable to any species undergoing a redox reaction at the upstream electrode. Detection limits of lo-' M hydroquinone can be achieved at scan rates up to 2.0 Vis. This report descrlbes the operational principles and characterization of this detector.
Liquid chromatography/electrochemistry (LCEC) has become a widely used technique for trace organic determinations because of the selectivity and low detection limits which can be achieved with this technique (1). Although most reports still describe the use of amperometric detectors with only a single working electrode, dual-electrode amperometric detectors are becoming more popular (2). The use of dualelectrode amperometric detectors provides advantages in selectivity, detection limits, and qualitative information content relative to single-electrode detectors (3-5). Voltammetric detectors with a single working electrode have also been reported (6-10). Voltammetric detectors provide improved qualitative information from chromatographic effluents relative to detection a t a single potential. Dual-electrode amperometric detectors have been used in a variety of configurations to achieve different modes of operation (2). In the series configuration the upstream electrode is used to modify the analyte for improved detection at the downstream electrode. This typically involves cycling of a chemically reversible redox species where the redox product can be more selectively detected than can the original molecule (3). Alternatively, the series dual-electrode configuration can be used with difference mode detection to eliminate base-line interferences when operating at extreme potentials (5). Dual-electrode amperometric detectors can also be used in the parallel configuration, in which case the chromatographic effluent can be monitored simultaneously a t two different potentials (4).The primary advantages of all of the dual-electrode techniques are improved selectivity and detection limits. T o obtain more complete voltammetric information than readily found with amperometric detection, various potential scanning (voltammetric) techniques have been developed for chromatographic and flow injection analysis detectors (6-1 0). These detectors typically use a pulsed waveform such as staircase or square wave to overcome the large charging 0003-2700/87/0359-0761$01.50/0
currents associated with potential scamkg-in thin-layer cells. Even with these pulse techniques, voltammetric methods have not been able to achieve the detection limits readily obtained by simple amperometric detectors. White et al. (IO) have recently described a voltammetric detector using a fiber microelectrode that exhibited greatly reduced charging currents permitting scan rates up to l V/s. However, this detector was limited to use with open-tubular chromatographic systems. The use of potential scanning techniques with a dualelectrode detector can provide the combined qualitative and quantitative advantages of these techniques while overcoming several of the problems. Voltammetric-amperometric detection is achieved by use of the dual-electrode transducer in the series configuration by scanning the potential at the upstream electrode while maintaining a constant potential a t the downstream electrode (11). The downstream electrode is used to monitor the redox reaction occurring at the upstream electrode without the charging current associated with scanning the potential. Therefore, voltammetric information can be obtained while maintaining the detection limits possible with amperometric detection. The downstream electrode can be operated either at a potential to reverse the reaction from the upstream electrode, known as collection, or at a potential to cause the same reaction as at the upstream electrode, known as shielding. These processes are illustrated in Figure 1along with typical upstream excitation signal and both upstream and downstream current responses. These processes will be described in greater detail in later sections. The principle of operation of the dual-electrode voltammetric-amperometric detector for liquid chromatography is the same as for the rotating ring-disk electrode system used with more classical voltammetric techniques. This report describes the operational principles and characterization of this detector. EXPERIMENTAL SECTION Reagents. All solutions were prepared from reagent grade chemicals and distilled, deionized water. Hydroquinone, vanillic acid, and ascorbic acid were obtained from Sigma Chemical Co. (St. Louis, MO). Potassium ferricyanide was purchased from Mallinckrodt, Inc. (St. Louis, MO). Standard solutions were prepared in 0.1 M sodium acetate buffer, pH 5. Chromatographic/Flow Injection System. The liquid chromatography/flow injection system consisted of a Model 396 Minipump (Milton Roy, Riviera Beach, FL), a flow-through pulse damper (Bioanalytical Systems, Inc., West Lafayette, IN), and a 7010 injection valve (Rheodyne, Berkley, CA). A 1.0-mL sample loop was used for flow injection analysis (FIA) and a 100-pL sample loop was used for liquid chromatographic analysis (LC). A Brownlee MPLC RP-18 column (4.6 mm X 10 cm) was used. For FIA the column was placed between the pulse damper and the injection valve to provide back pressure for proper operation of the pump and pulse damper. For chromatographic analysis the column was placed after the injection valve. A mobile phase of 0.1 M sodium acetate buffer, pH 5, was used in all experiments. The mobile phase was prepared from distilled, deionized water and was filtered through a 5-pm filter prior to use. A flow rate of 1.0 mL/min was used in all experiments unless otherwise noted. Electrochemical Apparatus. The bipotentiostat used in these experiments is controlled with an MC 6809 based micro0 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
Table I. Cell Characteristics scan rate,
-30 I I
collmctlon
I I I
I
_
IO I
0
I
-30
b - a t c ’ Flgure 1. Timing diagram for dual-electrode voltammetric-amperometric detection in the collection and shielding modes: E,, upstream potential; I,, upstream current; I,, downstream current; At, time delay between electrodes.
