Anal. Chem. 1986,58,647-650
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Dagnaii, R. M.; West, T. S.;Whitehead, P. Anal. Chim. Acta 1972, 60,25-35. Dagnall, R. M.; West, T. S.;Whitehead, P. Anal. Chem. 1972, 4 4 , 2074-2078. McLean, W. R.; Stanton, D. L.; Penketh, G. E. Analyst (London) 1973, 98, 432-442. Beenakker, C. I . M. Spectrochlm Acta, Part B 1977, 328, 173-187. VanDalen, J. P. J.; de Lezenne Couiander, P. A,; de Gaian, L. Anal. Chim. Acta 1977, 94, 1-19. Quirnby, B. D.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1978, 50, 2112-2118. Bonnekessel, J.; Klier, M. Anal. Chim. Acta 1978, 103,29-42. Brenner, K. S. J. Chromatogr. 1978, 167-365. Quirnby, B. D.; Deianey, M. F.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1979, 51,875-880. Windsor, D. L.; Denton, M. B. J . Chromatogr. Sci. 1979, 17, 492-496. Windsor, D. L.; Denton, M. B. Anal. Chem 1979, 5 1 , 1116-1 119. Pipes, D. T.; Baughrnan, K. W.; Goode, S. R. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, 1980; No. 155. Carnahan, J. W.; Muiiigan, K. J.; Caruso, J. A. Anal. Chlm. Acta 1981, 130, 227-241. Estes. S. A,; Uden, P. C.; Barnes, R. M. Anal. Chem. 1981, 53, 1336-1340. Estes, S . A,; Uden, P. C.; Barnes, R. M. Anal. Chem. 1981, 53, 1829-1837. Dingjan, H. A,; DeJong, H. J. Spectrochlm. Acta, Part B 1981, 368, 325-331 - - - - .. . Tanabe, K.; Haraguchi, H.; Fuwa, K. Spectrochlm. Acta, Part B 1981, 368, 633-639. Dingjan, H. A,; DeJong, H. J. Spectrochlm. Acta, Part B 1983, 388, 777-781. Eckhoff, M. A,; Ridgeway, T. H.; Caruso, J. A. Anal. Chem. 1983, 55, 1004- 1009. "ARL Model MPD 850",Information Bulletin: Applied Research Laboratories: Sunland, CA. "Kratos Model MPD 850", Information Bulletin; Kratos Scientific Instruments: Wertwood, NJ. Saitkavitz, K. J.; Uden, P. C. 1985 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, New Orleans, LA; Paper No. 618. Pauling, L. "General Chemistry", 3rd ed.; W. A. Freeman: San Francisco, CA, 1970; p 913. Fry, R. C.: Northway, S.J.: Brown, R. M.; Hughes, S.K. Anal. Chem. 1980. 52. 1716-1722. Maimstadt, H. V.;Erike, C. G.; Crouch, S.R. "Electronics and Instrumentation for Scientists"; Benjamin/Cummings: 1981; pp 176-177. "Motorola Power Data Book", 3rd ed.; Motorola, Inc.: 1982; pp 2.312-2.314.
.
A
B
Flgure 5. Plasma geometry and "nebulizer" argon flow rate effect: A, 0.5 L/min "nebulizer" argon: B, 1.0 L/min "nebullzer" argon; X, lower temperature axial central channel; Y, hotter ICP regions.
hotter regions (Figure 5A, y) long before the top of the plasma is reached. In this case, we would expect considerably more interaction between the CO sample and the hotter region y of the plasma. These plasma geometry and temperature gradient effects coupled with an increase in residence time (and number of bond breaking collisions) probably account for the dramatic differences in the extent of CO fragmentation observed a t 1.0 vs. 0.5 L/min of nebulizer argon flow rate. These preliminary results indicate significant promise for finding ICP experimental conditions conducive to complete molecular fragmentation and accurate elemental analysis of unknown organic compounds. In more general terms, the sample pair modulation technique introduced here should prove valuable in expediting future studies of molecular fragmentation in plasmas for a variety of different samples and experimental conditions. Registry No. Ar, 7440-37-1; CO, 630-08-0; 02, 7782-44-7.
LITERATURE CITED (1) McCormack, A. J.; Tong, S. C.; Cook, W. D. Anal. Chem. 1985, 37, 1470-1476. (2) Bache, C. A.; Lisk, D. J. Anal. Chem. 1967, 39, 786-789.
- I
-.
RECEIVED for review July 29,1985. Accepted October 10,1985. This work WRS supported in part by the National Science Foundation (Grant No. CHE-8219256) and was presented in part at the 1985 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, New Orleans, LA (paper No. 077).
