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Anal. Chem. 1981, 53, 1539-1541
AIDS FOR ANALYTICAL CHEMISTS Thin-Layer Spectroelectrochemical Cell for Nonaqueous Solvent Systems Rathbun K. Rhodes' and Karl M. Kadlsh" Department of Chemistry, University of Houston, Houston, Texas 77004
Many advancements have been made in OTTLE (optically transparent thin-layer electrode) cell design since the initial report of the glass slide-gold minigrid assembly ( I ) for spectroelectrochemistr,y. These include the capability for monitoring in the ultr,aviolet (2) and infrared (3) spectral regions, the ability to perform precise (4) and cryogenic (5) temperature studies, and the flexibility of several working electrode materials (6-8). In addition, cells have been specifically designed to allow for ease of solution deoxygenation (9),for reduction of uncompensated solution resistance ( I O ) , and for characterization of eluent compounds in a flowing chromatographic system (11). However, little has been reported concerning the (applicability of cells to a diversity of solvent systems. Historically, most thin-layer studies have been restricted to aqueous solutions (Ii?-I4) and to nonaqueous solvents like dimethylformamide and acetonitrile (2,3,15-19). While these solvents are extremely useful, they are not universal from either solubility or mechanistic points of view. It would be highly desirable to have a design which is both easy to construct and inert to solvents of a wide range of dielectric constant and binding properties. It is the purpose of this paper to report severall modifications to the basic OTTLE design ( I ) which make this possible. In past cell designs, the inability to use particular solvents usually resulted from disisolution of one of the cell components. This dissolution process either contaminated the solution of interest or caused severe leakage problems at one of the sealant junctions (20). Replacement of these more interactive components (Plexiglas, glue, silicon rubber, or epoxides) by inert components such as glws and Teflon removed these problems. Described herein is a design which has proved useful in a wide diversity of nonaqueouis solvents.
EXPERIMENTAL SECTION Cell Construction. The complete thin-layer spectroelectrochemical cell used in these studies is shown in Figure 1. The platinum auxiliary (1)and saturated calomel electrode (SCE) reference (7) are of conventional design. The gold minigrid working electrode (3) is formed by clamping a 1000 lines/in. gold minigrid (Buckbee-Mears Co., St. Paul, MN) that is 15 X 5 mm and a 0.25-mm Teflon spacer (Dupont Tefiel, Wilmington, DE) between two glass slides (25 X 9.5 X 1 mm). This assembly is then heated at 300 "Cin a muffel furnace for 4-7 min, thus sealing the Tefzel to glass. The resultant heat thinning gives an optical path length of from 0.05 to 0.10 mm. The Teflon lid (4) is 0.625 in. in diameter and 0.20 in. in height. A center cut (0.370 in. X 0.085 in. X 0.200 in.) is made to allow for insertion of the minigrid-slide assembly into the lid. In addition, a circular groove (0.515 in. o.d., 0.410 in. i.d., 0.080 in. deep) is cut into the lid bottom to give a tight fit with the solution reservoir, and a recessed cut (0.340 in. diameter X 0.080 in. deep) is made for easy removal of trapped air bubbles. The solution reservoir (2)-salt bridge assembly (6), separated by a cracked-glass frit (5), can be glassblown in less than an hour and a half. The dimensions of this Present address: Ohio Medical Products, Madison, WI 53707.
unit can be varied to accommodate the requirements of the spectrometer cell compartment. To maintain the integrity of the seals between the reservoir and lid and the lid and minigrid assembly, we applied several strpis of Teflon gasket tape. Devcon 5-min epoxy is then applied to the outer portions of the cell to stabilize the final positioning. Reagents, Iron tetraphenylporphinato chloride (TPPFeCl) was prepared by acid hydrolysis of the p-oxo-iron tetraphenylporphyrin dimer (TPPFe)20(Strem Chemical Co.) as per the method of Summerville and Cohen (21). Ethylene chloride (EtC12),dimethylformamide (DMF), dimethyl sulfoxide (MeaO), and pyridine (Py) (Aldrich Chemicals and Fisher Scientific) were purified by literature methods (22) and stored over 4-A molecular sieves prior to use. The supporting electrolyte tetrabutylammonium perchlorate (TBAP) was obtained from Eastman Chemical, recrystallized twice from an ethyl acetate-pentane mixture and dried in vacuo. Apparatus. The potential of the working electrode is controlled by a Princeton Applied Research 364 polarographic analyzer (Princeton, NJ) in conjunction with a Hewlett-Packard 3310B triangle wave generator (Palo Alto, CA). Spectra are obtained by externally focusing a 75-W quartz tungsten-halogen lamp (Oriel 6329 source, Stamford, CT) through the thin-layer cell to an optical Spectrometer, silicon diode-array detector (Tracor Northern 1223/1150/1710, Middleton, WI). Spectra result from the signal averaging of 100 sequential 5-ma acquisitions. Each acquisition represents a single spectrum from 325 to 950 nm at a resolution of 1.22 nm/channel. For spectrometers with a less intense spectral source, a more transmissive minigrid (100 lines/in.) can be used to obtain comparable spectra. Procedure. Solutions were made up to be 1.0 X M in porphyrin and either 2 or 5 X 10- M in TBAP. Dissolution of porphyrin was aided by an ultrasonic cleaner (Cole-Parmer, Chicago, IL). Since the TN-1710 is a single beam transmission spectrometer, a thin-layer solvent-supportingelectrolyte spectral reference was acquired and stored for future absorbance calculations. The reservoir was then emptied and rinsed twice with fresh solvent, the salt bridge filled, and the SCE inserted. Porphyrin solutions were deoxygenated in a 10-mL volumetric flask using dried nitrogen passed through heated copper turnings to remove residual oxygen. Solutions were pushed by a positive nitrogen puressure into the thin-layer reservoir (also under nitrogen), the auxiliary electrode was inserted, and solution was aspirated into the slide assembly. Spectra were then taken during the course of the thin-layer cyclic experiments.