processor. The construction of this instrument and its application to the anodic stripping voltammetry with collection of In3+have been previously described (12,13). This instrument can independently apply various waveforms and determine the resulting currents at two working electrodes. Possible waveforms include staircase, pulse, and constant potential. As this bipotentiostat was initially designed for a different purpose, it is not optimized for voltammetric-amperometric detection. For example, data acquisition is not directly synchronized with potential waveform application. An optimized bipotentiostat is currently being designed. The flow cell used for electrochemical detection was of the thin-layer design (TL-5A, Bioanalytical Systems, Inc.). The reference electrode was positioned downstream of the working electrodes and the auxiliary electrode was positioned directly across the thin-layer channel from the working electrodes. The working electrodes were glassy carbon and the auxiliary electrode was platinum. As with the bipotentiostat, this cell design is not optimal for this application. It was chosen to illustrate the usefulness of the voltammetric-amperometric detection mode using a commercially available electrochemical cell. An improved cell design is currently under construction. The BAS-100 electrochemical analyzer was used to determine the resistance associated with the thin-layer electrochemical flow cell. All potentials are vs. the Ag/AgCl reference electrode.
RESULTS AND DISCUSSION Characterization of the Flow Cell. Several flow cell parameters are important for optimal voltammetric-amperometric mode detection. As for any scanned potential technique, the cell resistance and capacitance are critical to good potential control at the scanned electrode. The degree of cross-talk between the two working electrodes is important a t low analyte concentrations. Because the two working electrodes are spatially separated, a flow-rate dependent delay will exist between the time when an “effluent packet” crosses the upstream electrode and when it reaches the downstream electrode. This time delay must be known in order to relate the downstream current to the upstream potential. Finally, the collection and shielding efficiencies of the series dualelectrode cell are important to successful operation. Electrical Characterization. The resistance of the thinlayer electrochemical cell was determined by studying the
V/s
i,, IA
0.2 0.5 1.0 2.0 5.0 10.0
0.61
1.20 2.85 5.56 14.4
25.6
Cdl,
IF
3.05 2.40 2.85
2.78 2.88 2.56
iR,mV
cross talk, nA shielding collection
5 9 22 42
0.4 0.7 1.4 2.9
0.7 1.8 3.6 7.1
109 195
8.9
17.9
17.9
35.7
variation in peak current and peak potential as a function of scan rate for a chemically reversible redox couple using cyclic voltammetry. The resistance was determined by using both the hydroquinone/benzoquinone and ferricyanidelferrocyanide couples to ensure that no chemical complications interfered with the resistance determination. The cell resistance was found to be 7.8 K using ferricyanide and 7.4 K using hydroquinone. This high resistance puts an upper limit on the scan rate which can be used without encountering significant iR drop complications. At a scan rate of 2 V/s, charging current was approximately 5 FA resulting in an iR drop of 40 