CORRESPONDENCE Diffractive Spectroelectrochemistry with a Continuum Source Sir: Diffractive spectroelectrochemistry is a technique that can be used to monitor chemical species in the diffusion layer (I). This method has been shown to possess some distinct advantages over previous spectroelectrochemical techniques. The most promising aspect of diffraction is that it shows potential for providing information about the distribution of chromophore within the diffusion layer much like X-ray diffraction provides information about molecular structure. In addition, the absorbance response is fast and the technique can be quite sensitive (2, 3). Thus far, the only light source used in diffraction experiments has been a laser. While previous work has illustrated the utility of the technique, the fact that a laser is required imposes limitations on the applicability. Single wavelength 0003-2700/86/0358-0647$0 1.50/0
lasers, although widely available, limit the range of species that can be examined to those that absorb at a particular wavelength. Even with a tunable laser, only one wavelength a t a time can be observed and it is necessary to repeat the experiment several times in order to obtain an entire spectrum, a process that can be difficult or impossible when dealing with complex reaction mechanisms. This paper describes the use of a white light source for diffraction experiments. In addition, the rather novel use of a minigrid electrode as both working electrode and diffraction grating is described. The results of some preliminary studies, presented below, indicate that this technique can be used to observe the entire spectrum of an electrogenerated chromophore. The absorbance response, observed in the diffraction 0 1986 American Chemical Society
648
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986 GLASS S L I D E S T vm
MINIGRID WIRES,'$
X
I
SOURCE
PMT
MINIGRID ELECTRODE
.
++--s,--.-------l Flgure 1. Schematic of experimental apparatus. L si
pattern, is found t o be proportional to concentration. EXPERIMENTAL SECTION Reagents. o-Tolidine (Fisher) was used without further purification. Aqueous solutions of orthotolidine are 1.0 M in hydrochloric acid and 0.5 M in acetic acid. Distilled-deionizedwater is used. Electrochemical Cell. The thin layer cell is similar in construction to that described in the literature ( 4 ) . The optically transparent electrode is a 1000 line-per-inch gold minigrid (Buckbee-MearsCo.). A silver/silver chloride reference electrode is used, and the auxiliary electrode is a platinum wire, isolated from the bulk solution via a medium porosity glass frit. The thickness of the cell was determined spectrophotometricallyusing ferricyanide to be 0.017 28 cm. Apparatus. A schematic of the apparatus is shown in Figure 1. The white light source is a 600-W tungsten lamp, powered by the 120-V ac line through a variable transformer. Lens 1 (L,) serves to collimate the light from the source and lens 2 (L,) focuses the light onto a 0.3-mm pinhole (PI) machined in an aluminum disk. This pinhole essentially serves as the entrance slit of the optical system. In order to avoid significant magnification of the image, the distances So and Siare both adjusted t o be equal to twice the focal length of lens 3 (L3). For these experiments, lens 3 has a focal length of 44.0 cm. The photomultiplier tube (RCA 1P28) is positioned in the image plane of lens 3 and is fitted with a pinhole 0.3 mm in diameter. This pinhole then serves as the exit slit of the optical system. The photomultiplier tube (PMT) is attached to two transitional stages, each equipped with a micrometer. The resolution of the micrometers is 0.0001 in. This allows for precise translation of the PMT in the xy plane. Output from the PMT is monitored by an Apple IIe microcomputer system. The helium/neon laser (Spectraphysics 0.5 mW) is used only to provide a reference point of accurately known wavelength within the diffraction pattern. For this purpose a mirror can be placed at point MI in the arrangement to direct the laser beam through lens 2, which then focuses the beam onto the entrance slit. Procedure. In all absorbance measurements, the tungsten lamp is employed as the light source. Absorbance can be monitored as a function of time after initiation of electrolysis by positioning the PMT at the desired location within the diffraction pattern and monitoring the output before and during a potential step applied to the working electrode. In order to obtain a complete spectrum, the PMT is moved across the diffraction maximum in incremental steps and the absorbance at each position is determined as a function of time. R E S U L T S AND DISCUSSION When an object is placed directly adjacent to a thin lens and illuminated by a spherical wave from the left, the Fraunhofer diffraction pattern can be observed in the image plane to the right (Figure 2). Theoretically, for Fraunhofer diffraction, both the light source and the plane in which the diffraction pattern is viewed should be a t infinity from the
IMAGE
I
MINIORID ELECTRODE
c-- so-
c--si
I
P
Figure 2. Optical arrangement for observing Fraunhofer diffraction
