Anal. Chem. 1989, 6 1 , 279-282 (23) Snopek, J.; Jeiinek, 1.; Smolkova-Keuiemansova, E. J . Chromatogr. 1888, 438, 211-218. (24) Qi, W. 6.; Zhu, L. Huaxue ShJi 1887, 9 . 208-211. Chem. Abstr. 1888, 708, 1423042. (25) Greatbanks, D.; Pickford, R. M a g . Reson. Chem. 1887, 2 5 , 208-21 5. (26) Francis, 0:J.; Kirschenheuter, 0.P.; Ware, G. M.; Carman, A. S.; Juan, S. S. J . Assoc. Off. Anal. Chem. 1888, 77, 725-728. (27) Bender, M. L.; Komlyama. M. C y c M x t d n Chemlshy; Springer-Verlag: New York, 1978; pp 2-9, and references therein. (28) Szejtli, J. Cyclodextrlns and Their Inclusion Complexes ; Akademiai Klado: Budapest, 1982; Chapter 1, and references therein. (29) Wiedenhof, N.; Lammers, J. N. J. J. Carbohyd. Res. 1888, 7 , 1-6. (30) Jorqlakowski, M. J.; Connors, K. A. Carbohyd. Res. 1885, 743, 51-59. (31) Cserhatl, T.; BoJarski, J.; Fenyvesi, E.; Szejtli, J. J . Chromatogr. 1888, 357, 356-362. (32) Tanaka, M.; Miki, T.; Shono, T. J . Chromatogr. 1885. 330, 253-281. (33) Friedman. R. B. Ger. Offen DE Patent 3,712,246, Oct., 1987. Chem. Abstr. 1888, 108, 96538d. (34) Imamura, T.; Kamiya, H.; Kurosaki, T. Jpn. Kokai Tokkyo Koho JP Patent 62,220,501, Sept 1987. Chem. Abstr. 1888, 709, 110838f. (35) Szefli, J.: Hdtilts, I.; Keszler. 6.; Govacs, G.; Fenyvesi, E. Hung. Teijes HU Patent 41,824, May 1987. Chem. Absb. 1888, 708, 77531e. (36) Szepli, J. J . Incl. Phenom. 1883, 7 , 135-150. (37) Pagington, J. S. Chem. Brn. 1887, 23, 455-458. (38) Harata, K.; Uekama, K.; Otagiri, M.; Hlrayama, F. J . Inclusion Phenom. 1884, 7 , 279-293. (39) Armstrong. D. W.; Jin, H. L. Anal. Chem. 1887, 5 9 , 2237-2241. (40) Kinoshlta, T.; Iinuma, F.; Tsujl, A. Chem. h r m . Bull. 1874, 22, 242 1-2426. (41) Kinoshita, T.; Iinuma, F.; Tsuji, A. Chem. Pharm. Bull. 1874, 22(11), 2755-2758, (42) The Merck Index, 9th ed., Windhoiz, M., Ed.; Merck 8 Co., Inc.: Rahway, NJ, 1976; p 1266.
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(43) Nakai, S.; Li-Chan, E. Hycl-ophoblc Interactlms in Food Systems; CRC Press: Boca Raton, FL, 1988; Chapter 1, pp 1-21, and references therein. (44) Melander, W.; Horvath, C. Arch. Biochem. Biophys. 1877, 783, 200-213. (45) Kuharski, R. A.; Rossky, P. J. J . Am. Chem. SOC. 1884, 106, 5794-5800. (46) Barone, G.; Castronuovo, G.; Eiia, V.; Muscetta, M. J . Solution Chem. 1888, 75, 129-140. (47) Van Etten, R. L.; Ciowes, G. A.; Sebastin, J. F.; Bender, M. L. J . Am. Chem. SOC. 1887, 8 9 , 3253-3262. (48) Herskovits, T. T.; Kelly, T. M. J . Phys. Chem. 1973, 77, 381-388. (49) Flohr, K.; Paton, R. M.; Kaiser, E. T. J . Am. Chem. SOC. 1875, 97, 1209-1 2 18. (50) Takemoto, K.; Sonoda, N. In Inclusion Compounds; Atwocd, J. L., Davles, J. E. D., MacNicoi, D. D., Eds.; Academic Press: New York, 1984: VOi. 2. ChaDter 2. DD 47-67. (51) Terabe, S.; Ozakl H.; Oiiuka, K.; Ando, T. J . Chromatogr. 1885, 332, 211-217.