mV. This was deemed to be the largest acceptable value. The capacitance of the thin-layer cell was also evaluated. The charging current a t various scan rates was determined and used to calculate the cell capacitance. A value of 2.8 MF was found. The cell resistance and capacitance combine to give a cell RC time constant of 21 ms. The results of the cell electrical characterization are shown in Table I. Cross talk is a nonfaradaic current occurring a t the downstream electrode due to a large current at the upstream electrode. Cross talk can arise in two ways but both are directly related to the cell resistance. Cross talk can arise from the iR drop at one electrode affecting the interfacial potential at the other electrode because of the manner in which dual-electrode potentiostats must be implemented. In addition, because of the high resistance of the thin-layer cell, a current path from the upstream working electrode to the downstream working electrode can compete with the current path to the auxiliary electrode. The absolute cross talk is a function of the current at the upstream electrode and therefore the scan rate. This is shown in Table I. The relative cross talk (id/iu) is independent of scan rate and is approximately 0.1 % for the electrochemical cell used in these experiments. While the iR drop puts an upper limit on the usable scan rate regardless of analyte concentration, cross-talk current can further limit the usable scan rate at low analyte concentrations. Background subtraction can be very effective in eliminating interference from cross talk. Figure 2 shows the downstream response at 2 V from a lo4 M hydroquinone solution scanned at 5 V/s a t the upstream electrode. Trace B is the raw data exhibiting cross talk from both faradaic and capacitance currents at the upstream electrode convoluted with the desired downstream faradaic response and is obviously unusable in this form. Trace A is the same data after background subtraction from a mobile-phase-only scan. The voltammetric response is readily seen in this trace. All subsequent experiments were background subtracted. Flow Characterization. Under the hydrodynamic conditions of the series dual-electrode thin-layer flow cell, there is a fiiite time delay between when any given segment of solution flows over the upstream electrode and when it reaches the downstream electrode. Knowledge of this time delay and its flow rate dependence is critical to using the voltammetricamperometric detection mode because the current response at the downstream electrode must be correlated to the potential at the upstream electrode. To determine this time delay between the electrodes, the potential of the upstream
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
763
Table 11. Collection and Shielding Efficiencies
A
flow rate, mL/min
collection
3.36 2.45 1.40 0.56 0.34
0.314 0.314 0.322 0.317 0.321 0.318
lOnA
shielding (id/iU)*
(id/iu)'
0.686 0.684 0.683 0.676 0.679
* 0.004
0.682 f 0.004
"Downstream potential = 1.0 V vs. Ag/AgCl. *Downstream potential = -0.2 V vs. Ag/AgCl. 0.5
- 0.5 VIS
-
..............
L e 0.0
V ~
J
"
"
0.2
"
J
"
"
J
0.4
"
~
0.6
0.2
-
0.1
-
Seconds Flgure 2. Background subtraction of cross talk from a voltammogram of 10" M hydroquinone scanned at 2 VIS: A, corrected current data: 6 , raw current data.
