Anal. Chem. 1907, 59, 2631-2637
2631
High-Sensitivity Normal and Resonance Raman Spectroscopy: Applications to Transient Electrochemistry Richard T. Packard and Richard L. McCreery*
Department of Chemistry, T h e Ohio State University, Columbus, Ohio 43210
Several spectrometer and detector combinations are compared for Raman spectroelectrochemistry, including single, double, and triple spectrometers with photomuitipiler or Intensified Hnear diode array detectors. Criterla for comparison included quantum yldd, throughput, collection efficiency, and ultimately performance for statlc or dynamlc examination of eiectrogenerated Raman scatterers. The most sensitive arrangement was a slngie monochromator wlth a flxed-wavelength photontounting detection system, which was able to detect an electrogenerated normal Raman scatterer after less than 10 ms of electrolysis (corresponding to 1 X lo-'' mol in the laser beam). The use of an array detector permits acquisition of spectra on a 10-ms time scale, such that tlme-reedved reactant, product, or intennediate spectra could be obtained during electrolysis. However, the multichannel advantage anticlpated for array detectors was not reailred, due to losses In quantum yield and detector area relative to a photomultiplier and poorer noise characterlstlcs. Appiications of these developments to the electrochemical oxidation of dopamlne, chlorpromazine, and hydroqulnone are described.
Since the initial development of UV-vis spectroelectrochemistry in the mid 196Os,novel spectral techniques designed to characterize events associated with electrochemical procmses have been almost continuously introduced and refined (1-5). The coupling of spectroscopy to electrochemistry has been driven by the need for greater selectivity and information content concerning the species associated with electrochemical charge transfer. A large fraction of the spectroelectrochemical techniques developed to date involve W-vis absorption, owing to its simplicity, rapid transient response (6),and sensitivity (7). However, the lack of structural specificity of UV-vis absorption spectroscopyhas spurred the growth of vibrational spectroscopy in conjunction with electrochemistry. This area, particularly Raman spectroelectrochemistry (SEC), has been reviewed recently (8-10). Unlike UV-vis absorption, Raman spectroscopy permits structural assignments and fingerprinting because of the sharp features apparent in its spectra. Raman spectroscopy has been used in combination with electrochemistry to elucidate the nature and behavior of species near an electrode in both static and transient experiments (11-16). Raman SEC is similar in concept to transient monitoring of photochemical processes (17) but the two approaches differ completely in the mode of generation of transient species. Recently, we demonstrated a technique for obtaining fixed-wavelength Raman intensity transients for normal as well as resonance Raman scatterers within the electrochemical diffusion layer on both long and short time scales (11). The fundamentally weak nature of Raman scattering demands an extremely efficient detection system, especially when dealing with the low concentrations of analyte encountered in electrochemical experiments. The recent popularity of multichannel detectors (generally referred to as optical
multichannel analyzers, OMA's) has extended into Raman spectroscopy (18-21) and has led to the development of spectrometer systems of particular value in time-resolved experiments (22, 23). Unfortunately, a large number of variables are involved in determining the amount of Raman light which is actually detected. Among these are variables associated with the collection optics, spectrometer, and the detector itself. Freeman et al. (241,Schwiesow (251, and Barrett and Adams (26)have discussed the effects of spectrometer and detector design on sensitivity for gas, liquid, and solid samples. Of particular relevance is the observation by Freeman et al. that commercial spectrometer designs often employ a tightly focused input laser beam to allow magnification onto the entrance slit, with an accompanying increase in collection efficiency. The objective of the present work is static and dynamic determination of Raman spectra of materials in the electrochemical diffusion layer. While the same techniques are useful for monitoring species on the electrode surface, the examples presented are limited to electrogenerated solution components. When Raman spectroscopy is combined with electrochemistry, three important experimental aspects differ significantly from most conventional spectroscopy in homogeneous solutions. First, the amounts of Raman scatterer involved are very small due to the heterogeneous nature of electrochemical generation. For example, electrolysis of a 5 mM solution for 100 me will mol of product/cm2 of generate approximately 5 X electrode area, or about 1 X lo-" mol in a typical sampling volume. Thus the sensitivity requirement is extreme compared to the concentrated solutions or neat liquids common to Raman spectroscopy. Second, the electrode surface and diffusion layer are easily disturbed or damaged by high laser flux, so a focused beam is not generally advisable. Sample damage is usually the ultimate factor that limits Sensitivity. Third, electrochemical generation of materials is dynamic, with typical time scales in the submillisecond to 30-5 range, and spectrum acquisition or fixed-wavelength monitoring in the millisecond range is desirable. These factors must be considered when designing instruments for Raman SEC. For the case where the laser spot on the electrode overfills the spectrometer aperture, the signal will be proportional to the electrode area sampled by the spectrometer, denoted here as AD. While AD is ultimately determined by spectrometer and detector characteristics, its effect on sensitivity is best appreciated by recognizing that only those Raman photons originating in AD can reach the detector. The Raman signal for the case of an overfilled spectrometer is given by eq 1 (11). An important consideration in spectroelectrochemical applications is to maximize AD, thus reducing laser-power density and minimizing solution heating or sample damage 4Po/3Nb(Dt)'1' 0TQAD S= (1) a2 where S = Raman signal, counts s-l; Po = input laser power, photons 5-l; /3 = differential Raman cross section, cm2 molecule-' sr-l; Nb = bulk number density of starting material, molecules cmy3;D = diffusion coefficient of reactant, cm2 s-'; t = generation time, s; 0 = collection efficiency, sr; T =
0003-2700/87/0359-2631$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 21, NOVEMBER 1, 1987
2632
Table I
system
spectrometer type
number
reciprocal linear dispersion, cm-’/mm at 515 nm
A.
