Anal. Chem. 1989, 6 1 , 1977-1980 (1 1) Franchini, G. C.; Marchetti, A.; Tassi, L.; Tosi, G. J. Chem. Soc., Faraday Trans. 1 1988, 84. 4427-4438. (12) Handbook of ChemisWy and Physics, 66th ed.;Weast, R. C., Ed.; The Chemical Rubber Co.: Cleveland, OH, 1985-1986. (13) Fuoss, R. M.; Hsia, K. L. J. Am. Chem. SOC. 1968, 9 0 , 3055-3060. (14) Goffredi, M. J. Chem. Soc.,Faraday Trans. 11987, 8 3 , 1437-1447. (15) Roberts, J. H. I n The Chemistry of Non8queous Solvents; Lagowski, J. J., Ed.; Academic Press: New York, 1976. (16) Davis, M. M.; Paabo, M. J. Res. Natl. Bur. Std. 1963, A 6 7 , 244-246. (17) Wilii, A. V.; Mori, P. Helv. Chim. Acta 1964, 47, 155-156. (18) Kortum, G.; Vogei, W.; Andrussow, K. I n Dissociation Constants of Organic AcMs in Aqueous Solution; Butterworths: London, 1961. (19) Ives, D. J. G.; Moseley, P. G. N. J . Chem. SOC. B 1966, 13, 757-761. (20) Shamim, M.; Spiro, M. J. Chem. SOC.,Faraday Trans. 1 1970, 66, 2863-287 1. (21) Marchetti, A,; Picchioni, E.; Tassi, L.; Tosl, G., unpublished work. (22) Time series Processor-TSP-User’s Guide; Hall, Bronwyn H., Ed.: TSP International: Stanford, CA, July 1983.
1977
(23) Accascina, F.: Petrucci, S.;Fuoss. R. M. J. Am. Chem. SOC. 1959, 8 1 , 1301-1305. (24) Bdenseh, H. K.; Ramsey, J. B. J. Phys. Chem. 1063, 6 7 , 140-143. (25) Frolich. H. I n Theory of Dielectrics; Clarendon Press: Oxford, U.K., 1958. (26) Green. J. R.; Margerison, D. I n Statistical treatment of experimental data ; Eisevier Publ.: Amsterdam, 1977. (27) Shorter, J. I n Correlation analysis of organic reactivky; J. Wiley and Sons, Ltd.: Letchworth, U.K., 1982. (28) Mandelbrot, B. B. I n Fractals, Form, Chance and Dlmension; W. H. Freeman and Co.: San Francisco, CA, 1977. Richter, P. H. I n The beauty of fractals; Springer-Ver(29) Peitgen, H. 0.; lag: Berlin, 1986. (30) Mandelbrot, B. B. I n Gli oggetti Frattali; Einaudi: Torino, Italy, 1987.
RECEIVED for review January 17,1989. Accepted May 22,1989. The Minister0 della Pubblica Istruzione (MPI) of Italy is acknowledged for financial support.
CORRESPONDENCE Voltammetric Detection with Gradient Elution for Open Tubular Liquid Chromatography Sir: Electrochemical detection has become an important tool in liquid chromatography. However, the acceptance of this detection method for LC has been hindered by the belief that it does not work well with gradient elution (1-3). As recently as 1980, it was thought that “gradient elution cannot be used with electrochemical detectors” (2). Since that time many researchers have found that the two can be compatible (4-14). To our knowledge, all of the published work on this topic has involved amperometric detection. While data obtained at a single potential can be very valuable, voltammetric detection can yield a great deal more information since it provides electrochemical as well as chromatographic data. This can be useful for complex mixtures, in which analytes may not be resolved chromatographically but do have different oxidation or reduction potentials. They can thus be resolved voltammetrically but not amperometrically under the same chromatographic conditions. An excellent example of the utility of voltammetric detection is found in the analysis of individual snail neurons (1516). The added resolution obtained by combining voltammetry with chromatography allowed for the quantitative determination of several neurotransmitters present in single cells. Without voltammetric detection it would have been impossible to resolve all of the compounds present. Unfortunately, a gradient was not available to these workers, and they were thus affected by the general elution problem. The combination of a gradient with voltammetry in this case would have provided an even more powerful technique. This report describes the use of voltammetric detection with gradient elution in an open tubular LC system. The value of the technique is demonstrated by using a sample of brewed tea.
