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Richard L. Mccreery Depaltment of Chemistly The Ohio Stale University Columbus. OH 43210
Richard T. Packard E. I. du Ponl de NemowS and Co. Deepwater. NI 08023 Moet electrochemical phenomena and techniques are based on electron transfer between a conductor and a redox system, usually dissolved in a solution. Electrosrnthetic Drocesses such as metals production,electroplaring,electronnalysis, and electrochemical energy conversion account annually for more than $30 billion worth of produ c e (-1% of the U S . GNP) and contrihute to a range of industries worth much more ( I ) . All of these processes rely on the direct interconversion of electrical current and chemical energy in the form of a redox system. For problems in analytical chemistry,the heterogeneous electron transfer phenomenon results in certain important consequences (2).First, electroanalytical measurements can be extremely sensitive, with measurable currents corresponding to minute amounts of analyte. For example, easily measured currents of 10-l2 amp equal IO-" molls of a one-electron redox system, and state-of-the-art techniques are approaching detection limits corresponding t o single redox events. Even small currents can be measured with high precision and accurncy, resulting in electroanalytical techniques with similarly high quantitative accuracy. A second consequence of particular relevance to this article is the continuously tunable driving force available for electron transfer, in the form of the 0009-27001891036 1-775Al$O 1.5010
0 1989 American Chemical Society
electrode potentid. Variations in potential correspond to changes in electron energy, and the free-energy change associated with a redox reaction can therefore be controlled. The electrode behaves like a tunable redox reagent that can be made, at will, as oxidizing as fluorine or as reducing as lithium. The ability to impose extreme redox potentials is the reason that aluminum, alkali metals, fluorine, cblorine, and so forth me produced electrochemically. Furthermore, changes in electrochemical driving force can he executed rapidly &e., eiectrode potential changes can be made on a submicrosecond time scale). An analogous
approach is stopped-flow kinetics, except the time scale is longer (- milliseconds) and one is constrained to redox potentials available with known chemical reagents. E~~ initlath ol a readion In soldution
Consider an electrode immersed in a solution containing the stable, reduced form of a redox couple, denoted Red. If is negative the applied potential, Eapp, relative to the standard potential, E", for the iedox system, electrons are more stable in Red than in the electrode, and no current flows. If E, is changed to a value that is positive relative to Eo (rapidly if desired), electrons will be more stable in the now positive electrode, and heterogeneous electron transfer will occur fromRedto theelectrode, thereby generating the oxidized form, Ox. If Ox is unstable in solution,
it may undergo homogeneous renctions to form new products, some of which may themselves be electroactive. Various electrochemical methods have long been used to monitor such electroinitiated reactions. This is usually done by monitoring the perturbation of the observed current caused by the homogeneous reaction (3,4).An example of the ECE mechanism, in which a homogeneous chemical reaction occurs between two heterogeneous electron transfer reactions, is shown in Figure 1and involves the oxidation of the neurotransmitter dopamine in HBr solution (5-7). The additional current resulting from the reaction in Equation 3
is directly related to the rate constant,
k,. Not only has the electrode reaction (Equation 1) provided a convenient means to produce the reactive orthoquinone, it has done so on a fast time scale, submilliseconds if necessary. Because the whole process occurs within the diffusion layer, extremely small amounts of dopamine are consumed (-30 ng for typical conditions and a 1-s experiment). The excellent time resolution and tunable driving force inherent in electrochemical generation of reactive species have resulted in widespread application. However, electrochemical monitoring of the homogeneous renction, usually via the current, has two drawbacks. First, current measurements are rather nonselective compared with analogous spectroscopic observations, leading to difficulties with complex reaction sequences. For some mecba-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1. 1989
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INS7RUMENTA7lON amounts of material in the electrochemical diffusion layer. The majority of techniques commonly used for structure determination, such as NMR spectroscopy, MS, EXAFS, and so forth, lack either the sensitivity or the t i e resolution required for SEC applications. Recall that the amount of material generated in a typical diffusion layer is small, on the order of 10-10 moVcm2 for a 1-ms experiment. In the case of FT-IR reflectance, the required sensitivity is available with sufficient signal averaging, but it is difficult to mate the instrumental requirements of an FT instrument with the transient nature of electrochemical reactant generation.
