Anal. Chem. 1991, 63, 2665-2668
limits which were lower than those obtained with the TNMS by a factor of 2-5. When cryogenic desolvation was used in tandem with the USN, detection limits were degraded by a factor of 2-3, except for Ce for which the detection limit was improved only by a factor of 2. The lowest ion kinetic energies (5.0-7.7 eV) and oxide levels were obtained when USNcryogenic desolvation or the TNMS was used.
ACKNOWLEDGMENT Special thanks to L. West, J. Ott, and R. Spangler of RF Plasma Products, Inc., M. L. Vestal, J. Dixon, C. R. Blakley, and J. Wilkies of VESTEC Corp., and W. B. Sisson, J. A. Easterling, and W.F. Syner of the Food and Drug Administration for significant contributions during the course of this work. We thank R. H. Clifford, S. P. Dolan of our group, R. S. Houk of Iowa State University, and S. Chan of CETAC Technologies, Inc. for their helpful suggestions during the course of this research and D. W. Golightly of Ross Laboratory for his constructive comments in the preparation of this manuscript. LITERATURE CITED Oustavsson, A. I n Inductively -bad plssmas h Analytfcel Atomic Spectrometry: Montaser. A., Oollghtly, D. W.. Eds.; VCH New York, 1987. Chapter 11 (see also references cited therein). Broekaert. J. A. C.; Boumans, P. W. J. M. I n Inductvely Coupled &Sma ~ ~ Spectroscopy; b n Boumans, P. W. J. M., Ed.; John WC ley 8 Sons: New York, 1987; Voi. I,Chapter 6 (see also references cited therein). A p p h l k n of I n d u c w coupled plssma Mess Spectrometry;Date, A. R., (Lay, A. L., Eds.; Blackie and Son Ltd.: London, 1989 254 pp. %en, W. L.: Caruso, J. A.; Fricke, F. L.; Satzger, R. D. J. Anal. At. Spectrom. 1990, 5 , 451-455 (see also references cited therein). W l n , D. R.; Smith, F. G.; Houk, R. S. Anal. Chem. 1991, 63. 219-225 (see ais0 references cited therein). Mderen, J. W.; Lam, J. W.; Qustavsson, A. SpectrocMm. Acta 1990, 458, 1091-1094 (see also references cited therein). RoyChowdhwy, S. 6.; Koropchak, J. A. Anal. Chem. 1990, 62, 484-489. Koropchak, J. A.; Aryamanya-Mugisha, H.; Winn, D. H. J. Anal. At. Spectrom . 1988.3, 799-802. Meyer. Q. A.; Roeck. J. S.:Vestal, M. L. ICP Inf. News/. 1985, 10, 955-963. Qustavsson. A.; Hletala, P. Spectrochlm. Acta 1990, 4 5 8 , 1103-1108. Backstrom, K.; Oustavsson, A.; Hietala, P. Spectrochim. Acta 1989, 448, 1041-1048. QustavsSon, A. Specfrochlm. Acta 1988, 438, 917-922. Mderen, J. W.; Lam, J. W.; Gustavsson. A. Spectrochfm. Acta 1990, 458, 1091-1094. Zhu, G.; Browner, R. F. J. Anal. At. Specfrom. 1988B3 , 781-789. Hutton, R. C.; Eetm. A. N. J. A&. A t . Specbom. 1987,2.595-598. Tsukahara, R.; Kubota, M. Spectrochlm. Acta 1990, 458, 581-589. Montaser, A.: Ishii. I.; Clifford, R. H.; Sinex. S.A.; Capar, S . 0. Anal. them. 1988, 67, 2589-2592. Wlederin, D. R.; Houk. R. S.;Winge. R. K.; DSihra, A. P. Anal. Chem. 1990. 62, 1155-1160. Akin, P.; Mawas, Y.; Douge. C.; Jaunauit, L.; Delaporte, T.; Beaugrand, C. Analyst 1990, 175, 813-815. Horlkk, G.: Tan, S.H.; Vaughan. M. A.; Shao, Y. I n Inductively CouM ~ s m a In s Anaslytfcel Atomic Spectrometry; Montaser, A., GoIlghtly. D. W., Eds., VCH New York, 1987; Chapter 10 (see also references clted therein).