electrode was pulsed between potentials where hydroquinone is not electroactive (0.0 V) and where it is oxidized at a diffusion limited rate (+1.0 V) while the downstream electrode was held at a potential to detect any benzoquinone formed upstream (-0.2 V). A sample of hydroquinone solution was then injected into the system, and the upstream potential and downstream current were recorded. The time delay between the electrodes could then be determined from the time between a potential step a t the upstream electrode and the resulting downstream current response. These experiments revealed that the effective delay time is inversely proportional to the flow rate. A plot of the inverse of delay time (ms-I) vs. flow rate (pL/s) gave a straight line of slope 0.162 pL-l with an intercept of 1.76 8; the correlation coefficient (r2)was 0.997. This relationship is used in all subsequent experiments to correlate the downstream current to the upstream potential. Electrochemical Characterization. The first electrochemical characteristic of the cell to consider is the conversion efficiency for the analyte. The cell channel thickness is 0.0127 cm which gives a diffusion time from the electrode to the opposite wall of approximately 8 s (assuming a diffusion coefficient of 10" cm2/s). Therefore, at scan rates of 0.2 V/s and greater, a molecule cannot diffuse across the channel width. In addition, at a flow rate of 1mL/min the residence time in the cell is approximately 500 ms. This means that not all molecules entering the cell will be oxidized or reduced and that those molecules which do undergo an electrochemical reaction cannot cross the channel to react again at the auxiliary electrode. Mass-transport-controlled hydrodynamic voltammograms should be obtained with this cell. The collection and shielding efficiencies of the dual-electrode cell were determined by using dual amperometric operation as previously described (5). Collection efficiency is the ratio of downstream current to upstream current when the downstream electrode is operated at a potential to reverse an upstream redox reaction (i.e., when redox cycling). Shielding efficiency is the corresponding ratio under equipotential operation. For a given electrochemical cell, the collection and shielding efficiencies should add to unity for a reversible redox couple. Values for the collection and shielding efficiencies using hydroquinone as the test compound are listed in Table 11. As shown, both collection and shielding
I
I
I
0.9
0.7
0.5
1.0 v/s
I
0.3
0.1
E~o~ts)
Flgure 3. Effect of scan rate on collection voltammetry of 2 X M hydroquinone: downstream potential, -0.2 V.
are flow-rate independent. The values found for this study agree well with previously reported values for a similar cell
(3,5). Voltammetric-Amperometric Detection. Two modes of operation are possible with voltammetric-amperometric detection. In the collection mode the downstream electrode is operated at a potential to reverse the redox reaction probed at the upstream electrode. This mode of operation therefore depends on the chemical reversibility of the analyte being studied and can be prone to chemical complications from following reactions (i.e., EC reaction mechanisms). However, the potential at which the downstream electrode can be operated in this mode is often near optimal for amperometric detection, thus providing low detection limits. The second mode of operation is termed shielding and involves operating the downstream electrode at a potential where the redox reaction being studied at the upstream electrode occurs at a diffusion limited rate. Reaction at the upstream electrode depletes analyte from the solution at the electrode surface; therefore the downstream electrode experiences a decrease in analyte concentration. In the shielding mode a decrease in response at the downstream electrode is associated with the occurrence of a redox reaction at the upstream electrode. This operational mode does not depend on the electrochemical behavior of the analyte but is a mass transport phenomenon and therefore is applicable to any electroactive compound regardless of reversibility. Collection Mode. To evaluate the utility of voltammetric-amperometric detection in the collection mode, several parameters are of interest. These include the maximum usable scan rate, detection limit, response linearity, and selectivity. Unfortunately, all of these parameters are not independent. In particular, the scan rate and detection limit are related through the amount of cross talk observed with the system. As described earlier, the absolute limit on scan rate is determined by the amount of iR drop that can t e tolerated. Figure 3 shows hydrodynamic voltammograms of a 2 X
764
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
i,, nA scan rate, VIS
2
459 471 462 473 463 461
0.2 0.5 1.0 2.0 5.0 10.0
25.0 25.1 22.6 23.1 21.9 23.2
465 f 6 25
.-...Vanillic Acid
i, nA 10-7 M
i, nA IO4 M
M
X
-Hydroquinone
r
Table 111. Variation of Plateau Current with Concentration and Scan Rate for Collection Mode
2.37 2.61 2.25 2.19
I
23.5 f 1.0
2.36 f 0.19
1.0
0.e
LO
0.8
I
'-.1--.-
I
I
I
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.o
I
r
E(volts) I
1.0
I
I
I
I
I
0.5
I
1
I
0.0
Figure 5. Collection voltammetry of a mixture (bottom voltammogram) of 10" M hydroquinone and 7.5 X lo-' M vanillic acid: scan rate, 1.0 VIS; downstream potential, -0.2 V. Top voltammograms are for
individual components. E(votts) Flgure 4. Detection of lo-' M hydroquinone by 0.5 V/s; downstream potential, -0.2 V.