Figure 1. Reaction mecnanism for the oxidation of dopamine, HGA, in 1 M HBr
solution. k, is me pseudo f l r s t d r rate constam lor HBr addlion; & is me equilibriumconstant tw me redox cross reaction in Equation 4. HDOQ = dODBmine orthoquinone. HPABr = 8-bramdopamine, mQBr = e-tmmodopamine wmoquinone
nisms, the homogeneous reaction does not perturb the current at all, making measurements of current uninformative. A second problem is the paucity of structural information available from the current response. Although E’ is sensitive to structure in many cases, there is no systematic procedure to deduce structure from electrochemical currents or ohserved E”. Chemists who study homogeneous solutions are used to deducing structures from NMR spectroscopy, mass spectrometry, vibrational spectroscopy, and the like, but electrochemista cannot obtain this type of structural information if they are constrained solely to electrochemical measurements. The desire for greater selectivity and molecular information led to the development of hybrids of spectroscopic and electrochemical techniques under the general label of spectroelectrochemistry (SEC) (2, 8-12). A common SEC experiment might involve reflection of a spectrophotometer beam off a planar electrode, thus permitting electrochemical reactant generation and spectroscopic monitoring. UV-vis absorption techniques have been developed extensively and are characterized by excellent quantitative accuracy and time resolution in both transmission and reflection modes. Although UVvis techniques have been applied to many problems with valuable results, 776A
they are still constrained by the nature of the spectroscopic transition. Vibrational spectroscopy, magnetic resonance, or other techniques commonly used to probe homogeneous reactions provide structural information that UV-vis absorption does not. Furthermore, electronic absorption spectra of solution species have very broad linewidths, often making it difficult to monitor several species in a possibly complex reaction path. Thus the driving force for developing vibrational spectroscopy of electrode reactions is the need for high-resolution. narrow-linewidth spectra to improve selectivity and provide structural information. Given that electrochemistry permits easy generation of a reactive species by a charge transfer reaction, vibrational spectroscopy is an informative means to monitor the fate of an electrogenerated material. Specifically, the spectroscopic probe must provide information about molecular structure, have sufficient resolution to permit selective monitoring of more than one species, have time resolution comparable to or better than the electrode process that generates the reactive species (spectral information must be obtained on a rapid time scale, to exploit the advantages of electrochemical generation), and possess sufficient sensitivity to detect and monitor the very small
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY
l, 1989
Ram-Y Starting in the mid-1970s. Van Duyne reported the use of resonance Raman spectroscopyfor SECapplications (1315).Raman spectroscopy involves scattering of incident light from a laser beam (huo) reflected from the electrode. The vast majority of laser liiht is scattered without a change in frequency and carries little chemical information. Such Rayleigh scattering is responsible for the blue sky, because it is more efficient at shorter wavelengths. Raman scattered liiht has lost energy to the sample molecule through vibrational excitation and is shifted in energy from the input laser light by a vihrational quantum (hul). If the incident light wavelength is close to an electronic absorption hand of the sample molecule, the scattering is more intense (by factors of 10-106) and the process is called resonance Raman Scattering. Being a vibrational spectroscopy, Raman produces high-resolution spectra with significant structural information. Because there are usually many Raman lines, comparisons to known spectra may allow structure identification in “fingerprint” mode. Changes in normal modes or bond strengths may he inferred from changes in the Raman spectrum accompanying a redox reaction. Raman peaks are narrow compared with UV-vis absorptions, so it may he possible to selectively monitor one component in a reaction sequence. For the case of resonance Raman, one species may have a much higher scattering crosa section than others, thus enhancing its Raman spectrum. With suitable choice of input Laser wavelength, the enhanced species may be monitored with high sensitivity and selectivity. Several aspecta of resonance enhancement will be discussed later. To exploit the significant advantages in selectivity and information content of Raman SEC over UV-vis SEC, the instrumentation must address a funda-
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mental issue regarding sensitivity. Raman scattering is a weak effect, and cross sections are much lower than those common to UV-vis absorptiot Consider the experiment depicted i Figure 2, in which a laser beam reflecl off the electrode surface and the Rr man scattering is collected normal t the electrode plane. Given the sma amount of scatterer generated by transient electrochemical experiment, instrument sensitivity is crucial to successful acquisition of spectra. The usual case of a species generated at a diffusion-controlled rate by a potential step is depicted in Figure 3. Although the current exhibits a transient response with a t-Il2 decay, the Raman scattering results from the integrated concentration of electrogenerated scatterers in the laser beam. For a stable species, thii integral is proportional to tin, yielding a parabolic shape of Raman intensity versus time (I3,16,17). The predicted Raman signal is given by Equation 5
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Figure 2. Geometry of typical Raman SEC experiment for monitoring solution species generated at a planar electrode. Incident beam (bud enters me call and reflens off me electrode, and scattered llm Is wI!acted normal m
me electrode swfacs. me elecDochemlcaldiwuslm layer, which 18 tvplcally 1-100 pm mla. mntalns a s01u11m scatterer.
