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different column types are connected by valves in an array structure. At the appropriate times a computer switches the valves to steer the desired component through the array to the detector. It is extremely unlikely that any other substance would have the same retention on all the different column types in the array. The combination of a set of precise timing signals with a corresponding array of columns is the key to this method. Similar revolutionary ideas can be found in the literature of most analytical methods. Not all of these will be practical, but many will and signal processing in develand related concepts718 will be instrumental oping and understanding them.

References 1 Fenimore, D. C. and Davis, C. M. (1981) Anal. Chem. 53,253A

Inelastic

electron

Reese, C. E. ( 1980) J. Chromatogr. Sci. 18, 249 Gates, S. C., Smisko, M. J., Ashendel, D. L., Young, N. D., Holland, J. F. and Sweeley, C. C. (1978) Anal. Chem. 50, 433 Griffiths, P. R. (Ed.) (1978) Transform Techniques in Chemistry Plenum Press, New York Phillips, J. B. (1980) Anal. Chem. 52, 468A Freeman, D. H. (1981) Anal. Chem. 53, 2 Eckschlager, K. and Stepanek, V. (1979) Znjrmation Theory as Applied to Chemical Analysis,John Wiley and Sons Ltd., New York Beauchamp, K. and Yuen, C. (1979) Digital Methods for Signal Analysis, George Allen and Unwin, London

John B. Phillips received his B.A. degree from the University of California, Irvine in 1970 and a Ph.D. in Analytical Chemistry from the University of Arizona in 1977. He is currently Assistant Professor in the Department of Chemistry and Biochemistry at Southern Illinois Universily, Carbondale, IL 62901, U.S.A. His research interests include chemical instrumentation, chromatographic processes, and applications of laboratory computers.

tunneling

spectroscopy

Inelastic electron tunneling spectroscopy (IETS) is a relatively new form of vibrational spectroscopy which is able to address problems previously unsolved by either IR or Raman. It is particularly useful for surface analysis. Hawey S. Gold and Lisa J. Hilliard University of Delaware, U.S.A.

The vibrational triumvirate Jaklevic and Lambe discovered in 1967, spectroscopy (IETS) Josephson junctions at the Ford conventional Josephson iunction insulator/metal) yields a linear applied bias voltage. However,

IR

RAMAN The Vibrational

0 165.9936/82/oooO-0/$02.7.5

inelastic tunneling whilst working on Motor Companyl. A (of the form metal/ ‘plot of current-vwhen an insulator

Triumvirate.

IETS

contaminated with organic molecules was used a change in the slope of the graph was observed at each voltage corresponding to the energy required to excite a vibrational mode of the molecule. Second derivative techniques made it possible to observe peaks in the plots at these locations. IETS is now the third form of vibrational spectroscopy available to the scientific community - the others are infrared (IR) and Raman. In contrast to IR and Raman, IETS is a non-optical technique. It involves the tunneling of electrons (in the quantum mechanical sense) from one metal to another through a barrier (an insulating oxide layer) containing sample molecules. Energy from the electrons is transferred inelastically to the vibrational modes of the sample molecule. There are several advantages and disadvantages associated with each of the vibrational spectroscopies; these are summarized in Table I. However, a few points regarding IETS should be emphasized. First, there are no quantum mechanical selection rules to restrict IET, with the result that all vibrational modes are active. An IET spectrum thus consists of IR-active modes, Raman-active modes, and optically forbidden modes, Second, the limit of detection for IETS is extremely low: as little as 5 X 10-s monolayer (or approximately 2 X 1010 molecules) of a substance can be detected on the barrier surface. This compares favorably with the most sensitive spectroscopic tech-. niques and is orders of magnitude more sensitive than either IR or Raman. 0 I962 ElJcvicr S&milk Publishing Company

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FERMI

-________

LEVEL

-.