double/PMT
7.8
12.5
cooled RCA 31034 photomultiplier
0.15
B.
single/PMT
5.2
26
cooled RCA 31034 photomultiplier
0.15
C.
single/OMA
5.2
26
triple/OMA
9
35
PARC OMA III/1421B, 1000-channel, intensified diode array PARC OMA II1/1420RB 700-channel, intensified diode array
0.067
D.
Spex1403, additive dispersion double Czerny-Turner 1800 lines/mm Instruments SA 640 mm, Czerny-Turner, 1800 lines/mm Instruments SA 640 mm, Czerny-Turner, 1800 lines/mm Spex 1877B “triplemate” Czerny-Turner spectrograph with subtractive dispersion double prefilter
f
detector
detector Q, cts/photona
0.067
Manufacturer’s mecification. transmission of spectrometer, unitless; Q = detector quantum yield, counts photon-’; A D = electrode area sampled by the spectrometer and detector, taking any magnification into account; a = beam radius at electrode. There are many variables to consider when designing and optimizing a spectrometer for Raman SEC, but they may be divided into three groups, dealing with light collection, spectrometer throughput, and detector characteristics. Light collection is determined by fl and A D , and is usually limited by the spectrometer or detector rather than the collection optics for a well-designed system. Throughput (7‘) is a function of spectrometer design and will differ substantially for single vs double vs triple spectrometers. The important detector parameters are Q and noise characteristics and whether one or many wavelengths are monitored. The overall objective of this paper is to compare various approaches to improving Raman SEC performance by using several criteria. A D , Q)will be First, spectrometer and detector variables (T, compared for standard light sources. Second, the performance of both single and double spectrometers will be compared for spectroelectrochemical conditions. Third, examples of performance for monitoring electrogenerated Raman scatterers will be presented.
EXPERIMENTAL SECTION Four combinations of commercial spectrometers and detectors were compared, as listed in Table I. A single photomultiplier and housing was used in both systems A and B. The relative sensitivities of the four systems for an ordinary radiating source were compared by monitoring the output of a weak tungsten or neon lamp operated at controlled current and positioned 50 cm from the spectrometer entrance slit. The lamp’s light entering the spectrometer is nearly collimated, so the f number of the spectrometer is unimportant. The signal of each spectrometer/detector combination is therefore a measure of the product of AD for the spectrometer, Q and T ,but not $2. T was estimated independently by the attenuation of a weak 515-nm laser line measured inside the entrance slit and just prior to the exit slit. These measurements provide a comparison of the throughput and detector sensitivity, independent of collection efficiency. The optics for collecting Raman spectra of intensity transients of electrogenerated materials were similar to those reported previously (11). When fiber-optic collection of the Raman light was desired, the incident laser beam was focused onto the horizontally mounted working electrode, and the resulting signal was gathered by a 3 by 9 fiber-optic array. The image of the distal ends of the fibers was formed on the entrance slit by a single f / l lens. The fiber-optic arrangement is convenient because the entrance slit image need not be coincident with the electrode and input laser beam. However, a lens system, which is less convenient, ultimately gathers more light because there are no spaces between fibers. For most of the work reported here, the lens arrangement
s
L3
Figure 1. Configuration of hser beam, electrode, and collection optics. The input beam is directed toward the electrode at angles of 85-87’ to the normal by steering mirror M. Diffusion of electrogenerated products occurs horizontaily as shown. See text for further details.
of Figure 1 was employed. The beam-steering mirror below a Lucite cell having glass windows was mounted on an adjustable stage so that the angle at which the focused beam was incident upon the working electrode could be readily changed. It was important to make sure that the electrode surface was positioned in the focal planes of both L1 (f = 15 cm) and L2 (f = 50.8 cm) while remaining on the optical axis of the spectrometer. L3 (f = 101.6 cm) acted to focus the light onto the entrance slit, after passing through a band-reject filter, F (Pomfret Research Optics, Inc., Orange, VA). The working electrode was fashioned from a Bioanalytical Systems (West Lafayette, IN) platinum-disk voltammetry electrode of 1.5-mm diameter. In order to obtain a high-quality flat surface, the electrode was secured in a 2iwdiameter stainless steel cylinder and polished by the same procedure as described previously (27). Raman spectral data were acquired in one of three modes, depending upon whether static spectra, time-resolved spectra, or single-wavelength transients were desired. To obtain static spectra of an electrogenerated species, a low-frequency square wave was applied to the electrochemical cell and the resulting steady-state concentration of the electrogenerated scatterer was monitored with spectrometer C. The OMA parameters were selected as they would be for a solution species, since the electrogenerated species concentration is at steady state. Any time an OMA is used, the noise due to the variations of sensitivity among pixels of the diode array can become quite evident. This noise was reduced by dividing the spectra by a “pixel gain profile” obtained by illuminating the array with a diffuse white-light source (28).