2885
(21) Blakley, C. R.; Vestal, M. L. Anal. Chem. 1983, 5 5 , 750-754. (22) Vestal, M. L. Science 1984, 226, 275-281. (23) Schwartz, S. A,; Meyer, 0. A. Spectrochim. Acta 1988, 418, 1287-1298. (24) Peng. R.; Tiggelman, J. J.; de Loos-Voilebregt, M. T. C. Spectrch". Acta 1990. 458, 189-199. (25) Margaretha, T. C. D.; Johan, J, T.; Pim, C. 6.; Christian, D. J. Anal. At. Spectrom. 1989, 4 , 213-217. (26) Barnes, R. M.; Wang. X. J. Anal. At. Spectrom. 1888, 3 , 1083-1089. (27) Wang, X.; Barnes, R. M. J. Anal. At. Specfrom. 1988, 3 , 1091-1095. (28) Fulford, J. E.; Douglas, D. J. APPI. SpeCtr~sc.1986, 40. 971-974. (29) Vickers, G. H.; Wilson, D. A.; tiieftje, G. M. J . Anal. At. Spectrom. 1889, 4 , 749-754. (30) Date, A. R., Gray, A. L., Eds. The Application of Indudvely Coupbad p l s m Mess Specfrmfw; Chapman 8 Hall: London, 1988; 224 pp. (31) Takahashi, J.; Hera. R. Anal. Sci. 1988, 4 , 331-333. (32) Koropchak, A.; Winn, D. H. Appl. Spectrosc. 1987, 4 1 , 1311-1318. (33) Elgersma, J. W.; Maessen, F. J. M. J.; Nkssen, W. M. A. Spectrochin?. Acta 1988, 478, 1217-1220. (34) Browner, R. F.; Tarr, M. A.; Nwogu. V.; Ruiz, A.; Zhu, 0. Sample Introduction Systems in Plasma Spectrometty: The Current Status and the Future Directions. Presented at the 1990 FACSS Meeting, Cleve land, OH. (35) Boumans. P. W. J. M.; De Boer, F. J. Spectrochim. Acta 1976. 318. 355-375. (36) Bradshew, N.; Hall, E. F. H.; Sanderson, N. E. J. Anal. At. Specbwn. 1989. 4 , 801-803. (37) Huber, K. P.; Hetizberg, G. Constants of Diatomic Molecules; Van Nostand Reinhold Co.: New Ywk, 1979. (38) Ackermann, R. J.; Raut, E. 0.;Thom, R. J. J . Chem. fhp. 1988, 65, 1027-1031. (39) Lam, J. W.; Mclaren, J. W. J . Anal. At. Spectrom. 1990, 5 , 419-424. (40) Rauh, E. 0.;Ackermann, R. J. J. Chem. mp.1974, 60, 1396-1400. (41) Tsukahara, R.; Kubota. M. Specfrmhim. Acta 1800, 458, 779-787. (42) Doughs, D. J.; French, J. 6. SpeclrocMm. Acta 1986, 418. 197-204. (43) Gray, A. L.; Houk, R. S.;Wiiliims. J. G. J. Anal. At. S p e c m . 1987, 2, 13-20. (44) Jakubowski, N.; Raeymaekers, B. J.; Broekaert, J. A. C; Stuewer. D. Spectrochim. Acta 1989, 448, 219-228. (45) Misekl, K. U S . Pat. No. 4 804 838, Feb 14, 1989. (46) Lim, H. 6.; Houk, R. S. Spectrmhim. Acta 1980, 1 5 8 , 453-461. (47) Grain, J. S.; Smith, F. G.; Houk, R. S. Spectrochim. Acta 1890, 458, 249-259. (48) Kim, Y.; Kawaguchi, H.; Tanaka, T.: Miiuike, A. S p e c t " . Acta IWO, 458, 333-339. (49) Lam, J. W. H.; Horlbk, G. Spectrochim. Acta 1990, 458, 1313-1325. (50) Evans, E. H.; EWon, L. J. Anal. At. Spectrom. 1989, 4 , 299-300. (51) Evans, E. H.; Ebdon, L. J. A M I . At. Spectrom. 1990, 5 . 425-430. (52) Beruchemin, D.; Craig, J. M. R o ~ e e a Y ~ofs Lhe 3rd S w y Conference on Plasma Source Mess Spectrometry; Royal Society of Chemistry: London, 1990; No. 85, pp 25-42. (53) Beauchemin, D.; Craig, J. M. Spectrochim. Acta 1981, 468. 603-6 14.