EXPERIMENTAL SECTION Apparatus. The chromatographic system used in this work has been described in detail elsewhere (14, 17). Briefly, the gradient was provided to the column by a Waters 600E Multisolvent Delivery System used at a flow rate of 1 mL/min. The flow rate on the column is typically 60 nL/min, so a splitting system was employed to divert the majority of the mobile phase 0003-2700/89/0361-1977$01.50/0
to waste. The analytical column had an inner diameter of 15 pm, was 150 cm long, and had octadecylsilane bound to its porous glass walls. The electrochemical detector used here has also been described previously (17,19,20). A carbon fiber, 0.4 mm long and 9 fim in diameter, serves as the working electrode. It is inserted directly into the outlet end of the capillary column. The electrodepotential was scanned from 0 to +1.5 V vs a AgjAgCl reference electrode at a rate of 1 VIS. Data were acquired at 100 points/s through the use of a microcomputer. A Model 427 current amplifier (Keithley Instruments, Inc., Cleveland, OH) with a 30 ms rise time and a Model 3341 low pass filter (Krohn-Hite Corp., Avon, MA) set at 40 Hz were also used. Materials. HPLC grade acetonitrile and methanol (Fisher Scientific Co., Fair Lawn, NJ) were used as received. All water used was purified by a Milli-Q Water Purification System (Millipore,Bedford, MA). Standards were purchased from Sigma (St. Louis, MO). Naphthalene-2,3-dicarboxaldehyde (NDA) was purchased from Molecular Probes, Inc. (Eugene, OR), while reagent grade sodium cyanide was obtained from Aldrich (Milwaukee, WI). Phosphate buffer (0.1 M), pH 3.1,was mixed with acetonitrile to form the mobile phases. All mobile phases were filtered before use. Sample solutions were made by using the initial mobile phase as the solvent and were prepared fresh each day. Procedures. Preparation of Tea. Tea samples were prepared by boiling 2.4 g of tea leaves (Nifda Golden Brew Tea) in 60 mL of 0.1 M phosphate buffer for 5 min. After cooling, the solution was filtered through a Whatman No. 42 filter (Whatman Paper, Ltd., Maidstone, England), and then through a 0.45-pm filter (Micron Separations, Inc., Westboro, MA). Derivatization of Leucine. Leucine was derivatized to form an electroactive compound using NDA (14,21-23). To 100 pL of lo-’ M leucine was added 100 pL of 0.1 M cyanide, followed by 200 pL of 0.01 M borate buffer (pH 9.5) containing 20% methanol. Finally, 100 pL of 0.1 M NDA (dissolved in methanol) was added. The reaction was allowed to proceed at room temperature for 10 min and was then diluted 1:l with the initial mobile phase.
RESULTS AND DISCUSSION The viability of voltammetric detection with gradient elution was determined by using a set of standards. Figure 1 0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1 , 1989
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Figure 1. Chromatogram at +1.2 V vs AglAgCl of an equimolar mixture (0.1 mM) of tyrosine (A), catechol (B), 4-methylcatechol (C). 2.3dihydroxynaphthalene (D). and NDA-tagged leucine (E).
A
1.
Figure 3. Chromatograms of four voltages from a single blank run: (A) chromatogram at +1.5 V YS AgIAgCI; (B) chromatogram at +1.1 V: (C) chromatogram at +0.7 V; (D) chromatogram at +0.3 V. togram of this group of standards demonstrates that the base
1.