Raman intensity transients were obtained at fixed wavelength as described previously (11, 12), using spectrometer B. The
ANALYTICAL CHEMISTRY, VOL. 59, NO. 21, NOVEMBER 1, 1987
2633
Table 11. Spectrometer/Detector Performance for Standard Light Sourcea
AD
(HX W , cm)
A
B
C
D
0.2 x 0.021
0.2 x 0.01 0.32 0.15 9.6 x 10-5
0.2 x 0.01 0.32 0.067 4.3 x 10-5 0.45 0.31 0.31
0.2 X 0.0075 0.11 0.11 0.040 0.044
1.0
0.33
0.054 0.60
0.0039
Tb
0.10
8'
0.15 6.3 X 0.66 0.24 0.56
ADTQ, cm2 cts photon-', predicted ADTQ,re1 to B ADTQ, measd, re1 to Bd ADTQ,line sourcee Q, sr, re1 to B QADTQ,predicted' QADTQ(MCA)fa
0.44
0.28 0.28
1.0
1.0 1.0 1.0 1.0 1.0
0.067 1.1 x 10-5
0.043
Manufacturer's values. dTungsten lamp, 550 nm. e Neon lamp, a Fixed resolution of 2.6 cm-' for all cases. *Collimatedbeam, 515 nm. 540.1 nm. 'Slit height increased to 2.5 mm for C and D, 15 mm for A and B. 8MCA = measured multichannel advantage.
I
I #.'
chemical oxidation of CPZ in 0.1 M HC1 solution is stable on the however, 42% time scale of the experiments performed (29,30); methanol (w/w) was added to prevent CPZ adsorption on the electrode surface. HQS was chosen over hydroquinone because of the higher water solubility of both its reduced and oxidized forms.
RESULTS AND DISCUSSION Equation 1 may be restated (11) as eq 2, in which the variables affecting the specific intensity (photons s-l cm-2 sr-l) of the Raman light are collected into a single quantity Lsol.
Flgure 2. Block dhgram of translent-spectra collection system based on system C. Timing for the experiment Is controlled by the OMA 111.
spectrometer was positioned at the desired wavelength, then a Tracor Model 1710 multichannel analyzer was used to monitor photon counts during a series of time windows with widths from 10 fis up. A wavelength adjacent to a Raman peak was recorded in a separate experiment and subtracted from the Raman transient to correct for any changes in broad-band emission. An Apple microcomputer was used to control potentiostat and detector timing. Transient Raman spectra were obtained by using spectrometer C by repeatedly rapidly scanning the diode array after a potential step that generated or consumed a Raman scatterer. The result was a series of up to 1000 spectra obtained over 10 ms or longer intervals after the beginning of electrochemicalgeneration. The optical and electronic configuration used to obtain transient spectra with multichannel detection is shown in Figure 2. The experiment was controlled by the Princeton Applied Research Corp. (Princeton, NJ) OMA I11 (1460) system, with timing linked to the diode array integration period. The diode array could be scanned in 10-17 ms at the end of a given integration period, repeatedly if desired. At the beginning of the experiment, the OMA I11 console triggered a Tektronix Model PG 507 pulse generator, which produced a pulse lasting 5-10 integration periods. The pulse controlled a PAR Model 173 potentiostat, which applied a potential pulse to the cell lasting as long as the PG 507 pulse. The OMA then scanned and stored the results of 200 sequential integration periods in different memory locations. The process was then repeated as desired to allow signal averaging, with the duty cycle for the applied potential never exceeding 3%. The end result was 200 spectra, each signal averaged if desired, and each representing a different time increment after the beginning of a potential pulse which lasted for 5-10 spectra. For the resonance Raman scatterer chlorpromazine cation radical, where the input beam was attenuated, the spectra were normalized to the 1026-cm-' MeOH band. Chlorpromazine hydrochloride (CPZ) and 3-hydroxytyramine hydrochloride (dopamine, H,DA) were obtained from Sigma Chemical Co., and hydroquinonesulfonic acid, potassium salt (HQS), was obtained from Aldrich Chemical Co. All were used as received, but solutions were passed through a O.l-pm filter (Millipore Corp.) prior to use to reduce scattering from small insoluble particles. The cation radical formed upon electro-
Lsolis determined by the chemical system, time scale, beam parameters, and diffusion parameters, whereas AD,0,T , and Q are determined by spectrometer variables. Table I1 presents comparative values of AD, T , and Q for the four spectrometer/detector combinations investigated, as determined for the case of a constant source placed distant from the entrance slit. The spectral resolution was constant in all cases (2.6 cm-') and the collection efficiency, R, was unimportant due to the near collimation of source light. As expected, the throughput, T , decreases as the number of gratings and mirrors increases. Q for the OMA's is lower than that for a high-quality photomultiplier tube (PMT), and the variation in Q across the array can contribute to noise. For the first seven rows of Table 11, the slit height was kept constant at 2 mm, but in actual experiments the P M T will have a larger AD because a slit height of 1-2 cm may be used. For