OXIDE BARRIER METAL

I

EMPTY

l= METAL

LEVEL

FERMI

LEVEL

II

Fig 1. Schematic diagram of the Fermi levels of two metals and the insulating barrier between them in an IET junction. The Fermi levels are shown separated by some particular bias voltage. Elastic tunneling (indicated by the horizontal dashed line) occurs without loss of energy. Inelastic tunneling (indicated by the diagonal dashed line) occurs between the Fermi level of one metal and an unoccu/ried level in the other metal.

voltage is impressed across the metals, electrons can tunnel in two ways: elastic tunneling, indicated by the horizontal dashed line in the figure, and inelastic tunneling, indicated by the diagonal dashed line. Inelastic tunneling proceeds via the transfer of energy from the electrons to the vibrational modes of the sample molecule. Electrons can tunnel inelastically whenever the bias voltage is greater than or equal to the threshold energy requirement of a particular vibrational mode. As one increases the bias voltage, and thus the gap between the Fermi levels of the two metals, further empty metal energy levels become accessible to the tunneling electrons, and further vibrational modes are available to be excited. After undergoing inelastic energy loss, the electron continues across the junction; this flow of electrons (current) is monitored as a function of the bias voltage and is characteristic for each sample species.

Junction preparation

Instrumental techniques

The sample to be studied is prepared in the following manner: the first metal is deposited, to a thickness of 100-300 nm, on a thoroughly cleaned glass substrate (usually a microscope slide) contained in a vacuum. The metal most commonly used is aluminum. An oxide is then formed on this metal (2.5-3.0 nm in thickness) in one of two ways - by exposure to the atmosphere at an elevated temperature, or by glow discharge ofoxygen. (Owing to the sensitivity of IETS, contamination of the junction is often a problem during oxidation by the first procedure.) The sample is allowed to adsorb onto the oxide and is introduced into the vacuum chamber as a vapor, or as drops of solution at atmospheric pressure, with the excess solvent spun off. Finally, the top metal is deposited. This is most commonly lead, and, as before, the thickness of this metallic layer is in the range of 100-300 nm.

Experimentally, IETS is performed as a second derivative technique in order to maximize its sensitiv-

Theoretical basis of the experiment Fig. 1 is a simplified representation of the Fermi levels of two metals in an IET junction separated from one another by an arbitrary bias voltage. When a bias TABLE Infrared Advantages 1 commonly

2 relatively inexpensive 3 fast spectral scans are routine 4 good resolution

Disadvantages 1 non-transparent samples are troublesome or impossible 2 cell design is restrictive

-

( , jc-yy

+* OSClLLATOR

tJulJlm--

Fig. 2. Instrumentation involved in an IETS experiment. Included are a ramp generator for providing a bias voltage, an oscillator or wave generator for providing the modulation voltage, a lock-in amplifier for detection of the second harmonic of the modulation voltage (which is proportional to the second derivative of the tunneling current with respect to voltage) and a recorder for plotting spectra.

I. The vibrational

triumvirate IETS

Raman

(IR)

available

I

1 liquids, solids, and gases are easily studied 2 little sample preparation 3 surfaces are prime candidates 4 sample cell design is easy 5 permits study of ‘working systems’

1 extreme sensitivity 2 3 4 5 6

adaptable to a wide range of molecules temperature programmable after the fact doping is possible good resolution no selection rules

1 poor sensitivity

1 artificial environments

2 slow

2 presently not adaptable to flow-through systems 3 requires cryogenic facilities

