Anal. Chem. 1988, 60,1987-1989
examination on CSP 1. The results of the present study demonstrated that the hydrogen-bond associations related to these CSPs function well as diastereomeric molecular associations leading to the optical resolution of many different kinds of enantiomers in LC.
(7) Frank, H.; Nicholson, G. J.; Bayer, E. J . Chromatogr. Sci. 1977, 15, 174. (8) Koppenhoefer, B.; Bayer, E. J. Chromatogr. Libr. 1985, 3 2 , 1.
Akira Dobashi Yasuo Dobashi Kyo Kinoshita Shoji Hara*
LITERATURE CITED (1) (2) (3) (4) (5) (6)
Blashke, G. Angew. Chem., Int. Ed. Engl. 1980, 19, 13. Hara, S., Cams, J., Eds. J. Liq. Chromatogr. 1988, 9 , 241. Dobashi, Y.; Hara. S. J. Org. Chem. 1987, 52, 2490. Dobashi, A.; Oka, K.; Hara, S. J. Am. Chem. SOC.1980, 102, 7122. Dobashi, A.; Hara, S. J. Chromatogr. 1985, 349, 143. GII-Av, E. J . ChrOmatogr. Libr. 1985, 32, 111.
1987
Tokyo College of Pharmacy 1432-1 Horinouchi, Hachioji, Tokyo 192-03 Japan RECEIVED for review February 2,1988. Accepted May 24,1988.
Surface-Enhanced Raman Spectroscopy at a Silver Electrode as a Detector in Flow Injection Analysis Sir: Surface-enhanced Raman spectroscopy (SERS) was first reported in 1974 by Fleishman and co-workers (1). The intensity of the Raman signal at or near a rough metal surface is dramatically enhanced with respect to the solution, often by as much as 5 or 6 orders of magnitude (2). A number of studies have been carried out on electrode surfaces, such as silver electrodes, silver island films, as well as on silver sols (3-11). In general, SERS has been used to study the interactions between molecules and the metal surface, and only recently has the technique been used for analytical purposes (12-16). In essentially all these studies, the measurement has been carried out under static conditions, i.e. no solution flow. Berthod, Laserna, and Winefordner have recently applied SERS on silver sols to flow injection analysis (17). Freeman, Hammaker, Meloan, and Fateley have reported on SERS on silver sols and its application to flow injection analysis and high-pressure liquid chromatography (18). If the analyte of interest can be reproducibly adsorbed and desorbed from the silver surface, then SERS a t a silver electrode has the potential to be exploited as a detection technique for measurement in flowing aqueous streams. Since the Raman spectrum is analogous to infrared in that it contains information on molecular structure, it is a powerful analytical tool for identification of compounds in mixtures or as single components as they elute past the detector. When SERS is carried out at a silver electrode, an additional parameter that can be controlled is the potential applied to the electrode. When the analyte of interest adsorbs onto the surface of the electrode from an ionic medium, there is a competitive equilibrium between the analyte and the ions composing the supporting electrolyte. Maximum adsorption is generally observed a t the point of zero charge (19). If the electrode is held at this potential, maximum enhancement is observed. If the potential is then adjusted to a different value, the analyte can be selectively desorbed from the electrode surface. Therefore, if the potential applied to the silver working electrode is synchronized with the scanning of the b a n spectrum, and is then adjusted to a potential to desorb the analyte after the completion of the scan, the surface can be reused as eluents flow past the detector. In this work we report on the use of SERS a t a silver electrode for the detection of eluents in flow injection analysis. Pyridine was used as a model compound throughout this study because it has been extensively studied at electrode surfaces and its behavior and Raman spectrum as a function of applied potential have been well characterized (2, 3). EXPERIMENTAL SECTION Instrumentation. Raman spectra were obtained with a Spex
Model 1401 double monochromator. The slits on the monochromator were set for approximately 8-cm-' resolution. The detector was a thermoelectrically cooled RCA C31034 PMT operated in the photon counting mode and processed with a Spex Industries photon counting system. The excitation source was a Spectra Physics Model 164 argon ion laser powered with a Model 265 exciter. The laser was tuned to the 5145-A line and delivered 50 mW of optical power at the sample. The outmt from the photon counter was processed by a DEC micro PDP 11/73. The software to operate the Raman system was written in FORTRAN and was developed in-house by the technical support staff at the MIT regional laser facility. Spectra were stored on floppy disk and plotted on