the constant height case, the predicted product of ADTQ is highest for system B and varies for the four systems by a factor of 9. The measured values of ADTQ shown in Table I1 were determined from comparisons of the signal obtained with controlled sources, at equal resolution and slit height. The results are qualitatively consistent with the predicted trends, with system B being most sensitive, followed A, C, and D in that order. Disagreement between predicted and observed results are most likely attributable to variation in Q from manufacturer's specifications. Table I1 also shows predicted sensitivity after the collection efficiency is taken into account and slit height was increased to the maximum allowed by the detector (2.5 mm for C and D, 1.5 cm for A and B). The values listed as QADTQare the anticipated relative signal strengths for various spectrometers obtained at fiied wavelengths for samples of constant Lml,with optimum Q, T , Q,and AD taken into account. Several points determine the relative performance of each spectrometer/ detector combination. First, the single monochromator has a higher Q and T than a double or triple, improving the photon flux a t the detector. Second, the pixel area of an OMA is smaller than a P M T by about a factor of 6, thus exacerbating an already low Q. Third, the product of QADTQis much lower for system D than B, but system B will have poorer stray light rejection. Fourth, the higher dispersion of the double
2834
ANALYTICAL CHEMISTRY, VOL. 59, NO. 21, NOVEMBER 1, 1987 ~~
~~
Table 111. Raman Signals for Electrochemical Conditions
spectrometer/ detector
Raman signal,"Hz
noise,b
Hz
signal/ noise
A B B with filter
12 400
85 500
42.0 319 69.3
295 268 936
64 900
"981-cm-' band of 1 M Na2S04,515-nm laser. bStandarddeviation of noise at 800 cm-l.
700
ado
900 1000 Raman S h i f t (cm-'1
i io0
i200
Figure 3. Raman spectra for 0.1 M (",),SO, in water obtained by using an unfocused 100-mW 514.5-nm laser line. Resolution (2.5 cm-'), entrance optics, spectrometer, and total acquisition time were identical for both spectra. Upper spectrum, system B, 250 steps of 2.2 cm-' size, 1-s photon counting integration for each step. Lower spectrum, system C, 10 25-s OMA integration periods were added.
monochromator allows a wider slit than the single for a given resolution, increasing AD and partially compensating for lower T and 0. The entries in all but the last row of Table I1 compare sensitivities for single-wavelengthmonitoring and ignore the multichannel advantage predicted for OMA detectors. When comparing complete spectra obtained over a fixed total time interval, the OMA should have an improved signal-to-noise ratio over a scanned single-channel system equal to MIzwhere N is the number of resolution elements in the spectrum. For our OMA detector, blooming limits the useful number of resolution elements to about 250, leading to a predicted signal-to-noise improvement of about 16. This value was tested experimentally by monitoring a CC14 sample with low laser power. The single monochromator, collection optics, sample, and slit height (2.5 mm) were identical for both cases, so the only differences between the photomultiplier and OMA being compared were Q and noise characteristics. The OMA signal was integrated for 25 s, and individual wavelengths were monitored with the PMT for 0.1 s. The ratio of the signal to the standard deviation of the noise was a factor of 5 better for the OMA vs the PMT, one-third the expected value of 16. This shortfall of the multichannel advantage was caused by the low quantum yield and noise characteristics of the OMA, some of which is readout noise. The last row of Table I1 indicates the relative sensitivities of the four spectrometer/detector combinations when obtaining complete spectra, and includes measured multichannel advantage. Note that system C is still inferior to B, even though the multichannel advantage should apply. The advantage is lost partly because of the noise characteristics of the OMA but also because of its smaller AD and Q. Thus for obtaining spectra for a stable sample and laser power, a scanned P M T system is predicted to be comparable to an OMA in terms of signal-to-noise ratio for weak signals and equal total acquisition time. This prediction is verified in Figure 3, which compares spectra of an (NHJ2S04 solution obtained with systems B and C. The laser power, entrance optics, and spectrographs were identical, with only the detectors and slit heights differing for the two spectra. The PMT was scanned over 250 wavenumber increments (2.2 cm-' each) during 250 s, and the OMA was integrated for 10 25-9 periods. Under these conditions, a 16-fold improvement in S / N would be predicted for the OMA from the multichannel advantage. In fact, the S/N for the PMT was 93 while that for the OMA