3 expensive 4 sample decomposition

are required

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ity. An additional bonus is that an IET spectrum obtained in this manner is visually similar to a conventional IR or Raman spectrum. In order to obtain the second derivative, a modulation voltage, of about 1 mV rms (root-mean-square), driven at a frequency of about 50 kHz, is superimposed upon the bias voltage. The second harmonic (at 100 kHz) of this modulation signal is proportional to the second derivative of the tunneling current with respect to voltage and is detected using a lock-in amplifier. The spectral region from 0 to 500 mV, corresponding to O4000 cm-i, is generally scanned. A schematic diagram of the instrumentation involved is shown in Fig. 2. For a hypothetical one-peak spectrum, the currentv-voltage curve, depicted in Fig. 3a, shows a change in slope when a vibrational mode’s energy requirement has been met by the applied bias voltage. Thus, the first derivative (Fig. 3b) shows a step, while the second derivative (Fig. 3c) shows a peak located at the threshold voltage necessary to excite the particular vibrational mode. The limiting resolution of an IET spectrum is proportional to 5.44 kT (where k is the Boltzmann constant and T is the absolute (Kelvin) temperature). However, if a superconducting metal is used, the resolution can be enhanced. This is one reason why lead is often used as the ‘cover’ metal in an IET junction. Lead becomes superconducting at a relatively high temperature (approximately 7 K). To achieve good resolution, the experiment is routinely performed at liquid helium temperature (4 K). There are some shifts in vibrational frequency in an IET spectrum (compared to an IR or Raman spectrum) owing to image-dipole effects, but peak locations can now be accurately predicted from theoretical calculations’.

IETS as a probe of surface orientation A typical molecular IET spectrum is shown in Fig. 4. The spectrum is of a simple system - benzoic acid on alumina. The rising background is due to the elastic tunneling of electrons described above. Recent developments in instrumentation enable this background to be subtracted from the whole by using differential IETSs. The comparability of IETS with IR and Raman can easily be ascertained by noting the prominent 1600 cm-i band caused by carbonyl vibra-

Benmic

i

I

0

0.1

0’

800 I

Acidon-

I

I

i

0.2

0.3

-0.4

1600 1

2400 1

3200 1

i 0.5

eU

4000

an-’

Fig. 4. IET spectrum of bencoic acid on alumina.

tion. Benzoic acid was one ofthe first molecules studied which showed that IETS can probe the orientation of molecules relative to the oxide surface. Hansma et al.4 have shown that this occurs because tunneling electrons preferentially couple to vibrational modes which have their oscillating dipole moment perpendicular to the oxide surface, i.e. parallel to the direction of tunneling. This is exemplified in the benzoic acid spectrum, which lacks the asymmetric stretching peak found at 1560 cm-i in the IR. Thus, benzoic acid, which actually adsorbs on the oxide as the benzoate ion, is oriented such that the COO- group is parallel to the alumina surface.

Current problems addressed by IETS

(a)

(b)

(cl

Fig. 3. A hypothetical ‘one-peak’IETspectrum illustrating the effect of using second derivative techniaues. (a) shows a chance in slobe at the threshold tunruling voltage, (b) shbws a &p, and (c), the c&ventioial IETplot, shows a peak.

A broad range of applications have already been found for IETS; a few of which are described below. The reader is also directed to several other review articles describing the past and present capabilities of IETS”7. IETS has been used in the study of the corrosion of aluminum by carbon tetrachloride. The oxide layer used in an IET junction is of the same thickness as that which forms on aluminum when it is exposed to the atmosphere. Carbon tetrachloride is deposited as drops on the oxide in the junction and penetrates through pinholes in the oxide layer to corrode the

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aluminum. IETS was used to verify reaction intermediates and products proposed in a free radical mechanism. A large number of biological molecules have been examined using IETS; amino acids, fatty acids, nucleotide units of DNA and RNA, and hemoglobin. In the case of the nucleotides, intensity information was found to facilitate sequencing. Important research is being carried out in the area of U.V. radiation damage to nucleotides. It was found that large molecular weight compounds gave spectra in which individual modes were difficult to resolve, but research is now being carried out to see ifintensity enhancement through the selective substitution of sample molecules will help overcome this problem. Perhaps the most exciting area of IETS application is in the study ofheterogeneous catalysis. Although the research here is still in its early stages, it is evident that IETS is a powerful tool for studying the interaction between a supported metal catalyst and intermediates and products formed in Fischer-Tropsch reactions - to cite just one example. Using IR and Raman it was impossible to observe metal-carbon vibrational modes due to strong absorption by the support (in the regions from 2OOG300 cm-l). However, Hansma has studied a model system using IETS - the absorption of carbon monoxide to rhodium supported on alumina - and has discovered that as the coverage of rhodium is increased, new modes appear in the IET spectrum. These correspond to the formation of a new type of surface composed of larger clusters of rhodium with bridging sites for carbon monoxide. At lower rhodium coverages, only linear bonding sites are available.