a Tektronix Model 4662 digital plotter. A single Raman spectrum from 990 to 1050 cm-' could be obtained, with a step size of 2 cm-' per data point, in 30 s. The data could be collected and stored on disk and the monochromator reset to begin another scan in 46 s. Time-dependent data were transferred to an IBM PC/XT compatible computer for analysis. The flow-through Raman cell was constructed from a 14/20 standard taper Pyrex thermometer fitting and 14/20 standard taper outer joint (Ace Glass). The outer joint was cut off perpendicular to the joint and served as the main body for the cell. Small grooves were filed in the end of the joint on opposite sides to accommodate 2.5 cm lengths of 20-gauge hypodermic syringe needles. A third groove was filed at 90' to accommodate a small loop of platinum wire that served as the auxiliary electrode. A Pyrex window cut from a microscope slide was then cemented with silicone cement onto the end of the joint with the needles and platinum wire in place. The Ag working electrode and reference electrode were sealed with epoxy cement into a short length of 7 mm 0.d. Pyrex tubing. The working electrode was a 3-mm Ag disk mounted in the center of the tube. The reference electrode was a Ag ring 5 mm in diameter around the center electrode, with a 1-mmspace between the two electrodes. This electrode a Kmbly was then polished smooth with progressively finer silicon carbide paper and finally with diamond paste on a polishing cloth (Buehler Corp.). The final polish was with 1-pm diamond paste. The electrode assembly was washed with distilled water and methanol between each polishing step. Before the beginning of a set of runs, it was only necessary to polish with either 1-pm diamond paste or 3-pm followed by 1-bm paste to obtain a fairly good mirrorlike finish to the electrodes. The 7-mm tubing was sealed in Teflon heat shrink tubing to fi the void between the thermometer holder, and reduce the cell volume. This arrangement was easy to reproducibly assemble and disassemble and has a cell volume of about 200 pL. A schematic diagram of the cell is shown in Figure 1. The potential on the Ag working electrode was controlled with a Princeton Applied Research Model 174 polarographic analyzer and a Model 175 potentiostatic controller. The Model 175 can be set to a maximum of four predefined potential settings. To prepare the cell for use before a set of runs the Ag ring was fist coated with a thin layer of AgCl by anodizing the ring at +0.4 V vs SCE in 0.1 M KC1. This step produced a Ag/AgCl reference electrode with a potential around +0.190 to +0.200 V vs NHE
0003-2700/8S/0360-1987$01.50/00 1988 American Chemical Society
1988
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988 40
Flgure 1. Schematic diagram of the Raman cell. The various parts of the cell are as follows in section A: (a) Pyrex window; (b) platinum auxiliary electrode; (c) 20-gauge hypodermic needles; (d) 14/20 standard taper outer joint; (e) Ag ring reference electrode; (f) Ag disk working electrode; (9)Teflon heat shrink tubing; (h) O-ring; (i) 14/20 standard taper thermometer fitting; (j) plastic tightening nut; (k) 7 mm 0.d. Pyrex tubing; (I) separate leads connecting to the Ag ring and Ag disk electrodes. Not shown is a plastic spring clip that holds the thermometer fiing and outer joint together. Inset B: end view of the electrode face, (a) 3-mm Ag disk working electrode; (b) Ag ring reference electrode; (c) Teflon heat shrink tubing; (d) 7 mm 0.d. Pyrex tubing.
in the 0.1 M KC1 that was used as the mobile phase and as the supporting electrolyte. The potential of the reference electrode would vary by about A10 mV from one run to another but was stable at a fixed potential during the run. The laser beam was brought into the cell at a 45' angle, and the Raman scattered light was collected at 90' to the surface of the working electrode. Chemicals. Working solutions were prepared by serial dilution from neat pyridine (Fisher Scientific). The pyridine was reagent grade and was used as received. The 0.1 M KCl used as the supporting electrolyte and mobile phase was prepared from reagent grade KC1 (Fisher Scientific). The mobile phase was pumped through the cell from a pneumatic reservoir pressurized to 10 psi from a nitrogen tank. The flow was controlled with a glass and Teflon needle valve and monitored with a Gilmont flowmeter tube. The pyridine samples could be injected with a syringe through a neoprene septum mounted above the flow tube. A 1-mL injection volume was employed in this study. All the tubing used to connect the system was 0.3 mm i.d. polyethylene tubing.