- Hydroquinone
collection: scan rate,
M hydroquinone solution obtained at several scan rates, including amperometrically using the multiple injection technique (12). The curves are in good agreement below scan rates of 2 VIS, above this scan rate iR perturbations are quite noticeable. These scans all exhibit well-defined hydrodynamic voltammetric behavior under semiinfiiite diffusion conditions. This is an improvement over the system of White et al. (IO) where mixed semiinfinitelfinite diffusion was observed. In addition to the voltammetric information contained in the voltammograms, quantitative information can be obtained from the plateau current data. As shown in Table 111,plateau current is independent of scan rate and directly proportional to analyte concentration. Therefore, scan rate can be optimized for a particular voltammetric analysis and the diffusion-limited current still compared to experiments performed at other scan rates. Voltammograms of solutions of less than M can be obtained with this technique. Figure 4 shows the voltammogram of a lo-' M hydroquinone solution obtained at a scan rate of 0.5 VIS. As can be seen from the figure, it should be possible to obtain reasonable voltammograms from even more dilute solutions. However, a t present bipotentiostat and interface limitations, instead of noise considerations, control the low concentration cutoff for obtaining voltammograms. At low concentrations, the current due to cross talk becomes larger than the faradaic current. Background subtraction can eliminate the interference from cross talk but the resulting signal contains more noise. To achieve the lowest detection limits, scan rates below the maximum imposed by iR considerations alone must be used. It is anticipated that an improved electrochemical cell design with lower resistance (and therefore less cross talk) will be capable of both faster scan rates and lower detection limits. The final consideration for voltammetric-amperometric detection with collection is selectivity. This mode provides two types of selectivity. First is the voltammetric discrimi-
--- ASCOrbiC
Acid
Downstream
\
- 0.1
E (volts)
- 0.5
1.5
Figure 6. Collection voltammetry of 2 X M hydroquinone and 1.6 X M ascorbic acld scanned at 1.0 V I S : downstream potential, -0.2 v.
nation from the upstream electrode. Second is discrimination of chemically irreversible redox compounds by the downstream electrode. Voltammetric selectivity is shown in Figure 5 which is the voltammetry of a mixture of lo4 M hydroquinone and 7.5 X M vanillic acid. Two voltammetric waves can be seen in this voltammogram and the relative plateau currents are proportional to analyte concentration. The voltammetry at both the upstream and downstream electrodes of 2 X M hydroquinone and 1.6 X M ascorbic acid is shown in Figure 6. Because the oxidation of ascorbic acid is chemically irreversible, no current is detected at the downstream electrode
ANALYTICAL CHEMISTRY, VOL.
E(Volts)
E (volts) 1.0 1.0
I
0.8 I
0.6 I
59, NO. 5, MARCH 1, 1987 785
0.4 I
0.2 I
0.5 I
1
0.0
I
I
I
I
I
I
L.
r
-13.0
..-..._ 10.0v/s Figure 8. Detection of lo-' M hydroquinone by shielding: scan rate, 0.5 V/s; downstream potentlal, 1.0 V.
Arnperornetric
Figure 7. Effect of scan rate on shielding voltammetry of 2 M hydroquinone: downstream potential, 1.O V.