where S = Raman signal, counts s-I Po = incident laser power, photons s-l j3 = differential Raman scattering crow section, cm* sr-l molecule-’ (often denoted doldn); 81 indicates steradian, unit of solid angle Nb = bulk number density of starting material (Red in the discussion so far), moled e s em+ D = diffusion coefficient of Red, cm*s-1 t = time from beginning of diffusion-controlled oxidation, 8
Q = collection efficiency of spec-
trometer and collection optics, 81 T = spectrometer and optics transmission, unitless Q = detector quantum yield, counts photon-1 AD= electrode area sampled by the spectrometer, cm2 a = laser beam radius at electrode, cm Equation 5 mumes the laser spot on the electrode overfills the spectrometer and detector. The many variables in Equation 5 can be classified into four subgroups. First, the laser power Po is limited by the laser employed and the damage threshold of the electrode surface or the diffusion layer. Second, the cram section j3 varies greatly (at least a factor of 106) for different molecules and ultimately controls signal-to-noise 778A
Flgure 3. Electrochemical generatlon of a Raman scatterer by oxidation of a stable reduced material. (a) Potential. (b) anent, and (0)Raman Scattering waveforms.
ANALYTICAL CHEMISTRY, VOL. 61. NO, 13, J U Y 1, I989
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(S/N)ratio and sensitivity. Third, the product Nb(Dt)”2a, equals the number of electrogenerated molecules observed by the spectrometer. As the diffusion layer thickens with time, this quantity increases with tln, yielding the parabolic response of Raman signal versus time. Fourth, the product QTQl +‘Wis an indication of the spectrometer’s performance and permits comparison of different instruments. Several conclusions relevant to the problem of detecting electrogenerated species can be drawn from Equation 5. First, a high p permits the observation of low concentrations or short-lived species. As 0 decreases, the experimental time scale or the concentration will have to increase to maintain the same signal. Second, improvements in the instrument (QTQ)will permit experiments on weaker scatters, shorter times, or lower concentrations. Third, the ability to monitor a larger AD is important, particularly for SEC applications. Surface damage or thermal disturbance of the diffusion layer limits the acceptable power density, so a larger AD yields a larger signal before the damage threshold is reached. Finally, spectral monitoring of normal Raman scatterers will be much more difficult than observing resonance Raman scatterers because of the 10-106fold difference in 0. As with any spectroscopic method, the quality of the spectra and detection limits are determined by S/N ratio, not just signal. Noise sources include shot noise from both sample and background scattering, detector dark noise, and detector readout noise. In most casea, the background scatter from an electrochemical solution is large com-
pared with detector shot or readout noise. In this limiting but often valid case, the S/N ratio is proportional to the square root of the signal given in Equation 5. If a multichannel detector having N channels is employed (as described below), a factor of NLIZis added to the SM ratio expression. After reducing the SM ratio equation to include only terms involving the spectrometer and detector, Equation 6 is obtained S/N = K(A,TQN)’/’ (6) for the case of a photon shot noise limited experiment and a fixed total measurement time (18).K is a constant, derived from concentration, laser power, and other factors and is related to Raman scattering intensity. The iW2 enhancement in SM ratio is predicted for multichannel Raman detectors and will be beneficial under certain conditions, as discussed below. l n S t N ~ t b
Now that the variables of Raman SEC have been defined, the instrument necessary to obtain spectral information about electrogenerated species, as well as two data acquisition modes, can be described. First, it is usually desirable to obtain complete spectra after electrolysis begins (preferably time resolved) to produce a 3D data set of Raman intensity versus Raman shift and versus time. Such data will reveal vibrational spectral changes following electrochemical generation and will provide the most structural information. Once time-dependent spectral changes are observed, it is often desirable to conduct fixed-wavelength experiments in which a particular Raman
WT Is a slngl-hdon phOtomunlpiler tube. Working. reference. and auxiliary eleclrodes (W. R, and A, respectively)are immersed in the 801Ulimof imerest.