RESULTS AND DISCUSSION One criterion that must be met for SERS to be of practical use as a detector for flow analysis is that the analyte be removable from the electrode surface under controllable conditions. Figure 2 illustrates the time dependence for a 1-mL injection of 0.0500 M pyridine into 0.1 M KC1 flowing at 2 mL/min with the potential held at -0.6 V vs Ag/AgCl. The initial Ag surface was conditioned with a single oxidation reduction cycle (ORC) by stepping the potential to +0.4 V for 20 s and then back to -0.6 V just before the injection and scan were initiated. If the potential was stepped to -1.2 V immediately after a scan was completed and held there for approximately 60 s and then stepped back to -0.6 V and the Raman spectrum scanned again, no evidence of a pyridine signal could be observed. This would indicate that the slow desorption of pyridine from the electrode surface can be increased by appropriately modulating the electrode potential. It has been shown that there is a competitive equilibrium between the pyridine and the supporting electrolyte at the electrode surface (19). Since the supporting electrolyte is continuously flowing past the electrode surface, and the pyridine is not being replenished, a gradual loss of pyridine from the surface at constant applied potential is not unexpected. The scanning of the Raman spectrum was initiated immediately after injection of a pyridine sample, so all conditions for repeat in-
0
i 200
'00
C
j
seconds )
Time-dependent trace of the Raman intensity of the peak of the 1009-cm-' band for a I-mL injection of 0.0500 M pyridine into 0.1 M KCI flowing at 2.0 mL/min. The potential was held at -0.6 V vs Ag/AgCI. Figure 2.
,
990
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990 1050 W a v e r d r r b e - S- 11 c - -
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Plot of the reproducibility of the first scan of four different 1-mL injections of 0.0500 M pyridine into 0.1 M KCI flowing at 2 mL/min. The potential was held at -0.6 V vs Ag/AgCI. The four injections are representative of the stability over approximately a 4-h Figure 3.
time period. See the text for details. jections are internally consistent. Therefore, quantification of peak intensities is possible from one injection to another. One measure of the utility of SERS as a detector is the reproducibility of the signal for repeated injections. Figure 3 illustrates the reproducibility of the Raman signal for the first scan of four different injections. The measured reproducibility was found to be 7% (relative standard deviation). This is in good agreement with the 5% reproducibility reported by Berthod and co-workers for SERS on p-aminobenzoic acid (17). Freeman and co-workers found a reproducibility of approximately 1%for the integrated peak area for the 1593-cm-' band of pararosaniline hydrochloride (18). At the beginning of each injection, the Ag surface was regenerated with a short ORC to +0.4 V for 1 s and then the potential was held a t -0.6 V during the scan. There was no evidence of loss of sensitivity over the course of a day of continuous injections. The four different injections illustrated in Figure 3 span approximately 4 h of analysis. The surface was polished and conditioned at the beginning of the day and was used without repolishing and reconditioning for the rest of the day. However, if the surface was not subjected to a short (1s) ORC at the beginning of an injection, the SERS signal would be significantly reduced in intensity. This is in
Anal. Chem. 1988. 60. 1989-1990
1989
agreement with studies reported by Pettinger and Wetzel(20) indicating that, in quiescent solutions, the SERS signal could be regenerated with only about one monolayer of Ag once the surface is formed. If the surface that had been eluted free of pyridine in flowing KC1 was subjected to a 1-s ORC and the Raman spectrum scanned, in the absence of an injection of pyridine, there was no evidence of a residual pyridine signal. To determine the sensitivity of the present configuration, progressively more dilute samples of pyridine were injected. The intensity of the 1009-cm-' band was found to be linear over 3 orders of magnitude with a detection limit of about 250 nmol. This work has clearly demonstrated that surface-enhanced Raman spectroscopy at a Ag electrode can be utilized as a detection system for flow analysis and is both qualitative and quantitative. With a reduction in the cell volume, it will be possible to extend the utility of SERS at Ag electrodes as a detector for chromatographic applications. The present study was limited by the fact that the Raman spectrometer employed was a scanning instrument, and analysis time can be greatly reduced if the PMT were replaced with a multichannel detector such as an intensified diode array detector. Likewise, a multichannel detector should greatly improve the sensitivity and limits of detection, since the signal could be time integrated. Work is presently under way on amino acids and other compounds of biological interest.