E(volta)
X lo-'
for this compound. Therefore, although hydroquinone and ascorbic acid are not voltammetrically resolved at the upstream electrode, only hydroquinone produces a response at the downstream electrode. Shielding Mode. In the shielding mode the downstream electrode is held at a potential such that the redox reaction being investigated occurs at a mass transfer limited rate. The current at the downstream electrode is then determined by the amount of analyte reaching the electrode. Any reaction at the upstream electrode which reduces the concentration of the analyte in the mobile phase will cause a decrease in the current at the downstream electrode. Therefore, scanning the potential at the upstream electrode will produce a voltammetric response at the downstream electrode although it is maintained at a constant potential. The shielding mode differs from the collection mode in that shielding is a mass transport phenomena instead of an electrochemical phenomena. However, many of the limitations of voltammetric-amperometric detection encountered in the collection mode are also significant in the shielding mode. In particular, the scan rate limitation imposed by iR drop considerations is independent of whether shielding or collection is being used. This is because the iR drop occurs at the upstream scanning electrode which also does not depend on shielding or collection. Figure 7 shows shielding voltammograms of hydroquinone obtained a t various scan rates. The effect of iR drop on these voltammograms is the same as that found with the collection mode (Figure 5). Figure 7 illustrates another important characteristic of the shielding mode. Instead of starting a t zero current at potentials were the compound is not electroactive and increasing to a mass transport limited value, the current begins at a large value and decreases as the potential is increased to where the compound is electroactive. This is because the voltammetric information is contained in the depletion of analyte from the diffusion layer. The identical voltammetric information is contained in these data as in the collection experiment; it is just not in the typical format (in essence the voltammogram is inverted and offset). In general, the detection limit that can be achieved with the shielding mode is the same as with the collection mode. Although the amount of cross talk is slightly lower at the downstream potential of +1.0 V for shielding (Table 111), this gain is offset by the inherently higher background current encountered at the more extreme potentials necessary for the shielding mode. Figure 8 shows a voltammogram of a lo-' M hydroquinone solution obtained at a scan rate of 0.5 VIS using
0
.
0
0.8 , l
0.4
0.6 I
,
I
0.2 I
1
,
!
I
1.0
-
15
- 2.0
Figure 9. Shielding voltammetry of 2.6 X M ascorbic acid scanned at 1.O Vls: downstream Dotential, 1.O V. UDstream current axis is left. E(volts)
I;O
Oi8
Oi6
,
Oi4
,
Oi2
,
I
Figure 10. ShieMing voltammetry of a mixture of lo-' M hydroquinone and 7.5 X lo-' M vaniillc acid: scan rate, 1.0 V/s; downstream potential, 1.O V. the shielding mode with the downstream electrode held at +LO V. If background-related noise limits the detection of the analyte as is typical for amperometric detectors, the detection limit for the shielding mode should be somewhat higher than for the collection mode. This is because the shielding mode typically requires larger potentials which causes higher background currents and noise. Because of instrumental limitations we have not reached this point at this time; the lowest concentrations used in this study were the same for both shielding and collection, Because the shielding mode is based on mass transport phenomena instead of electrochemical phenomena, this mode is applicable to any compound that is electroactive at the upstream electrode regardless of chemical reversibility. This is illustrated by the voltammetry of ascorbic acid obtained
766
Anal. Chem. 1087, 59, 766-769
1
\
m~nutes
Chromatographic Detection. This detection mode can also be used in combination with liquid chromatography. Since it is the purpose of this work only to demonstrate the principles of this detection mode, most of the data were obtained by using flow injection analysis. Figure 11 shows the voltammogram of a chromatographic peak from a 1OO-fiL injection of M hydroquinone obtained in the collection mode. This voltammogram is identical with those obtained by using flow injection analysis. The chromatogram of the sample obtained at a fixed potential of +0.7 V and the timing of the voltammetric scan are shown in the insert. Hardware and software improvements are currently being developed which should both improve the detection limits which can be achieved and provide for more useful chromatographic detection. In particular, the software is being modified so that the potential can be repetitively scanned during the course of a chromatogram.
E(votts)
LITERATURE CITED
Figure 11. Collectton voltammetry of a chromatographic peak from the injection of 100 ML of lo-' M hydroquinone: scan rate, 1.0 V/s; downstream potential, -0.2 V.
in the shielding mode as shown in Figure 9. Because the oxidation of ascorbic acid is chemically irreversible, the voltammetry could not be obtained in the collection mode. The shielding mode still provides the voltammetric selectivity from the upstream electrode as shown in Figure 10 for the voltammetry of a mixture of hydroquinone and vanillic acid. While it might appear that the shielding mode and collection mode represent competing techniques for voltammetric-amperometric detection, they are actually complementary methods. The shielding mode provides a universal method of obtaining voltammetric information from a chromatographic effluent while the collection mode provides improved selectivity for certain applications. The hardware and software requirements are identical for both methods and they can therefore both be used with the same sample on alternate chromatograms.