was 145, a factor of only 1.56 improvement. Thus the OMA exhibits only a slight multichannel advantage, due to its smaller detector height and quantum yield. This conclusion differs from that of Freeman et al. (24) who observed a large S/Nimprovement for the OMA under similar conditions. The S/Nfor the OMA in Figure 3 is about twice that of Freeman's experiment, but our PMT S / N is much better than his. A probable reason for the discrepancy is laser-power fluctuation in Freeman's experiment, which worsened the PMT results. Whether or not an OMA will result in improved S / N ratio is strongly dependent on experimental parameters. When radiation damage is a possibility, as in many spectroelectrochemical experiments, it is important to maximize AD and an OMA will suffer from small pixel area. However, for a tightly focused beam or fluctuating laser power, the OMA may outperform a scanned PMT in terms of S / N ratio. Since our principal motivation for improving instrumentation is spectroelectrochemistry, we next compared systems A and B for ultimate sensitivity under electrochemical conditions. Fiber-optic collection (11)was employed to equate the entrance optics, and the spectrometers were operated at equal resolution (5 cm-l) and acquisition time. Na2S04(1.0 M) served as a static sample to eliminate any electrochemical variables, but the electrode, cell, etc. were present as they would be for an SEC experiment. This approach permitted comparison of the two systems under identical conditions as far as the electrochemical cell is concerned. Table I11 shows the signal and noise determined for the 981-cm-' band of SO-: and an adjacent wavelength, 800 cm-'. As expeected from Table 11, the signal is larger for the single monochromator, but the noise is also larger due to stray light from the highly scattering electrode surface. A notch filter centered at the laser line between the sample and system B reduced the signal by 24% but the noise by 89%, leading to a S/Nimprovement of a factor of 3.5. The increased signal for system B is due to higher T and higher 0, accompanied by lower dispersion and narrower slits. For an electrochemical experiment, the factor of 3.5 improvement in signal-to-noise translates into a factor of 10 less signal averaging or the ability to monitor weaker Raman scatterers. The spectroelectrochemical performance of several spectrometer/detector combinations was evaluated by using three chemical systems in different data acquisition modes. In the first case, system B was used to monitor the oxidation of hydroquinonesulfonate by a diffusion-limited potential step. As indicated in eq 1,the Raman scattering at fixed wavelength should have a t l I z dependence. On the basis of Table I1 system B should have the highest S / N ratio of all four systems for single-wavelength monitoring. As shown in Figure 4, it was possible to obtain fixed-wavelength SEC transients in the millisecond time scale without resonance enhancement. Comparable transients were unobtainable with the other spectrometer systems at these time scales. The slope of the intensity vs t112plot of Figure 4B is 9 kHz We have shown that the OMA is not advantageous for single-wavelengthmonitoring both because of low sensitivity and poorer time resolution relative to a single-channel PMT. While we do not predict that an OMA will be better than a
ANALYTICAL CHEMISTRY, VOL. 59, NO. 21, NOVEMBER 1, 1987
2635
E
z
0
A
.M 41
$
m
-
C
i
Y
C
n
m
u
Y
C
1020
1100 1180 Remen Shift (cm-’)
1260
Flgure 6. Transient resonance Raman spectra of CPZ’ generated by a 50-ms potential pulse of 0.3-0.85 V vs Ag/AgCi. Solution and input beam conditions are the same as those in Figure 5. Five hundred channels of the array were utlllzed, and MeOH peaks were subtracted. Integration time is 10 ms with 1000 cycles signal averaged. Time of array readout: (a) 0 ms after potential step; (b) 10 ms; (c) 20 ms; (d) 50 ms.
0.1
0.0
0.2
0.3
tl” (sect”)
Flgure 4. Normal Raman transient intensity for the 1670-cm-’ band of benzoquinonesulfonate generated by a 0.0-0.85 V vs Ag/AgCI potential pulse. Initial HQS concentration was 20 mM in 1.0 M HCi. 100-ms potential pulses repeated every 2.5 s were signal averaged for a total of 1000 runs. A T r a m Model 1710 muMchannei scaler wlth a dwell time of 2 ms was used to sample the Raman intensity. Curve A is the averaged transient, curve B is a plot of intensity vs t ”* for the duration of the pulse. Spectral band-pass was 10 cm-’. Input beam was 75 mW, 514.5 nm, incident angle 86O, system B.
(D
2
‘[
z
Y
500 Hz
CI
m C
m9
Y
C
2
U
I
.x..” m
b 9
,
1050
4
1200
1350
1500
Remsn Shift Icm-’l
Figure 5. Steady-state resonance Raman spectrum of electrogenerated CPZ’ from a solution of 2.0 mM CPZ in 1.0 M HCI in 42% MeOH/H,O obtained with system C. The slgnal is the sum of 25 array integration periods of 4-s duration. Applied potential waveform was a 20 Hz, 50% duty cycle square wave with limits of 0.0-0.85 V vs Ag/AgCi. Spectral band-pass was 5 cm-’. Beam incidence angle was 87O, power at the sample was 40 mW, 514.5 nm. MeOH peaks from a blank spectrum were subtracted.