Flgure 2. Chromatovoltammograms of ihe standard run shown in Figure 1: (A) includes the first 20 min: (6)covers the remaining 30 min. Labeled peaks correspond Io those given in Figure 1 shows results that were obtained voltammetrically but with only the current from a single potential (+1.2 V) plotted. The mixture consists of 1 P M tyroaine, catechol, 4-methylcatechol, 2,3-dihydroxynaphthalene,and NDA-leucine. The chroma-
line is relatively flat over a wide range of organic content. The gradient used here began with 100% phosphate buffer and changed linearly to 80% phosphate with 20% acetonitrile over the first 20 min. During the following 30 min the mobile phase composition changed Linearly to 30170 phosphate/acetonitrile. Chromatovoltammograms of two parts of the standard run are given in Figure 2. Prior to converting all data to the three-dimensional domain, the mathematical derivative was taken of each electrochemical wave (20). Figure 2a is taken from the first 20 min (0to 20% acetonitrile) and shows a few ripples but is, in general, very flat. Figure 2b contains the part of the run that involves an increase in acetonitrile content of the mobile phase from 20 to 70%. While a rise in the base line is evident a t very low potentials, again the background is quiet. The valleys seen behind the peaks in the chromatovoltammograms are a result of taking the mathematical derivative of the peaks and are not due to the gradient or the detection method (20). Evaluation of the extent of base-line drift due to the gradient was done by means of a blank run, in which no solutes were injected. Data-taking was initiated when the gradient
ANALYTICAL CHEMISTRY, VOL.61. NO.17. SEPTEMBER I. 1989. 1979
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TIME IminYIco)
Flgue 4. Chomatogam at +1.3 V vs AgIAm of a s a m w of brewed tea. Peak labeled "T" is theophylline.
began. The gradient used was a linear change from 0 to 40% acetonitrile in phosphate buffer over 50 min. Figure 3 shows the base line obtained a t four different potentials. The background current is the highest a t +1.5 V, as might he expected. The large dip present from approximately 15 to 20 min is thought to he due to electroactive contaminants present in the mobile phase that build up a t the head of the column during the early part of the gradient. As the organic content increases, these contaminants are eluted. Certainly the base-line drift seen here would he detrimental if very low andyte concentrations were used, hut for routine analyses the change in base line is acceptable, as demonstrated by the standard run shown in Figure 1. The detection limit for voltammetric detection with gradient elution was estimated for catechol. The signal to root mean square noise ratio for catechol at a concentration of 1.9 X lod M was found to he 47. A concentration of 1.2 X 10-6 M would therefore yield a signal to noise ratio of three. This is equivalent to approximately 7 fmol injected onto the column. This estimated detection limit is about 100 times higher than that found for voltammetric detection without the gradient, which is ahout 10- M (20). However, the increased analytical utility of the combination of the gradient with voltammetry will certainly outweigh the disadvantage of a higher detection limit in many cases. The resolving power of voltammetric detection in conjunction with gradient elution is demonstrated for tea, a complex sample containing many electroactive species. Figure 4 is a chromatogram of brewed tea, obtained voltammetrically hut with only the current from +1.3 V plotted. The gradient used in this case is the same as that for the blank run, a linear change from 0 to 40% acetonitrile over 50 min. Two major components of tea, theophylline and caffeine, should he present in the chromatogram. Theophylline is the first peak to elute. The voltammetric wave of this compound peaks at approximately +1.5 V. Caffeine, however, has a voltammetric wave that peaks very sharply a t +1.7 V. It cannot he determined until this potential is reached, and so is not evident in Figure 4. When the potential was scanned up to +1.7 V in order to oxidize caffeine, severe disturbances in the base line were produced due to the massive oxidation of water that occurs a t this potential. Caffeine was found to elute a t approximately 22 min under these chromatographic conditions, however. Chromatovoltammograms of this tea sample are given in Figure 5. The first 15 min of the chromatogram are contained in Figure 5a. Theophylline is the first peak seen. Figure 5h
oo.,; Figure 5. Chromatovoltammograms of portions of Figure 4 (A) includes the Rrst 15 min of the run: (6) covers the second 15 mln of tha chromatogram. Theophylline is labeled "T".
covers the second 15 min. Many peaks present a t lower potentials are observed in the chromatovoltammogram that cannot he seen in Figure 4, which is plotted at a single potential (although obtained voltammetrically). Much more information and greater resolving power are available due to the use of voltammetric detection rather than amperometric detection. Voltammetric detection with gradient elution has thus been shown to he a valuable technique for use with liquid chromatography. I t provides important electrochemical information that cannot he obtained with amperometric detection, and excellent chromatographic and voltammetric resolution are both possible.