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ANALYTICAL CHEMISTRY, VOL. 61. NO. 13, JULY I . 1989
peak intensity is monitored as a function of time. The f i s t Raman SEC experiments employed an instrument similar to that in Figure 4 (14,16).In addition to the considerations of designing a Raman spectrometer, the cell design and optics are crucial to assure that the region sampled by the spectrometer coincides optically with the electrode surface and diffusion layer. The fvst reported Raman SEC experiments were conducted at steady state, with a square wave potential perturbation (14, 19).Eappwas oscillated between values above and below E O , thus establishing a concentration of electrogenerated products. In this condition the Raman spectrometer could be scanned slowly relative to the potential perturbation frequency, producing a spectrum of intensity versus Raman shift. As far as the spectrometer is concerned, the experiment is conventional, whereas in fact the advantages of electrochemical generation are being exploited to produce a possibly reactive sample. Because of the steady-state nature of sample production, the steadystate scatterer concentration will decrease for shorter lived species, thus degrading the SM ratio. For this reason and because of certain instrumental factors, the initial experiments examined resonance-enhanced scatterers with relatively large cross sections. For stable electrogenerated products, this relatively simple, static acquisition mode and related procedures proved particularly valuable for identifying molecular structures, and excellent examples are available in the literature (14, 20, 21). A sample spectrum obtained in our lab with this acquisition mode appears in Figure 5. To obtain time-resolved spectra following electrochemical generation, an important change in spectrometer design is required. Not only is scanning impractical on a short time scale (a few seconds or less), but transient signals are weak as a result of the t1I2factor of Equation 5, and high sensitivity is required. Consider the apparatus of Figure 4, after replacement of the photomultiplier tube (PMT)with a multichannel detector. The detedor consists of many (500-1000)photosensitive elements, each monitoring a different wavelength at the focal plane (22).The most important benefit of such a change in spectrometer design is the operation of the spectrometer as a spectrograph rather than a scanning monochromator. Instead of scanning wavelength (and Raman shift) across the exit slit, the dispersed spectrum is focused onto a multichannel detector at the focal plane. An intensified pho-
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Flgura 5. Resonance Raman spectrum of chlorpromazine cation radical (CPZ") generated elechchemically in steady-state mode from reduced CPZ. Law wavelnrm was 514.5 nrn CPZ 16 not rewnance-enhanced and acanas 100 weakly IOinterfere wim mS CPZ' specburn ' dBm1es wivenl W P (Adapted horn RetwenCd 16 )
todiode array (IPDA) positioned at the focal plane can monitor 1000 wavelengths simultaneously, in principle producing a complete spectrum in as little as 10 ns. Each channel of the IPDA is physically a 25 pm X 2.5 mm rectangle and acta aa a detector analogous to the PMT used for the scanning experiment. A range of wavelengths is monitored at all times, thus lengthening the time monitored for each wavelength and providing S/N ratio enhancement. The width of the spectrum typically monitored by our spectrometer is 600 an-'. Although the PMTs normally employed for Raman spectroscopy operate in a single-photon counting mode, an IPDA is inherently an integrating detector. Each element of the IPDA stores charge proportional to the integral of the number of photons striking the element. At intervals of 10 ms or more, the stored charge from all pixels is read out, and the pixel is reset. With repetitive integration and readout, it is possible to generate a series of spectra, each of which represents Raman scattering during a IO-ms segment of time after electrcgeneration begins. The sequence of events for the on-the-fly ac-
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ANALYTICAL CMMISTRY. VOL. 61,
NO. 13, JULY 1, 1989
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INSrRUMENT A l O N quisition of a series of Raman spectra is shown in Figure 6, and a series of spectra obtained in this manner is shown in Figure 7. The 3D data set of Raman scattering versus Raman shift and time contains several types of information of signifi-