(1) Shoup, R. E., Ed. Sibliogrsphy of Recent Reports of Electrochemical Detection; BAS Press: West Lafayette, IN, 1982. (2) Roston, D. A.; Shoup, R. E.; Kissinger, P. T. Anal. Chem. 1982, 5 4 , 1417A. (3) Roston, D. A.; Kissinger, P. T. Anal. Chem. 1982, 5 4 , 429. (4) Lunte, C. E.; Kissinger, P. T. Anal. Chem. 1983, 55, 1458. (5) Lunte, C. E.; Kissinger, P. T.; Shoup, R. E. Anal. Chem. 1985, 5 7 , 1541. (6) Wang, J.; Dewald, H. D. Anal. Chlm. Acta 1983, 753,325. (7) Scanlon, J. J.; Flaquer, P. A.; Robinson, G. W.; O'Brien, G. E.; Sturrock, P. E. Anal. Chim. Acta 1984, 758,169. (8) Last, T. A. Anal. Chem. 1983, 55, 1509. (9) Caudill, W. L.; Ewing, A. G.; Jones, S.; Wightman, R. M. Anal. Chem. 1983, 55, 1877. (IO) White, J. G.; St. Claire, R. L., 111; Jorgenson, J. W. Anal. Chem. 1988, 58, 293. (11) Lunte, C. E.; Wong, S.; Chan, K. W.; Ridgway, T. H.; Heineman, W. R. Anal. Chim. Acta, in press. (12) Wise, J. Ph.D. Dissertation, University of Cincinnati, 1983. (13) Oth, M. Ph.D. Dissertation, University of Cincinnati, 1983.
RECEIVED for review June 25, 1986. Accepted November 3, 1986. Support for this project was provided by National Science Foundation Grant CHE-8217045 and Bioanalytical Systems, Inc.
1,3-Bis(8-quinolyloxy)propane Derivatives as Neutral Carriers for Lithium Ion Selective Electrodes Kazuhisa Hiratani,* Tatsuhiro Okada, and Hideki Sugihara
Industrial Products Research Institute, 1 - 1 -4, Yatabe-machi Higashi, Tsukuba-gun, Ibaraki 305, Japan
Poly(v1nyi chlorlde) membrane electrodes based on 1,g-bis(8qulndyloxy)propane derivatlves as carriers exMM appre clabk selecthflty for Ll' relative to Na', ,'K and Ca2'. The Li' selectivity can be slgnmcantly Improved by modllcatlon of the chaln structure of the carriers. 2,2-MethyI-1,3bls(s-gulnolyloxy)pane has the bsd selectivity coefflclents (log &'@(Li,M)) for L1+ over Na', up to -1.6, over K', up to -2.3, over Mg2', up to -2.6, and over Ca2+, up to -2.5. The detection limit for LI' was about 5 X lo4 mol/dm3.
w+,
Much attention has been focused on neutral carrier Li+selective electrodes for monitoring Li+ activities in biological systems, e.g., in the therapy of manic depressive psychosis
(1-3). Several neutral carriers for Li+-selectiveelectrodes have been reported so far ( 4 ) ,such as noncyclic polyether amides (5-7) and 12- or 14-crown-4 derivatives (8-13). Because of the high requirement with respect to the Li+/Na+ selectivity and the electrode potential stability (7), work has been done both on the synthesis of the new types of neutral carriers and on the improvement of ion selectivity (14). Various requirements must be met if the neutral carriers are to be successful: the carriers must possess good coordination sites (cavities) which fit the sizes of primary ions, but their structures should also be flexible enough to interact reversibly at the ion-selective membrane/solution interface; obviously, the carriers must be sufficiently lipophilic to be soluble and/or mobile in the membrane. From these points of view, noncyclic or sandwich-type polyether derivatives (5-7, 15) would be
0003-2700/87/0359-0766$01.50/00 1987 American Chemical Society