ACKNOWLEDGMENT The authors wish to thank Waters Associates (Milford, MA) for the gift of the 600E Multisolvent Delivery System. LITERATURE CITED (1) Lwes, E. M.: Bristoi. D. W.: Moseman, R. F. J . h m t o g r . Sol. 1978, 16, 358-362. (2) Rucki. R. J. Takn1.a 1980. 27. 147-156. (3) Stuiik. K.; Pacakova. V. CRC Crii. Rev. Anal. chem. 1984, i 4 , 907-.2&,
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(4) Khaledi. M. G.: Dorsey, J. G. Anal. Chem. 1985. 57, 2190-2196. (5) Maiiiaros. D. P.: DeBenedeno. M. J.: Guy, P. M.: T o w s . T. P.: Jahngen, E. G. E. A p_. t . R;firhm 0111-, n l69 ... 1._ . , 101-11< . . . .. 16) . . Hadi-Mohammadi, M. R.; Ward. J. L.: m y . J. G. J . Llq. Muomatog;. 198: 3. 6 . 511-526. (7) Gunaringtlam, H.: Tay. B. T.: Ang. K. P.; Koh, L. L. J . chmmamg. 1984. 285. 103-114. (8) Tjaden. U. R.: deJang. J. J . Llq. Chmmetqc 1983. 6. 2255-2274. (9) Drumheiier. A. L.: Bachelard. H.: St-Piene. S.: Joiicoeew. F. B. J . Liq. chromatogr. 1pa< n", ,n?a-,pld? (10) Meinsma. D. A .: Radzik. D. M.: Kissinger, P. T. J . Llq. chromatog. *a*. 6 , 2311-2335. (11) SI.(,laire, R. L., 111: Ansari. 0. A. S.: Abeii, C. W. Anal. Chem. 1982. 54 186-1 89. (12) Allison. L. A,: Mayer. G. S.: Shoup. R. E. Anal. Chem. 1984. 56, 1085- .*** ,"SO
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Anal. Chem. 1989, 61,
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(19) (20) (21) (22) (23)
Pallister. D. C u r . S e p . 1987, 8 , 53-57. Oates, M. D.; Jorgenson, J. W. Anal. Chem. 1989, 61,432-435. Kennedy, R. T.; Jorgenson, J. W. Anal. Chem. 1988, 6 0 , 1521-1524. Kennedy, R. T.; Jorgenson, J. W. Anal. Chem. 1989, 61,436-441. Knecht, L. A.; Guthrie, E. J.; Jorgenson. J. W. Anal. Chem. 1984, 56, 479-482. St. Claire, R. L., 111 Ph.D. Thesis, University of North Carolina, Chapel Hill, North Carolina, 1986. White, J. G.: St. Claire. R. L.. 111: Jorwnson. J. W. Anal. Chem. 1986, 58, 293-298. Whle, J. G.; Jorgenson, J. W. Anal. Chem. 1986, 58,2992-2995. Roach, M. C.; Harmony, M. D. Anal. Chem. 1887, 59,411-415. de Montigny, F.; Stobugh, J. F.; Givens, R. S.; Carlson, R. G.; Srinivasachar, K.; Sternson, L. A.; Hlguchi, T. Anal. Chem. 1987, 59, 1096-1101. Matuszewski, B. K.: Givens, R. S.;Srinivasachar, K ; Carison, R. G :
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1980-1983 Higuchi, T. Anal. Chem. 1987, 59, 1102-1105.
Mary D. Oates J a m e s W. Jorgenson* Department of Chemistry University of North Carolina Chape1 North 27599-3290 RECEIVED for review December 12, 1988. Accepted May 19, 1989. Support for this work was provided by the donors of the Petroleum Research Fund, administered by the American Chemical Society.