cant value for diagnosing the fate of electrogenerated species. The most obvious is the sensitivity of the spectra to structural changes evolving with time. Because certain Raman peaks correspond to particular normal modes of the intermediates of interest, changes
Flgure 6. Relationship between E-, Faman signal, and timing of detector electronics for time-resolved spectral acquisition and singiswaveiength monitoring. r he mid wavefam hwn me top s b w s sampling by the intensified phdodiodem y (IWA) detector. wherein each readart prcdum a specbM over wrne wavelem lntervbl. The foum wavetam Is me multichannel W l e r (MCS) trigger, and lhe fifth bace shows Which MCS channels are active efler me trigger.
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13. JULY 1. 1989
in peak intensities with time reflect concentration versus time behavior and provide implications about reaction mechanism. An informative example is the oxidation of dopamine (H3DA) in the presence of HBr, shown in Figure 8. HJIA is a relatively weak Raman scatterer, so the initial spectrum (Figure Sa) shows no features in the 1300-1800 an-' region. After the initial spectrum was acquired, the electrode potential was stepped to a value that oxidized H3DA at a diffusion-controlled rate. Raman peaks at 1572 cm-I and 1672 cm-' appear rapidly and are assignable to the dopamine orthcquinone (HDOQ). If HDOQ were stable (e.& in HC104), these peaks would grow with the t1I2dependence expected from Equation 5. However, it is obvious that the 1572 cm-I band is weakening with time as a strong new band at 1540 cm-I appears. At least two products are formed, one an intermediate and the other a more stable product. Less obvious but similar changes also occur in the 160&1750 cm-I range. These spectral changes have been assigned to particular species in the reaction mechanism, and the mechanism of Figure 1has been confvmed (17). The time-resolved spectra from the multichannel spectrometer provide valuable qualitative information about reactive systems, on a millisecond to several second time scale. In principle, one could construct quantitative intensity versus time transients for particular peaks from the 3D data set, thus permitting quantitative kinetic analysis. In practice, better S / N ratio and time resolution are possible with a single-channel detector (PMT in this case), because the integrative nature of the IPDA and readout requirement are avoided. In single-wavelength mode, one is sacrificing the structural information available from complete spectra such as those of Figure 8, but one is still exploiting the high resolution of Raman spectroscopy to monitor individual species. As an indication of the resolution and narrow linewidth of Raman features compared with UV-vis absorption spectra, consider the 1540 cm-' and 1572 an-' peaks in Figure 8. These two features and the species to which they correspond are easily resolved on the Raman spectrum, yet the difference in Raman shift expressed in wavelength is only 1 nm. The UV-vis spectra of the two quinones are severely overlapped, and independent quantitative monitoring of them would be difficult. The timing for single-wavelength transient Raman spectroelectrochemistry is shown in the bottom two traces in Figure 6. With the exception of the
counting electronics and potential waveform, the spectrometer is the same 88 the scanning system of Figure 4. The spectrometer is set and held at the wavelength of interest (e.g., 1572 cm-') for the -A oxidation. Photons detectad by the PMT are counted, 88 they were for a scanning system, except the counts are stored in a multichannel
scaler (MCS). The MCS consists of many (-1OOO) channels, each of which is assigned to a different time interval foUowing a trigger. As a shown in Figw e 6, photon counts arriving after the trigger are stored in MCS channels, according to arrival time. The width of a channel varies from a microsecond up to several seconds. If the potentid step
ryure 8. Timeresolved spectra of dopamine oxidation in 1 M HBr soiution obtal.., wkh a multichannel spectrometer. n- dtw ~nmatknof o x w m(a) o m,(b) &50 ms, (e) 100-150 ms, (d) 200-250 ms, ~d (e) 400-450 m. Ints(raa0n pedal wa 50 m;-Ivd specus are dlaplacad u p w d for clarnv (Adapted horn Rerwhnm 17.)