RECEIVED for review July 8,1988. Accepted October 31,1988. This work was supported in part by the National Science Foundation (CHE-8215508), a Susan Greenwall Foundation, Inc., Grant of the Research Corporation, and Wake Forest University. Z.S.F. acknowledges support in the form of a fellowship from the American Chinese Education Foundation of Indiana. This work was presented in part a t the 1982 Pittsburgh Conference, Atlantic City, NJ, March 1982 (Abstr. No. 265).
Chemically Modified Carbon Paste Electrode for Chronoamperometric Studies. Reduction of Oxygen by Tetrakis(p-2-anilinopyridinato)dirhodium( I I , I I I ) Chloride Chao-Liang Yao, Kwang Ha Park, and John L. Bear* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 We recently reported that dioxygen could be catalytically reduced by tetrakis(pl-2-anilinopyridinato)dirhodium(II) at an applied potential of -0.48 V in aprotic solvents (I). In order to extend these studies to aqueous solutions, a working electrode material was needed to disperse the water-insoluble dirhodium catalyst for electrochemical measurements. In this regard, carbon paste has several distinct advantages. Carbon paste electrodes containing various electroactive compounds have been extensively characterized in the literature (2,3). Their use in evaluating electrochemical processes ( 4 , 5 ) and, more specifically, the electrocatalytic mechanism (5-7) has been frequently reported. The most common preparation of paste electrodes is thoroughly mixing graphite powder and Nujol in a 5 g to 3 mL ratio followed by blending the desired weight of electroactive reactants. The paste is then packed into an electrode assembly consisting of two concentric lengths of glass tubing in a pistonlike configuration (8). All carbon paste electrodes described in the literature have been designed for voltammetric studies and therefore have a relatively small electrode surface area. To our knowledge, no studies on the electrochemical behavior of large surface area paste electrodes have been reported. In order to conveniently analyze the bulk solution components generated by the electrode reaction, and at the same time investigate the mechanism of O2reduction by the dirhodium catalyst, a paste electrode with a large surface area is desirable. This is difficult to achieve by simply enlarging the cross section diameter of the electrode due to the weak mechanical strength of the pasting material. Even though electrodes with a large surface 0003-2700/89/0361-0279$01.50/0
area may be prepared by hand-coating paste material directly on a graphite plate of the desired surface area, this type of design is not convenient for chronoamperometric studies because of the work required to prepare a fresh surface and poor reproducibility of the electrode behavior. In this study, we report the design and characterization of a chemically modified carbon paste electrode with a large geometric surface area that is easy to construct and convient to use for chronoamperometric studies. The electrode is used to investigate the catalytic reduction of O2in aqueous solution by the electroactive dirhodium complex and allows the easy analysis of the generated H20z in aqueous solution.
EXPERIMENTAL SECTION Electrode Design. The design of the carbon paste electrode for controlled potential electrolysis is shown in Figure 1. The electrode consists of a 60-mL disposable syringe cylinder (Plastipak)with the needle end cut off leaving a 3 mm wide edge. A Teflon disk of 0.5-cm thickness that contained 44 holes of 0.235-cm diameter is then sealed to the open end of the syringe with epoxy glue. The electrode has a calculated geometric area of 1.91 cm2. Close attention should be given to the size of the holes and the thickness of the Teflon disk in order to maintain the machanical strength of the electrode surface and allow easy extrusion of fresh carbon paste. The paste is then packed into the syringe cylinder and extruded through the honeycomb holes. The carbon paste was prepared by thoroughly mixing 5 g of graphite powder (UPS grade, Ultra Carbon, Inc.) and 3 mL of Nujol oil (Aldrich). A fresh surface can be obtained quickly by extruding fresh carbon past followed by polishing on wax paper. The carbon paste electrode without electroactive reactanta is used 0 1989 American Chemical Society
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:I
- 0.4 .