intensity in both doped and undoped junctions. It was suggested that normalization of peaks could be applied using this internal standard’*. IETS is a valuable technique which can provide molecular information unique to vibrational spectroscopies, although it is much more sensitive than either IR or Raman. Other surface techniques do not provide similar structural or orientational information. The development of IETS as a quantitative tool, using proper cali bra tion methods, promises to greatly expand the usefulness of the technique. However, the present semiquantitative capabilities are such that IETS is already emerging as a powerful spectroscopic method for the analysis of systems ranging from ‘small’ molecules, relevant to catalysis, to very large and complex species of biophysical importance. While no one technique can solve all the problems, IETS provides the surface scientist and the vibrational spectroscopist with heretofore unattainable sensitivity and applicability. IETS is a powerful weapon in the analytical chemist’s arsenal.

GLOSSARY Quantum Mechanical Tunneling: A

C

El

Prospects for the future Although IETS is an excellent qualitative probe of the structure and orientation of molecules on surfaces, using peak intensities, is a quantitative analysis, problem which has not been sufficiently resolved. Peak intensities are not only dependent on sample concentration, but also upon junction treatment and storage methods. Water from the atmosphere, for example, can penetrate the junction after it has been made. Thus, one can compare relative intensities of different functional group modes for the same molecules, but a comparison of absolute intensities is difficult. Langan and Hansma have performed experiments which measure surface concentration v spectral peak intensity for solution-doped junctions, using radioactively labeled benzoic acid and other species. Optimal peak intensities were obtained using solution concentrations which were just great enough to produce saturation coverage of the oxide layer. These results indicate that peak intensity is not linearly dependent on sample concentration, but decreases more rapidly than surface concentration once the concentration is lowered below saturation’ l. One attempt by Skarlatos to quantitate IETS for trace analysis has used the observation that the hydroxyl stretching mode at 450 mV is of equal

An electron wave ofkinetic energy E is incident upon a square potential barrier of height H > E and width w.

Assume that a particle,

e.g., an electron, comes from the left with a certain kinetic energy E < H and strikes the barrier. In classical mechanics, the result is simple. The electron would experience an elastic collision with the barrier and be reflected back. Where E < H, the probability that the electron could escape either through or over the barrier would be zero. However, the quantum mechanical result indicates that when the barrier is neither infinitely high nor infinitely wide, there is always a finite probability that the electron will penetrate the barrier and continue on its path in the X direction, beyond X = W. This phenomenon

is called the tunnel

effect.

Fermi level: The highest occupied electronic energy level in a partly filled band at a temperature of absolute zero. All energy levels below this will be occupied.

l

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References 1 Lambe, 2 Kirtley, 3 Colley, 4 Kirtley, -

J. and Jaklevic, R. C. (1968) Phys. Rev. 165,821 J. R. and Hansma, P. K. (1976) Phys. Rev. 13, 2910 S. and Hansma, P. (1977) Rev. Sci. Instrum. 48, 1192 J., Scalapino, D. J. and Hansma, P. K. (1976) Phys.