that initiates the electrochemical reaction is used an the trigger, the MCS will store the Raman intensity versus time at a given wavelength into the series of successive channels. Thus an intensity versus time transient is obtained at fied wavelength, with time resolution equal to the MCS channel width. Several intensity versus time transients are shown in Figure 9 for the dopamine oxidation monitored at 1540 and 1572 cn-'. The lines were calculated for the reactions of Figure 1;details of the approach appear elsewhere (17,18). The sensitivity constraints of normal Raman SEC are more fundamental than merely inatrumental limitations. Although instruments, crosa sections, and Laser powers vary greatly, a crude estimate of signal strength may be calculated from Equation 5. For the case of electrogenerated benzoquinone, a Raman signal of -10 kHz is predicted at 1s after a potential step. At t = 1 ms, this value is 300 Hz, or 3 photons in a IO-ms MCS channel. Even for ideal detectors and spectrometers (T= l, D = large), the count rate might he increased by a factor of 10-100, but one would still be d e a l i i with a few counts in each interval at short times. S i i a l averaging, digital fiitering, and the like can be used to enhance S/N ratio, but eventually one is always limited by the weaknew of Raman scattering. The performance of the instrument determines how closely this fundamental limit is approached. As noted earlier, any increase in scattering cross section (,¶ in Equation 5) resulta in improved S/N ratio or faster time resolution. Partly for this reason,
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w by a 1024 muiuchanns~ scaler wlth a dwell t~ms0130 ps,Potential pulse duration was 1.5 ms belore rehnning to 0.4v and was refor the 1.5 ms durnpeated WSry 30 ms I O allow Signal averaging for a total Of 50.000 NnS. Cvve a shwrs raw data. and CUNB b 18 a plot Of intensity versus tlon of the potemla1 pulse. (Adapted from Reference 16.1
resonance Raman SEC (RRSE) was developed before normal Raman SEC. Not only is 3, much higher (by factors of l(noS), but the resonance enhancement permits improved selectivity, because only one or a few components of a mixture will exhibit strong resonance enhancement. RRSE was presented initially in the mid-19708 (13),and micrmcond time resolution was demonstrated in the mid-1980s (16, 22). As shown in Figure 10, a resonance-enhanced electrogenerated scatterer can be monitored quantitatively, and the expected t"2 dependence at short electrolysis times is down to tens of microseconds. Unfortunately, strong resonance enhancement occurs only near an absorption band of the scatterer of interest, and the input laser power is necessarily attenuated as the beam passes through the diffusion layer. Because Equation 5 any assumes constant laser power (PO), attenuation will lead to a negative deviation from a t"2 dependence (17).This effect is shown in Figure 11for the case of electrogenerated chlorpromazine cation radical (CPZ+.) monitored with a 515-nm laser. It is obvious that attenuation is severe, at times greater than a few milliseconds, and quantitative deductions about kinetica or mechanisms would be invalid if attenuation were ignored. A nontrivial correction involving some mathematical complexity can be made to extend the useful time scale of RRSE from 50 ps to 5 8, but this correction must be applied with care for any but the simplest systems (17, 18).Thus RRSE pays significant benefits in sensitivity, time resolution, and selectivity, but at the cost of mathematical complexity and loss of quantitative accuracy. 784A
Electrode surface sbucture The discussion presented thus far has dealt with solution species generated electrochemically, which may be stable or reactive. A more difficult but equally important problem is the spectroscopic observation of electrode surface structure and dynamics. Because heterogeneous events are the basis of all electrochemical phenomena, it is particularly important to probe the electrodelsolu-
tion interface to determine its molecular structure, hoth statically and dynamically. To determine the relationship between interfacial structure and electrochemical behavior, it is essential for researchers to have informative structural prohea. As has already been noted, Raman spectroscopy can provide such information on a millisecond time scale for electrogenerated species in solution. Unfortunately, the sensitivity problem
Figure 11. Transient Raman intensity of the 1126 cm potential pulses from 0.2 to 0.85 V versus AgmAgCi.