RECEIVED for review June 7,1991. Accepted August 16,1991. This work was sponsored by the U.S. Department of Energy under Grant No. DEFG05-87-13659 and by the U.S. Geological Survey (through D.C. Water Resources Research Center) under Project No. 14-08-0001-G1554-03. The ultrasonic nebulizer and the ICP system used in this work were gifts from CETAC Technologies, Inc. and RF Plasma Products, Inc., respectively. The thermospray nebulizer was loaned by VESTEC Corp.
Fabrication and Evaluation of a Shielded Ultramicroelectrode for Submicrosecond Electroanalytical Chemistry Satoshi Nomura, Koichi Nozaki,' and Satoshi Okazaki* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606-01, Japan INTRODUCTION The development of electrochemical measurements with micro or submicrosecond time resolution is one of the most *Towhom correspondence should be sent.
Present address: Department of Chemistry, College of General Education, Osaka University, Osaka 560,Japan. 0003-2700/91/0363-2665$02.50/0
interesting topics in recent electroanalytical chemistry. These novel measurements, including fast scan cyclic voltammetry (FSCV) (1-4) and chronoamperometry (1-3) have been made possible by the special characteristics of ultramicroelectrodes (UME) (5). However, in transient techniques, observed Current signals are more sensitive to poor UME construction than in steady-state measurements (3, 6). In addition, 0 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 22, NOVEMBER 15, 1991
a.
b
a.
I"
b.
A
B
C-
C
d.
Flguro 1. Diagram of a shielded microelectrcde (SME) assembly (a) and the dlmensions (In mm) (b): (A) epoxy resin; (6)copper foil; (C) capillary tube (1.d. 0.85 mm, 0.d. 1.65 mm); (D) alumlnum foil: (E) microwke; (F) weld point; ((3) soft-glass tube (1.d. 2 mm. 0.d. 4 mm); (H) Au lead wire; (I) terminal for shielding; (J) terminal for working.
Wightman e t al. recently pointed out that the stray capacitance formed between the electrode material and the solution increases the background current in FSCV (7). Although various methods for fabricating UMEs have been reported (8-IO),little attention has been paid to the fabrication of UMES which meet the special requirements of these transient techniques. Difficulties in UME fabrication are among the most serious obstacles facing chemists seeking to make these transient techniques practical. Therefore, in a previous study, we developed a fabrication method for a UME which can be used for FSCV or chronoamperometry over a microsecond time scale (I). We now report further improvement of this UME. The fabrication procedure was improved to increase the reliability and durability of the UME. To remove the stray capacitance, the internal conductor was completely shielded and a shielded ultramicroelectrode (SME) was fabricated. With this SME stray capacitance was found to be less than 0.1 pF and significant practical advantages were obtained in FSCV and chronoamperometry .
EXPERIMENTAL SECTION System Configuration of FSCV. The details of the construction of the fast response potentiostat and the current transducer have been previously described (1). Electrode Materials and Chemicals. Platinum microwire (10 fim in diameter) and Wollaston wire (5 and 2 pm in diameter) (99.99%,purchased from Japan Lamp Industry Co., Ltd.) were used as electrode materials. The chemicals were prepared in the same way as previously described (I). Electrochemical Cell. A conventional 5-pm-diameter UME was fabricated by sealing the microwire using the electric furnace described below. Electrical contact between the microwire and lead wire was made by a mercury drop. The surface of the electrode was polished using 0.3- and then 0.05-pm alumina on suede before each measurement. A double-junctioned assembly of a Pt/(13-, I-) reference electrode was employed (11). Platinum wire, 0.5mm in diameter, was used 09 an auxiliary electrode. The temperature of the electrochemical cell was kept at 25 f 0.1 "C. Design and Fabrication of the SME. Figure 1 shows the configuration of the SME. Platinum microwire or Wollaston wire 2 mm in length was directly connected to an Au lead wire (50pm in diameter, 6 cm in length) by spot welding using a spark from a pair of graphite rods (8 mm in diameter) and a 2-5-V ac. A
C.
e.