Crystal-Face-Specif ic Response of a Single-Crystal Cadmium Sulfide Based Ion-Selective Electrode Sir: The approach to characterization of ion-selective electrodes (ISEs) has been to pull together thermodynamic and kinetic principles and apply them to response interpretation. The extensive studies by Buck, Eisenman, Simon, Morf, Pungor, Cammann and others have solved major thermodynamic and kinetic problems in the theory and mechanism of ISE responses (1-6). However, not much has been achieved in the way of microscopic characterization of ISEs, particularly of solid membrane ISEs. Although ionexchange reaction is considered to be one of the processes responsible for permselectivity and charge separation for solid ISE/electrolyte interfaces, conventional solid membrane ISEs such as pressed-pellet types do not necessarily satisfy rigorous analysis of experimental results in terms of the relation between surface structure on the atomic scale and potential response. It appeared to us that the study of the ion-selectiveresponse of a well-defined surface of a single-crystal ISE would provide direct atomic/molecular information for understanding the basic principle of potential generation. Since the introduction of LaF3 single-crystal ISEs (9, several attempts have been made to use single crystals for ISEs. However, the purpose for utilizing a single-crystal membrane has been limited to improving bulk electrical conductance and reducing the dissolution of solid matrix. Thus, no control of the crystallographic polarity has been attempted aiming a t the atomic/ molecular basis for the potential response (8-12). Here we report for the first time the crystal-face-specific response of single-crystal ISEs. As a representative example of the system, we have chosen CdS, which has been known for a long time as an active component of Cd(I1) ISEs (see, for example, ref 13). The lack of inversion symmetry in the [OOOl] direction of wurtzite CdS single crystals gives rise to a crystallographic polarity of this compound (14). One face (0001) terminates with Cd and the other (0001) with S. Fortunately, crystal-face controlled single crystals of CdS are easily available because of its use as a photocell component. Also, the historical background that CdS has been well studied in terms of solid-state physics, semiconductor electrochemistry, and photoelectrochemistry further justifies the choice of CdS for the above objective. Striking differences between these two faces have been observed in some of the physical, electrical, and chemical properties (15-20). To correlate the surface compositions with the response characteristics, the surface of ISE membranes should be characterized. X-ray photoelectron spectroscopy (XPS),
Auger, and Fourier transform infrared (FT-IR) techniques are often employed (21, 22). Another potential technique for characterization of ISEs seems to be ion-scattering spectroscopy (ISS) (23-25), particularly impact collision ionscattering spectroscopy (ICISS) (26,27). ISS gives stoichiometric information about an exclusively top layer (monolayer) of solid surfaces and is most suitable for this study. Here we demonstrate for the first time the use of ICISS for the surface characterization of a CdS single-crystal ISE. Theoretical analysis of the potential response of the (0001)Cd and (0oOi)S surfaces of a single-crystal CdS ISE suggests the imperfection of surface composition, which is confirmed by ICISS measurements. The results and approach of this study will be of value for the atomic/molecular level characterization and design of solid membrane ISEs.
EXPERIMENTAL SECTION CdS single-crystal slices (thickness 1 mm) cut perpendicularly to the c axis were purchased from Teikoku Tsushin Kogyo Co., Ltd. (Kanagawa, Japan). The crystals were not intentionally doped, and their carrier density was 1017~ m - Two ~ . types of electrodes were prepared. One has a (0001)Cd face and the other has a (000T)Sface as electrode surfaces, respectively. Two faces were distinguished by chemical etching in 6 M HC1 in this study. The difference in etching characteristics of these two faces has been noted many times (16-20). The (0001)Cd face is etched much faster than the (G00T)Sface and gives a brighter face by the etching (17). The faces thus determined were confiied by many methods including X-ray reflection (16), ISS (14), and piezoelectric (15) techniques. An ohmic contact was obtained by indium metal at the back face. Except for the front face, all other parts were covered with epoxy resin and shielded in a glass tubing. Sample solutions were prepared by using reagent grade chemicals (Wako Pure Chemicals Co., Ltd., Tokyo) and purified water (Milli-Qwater purification system, Millipore Corp.). Solutions used were to 10+ M CdS04 plus 0.1 M NaN03 and lo-* to lo4 M NazSplus 0.3 M CHBCOONHl (pH = 9.0). Since the pKa1 and pKd for sulfur species are 7.0 and 12.9, respectively, the solution species at pH = 9.0 is almost exclusively (>99%) SH-. Potentiometric measurements were carried out at room temperature in complete darkness by using a millivolt meter (Model COM-BOR, Denki Kagaku Keiki Co., Ltd., Tokyo) with Ag/AgCl as a reference electrode. Prior to each run, the electrode surface was etched in 6 M HCl followed by exhaustive rinsing. The measurements were carried out from dilute t o concentrated solutions unless otherwise stated. Flat band potentials were determined by analyzing measured impedance data (28). Impedance measurements were carried out
0003-2700/89/0361-1980$01.50/00 1989 American Chemical Society
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