- 0.2 . Flgure 1. Illustration of the design of the carbon paste electrode with honeycomb conflguratlon: (a)the working electrode connection; (b) syringe plug; (c)the plastic syringe cylinder; (d) the carbon paste; (e) the Teflon disk.
t
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as a blank to determine the background current. The working electrodes are prepared with the desired weight ratio of electroactive reactants (2.5% (w/w) for Rh2(ap),C1and 0.5% (w/w) for ferrocene) in identical separate electrode assemblies. The electric connection was made through a fine copper wire through the rear end of the syringe plug. The electrode for conventional voltammetric measurements consisted of a disposable 1-mL syringe (Plastipak,without needle) packed with the carbon paste. The measured geometric surface area of the electrode was 0.025 cm2. Instrumentation and Reagents. The cyclic voltammograms were recorded on an IBM 225 voltammetric analyzer with a three-electrode configuration. A saturated calomel electrode was used as potential reference. The controlled potential electrolysis was performed on a BAS Model SP-2 synthetic potentiostat. Stock buffer solutions (American Scientific) containing 0.05 M phosphate, with pH adjusted to 7 with sodium hydroxide, were used for all electrode characterization experiments. The Ar gas (Airco) used was 4.8 grade with less than 3 ppm 02,while oxygen (Airco) was ultrahigh purity grade. Tetrakis(~-2-anilinopyridinato)dirhodiumcomplexes were prepared by a previously reported procedure (9). The complex, Fth2(ap),C1, used in this study consists of a dirhodium unit bridged by four 2-anilinopyridinate ions with four anilino nitrogens bound to one rhodium ion and four equatorial pyridyl nitrogens and one axial chloride bound to a second rhodium ion.
RESULTS AND DISCUSSIONS Characterization of the Carbon Paste Electrode. As shown in Figure 1,the carbon paste mixture is extruded from each of the honeycomb holes in the disk attached to the end of the syringe cylinder. This arrangement has the advantage of achieving a large and rapidly renewable electrode surface area with increased mechanical strength. Under our experimental conditions, which involved rapid stirring of the solution with a magnetic stirring bar 0.5 cm away from the electrode surface for 4-6 h, the electrode does not show any detectable damage. An estimate of the effective area of the carbon paste electrode is obtained from the Cottrell equation (10) using the known diffusion coefficient of ferrocene in Nujol. Murray et al. (3)have suggested using the result from Eyring rate theory ( I I ) ,Dlql = D2q2,to estimate diffusion coefficients. In this equation, D1 and D2 are the diffusion coefficients of the electroactive compound in the first and second solvent media,
-I 0.2
, 0.4
I
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(time, s P 2 (a)Current-time curve of ferrocene doped in carbon paste electrode (0.5% (w/w)) by double-potentiil-steptechniques at an initiil potential 4-0.50V (curve a) and final potentlal -0.30 V (curve a'). Curves b and b' show the background current on a blank carbon paste electrode under the same conditions. The electrode area is 1.38 cm2. (b) Plot of current vs time-"' for the curve a data. Figure 2.