I
CUI
-
Figure 2. Fabrication procedure for the SME (A) Au lead wire; (e) weld point; (C) pt microwire; (D) Wollaston wire (silver coatlng was removed later; see text);(E) soft-glass tube; (F) capillary tube (1.d. 0.85 mm, 0.d. 1.65 mm); (G) aluminum foil; (H) soft-glasstube; (I) copper
foil.
soft-glass tube (i.d. 2 mm., 0.d. 4 mm) was drawn to give a fine pipet, as illustrated in Figure 2c. The Pt microwire connected to the Au lead wire was inserted into the pipet so that the microwire was positioned at the capillary part (i.d. 0.3-0.4 mm)and was then rinsed with 2 mL of acetone. The Wollaston wire was inserted into the pipet with its tip 2 mm outside of the capillary. The silver coating on the tip (0.5-1mm) was removed by dipping the tip into 50% nitric acid solution for 15 min. After etching the coated silver, the lead wire was pulled up so that the microwire was positioned at the capillary part. The microwire was rinsed with 2 mL of distilled water, dilute ammonia, distilled water, and acetone,in that order. After the tip of the capillary part was sealed using a methane flame, the capillary part was inserted into an electric furnace, which was fabricated by winding a Nichrome coil around a ceramic tube (i.d. 8 mm). The microwire was sealed into the capillary by gradually increasing the temperature of the furnace (20 OC/min) while the inside of the pipet remained under vacuum. The temperature was kept at optimum (670 f 10 "C) for 20 min and then lowered to 450 10 O C for annealing. After 10 min of annealing, the capillary was cooled down to room temperature. The sealed capillary was cut 5 mm above the sealed part (Figure 2d). The Au lead wire was inserted into another capillary tube (i.d. 0.85 mm, 0.d. 1.65 mm) so that the exposed Au lead wire was covered by a new capillary (Figure 2e). Then, the two capillaries were tightly wrapped with aluminum foil, keeping the tip of the microwire exposed. The whole assembly was inserted into another pipet of the same size (Figure 2e), and then, the unwrapped part of the inner capillary was again sealed using the electric furnace. Finally, the Au lead wire was soldered to a gold-plated terminal. The exposed aluminum foil was folded out and wrapped with copper foil (Figure 20. The terminal and the copper foil were fued using epoxy resin. Another gold-plated terminal was soldered to the copper foil. The microwire was exposed from the electrode surface using rough polishing paper. Scanning electron microscopy (SEM) was used to confirm that the microwire was completely sealed into the glass capillary. The electrode radius ( r , pm) was obtained from SEM.
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ANALYTICAL CHEMISTRY, VOL. 63,NO. 22, NOVEMBER 15, 1991
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Table I, Comparison between Apparent Double-Layer Capacitances (ChDD) and Calculated Double-Layer Capacitances (Cd1.o~) of the Conventional UME and Various Sizes of SMEs
I
electrode type
conventional UME 2.90
20nA
SME unshieldede shieldedf SME unshielded unshielded shielded shielded SME unshielded shielded
5.25 f 0.1
Cd~.pp,pF'
C ~ ~ , P F
6.15 f 0.126 11.8 f 0 . 9
2.6
10.0f 0.2' 8.35 f 0.2'
8.6
2.90 f 0.1
4.75 f 0.1' 5.25 f O.ld 3.05 f 0.16' 2.95 f 0.15d
2.6
1.23 f 0.1
2.54 f 0.05' 0.42 f 0.03'
0.47
a The radius as measured by SEM. * Values obtained from three independent measurements. 'The tip of the electrode was shallowly (ca. l mm) immersed into the solution. dThe tip of the electrode was deeply (ca. 10 mm) immersed into the solution. e Without keeping the internal aluminum foil of the S M E at the ground potential. f With the internal aluminum foil of the SME keot at the mound Dotentid.
0.5
0
r,pma
E/V vs l;,lFigure 3. Fast scan cyclic voltammograms of 1 mM ferrocene In AN wlth 0.0 M lEAPF( at 10 kV/s after backgwnd subtraction (solkl Ilne) and calculeted-ms after the ohmic drop compensatkn (0): (a) SME 5 pm and (b) 2 pm In diameter.
a.