scanned PMT for obtaining spectra of static samples, it can be used for such samples when sensitivity is not a prime consideration, as shown in Figure 5. An OMA has the important advantage over a scanned PMT of rapid spectral acquisition, allowing a complete spectrum to be obtained in 10-17 ms in nongated mode or 10 ns if gated. Provided enough Raman photons are available to produce acceptable signalto-noise ratios for these short acquisition intervals, the rapid
acquisition capability of the OMA can be useful for obtaining spectra of electrogenerated reactive species. A series of spectra obtained immediately following a potential step oxidizing CPZ to its radical cation is shown in Figure 6. The sensitivity of this technique is exemplified by the fact that at 10 ms after the potential step, there are only mol of scattering species contained within the about 4 x sampling volume adjacent to the electrode. For comparison, fixed-wavelength experiments with a photomultiplier permitted less than 100-ks time resolution. The limit on the minimum time resolution for the OMA is set by the time required to read the array, normally 17 ms for a 1024-channel array. However, by reducing the spectral window, some of the pixels may be ignored during the scan and shorter integration times may be achieved. Alternatively, the integration “window” may be defined by gating the intensifier at a specified time after the potential pulse. This allows data acquisition a t times much closer to the potential pulse, but restricts the data collection to include only 1time window/ trial. An experiment of this type was tried, and similar results to those shown in Figure 6 were obtained. Note that the lower limit on spectral acquisition time is determined not by the detection system, which can gate down to 10 ns, but by the rate of Raman photon production. Even at strong count rates of 10 kHz, only 10 photons will arrive at the detector in a 1-ms time window, thus requiring signal averaging to achieve acceptable statistics. Obtaining spectra of transient species that are not resonance enhanced is considerably more difficult, since the scattering cross sections are several orders of magnitude lower. Figure 7 shows time-resolved spectra taken during the conversion of hydroquinonesulfonate to benzoquinonesulfonate. The time windows used are considerably longer than those for resonance-enhanced CPZ’; lasting 1s rather than 10 ms. To our knowledge, these are the first examples of time-resolved spectra for electrogenerated normal Raman scatterers. A final example of Raman spectral monitoring of a reactive system is the oxidation of dopamine. The electrochemical behavior of catecholamines and their various derivatives have been the focus of a large amount of research over the last 20 years (32). It is well-known that the initial product of dopamine (H,DA) oxidation, the o-quinone (HDOQ), is a highly reactive species, which can readily undergo nucleophilic attack (32,33).While the most common nucleophilic addition is that of dopamine’s own side chain amine, the ring is also susceptible to intermolecular nucleophilic attack. The 1,4 addition of HCl
2636
ANALYTICAL CHEMISTRY, VOL. 59, NO. 21, NOVEMBER 1, 1987 m
I
r U
r
-c(
m
4J .I
C W 4J
m
W C
100 HZ
Y C
4J Y C
d C
b
- 1450
a 1550
1650
1750
Raman S h i f t (ern-') Rarnan S h i f t
(cm-')
Figure 7. Normal Raman transient spectra of HQS oxidation resulting from a 0.0-0.9 V vs Ag/AgCI potential step of 5-s duration. Solution conditlons are the same as in Figure 4. Laser power is 50 mW, 488 nm. Time after potential step: (a) 0 s; (b) 1 s; (c) 2 s;(d) 5 s. H and B denote hydroquinone and benzoquinone peaks, respectively. Spectra are the result of a single 25-point Savitsky-Goiay smooth. Six hundred points of the array were scanned, and 50 transients were averaged. m
2
1650
1750
(cm-')
Raman S h i f t
Figure 8. Rarnan spectra of 20 mM dopamine oxidation at 500 ms after a 0.0-0.9 V vs Ag/AgCI potential step. Curve A was obtained in 1.0 M HCI, curve B in 1.0 M HCIO,. Laser power was 75 mW, 488 nrn at the electrode. A water band at 1636 cm-' obtained from a blank was subtracted from each curve.
to electrogenerated o-quinone has been shown to proceed by the following modified ECE mechanism (34):
H,DA + HDOQ HC1+ HDOQ
-
+ 2e- + 2H+
(3)
HzDACl
(4)
-+
+ 2H+ HDOQ + HzDACl
H,DACl + DOQCl H,DA
+ DOQCl
resolution of Raman spectroscopy allows selective monitoring of individual components and also allows structural inferences from the vibrational spectrum.
CONCLUSIONS
1550
14'50
Figure 9, Transient Rarnan spectra of H,DA oxidation in a solution initially containing 20 mM H,DA in 1 M HCI. The potential pulse is 0.0-0.9 V vs. Ag/AgCI with a duration of 0.5 s. Laser power is 75 rnW, 488 nm. Time after potential step: (a) 0 ms; (b) 100 ms; (c) 200 ms; (d) 500 ms. The H20 peak at 1636 cm-' from a blank spectrum has been subtracted for each curve.