while and q2 are the viscosities of two solvents. By use of the diffusion coefficients of ferrocene in nitromethane, acetonitrile, water, and ethylene glycol (I2-14), an estimated diffusion coefficient of (5.1 f 0.7) X lo4 cm2/s was obtained for ferrocene in Nujol oil. The estimated effective area of the electrode was calculated from data obtained in potential step experiments using the Cottrell plot and the known concentration of ferrocene dissolved in Nujol (4.5X 10" mol/cm3). The effective area was found to be 1.2 times larger than geometric area. A similar result of 1.15 has been reported for the graphite/Nujol system (I@, while a value of 1.6 was found for graphite/ 1-nonanol electrode packing (3). Oxidation of Ferrocene on the Carbon Paste Electrode. A reversible oxidation of ferrocene (0.5% (w/w)) on a carbon paste electrode has been reported to occur at Ellz = +0.22 V vs SCE (8). A similar redox wave at E l j z = +0.23 V was observed in pH 7 buffer solution by using our voltammetric carbon paste electrode. Since the ferrocene couple has been well characterized, it was incorporated into the carbon paste and used to test the characteristics of the large surface area electrode. Figure 2 shows the current-time curve of ferrocene (0.5% (w/w)) on the honeycomb carbon paste electrode in a pH 7 buffer solution. The initial potential setting was a t +0.50 V (curve a, Figure 2a). The dotted line (curve b, Figure 2a) indicates the charge current at a blank carbon paste electrode at the same potential. The net anodic current for ferrocene oxidation is obtained by subtracting the background current from the total current. A plot of net current vs time-1/2gives a nearly straight line, which extrapolates close to the origin. This behavior is expected from the Cottrell equation (IO)for a linear, semiinfinite diffusion-controlled process for the rate of ferrocene oxidization. However, there is no net cathodic
ANALYTICAL CHEMISTRY, VOL. 61,
current observed corresponding to the reduction of ferricinium ion when the potential is stepped from +0.50 to -0.30 V (curve a’, Figure 2a). This irreversibility has not been reported for the studies preformed on the smaller voltammetric electrode. The observed residual current is caused by solution impurities, not by the ferricinium ion as shown by the superimposed cathodic currents observed at a blank electrode and at a ferrocene-doped electrode (curves a’ and b’, Figure 2a). The ferricinium ion produced at the electrode is either decomposing or removed from the electrode surface by dissolving into the aqueous solution. The latter case is more likely since the controlled potential electrolysis of ferrocene suspended in a pH 7 buffer solution at +0.50 V using a platinum electrode results in the disappearance of yellow ferrocene crystals (solubility of ferrocene in water = 5 X M) (14) and the appearance of a blue solution. The cyclic voltammograms measured at a carbon paste electrode are usually dependent on the previous conditions of the electrode and the reproducibility is achieved by mildly polishing the electrode surface before each potential scan. This type of histeresis is not observed when using the large surface area electrode for ferrocene oxidation. The currenttime curves of the oxidation are very constant on continuous double-potential-step cycles without polishing the electrode (repeated up to five times in our experiments). This is probably due to the lack of surface enrichment of the ferricinium ion resulting from its high solubility in water. Reduction of Dirhodium Complex on the Carbon Electrode. The dirhodium(I1,II) complex, Rhz(ap),, which can be generated from electroreduction of Rh,(ap),Cl or RhZ(ap),C1O4,has been shown to react with molecular oxygen to form a superoxide complex, RhZ(ap),(Oz),in aprotic solvents (I). This complex undergoes a one-electron reduction at -0.48 V to form [RhZ(ap),(Oz)]-, which will further react with CH2Clz or other superoxide scavengers to re-form the original dirhodium(I1,II) compound. In aqueous solution the reaction of [Rh2(ap),(O2)]-with H+ should facilitate superoxide dissociation in the form of HOz which disproportionates to farm HzOz and Oz. The cyclic voltammogram of Rhz(ap),C1on a carbon paste electrode in pH 7 buffer solution shows an irreversible reduction wave at E , = -0.4 V under Ar. When 1atm of Ozis bubbled through the solution, the intensity of the peak current is increased by a factor of 9. The direct reduction of Ozon a pure carbon paste electrode is not seen under the same condition up to -1.0 V, clearly indicating the electrocatalytical reduction of dioxygen by the dirhodium complex. The current-time curve of the controlled potential electrolysis of Rhz(ap),C1doped on a carbon paste electrode with a surface area 1.91 cm2 in a pH 7 buffer solution is shown in Figure 3. A small reduction current is observed under Ar (curve b, Figure 3). The net electrolysis current was obtained by subtracting the background current obtained on a blank carbon electrode (curve a, Figure 3) using the point-by-point subtraction method. A plot of net current vs time-’12 shows a straight line which has an intercept on the current axis (Figure 4a). The cause of the nonzero intercept is not fully understood at the present time. When the potential is switched from -0.40 to +0.30 V, the current due to oxidation of the reduced form of Rh2(ap),C1 at the electrode surface rapidly decreases and approaches zero (curve b’, Figure 3). The plot of current vs time-’12 again gives a nearly straight line which extrapolates close to the origin (Figure 4a). The oxidized and reduced forms of the dirhodium complex are known to be insoluble in HzO and the bulk solution does not contain the reduced Rhz(ap),C1. Therefore, this type of near Cottrell equation behavior is not due to a linear, semiinfinite diffusion process but may be caused by
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FEBRUARY 1, 1989
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200.