I
RESULTS AND DISCUSSION Parts a and b of Figure 3 show voltammograms of 1 mM ferrocene in acetonitrile (AN) containing 0.6 M tetraethylammonium hexafluorophoephate (TEAPFB)obtained at a scan rate of 10 kV/s with a 5- and 2-pm-diameter SME, respectively. Simulated voltammograms based on reversible electron transfer are also shown. In the case of the simulated voltammograms, ohmic potential drop was compensated by iterative calculations as described previously (1). There was excellent agreement between the observed and simulated voltammograms, which indicates that the SME were well constructed. This also confirms that there was no overcompensation of the ohmic potential drop, which indicates that there was no large stray capacitance (7). The apparent double-layer capacitance ( C w , pF) and the calculated double-layer capacitance (Ca.cal, pF) of the conventional UME and the SME were compared to confirm that shielding reduced the stray capacitance. Cdl.app values were obtained from the residual current (ic, A) observed at a scan rate of 10 kV/s in AN containing 0.1 M TEAPF6 according to eq 1 where u is the scan rate (V/s). Ca.d values were
4 = UCdl.app
(1)
obtained from rand the doublelayer capacitance per unit area in this media (9.9 pF/cm2). This value was calculated from the C h w of a O.bmm-diameter Pt electrode and was in good agreement with the typical value in AN containing 0.1 M electrolyte (10 pF/cm*) (7,12). The results are listed in Table
I. Positive deviations of C , J . ~from ~ ~ Cdl.4 and differences in Ca.appdepending on the portion of the electrode immersed into the solution observed with the conventional UME were greatly decreased with the SME. Furthermore with the SME, there was excellent agreement between Ca.appand Cdl.d, regardless of the depth to which the electrode was immersed when the internal shielding was used, i.e., keeping the internal aluminum foil on the SME at the ground potential. This shows that the internal shielding removed the stray capacitance of the UME. In FSCV, background current is significantly larger than in conventional CV. In such a condition, the reduction in
b.
I
I
c.
,
I
r/
L-.-.
0
2
-____ 4
6
8
Time / ps Flguro 4. Fast response chronoamperograms recorded from experiments In a high+esistance AN solution containing 0.01 M TEAPF, with various electrodes: (a) SME, 10 pm in diameter; (b) SME, 5 pm In diameter; (c) conventional UME, 5 pm in dlameter. background current due to a smaller Ca.app is very useful, because otherwise a large fraction of the dynamic range available for amplification is lost (7). A constant Caappregardless of the portion of the electrode immersed can give a reproducible background current value and is also useful in subtracting the background current. Figure 4 shows transient current responses in chronoamperometry measured with 5- and 10-pm-diameterSMEk (the internal shield was used) and with the coventional UME in which the electrode potential was stepped from -500 to -200 mV. In order to observe detailed profiles of the charging current, measurements were carried out in a high-resistance AN solution containing0.01 M TEAPFe As shown in Figure 4, the current response in chronoamperometry consisted of two components with different time constants (currents A and
B). Current A is the charging current for the stray capacitance of the UME. The apparent response of this current was delayed due to the band-pass limitation of the current transducer (0.2 ps) regardless of electrode size and design. However, the peak intensity of current A increases with the amount of stray capacitance of the electrode in such condition that the current response is delayed due to band-pass limitation. Therefore, the higher current A observed with the conventional UME can be explained by the presence of more stray capacitance. On the other hand, current B decayed exponentially with various time constants corresponding to the various electrode sizes, i.e., 2.8 and 1.5 ps at 10 and 5 pm in diameter, respectively. This can be attributed to the charging current for double-layercapacitance. Indeed, these time constants were in good agreements with the cell-time constants (7,s) predicted from eq 2, where p is the uncompensated specific resistance of the medium (650 Q cm), and R , (0)is the solution resistance. = RurCdl.cal = Cdl.dd4r (2) The presence of current A did not cause any delay in current B. However, as shown in Figure 4c,it is difficult to distinguish these components when the former is much larger than the latter, that is, when the stray capacitance of the UME is significantly larger than the double-layer capacitance. In such a case, current responses originating from stray capacitance can be mistakenly interpreted as electrochemicalresponses. Therefore, internal shielding of the UME is necessary for precise measurement of electrochemicalresponses including
faradaic responses in chronoamperometry. Another important property of the SME is its permanent, low-resistancejunction between the microwire and the Au lead wire, which is obtained by arc welding. This junction helps a great deal in increasing the reliability and durability of the SME and also in reducing the consumption of the expensive microwire to only a few millimeters. These additional feature of the SME can give practical benefits to many chemists who are interested in transient electrochemical techniques.