Rev. B 14, 3177 5 Hansma, P. K. (1977) Phys. Rep., Phys. Lett. (C) 30, 145 6 Weinberg, W. H. (1978) Ann. Rev. Phys. Chem. 29, 115 7 Wolfram, T., ed. (1978) Inelastic Electron Tunneling Spectroscopy, Proceedings of the International Conference and Qnrposium on Electron Tunneling, University of Missouri-Columbia, USA, May 2527, 1977,

Springer-Verlag, New York R. M., White, H. W., Godwin, L. M. and Wolfram, T. (1980) J. Chem. Phys. 72, 5291 9 Coleman, R. V., Clark, J. M. and Korman, C. S., in Wolfram, T. ed. ( 1978) Inelastic Electron Tunneling Spectroscopy, Proceedings of 8 Ellialtioglu,

the International Conference and Symposium on Electron Tunneling, Universily of Missouri-Columbia, U.S.A. May 2>27, 1977, p. 34,

Springer-Verlag, New York 10 Hansma, P. Kaska, W. and Laine, R. (1976) J. Am. Chem. Sot. 98, 6064

11 Langan, J. D. and Hansma, P. K. (1975) Surf Sci., 52, 211’ 12 Skarlatos, Y., Barker, R. C., Haller, G. L. and Yelon, A. (1975) J. Phys. Chem. 79, 2587

Harvey Gold is an Assistant Professor of Chemistry in the center for Catalytic Science and Technology, Department of Chemistry at the University of Delaware, Newark, DE 19711, U.S.A. He received a B.A. degree from Cornell University (as a double major in histol-yand in chemistry) in 1974, and a Ph.D. in 1978, from the University ofNorth Carolina at Chapel Hill. He is an analytical spectroscopist with research interests in vibrational and molecular fluorescence spectroscopy and computer treatment of spectroscopic data. Lisa Hilliard is presently completing her studies as a graduate student in analytical chemistry at the University of Delaware. She received a B.A. degree from Douglass College, Rutgers University (New Brunswick, N.J.) in 1978. The Explorer’s Club recently recognized her for her research on IETS by the award of a Student Scientific Exploration Grant.

Post cotumn derivatization methodology in high performance liquid chromatography (HPLC) Post column derivatization procedures represent a powerful analytical tool. The methodology is suitable for trace and ultra trace analysis and offers enhancement of both detectability and specificity compared to conventional HPLC methods. James T. Stewart University of Georgia, U.S.A.

Historical development Performing post-column chemical modifications as a means of improving our ability to detect a compound has been widely employed in classical column chromatography and in commercial amino acid analyzers. As HPLC gained popularity and analysts sought more sensitive detection systems suitable for trace and ultra trace analyses of compounds (especially in complex matrices) it seemed feasible that derivatization techniques could improve both detectability and specificity. So far, there have been two approaches to postcolumn derivatization methods. One is to perform the derivatization step on-Iine, i.e. in some type of reactor apparatus included in the HPLC system. In this manner, reagents needed for the derivatization reaction are pumped into the mobile phase post-column. The necessary reaction conditions such as mixing, heating, and extraction or combinations of them can be II165-99.~/82/oooo-00~/s~~.75

accomplished using commercially available or fabricated equipment before the stream is finally directed into an appropriate detector for measurement of the derivatized species. It is also possible to subject the mobile phase post-column to other conditions such as light irradiation or electrochemical methodology where the detectable species is created without the need for additional reagents. The second derivatization approach is termed offline, i.e. where the compound of interest is collected after separation on an HPLC column and subjected to chemical modification followed by detection. In this article, only the methodology connected with postcolumn on-line technology will be discussed. Advantages of the post-column on-line derivatization approach to HPLC analysis are that artefact formation is not critical and it is not necessary for the reaction to be complete or well defined provided it is reproducible. The major disadvantage concerns the interdependence between the mobile phase and the reaction medium. Other disadvantages include timetemperature restrictions on reaction conditions, the need for hardware modifications to suit the reaction, 0 1982 Elrwirr Srirntilir Publishing Company