Anal. Chem. 1988. 60. 1989-1990
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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, 26, 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 and James 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 superconductive material 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
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/
L
Level Of Liquid
\
Nitrogen
Figure 1. Schematic diagram of apparatus for demonstrating superconductivity.
If the material is a powder, a superconductor may be detected by sprinkling the powder into a container of liquid nitrogen with a magnet in or beneath it (8). The superconducting material avoids the region of high magnetic flux, leaving a bare spot. According to the authors, 2% of superconducting powder admixed with inert copper oxide can be detected in this way. This method requires the use of only simple, inexpensive equipment that, except for the liquid nitrogen, is likely to be available in most laboratories. It will be useful for the detection of superconductivity in known compounds.
EXPERIMENTAL SECTION Apparatus. The apparatus (Figure 1) consisted of a clean, dry wide-mouth Dewar flask filled to within 3 cm of the top with liquid nitrogen. (Caution: contact with bare skin can cause injury due to frostbite). An aluminum cup was fabricated by forming a disk of aluminum foil (Alcoa heavy duty) against the bottom of a round-bottom flask. The diameter of the disk and roundbottom flask were the same, and both were somewhat smaller than the inner diameter of the Dewar flask. A Teflon-coated Alnico V magnetic stirring bar 38 mm long by 7.9 mm in diameter (Dynalab Corp., Rochester, NY) was inserted into a piece of plastic tubing to serve as a magnetic probe. Procedure. The piece or powder to be tested for superconductivity was placed in the cup and floated on the surface of the liquid nitrogen. To ensure that the sample was thoroughly cooled, liquid nitrogen was added a few drops at a time to the cup, until a small amount remained. When the magnet was brought to within 1 cm of the sample, the presence of superconductivity was demonstrated by repulsion between the sample and the magnet. RESULTS AND DISCUSSION This procedure was used to detect the presence of superconductivity in samples of Y-BaXu oxide, prepared according
to a published method (9). The prepared material was found by X-ray diffraction to be primarily composed of the superconducting phase (IO). Bulk and powdered samples ranging down to 0.1 g gave a positive response. When the superconducting powder was mixed with powder of the same chemical composition that was not superconducting, the repulsive effect was observed down to 5% (w/w). Larger A1 cups were used to provide the necessary buoyancy with large samples. Repulsion was not observed with an empty AI cup or with a nonconducting material. When the experiment is properly conducted, even relatively weak repulsive effects can be observed because of the ease of movement of the floating Al cup. Some precautions are necessary to avoid extraneous effects that may either mask the repulsive motions or be mistaken for them. Obviously, air currents can cause motion. Furthermore, bubbles rising within the liquid nitrogen cause currents at the surface. The liquid bubbled vigorously when first placed in the Dewar flask, but the bubbling ceased, resulting in a quiet surface. After some use, when the liquid became contaminated with ice crystals, continuous streams of fine bubbles made it necessary to empty, clean, and refill the flask. The design of the AI cup was important in avoiding another interfering effect, sticking of the cup to the wall of the Dewar flask due to a film of liquid drawn up by capillary attraction. The flaring sides of the cup prevented a close approach and minimized this problem. In addition to experimental factors that can mask the presence of superconductivity, if the physical dimensions of the superconducting particles are too small relative to the London penetration depth, the Meissner effect will be suppressed (11). The suppression is more pronounced in a ceramic superconductor because of the low density of conduction electrons. If this apparatus is used as a quick check for superconductivity in unknowns, it must be remembered that false negative responses are possible. Therefore, a negative result does not necessarily prove the complete absence of superconductivity.
LITERATURE CITED (1) Daigani, R. Chem. Eng. News 1987, 65(19), 7-16. (2) Fisher, G.; Schober, M. Am. Cefam. SOC. Bull. 1987, 66, 1087-1092. (3) Hor, P. H.; et al. Phys. Rev. Lett. 1987, 58, 1891-1894. (4) Murphy, D. W.; et al. Phys. Rev. Lett. 1987, 58, 1888-1890. (5) Wang, H. H.;et ai. Inorg. Chem. 1987. 26. 1190-1192. (6) Juergens, F. H.: et al. J . Chem. Educ. 1987, 64(10), 851-853. (7) Kittel, Charles. Introductlon To Sol& State Physics, 3rd ed.; Wiley: New York, 1966; p 341. (8) Baker, R.; Thompson, J. C. J . Chem. Educ. lg87, 64(10), 853. (9) Cava, R. J.; et at. Phys. Rev. Lett. 1987. 58, 1676-1679. (10) Wong-Ng, W.; et al. A&. Ceram. Mater. 1987, 2, No. 38, Special Issue, 565-576. (11) Kittei, Charles. Introduction To SolM State Physics, 3rd eo'.;Wiley: New York, 1966: p 354.
RECEIVED for review October 6,1987. Accepted April 26,1988.
Electron-Ionization-Like Mass Spectra by Capillary Supercritical Fluid Chromatography/Charge Exchange Mass Spectrometry Edgar D. Lee, Shih-Hsien Hsu, and Jack D. Henion* Equine Drug Testing and Toxicology, New York State College of Veterinary Medicine, Cornell University, 925 Warren Drive, Ithaca, New York 14850 Coupling supercritical fluid chromatography with mass spectrometry (SFC/MS) has been suggested as a method for
solving various analytical problems that are not amenable to gas chromatography/mass spectrometry (GC/MS) or liquid
0003-2700/88/0360-1990$01.50/00 1988 American Chemical Society