2e-
(5) (6)
where H2DACkcorresponds to 6-chlorodopamine and DOQC1 to the substituted o-quinone. Figure 8 shows the results of a time-resolved Raman experiment in which a potential 200 mV past the oxidation peak potential for dopamine was applied to an electrode in a solution of 20 mM dopamine. In l M HC104, the Raman spectrum of oxidized dopamine after 500 ms shows two peaks in the 1400-1800 cm-' range, a t 1574 and 1672 cm-' (Figure 8). Both of these peaks grow with a t'12 dependence, indicating a stable species on this time scale. In 1M HC1, the spectrum is altered significantly, with peaks at 1548 and 1693 cm-'. As shown in Figure 9, the 1548-cm-' peak increases rapidly with time at the expense of the 1574-cm-' peak. Clearly the oquinone intermediate is observable but is converted to the chloroquinone on a 500-ms time scale. The high spectral
The choice of spectrometer and detector for Raman SEC will depend on the particular data acquisition mode. For single-wavelength monitoring, a single-grating spectrometer is superior to a double or triple in terms of signal-to-noiseratio by the factors listed in Table 11. A photomultiplier significantly exceeds the performance of a single channel of an intensified diode array because of larger detector area and quantum yield and lower noise. For acquisition of static spectra, for example a steady-state concentration of electrogenerated species, a scanned photomultiplier tube will provide comparable or better S / N ratio than an OMA system for the same total measurement time. The multichannel advantage expected for the array is lost due to lower quantum yield, smaller pixel area, and higher readout noise. In most spectroelectrochemical applications, the sensitivity is ultimately limited by disturbance of the sample by high laser-power density. In this case, AD should be maximized, the detector area becomes important, and OMAs are inferior to a scanned PMT. An OMA may perform better than a PMT in the case where laser-power fluctuations are significant, e.g. for pulsed systems. In this case the OMA may perform better than a scanned single-channel detector because it will accommodate source intensity fluctuations. An experiment where the OMA excels is fast acquisition of transient spectra where there is sufficient Raman light available. It is much simpler to obtain a series of spectra from a reactive system by using a multichannel detector than with a point by point approach employing a photomultiplier. In addition to detector parameters, a low spectrometer f number improves collection efficiency and signal-to-noise ratio. However, low f number usually implies shorter focal length and lower dispersion, requiring narrower slits and smaller AD for a given spectral resolution. This work demonstrates that the effect of major variables from eq 1 on Raman spectroelectrochemical performance is predictable and consistent with experimental observation. If proper attention is paid to these factors, excellent performance may be attained. The result is the first transient spectra of electrogenerated normal and resonance Raman scatterers on a 10-100-ms time scale and single-wavelength monitoring of normal Raman scatterers starting at a few milliseconds. Note Added in Proof. Replacement of lens L2 (Figure 1) with a good quality f/1.4 camera lens (Canon 50-mm focal
Anal. Chem. 1987, 59, 2637-263%
length) and L3 with a f/5,254-mm focal length achromat in system led to a factor Of approximately increase in The bulk of this improvement resulted from the camera lens and is probably due to more effective collection of marginal rays.
ACKNOWLEDGMENT
meauthors thank F, T~~~~~ Gamble of ~~~i~~~University and Prabir Dutta of Ohio State University for useful comments regarding this work. In addition, we thank Professor Dutta for the use of a Spex 1877B “triplemate”spectrometer.
LITERATURE CITED Kuwana, T.; Winograd, N. Eiectroanal. Chem. 1974, 7 , 1-74. Heineman, W. R. Anal. Chem. 1978, 50, 390A. Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980;Chapter 14. Helneman, W. R.; Hawkridge, F. M.; Blount, H. N. I n Electroanalytical Cl!emlstfy; Bard, A. J., Ed.; Marcel Dekker: New York, 1984;Vol. 13, P
1.
McCreery, R. L,, In physical Methods in Chem/sw; Rossiter, B., Ed.; Wiley: New York, 1986;Vol. 2,p 561. M ~ c R. ~L,; Robinson, ~ ~ ~R. ,S,J , ~~ectfoanal, Chem, Intertack/ Electrochem. 1985, 182, 61. Jan, C. C.; Lavine, K.; McCreery, R. L. Anal. Chem. 1985, 5 7 , 752. Van Duyne. R. P. I n Chemical and Biological Applicatlons of Lasers; Moore, L. B., Ed.; Academic: New York, 1979; Vol. 4, Chapter 4. (honey, R. P.; Mahoney, M. R.; McQuilkn, A. J. I n Advances In Infrared and Raman Spectroscopy; Clark, R, J., Hester, R, E,, Eds,; H ~ den: London, 1982;Vol. 9,Chapter 4. Fleischmann, M.; Hill, I. R. In Comprehensive Treatles of Electrochemistry; White, R. E. et al., Eds.; Plenum: New York, 1984; Vol. 8,Chapter 6. Schwab, s. D.; McCreery, R. L.; Gamble, F. T. Anal. Chem. 1988, 58, 2486. Jeanmaire, D. L.; Van Duyne, R . P. J . Electroanal. Chem. Interfac/& Electrochem. 1975, 66, 235.