generated b
The solid llne Is the slmylated result Bssumi~no Input lighl absorption by CPZ". Points are the experimental vaiw. CPZ mncenbetion was 5 mM in 1.0 M HCI in 43% (w/w) MeOH/H*O. Input laser beam was 20 mW at 458 nm. Ten transients were averam; spectral resaiulion was 7 cm-'. (Adapted lrm Reference 17.)
ANALYTICAL CHEMISTRY, VOL. 61. NO. 13, JULY 1. 1989
becomes even more severe for thin layers (perhaps submonolayer) of species on the electrode surface. For the case of an adsorbed layer of analyte or the interfacial layers of the electrode material, Equation 5 can be modified to produce Equation 7 (16,23,24)
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where Ssurf = Raman signal from surface species, counts 9-1 Nsurf = surface number density, molecules cm-2 a = incident beam angle, relative to surface normal Nsurf need not be restricted to a monolayer case; for multilayer film or finite laser penetration depth into the elecwill equal the total trode material, NSurf density of molecules sampled by the laser beam and spectrometer. An important difference between the surface and solution experiments is apparent after comparing Equations 5 For more information contact: and 7 . SSudis not time-dependent, beHoriba Instruments, Inc. cause the number of scattering mole1021 Duryea Ave., Irvine, Calif 92714 cules on the surface does not usually or call 1.8004 HORIBA increase with diffusional time. All else in Calif. call: 7141250-4811 being equal, a monolayer of a normal Raman scatterer will result in an Ssurf CIRCLE 63 ON READER SERVICE CARD roughly comparable to solution scatterer generated by a 1-ms potential step. Thus the surface signal is very small because so few molecules are sampled ( ~ 1 O - mol) l ~ and the Raman effect is so weak. For a normal scatterer with no enhancement mechanisms, a monolayer will produce a few counts per second of signal, and the experiment is very demanding. The consequence of this weak signal has been investigation of a variety of enhancement mechanisms and a comparative dearth of surface dynamic experiments. The current state of the art in instrumentation has resulted in examples of The fact is, there are more than 137,000chemists in this unenhanced surface Raman spectra in vital, career-enhancing society - the Zargest scientific an ultra-high-vacuum environment organization in the world! To learn those reasons, write, (24).However, the vast majority of surface Raman results have exploited suruse coupon or CALL TOLL FREE I -800-ACS-5558 face enhancement (SERS), which primarily involves an increase in Po of Equation 7 . Local electromagnetic effects on certain roughened electrode ’1 materials (e.g., Ag, Au, Cu) enhance the I American Chemical Society, 1156 Sixteenth St., NW, Washington, DC 20036 I Raman signal by factors of up to 106 I I would like to know 224 reasons to I and make detection and spectral charI acterization of even submonolayers join the American Chemical Society, I straightforward (23-26). (SERS was Please send free brochures to: I I the subject of a recent A-page article [27] and will not be discussed extenI Name I sively here.) Surface enhancement may I Address I also involve an increase in P through I I chemical interactions of the adsorbate I City State, ZIP I and metal. SERS has proven to be a very valuable technique for examining I Ag, Au, and Cu electrodes Jl l l l l l l linl lvacuum, l l l l l l l l l l l l l lL
most chemists
American Chemical society.
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
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INSTRLJMENIAIION solution, and so forth and has revealed significant new information about electrochemical interfaces. Less explored but still valuable approaches include surface resonance Raman spectroscopy (SRRS), which exploits the high 9, of adsorbed RR molecules to enhance the signal, and surface enhanced RRS (SERRS), which combines resonance and surface enhancement (13, 28). SERS, SRRS, and SERRS can exhibit strong signals that --- easily measured, but SERS and
SERRS have the added advantage of providing surface selectivity. Because both local field and cross section enhancementsare short-range (