Surface Enhanced Raman Scattering; Chang, R. K., Furtak, T., Eds.; Plenum: New York, 1982. Hembree, D. M., Jr.; OswaM, J. C.; Smyrl, N. R. Appl. Spectrosc. 1987, 41, 267. Weaver, M. J.; Hupp, J. T.; Barz, F.; Gordon, J. G., 11; Philpott, M. R. J. Electroanal. Chem. 1984, 760, 321. Weitz, D. A,; Garoff, S.; Gersten, J. I:Nitzan. A. J. J. Chem. Phys. 1963, 73, 5324. Chou, Y. C.; Liang, N. T.; Tse, W. S. J. Raman Spectrosc. 1986, 77, 481. Itoh, K.; Tsukada, M.; Koyama, T.; Kobayashi, Y. J. Phys. Chem. 1986. 90. 5286. Davies, J: P.; Pachuta, S. J.: Cooks, R. G.: Weaver, M. J. Anal. Chem. 1986, 5 8 , 1290. Suh, J. S.; Moskovits, M. J. Am. Chem. SOC. 1966, 708, 4711. Otto, C.; van den Tweel, T. J. J.; de Mul, F. F. M.; Greve, J. J. Raman Spectrosc. 1986, 17, 289. Kim, S. K.; Kim, M. S.; Suh, S. W. J. Raman Spectrosc. 1987, 78, 171. Vo-Dinh, T.; Hiromoto, M. Y. K.; Begun, G. M.; Moody, R. L. Anal. Chem. 1984, 56. 1667. Enlow, P. D.; Buncick, M.; Warmack, R. J.; Vo-Dinh, T. Anal. Chem. 1986, 58, 1119. Vo-Dinh, T.; Uziel, M.; Morrison, A. L. Appl. Spectrosc. 1987, 47, 605. Alak, A. M.; Vo-Dinh, T. Anal. Chem. 1987, 5 9 , 2149. Moody, R. L.; Vo-Dinh, T.: Fletcher, W. H. Appl. Spectrosc. 1987, 47, 966. Berthod, A.; Laserna, J. J.; Winefordner, J. D. Appl. Spectrosc. 1987, 41, 1137. Freeman, R. D.; Hammaker. R. M.; Meloan, C. E.; Fateley, W. G. Appl. Spectrosc. 1988, 42, 456. Van Duyne, R. P. Chemical and Biochemical Applications o f Lasers; Moore, L. B., Ed.; Academic: New York. 1979; Vol. 4. p 146. Pettinger, 8.;Wetzel, H. Surface Enhanced Raman Scatterlng; Chang. R. K., Furtak, T., Eds.; Plenum: New York, 1962; p 304.
ACKNOWLEDGMENT The author thanks Mark S. Wrighton for his helpful comments and for providing the space and resources necessary for the conduct of this work. R.K.F. conducted this work while he was on sabbatical leave as a visiting scientist at M.I.T.
Permanent address: Chemistry Department, University of Rhode Island, Kingston, R I 02881.
LITERATURE CITED (1) Fleishman, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 2 6 , 163.
'
R. Ken F o r d ' George R. Harrison Spectroscopy Laboratory Massachusetts Institute of Technology Cambridge, Massachusetts 02139 RECEIVEDfor review February 9,1988. Accepted May 18,1988.
TECHNICAL NOTES Apparatus for the Demonstration of Superconductivity at Liquid Nitrogen Temperature by Means of the Meissner Effect William 0.McSharry*J a n d J a m e s E. Phillips
Engelhard Corporation, 1 West Central Avenue, East Newark, New Jersey 07029 The recent increased interest in superconductivity (1,2), occasioned by finding this phenomenon at much higher temperatures than previously believed possible, has greatly stimulated attempts to synthesize new superconductive materials. In these experiments, superconductivity has been demonstrated by such methods as four probe resistance measurements (3,d), magnetic susceptibility (4,and radiofrequency penetration (5). These methods require specialized equipment, which is necessary for accurate, quantitative measurements but is not readily available in the average laboratory. Since the limits of these superconductors have yet to be determined, either experimentally or theoretically, the field can at this point benefit from widespread exploratory investigation. This exploration is inhibited if a laboratory has the capability of producing potentially superconducting materials, but lacks Present address: Engelhard Corp., Menlo Park, CN 28, Edison, N J 08818.
the equipment to evaluate the results. One property of superconductive materials, the exclusion of a magnetic field, or Meissner effect, can be observed with only a magnet and a refrigeration source. Levitation of a cooled pellet of superconducting material above a magnet or levitation of a magnet above the superconducting material (6) is commonly used as a means of demonstrating the presence of a superconductor. The effect depends on the perfect diamagnetism of a superconductor, which has a calculated theroretical magnetic susceptibility of -0.08 (7). The susceptibilities of ordinary diamagnetic substances are typically about 5 orders of magnitude weaker. Therefore, if the applied magnetic field is strong enough relative to the amount of superconductivematerial present in the sample, levitation can be observed. However, if the fraction of superconductor in the pellet is not great enough to overcome the force of gravity, the repulsive force may not be apparent without sensitive equipment.
0003-2700/88/0360-1989$01.50/00 1988 American Chemical Society