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(13) Jeanmaire, D. L.; Suchanski, M. R.; Van Duyne, R. P. J. Am. Chem. SOC. 1975, 97. 1699. (14) Clarke, J. S.;Kuhn, A. T.; Orville-Thomas, W. J. J . Electronanal. Chem. Interfacial Electrochem. 1974. 54. 253. (15) Van Duyne, R. P. J. Phys., Colloq. 1977, 5,i39. (16) Van Duyne, R. P. Haushalter, J. P. J . Phys. Chem. 1984, 88, 2446. (17) Beck, S.M.; Brus. L. E. J . Chem. Phys. 1981, 75, 4934. (18) Chao, J. L. Appl. Spectrosc. 1981, 35, 281. (19) Hug, W.; Surbeck, H. J . Raman Spectrosc. 1982, 13, 38. (20) Campion, A.; Brown, J.; Grizzle, W. M. Surf. Sci. 1982, 715, L153. (21) Chang, R. R.; Long, M. B. I n Topics in Applled Physlcs; Cardona, M., Guntherodt, G., Eds.; Sprlnger-Verlag: Berlin, 1982;Vol. 50, Chapter 3. (22) D’Orazio, M.; Hirschberger, R. Opt. Eng. 1983, 22, 308. (23) Asher, S. A.; Flaugh, P. L.; Washinger, G. Spectroscopy (SpringfleM, Oregon) 1968, 1 ,-26. (24) Freeman, J. J.; et al. Appl. Spectrosc. 1981, 35, 196. (25) Schwiesow, R. L. J . Opt. SOC.Am. 1989, 59, 1285. (26) Barrett, J. J.; Adams, N. I. J. Opt. SOC.Am. 1988, 58, 311. (27) Jan, C. C.; Lavine, B. K.; McCreery, R. L. Anal. Chem. 1985, 57, 752. (28) Howard* R.; M. Spectrosc. 1g86v 1245. (29) Cheng, H. Y.; McCreery, R. L.; Sackett, P. M. J . Am. Chem. SOC. 1978. 100. 962. (30) Mayausky,.~. s.;McCreery, R. L. J . Electroanal. Chem. Interfacial Electrochem. 1983, 145, 117. (31) Dryhurst, G.;et al. Biological Electrochemistry; Academic: New York, 1982;Vol. 1, Chapter 2. (32) Hawley, M. D.; Tatawawadi, S.V.; Piekarski, S.;Adams, R. N. J . Am. Chem. SOC. 1967, 89, 447. (33) Sternson, A. W.; McCreery, R. L.; Feinberg, B.; Adams, R. N. J. Electroanal. Chem. Interfacial Electrochem. 1973, 46, 313. ~ - (34) Adams, R. N.; Hawley, M. D.; Feldberg, S. W. J . Phys, Chem, 1987, 71, 851. 401
RECEIVED for review March 31,1987. Accepted July 20,1987. This work was supported by the Chemical Analysis Division ofthe National Science Foundation and by the Dow Chemical co.
CORRESPONDENCE Fourier Transform Infrared Photoacoustic Spectroscopic Study of Surface Texture in Brush and Polymeric Bonded Phases Sir: Liquid chromatographic bonded stationary phases are generally classified into two categories, “brush” and “polymeric”. They are produced by reacting monofunctional or polyfunctional silanes with the accessible surface silanols of silica gel (1-5). The effect of textural differences between polymeric and brush phase ligands on solute selectivity is of particular interest. Sander and Wise (2)suggest that changes in selectivity for polycyclic aromatic hydrocarbon mixtures is the result of surface coverage, bonded phase type, and hydrosilylation reaction conditions. Lochmuller et al. (6) synthesized a model stationary phase to test predictions based on a lattice-model, unified molecular theory for selectivity. Their results indicate that selectivity follows the order of rodlike solutes > platelike solutes > flexible chains. Chemically modified, liquid chromatographic stationary phases appear to have an active role in solute selectivity. Consequently, the examination of derivatized silica surfaces is essential to a better understanding of how bonded phases effect retention and selectivity (7-10). In a recent paper by Hunnicutt et al. (11),monofunctional pyrenesilanes covalently bound to a microparticulate silica were examined by Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS). In that paper it was suggested that
Table I. Percent Carbon for Brush and Polymeric Propylpyrene Bonded Phases Determined by Elemental Analyses % carbon
brush polymeric
1.19 1.36
1.49 1.99
3.11
3.76
5.56 5.11
6.95
7.24
for these “brush” phases, the enhancement of weakly allowed vibrational transitions between 1350 and 1725 cm-’ could be attributed to the interaction of the pyrene moiety with surface silanols. The observed decrease in intensity of these vibrational transitions with increasing surface coverage was attributed to steric constraints which limit possible conformational changes of the bound silane and, consequently, reduce the pyrene ligand/surface silanol interaction. The work presented here indicates that, for the surface coverages studied, polymeric phases interact with surface silanols to a lesser degree than brush phases. These results support a bonded-phase model currently under development in our laboratory, which suggests that covalently bound, polymeric stationary phases are motionally more constrained than corresponding brush phases.
0003-2700/87/0359-263780 1.50/0 0 1987 American Chemical Society