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.O
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0 10 20 30 time (sac)
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6.0
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T
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I0
Figure 3. Current-time curves of Rh,(ap),CI on the carbon paste electrode (2.5 % (w/w)) by double-potentiaktep techniques at an initial potential -0.40 V (curve b, Ar; curve c, 0,) and a final potential 0.30 V (curve b‘, Ar; curve c’, 0,). Curves a and a‘ are the background currents on a blank carbon paste electrode. The insert shows the charge-time curve for curves c and c’.
/
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(b)
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A
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rm,S)-l’2 Figure 4. Plot of current vs time-’” from data shown in Figure 3: (a) cathodic current (A)and anodic current (0)obtained under Ar; (b) cathodic current obtained under 0,. finite diffusion on a thin film where near Cottrell equation behavior can be approximated over a short time period (16). The current-time curve obtained in the presence of O2 at a reduction potential of -0.4 V is shown in Figure 3 (curve
282
Anal. Chem. 1989, 6 1 , 282-285
c). The plot of current vs time-'I2 gives a straight line that levels off a t a current intensity of 145 pA (Figure 4b). The steady-state current is largely due to the catalytic reduction of O2 by the dirhodium complex since the direct reduction of O2 on a blank electrode under the same conditions shows a residual current of only 20 FA. As the electrolysis potential is stepped to +0.3 V, no anodic current is obtained (curve c', Figure 3). This behavior indicates the irreversible nature of the catalytic reduction of 02.The insert of Figure 3 illustrates a charge vs time curve for reduction and reoxidation of Rh2(ap),C1 under the same conditions and the curve shows only a flat line when the potential is stepped from -0.4 to +0.3 V (the charge due to the oxidation of the reduced Rh2(ap),C1 is negligible). The increased electrolysis current using the large electrode also allows the easy and accurate analysis of Hz02produced by the electrode reaction. The enrichment of H202in the buffer solution determined by an iodometric method ( I 7) reveals a solution concentration of 3.5 x mol/L after electrolysis for 4 h which gives a 20% current efficiency. The low current efficiency is probably due to decomposition of much of the generated Hz02over the 4-h period. The current-time curves of the electrode containing the Rh2(ap),C1 complex show a steady increase after each double-potential-step scan under Ar or OF This increase is clearly due to the surface enrichment of the dirhodium complex after each cycle which results from the insolubility of both the oxidized and reduced form of the dirhodium compound in water. In summary, this study shows that the mechanical strength of a large surface area carbon paste electrode can be improved through the described honeycomb design. The electrode can
be easily used for bulk solution electrolysis and allows the convenient analysis of the soluble components generated from the electrode reaction. This type of electrode can also be used to evaluate the chronoamperometric, chronocoulometric, and chronopotentiometric behavior of electroactive substances doped in carbon paste electrodes and be a valuable tool for studying the kinetics and efficiency of electrocatalysis.