LITERATURE CITED (1) Nozaki, K.; Oyama, M.; Hatano, H.;Okazakl, S. J . Electroanel. Chem. Interfacial E k t ” . 1989, 270, 191-204. (2) Wipf, D. 0.; Wightman, R. M. Acc. Chem. Res. 1990. 23, 84-70. (3) Andrieux, C. P.; Haplot, P.; Saveant, J. M. Chem. Rev. 1990, 90, 723-738. (4) Andrleux, C. P.; Hapiot, P.; Saveant, J. M. Ektrwna&sh 1990. 2 , 183-1 93. (5) Roblnson, R. S.; McCreery, R. L. Anal. Chem. 1981, 52, 997-1001. (8) Wightman, R. M.; Wlpf, D. 0. In Ektroana3/tlcel Chemlsfry; Bard, A. J., Ed.; Marcel Dekker Inc.: New York, 1989; Vol. 15, pp 288-353. (7) Wipf, D. 0.; Michael, A. C.; Wightman, R. M. J . E l e c t r ~ e ~Chem. l. Interfacial Electrochem. 1989. 269. 15-25. ( 8 ) Bond. A. M.; Flelshmann, M.; Roblnson, J. J . Ektroenal. Chem. Interfacial Electrochem. 1984, 168, 299-312. (9) Flelshmann, M.; Pons, S.; Rollnson, D.; Schmidt, P. U k ” bodes; Detatech Systems: Morganton, NC, 1987. (10) b a r , C. D.; Stone. N. J.; Swelgatt, D. A. Anal. Chem. 1988, 60, 188-191. (11) Coetzee, J. F.; Gardner. C. W., Jr. Anal. Chem. 1982, 51, 2530-2532. (12) Howell, J. 0.;Wightman, R. M. Anal. Chem. 1984, 56. 524-529.
RECEIVED for review June 3,1991. Accepted August 23,1991.
Long Optical Path Thin-Layer Spectroeiectrochemistry in a Liquid Chromatographic Ultraviolet-Visible Absorbance Detector Cell Thomas R. Nagy and James L. Anderson* Department of Chemistry, School of Chemical Sciences, University of Georgia, Athens, Georgia 30602 Spectroelectrochemical flow cell (SEFC) detectors of small volume offer novel possibilities for enhanced analytical selectivity as well as characterization of reaction pathways in applications such as high-performance liquid chromatography (HPLC) or flow injection analysis (FIA). Although numerous stationary spectroelectrochemical cells have been reported (1-331, with both short (1-12) and long (13-33) optical paths, only several applications to flowing solutions have been reported, with short (3) or long (13, 14,21,25, 30-32) optical path lengths. Occasional reports have also appeared in which separate electrochemicaland spedroscopic detectors have been used in series in a single flow stream (34). Electrochemical flow detector design and applications have been reviewed (35,
36). T w o general classes of spectroelectrochemicalcells have the small volume which is needed for use in a flow stream. One class is based on optically transparent thin-layer electrode (OTTLE) cells, in which light passes along a short path perpendicular to an optically transparent electrode (OTE) consisting of a transparent conductor ( 1 , 4 ) or a meshed or porous conductor (7-9). Cells based on specular reflection with a short optical path would also fall in this category. The short optical path length of OTTLE cell limits optical sensitivity and detection limits. A second class of spectroelectrochemical cells is based on a long optical path thin-layer electrode (LOPTLE) with the optical beam parallel (or nearly so) to the surface of the electrode. Both planar (20-23,25, 27-33) and tubular (24) 0003-2700/91/0383-2888$02.50/0
geometries have been successfully applied, as has a multiple specular reflectance cell geometry (18). The LOPTLE design can have significantly higher optical sensitivity due to a much longer optical path than feasible in the OTTLE design. The OTTLE and LOPTLE cells reported to date have exhibited various problems of material compatibility, optical alignment, and optical signal/noise ratio. We report here a tubular flow-through spectroelectrochemical LOPTLE cell which overcomes many of these limitations by incorporation of a tubular working electrode into a conventional HPLC UV-visible absorbance detector cell. This approach exploits the optimized optical alignment and signal/noise ratio of modern HPLC detectors. We demonstrate simultaneous electrochemical and optical absorption detection of analytes by FIA and discuss factors governing signal/noise ratio and interferences due to interaction between detectors in a flow stream. We also describe application of the LOPTLE cell for stopped-flow spectropotentiostatic titrations.
EXPERIMENTAL SECTION The spectroelectrochemical cells were fabricated for these experiments by modifying a UV-visible HPLC detector cell of standard z-type configuration. The modified cell is shown in Figure 1. A tubular platinum working electrode (WE)(AESAR, Seabrook, NH, inner diameter 1.32 A 0.03 mm, outer diameter 1.57 0.03 mm, length 10.0 f 0.1 mm) was press-fit into the optical path of a Kel-F flow cell (Schoeffel, Westwood, NJ), which had been dried out to a diameter ca.0.03 mm undersize to ensure a tight leak-free fitting. Electrical contact to the WE waa made
*
@ 199 1 American Chemical Society