pattern. Electrochemical cell is positioned as close to lens as possible. object. However, the same effect can be obtained through the use of lenses. A conventional two-lens system that serves to collimate the light before and after the object could be used, or the function of both lenses can be combined in a single lens. For simplicity in the optical arrangement, a single lens was used in this work. That the single lens produces a result equivalent to the infinite distance situation can be shown mathematically and will be presented in a later paper. When a minigrid electrode is used as the diffracting object and the source is a white light source, dispersion and diffraction are obtained in one step. The diffraction pattern is observed to be a two-dimensional array of maxima, each dispersed into the component frequencies of the source. If the interference pattern produced by this optical arrangement is truly the Fraunhofer diffraction pattern, the relationship between the wavelength of diffracted light and distance in the diffraction pattern will be given by the equation x = mXSi/2L
(1)
where x is the distance from the zero-order maximum, Si is the distance from the lens to the image plane, X is wavelength, m is the diffracted order, and 2L is the distance from the center of one aperture in the minigrid to the center of the next (5). Si is precisely determined by irradiating the minigrid with the laser instead of the continuum source. With solution in the thin layer cell, the distance between two adjacent diffraction maxima is determined. (A relative maximum in PMT output is assumed to indicate the position of the diffraction maximum.) Sican then be determined from eq 1,since the other parameters are accurately known. I t is important to note that the spacing (2L) must be constant. If the grid is not uniform, the diffraction pattern may be smeared or ghosts may appear. When a chromophore is generated electrochemically a t the minigrid, light in the diffraction pattern is attenuated a t wavelengths corresponding to the visible spectrum of the chromophore. The spectra obtained for the oxidation product of o-tolidine is shown in Figure 3 (solid line). Second-order diffracted light was monitored. With the present experimental configuration, this requires that the PMT be positioned a t about 3 cm off the optical axis. The precise value of x was measured via micrometer and then eq 1was used to make the (438.0 nm) the molar abwavelength assignments. At A,, sorptivity obtained using diffraction differs by only 1.1%from that obtained using transmission spectroelectrochemistry (Figure 3, dashed line). For both spectra, measurements were made a t times long enough to ensure complete electrolysis in the thin-layer cell. The path length for absorbance for both methods is equal to the physical thickness of the thin-layer
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
649
1.2 6.0
5.0
0.8
ExlO-4
ABSORBANCE
(M-I cm-(1 4.c
0.4
3.c
2.( 420
440
CONCENTRATION (mM)
460
WAVELENGTH (nm)
Figure 3. Molar absorptivity (e) as a function of wavelength: solid line (data shown as 0)obtained from diffraction, dashed line (data shown as A) from transmission spectroelectrochemistry.
cell and the molar absorptivity ( E ) can be calculated from Beer’s law. The peak widths at half height for the transmission and diffraction spectra (59 and 60 nm, respectively) are the same within experimental precision. This indicates that resolution provided by the optical system is sufficient for obtaining spectra of broad band absorbers such as o-tolidine. The limit on resolution is imposed by the effective spectral bandwidth, which is 4.3 nm for second-order radiation. This value is found by setting n in eq 1 equal to 0.3 mm (i.e., the diameter of the exit slit). To improve resolution, the band-pass could be decreased through use of a smaller exit slit. Alternatively, Sicould be increased or higher orders of diffracted light could be monitored. From eq 1 it can be seen that interfering signals will be present from diffracted light a t other wavelengths of other orders. For example, first-order light of 800 nm will be superimposed on the second-order signal at 400 nm; second-order light of 600 nm will be superimposed on the third-order signal at 400 nm, and so on. For these experiments the interferences are insignificant since the P M T is unresponsive to those frequencies that overlap in second order. At higher orders, the interferences should be eliminated through use of a cutoff filter positioned between the light source and the minigrid electrode. Absorbance was measured as a function of concentration in order to test the validity of using Beer’s law for diffraction. It can be seen from Figure 4 that the relation is linear, even a t relatively short times. At times long enough to assure complete electrolysis within the thin layer cell (tJ, the slope of the line obtained (1.05 X lo3) is in good agreement with the product of the molar absorptivity and the path length of the cell (1.07 X lo3). At shorter times the path length for absorbance is not as easily determined but is expected to be related to the thickness of the diffusion layer extending out into the solution on both sides of the gold minigrid (Figure 2, along the z axis). Therefore, absorbance for any one concentration should increase linearly with t1I2and this is observed experimentally (Figure 5). At short times, (less than 0.7 s), absorbance is not linear with This has been observed for transmission and is found to correspond to times where the diffusion layer thickness is small relative to the dimensions of the holes and wires of the minigrid (6). In this case the path length for
2e
Figure 4. Absorbance as a function of concentration for oxidation of o-tolidine measured at 438.0 nm. Time after initiation of electrolysis as indicated. Thickness of thin layer cell equal to 0.01728 cm.