LITERATURE CITED Bear, J. L.; Yao, C.4.: Cacdevielle, F. C.: Kadish. K. M. Inom. Chem. 1988, 27, 3782. Korfhage, K. M.; Ravichandran, K.; Baldwin, R. P. Anal. Chem. 1984, 56, 15-14. Takeuchi, E. S.; Murray, R. W. J. Electroanal. Chem. 1985. 768, 49. Castro, M. D. L.; Valcarcel, M.; Albahadily, F. N.; Mottola, H. A. J . Electroanal. Chem. 1987, 219, 139. Halbert, M. K.; Baldwin. R. P. Anal. Chem. 1985, 57, 591. Geno, P. W.; Ravichandran, K.; Baldwin, R. P. J. Electroanal. Chem. 1985, 783, 155. Kamin, R. A.; Wilson. G. S. Anal. Chem. 1980, 52, 1198. Ravichandran, K.: Baldwin, R. P. J . Elecfroenal. Chem. 1981, 726, 293. Bear, J. L.; Liu, L.-M.: Kedish, K. M. I n w . Chem. 1987, 26, 2927. Bard, A. J.; Faulker, L. R. ElectrochemicalMethods;Wiley: New York, 1980; p 143. Tyrrell, H.J. V. Dlffusbn and Heat Flow In Liquids; Butterworths: London, 1961; p 138. Lacaze, P. C.: Minh, C. P.; Delarnar, M.; Dubois, J. E. J. Elecfroanal. Cham. W80. ..., 708. .- 9. Chin, J. W.; Georges, J. J. Electrottnal. Chem. 1986, 270, 205. Georges, J.; Desrneftre, S. Electrochlm. Acta 1984, 2 9 , 521. Ravichandran, K.; Baldwin. R. P. Anal. Chem. 1983, 55, 1586. Murray, R. W. I n phvsicel Methods of Chemlstry; Rossiter, B. W., Hamllton, J. F., Eds.; Wiley: New York, 1986; Vol 12, p 557. Vogel, A. I. A Text-Book of Quanflfaflve Inorganic Analysls; Wiley: New York 1961; Vol 59, p 363.
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RECEIVED for review July 6,1988. Accepted October 31,1988. The authors thank the Robert A. Welch Foundation (Grant No. E-918) for financial support of this work.
Molecular Fluorescence Measurements with a Charge-Coupled Device Detector Patrick M. Epperson, Rafi D. Jalkian, and M. Bonner Denton* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 The sensitivity of a fluorescence measurement depends upon a number of factors including source intensity and stability, excitation and emission collection efficiency, background fluorescence from solvent and cuvettes, and the detector sensitivity. In the last decade, the majority of work aimed at increasing sensitivity focused on the development of more intense excitation sources. The most significant results were achieved by the use of laser excitation sources (1-3). Whereas impressive limits of detections were achieved by such systems, the cost and complexity of such sources make them impractical for use in most routine analytical instrumentation. Additionally, the use of powerful excitation sources can result in sample heating and photodecomposition. An alternate method of increasing the sensitivity of a fluorometer involves the more efficient detection of the available fluorescence emission from the sample by increasing the detector efficiency. Multichannel array detectors are actively being investigated as a means of improving upon the sensitivity of the single-channel photomultiplier tube (PMT). One- and two-dimensional array detectors such as vidicons, photodiode arrays, and intensified photodiode arrays are being investigated for use in many areas of atomic and molecular spectroscopy (4-8). These detectors have a demonstrated multichannel advantage over PMT-based instrumentation resulting in improved signal-to-noise (S/N) ratios in situations 0003-2700/89/036 1-0282$01.50/0
requiring the measurement of a complete spectrum in a short period of time (9-12). However, these detectors suffer from relative high detector noise and dark current levels that limit their sensitivities when measuring low light level signals. Currently, the P M T is still the detector of choice when measuring extremely low photon fluxes of less than a few thousant photons at a single fixed wavelength for quantitative fluorescence spectroscopy. Another type of multichannel array detector, the chargecoupled device (CCD), shares the same multichannel advantages as the above detectors but has a much lower detector noise level and dark current (13-15). The low detector noise and the ability to integrate photogenerated charge for several minutes make the CCD as sensitive as a photon-counting PMT for measuring extremely low light levels. Additionally, the CCDs ability to combine photogenerated charge packets from adjacent detector elements via a process known as binning allows the performance of the detector to be tailored to the needs of the spectroscopic measurement (14,16). One- and two-dimensional CCDs were evaluated for use in UV-vis absorption, atomic emission, and Raman spectroscopy; however, to date CCDs have not been evaluated as detectors for quantitative molecular fluorescence. Given both the multichannel advantages of the CCD together with the ability to detect low photon fluxes that is equal or superior to PMTs, 0 1989 American Chemical Society