0.30
0.20
ABSORBANCE
0.10
0.0
Figure 5. Absorbance as a function of t”*, measured at 438.0 nm for the oxidation of 0.75 mM o-tolidine. Width of the thin layer cell is equal to 0.01728 cm.
absorbance no longer is related only to the thickness of the diffusion layer along the n axis but also is a function of position in the x,y plane for both transmission and diffraction, and linearity with t1/2cannot be expected. Complete spectra can be obtained at times shorter than that required to completely electrolyze the solution within the thin layer cell (Figure 6). The wavelength of maximum absorbance is within 4 nm of the expected value for all spectra. The noise in the spectra can be attributed to fluctuations in the tungsten source, which become especially significant when absorbance values are small. This could be minimized by an optical feedback mechanism to improve source stability. Alterna-
650
Anal. Chem. 1986, 58,650-653 0.20
lamp, can be used. Second, the minigrid provides an electrode geometry that is centrosymmetric. Therefore, it should be possible for mathematically manipulate the experimental diffraction pattern (using an inverse Fourier transform) to obtain spatial information about the distribution of chromophore within the diffusion layer. The transform relationship between the diffusion layer and the diffraction pattern has already been shown (7) although the inversion was not accomplished. For a nonsymmetric electrode geometry like that used in the previous work, the inversion requires knowing the phase of light in the diffraction pattern. This information is difficult or impossible to obtain experimentally. However, the phase factor is zero for a centrosymmetric geometry like that provided by the minigrid. This greatly simplifies the inversion. Information such as that which would be obtained by inverting experimental diffraction patterns, would be extremely useful in the study of complex electrochemical reactions.
0.15
ABSORBANCE
0.10
ACKNOWLEDGMENT We thank R. L. McCreery (Ohio State University) for helpful discussion and Michael Mickelson and Lee Larson (Denison University) for technical assistance. Registry No. Au, 7440-57-5; o-tolidine, 29158-17-6.
0.05
4jO
440
450
WAVELENGTH (nm)
Figure 6. Absorbance as a function of wavelength at 1.3, 1.0, 0.8, and 0.4 s (listed from top) during oxidation of 1.OO mM o-tolidine. Width of thin layer cell is equal to 0.017 28 crn.
tively, time averaging could be employed. (All spectra shown are single runs.)
CONCLUSIONS The results presented here show that spectra of adequate resolution can be obtained by using 1000 lines-per-inch micromesh as both electrode and diffraction grating. In fact, the spectral band-pass employed in these experiments is comparable to that provided by many commercially available spectrophotometers when operated a t high scan rates. Extremely fast acquisition of spectra would be possible using diffraction if the P M T was simply replaced with an array detector. This work extends the potential of diffractive spectroelectrochemistry in two important ways. First, any readily available white light source, such as a tungsten or xenon arc
LITERATURE CITED (1) Rossi, P.; McCurdy, C. W.; McCreery, R. L. J . Am. Chem. SOC. 1981, 103, 2524. (2) Rossi, P.; McCreery, R. L. J . Electroanal. Chem. 1983, f 5 f , 47. (3) Chwu-Ching, J.; Lavine, B. K.; McCreery, R. L. Anal. Chem. 1985. 5 7 , 752. (4) Murray, R. W.; Heinernan, W. R.;O’Dom, G. W. Anal. Chem. 1967, 3 9 , 1666. (5) Klein, M. V. “Optics”; Wiiey: New York, 1970; Chapter 7 . (6) Petek, M.; Neal, T. E.; Murray, R. W. Anal. Chem. 1971, 4 3 , 1069. (7) Rossi, P. Ph.D. Thesis, Ohio State University, 1982.
Rick A. Fair Daniel E. Ryan Peter K. Smith Paula Rossi Melaragno* Department of Chemistry Denison University Granville, Ohio 43023
RECEIVED for review July 1, 1985. Resubmitted November 15, 1985. Accepted November 15, 1985. This work was supported by grants from the Research Corporation, the Apple Education Foundation, and Denison University Research Foundation.
Potentiometric Gas Sensor for the Determination of Free Chlorine in Static or Flow Injection Analysis Systems Sir: Chlorine is extensively used in many industrial processes and as a biocide to destroy harmful bacteria in drinking water and other types of water, such as in swimming pools and plant effluents. The speciation of chlorine in such media can be highly complex ( I ) . I t is customary to classify chlorine-containing species as “free chlorine” (chlorine plus hypochlorous acid) or “combined chlorine” (chloramines, etc.). Numerous methods of widely different efficacy and convenience have been developed for the determination of chlorine in aqueous samples. In many of these methods “combined
chlorine” (total oxidizing power) is determined, rather than free chlorine alone ( I , 2 ) . The most commonly used methods involve colorimetric, potentiometric or amperometric procedures, each of which has particular advantages and limitations. A typical colorimetric method is based on the reaction of chlorine with N,N-diethyl-p-phenylenediamine producing a red color which, however, is not ideally stable ( I , 2). The so-called “residual chlorine electrode” (3) is a potentiometric sensor for iodine liberated from excess potassium iodide; many oxidizing agents interfere. Finally, a widely used amperometric
0003-2700/86/0358-0650$01.50/00 1986 American Chemical Society