RESULTS AND DISCUSSION Synthetic iron porphyrin electrochemistry has been studied in this laboratory because of the relevance to biologically significant model compounds. The electrochemical mechanism for TPPFeCl is known to be highly solvent dependent (23)and thus provides a good test for cell performance as a function of solvent. That it is also highly oxygen and impurity sensitive (24) provides a stringent test for the provision of a clean, oxygen-free environment. A typical thin-layer cyclic for TPPFeCl reduction in MezSO is demonstrated in Figure 2. In theory, there should be no peak spearation between the oxidation and reduction waves. In practice, however, the observed peak separation in the cyclic voltammogram is due to uncompensated solution resistance,
0003-2700181/0353-1539$01.25/00 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
Figure 1. Diagram of the thin-layer spectroelectrochemical cell: (1) Pt auxiliary electrode, (2)solution reservoir, (3) Au minigrid working electrode, (4) connecting Teflon lid, (5) Pyrexlsoft glass cracked frit, (6) solvent-supporting electrolyte salt bridge, (7) SCE reference electrode.
0.2
0.0
-02
- 0 4 -0.6 - 0 8 POTENTIAL
-10
-1.2
-14-16
l v vs. S C E I
Figure 2. Thin-layer cyclic voltammogram of 1.0 X lo-' M TPPFeCl and 2 X lo-' M TBAP in Me2S0. Scan rate is 2 mV/s. a problem inherent to all thin-layer designs (I,10,15).TBAP concentrations of 0.2 M in high dielectric constant solvents and 0.5 M in low dielectric constant solvents are sufficient to keep the peak spearations to 250 mV or less (at a scan rate of 2 mV/s). Superimposable cyclics and spectra are found with multiple sweeps. These spectra show well-defined isosbestic points as expected for complete conversion of one species to another and are demonstrated elsewhere (25). Comparable results are found in the other solvents tested (EtC12,DMF and Py). In discussion of the utility of any thin-layer design, questions must be answered about its ease of construction, ease of cleaning, permeability to atmospheric oxygen, solution residence time, and cell lifetime. In this case, the three principal components of the cell (slide-minigrid, Teflon lid, and reservoir-salt bridge) can each be constructed in less than 2 h. The assembly into a complete unit requires about 30 min with an additional 30 min for the final drying of the epoxy. The largest part of the 30 min assembly time deals with the proper cutting and seating of the Teflon tape gaskets which prevent solution leakage. A minimal amount of high-vacuum silicon stopcock can also be used with the gaskets to further minimize leakage. This has never been observed to change the electrochemistry of any system tested in this laboratory and has beneficial effects on cell lifetime. After completion of the experiment, the cell can be easily broken down into its components. The reservoir is cleaned by conventional means, the lid rinsed with solvent, the minigrid-slide flushed several times with solvent, and the gaskets discarded. This quick breakdown allows for easy interchange of minigrid-lide assemblies to avoid cross contamination from different compounds and solvents. This cell also proves to be easy to protect from oxygen. After the solution is initially deoxygenated, experiments can be run in the open for an hour and a half before appreciable
oxygen (2 X lo4) diffuses back into the minigrid region from the surrounding atmosphere. When necessary, a nitrogen purge box can be used to lower this level of residual oxygen. In theory, the solution being spectrally monitored remains in the minigrid region indefinitely. In practice, however, a finite solution residence time occurs due to evaporative convection in the slide assembly. The time for this effect to occur can be quantitated by multiple cyclic superimposition and/or open circuit spectral monitoring of a stable electrochemically generated species. With this cell, residence times of over an hour are found for nonvolatile solvents like DMF, Me2S0, and Py. A more volatile solvent like EtCl2 requires shielding of the electrode from the spectral source (except during the short spectral acquisition times) to maintain a residence time of over an hour. From other studies with this same cell (26)we know that extremely volatile solvents like methylene chloride are difficult to work with for more than 10 min, even with light shielding and a solvent saturated atmosphere. Thus, these most volatile solvents can only be used with great care. The useful lifetime for this cell is ultimately limited by solution leakage around one of the Teflon gaskets (usually the slide-lid gasket). However, even with no applied silicon grease, the cell remains solution tight for at least 2 h in all solvents tested. This is easily enough time to complete most series of thin-layer experiments. When a little silicon grease is used on the slide lid gasket, cell lifetimes of over 20 h are not uncommon, and the electrochemistry remains unchanged. Thus, the spectroelectrochemical cell described herein is suitable for work in a wide variety of nonaqueous solvents. These include: EtC12, DMF, Me2S0, and Py (this work) and methylene chloride, acetone, acetonitrile, and methanol (26) and will undoubtedly be extended to further solvents in future studies. The electrochemical response for the cell is comparable to other minigrid designs; and this design allows for ease in construction and cleaning, a high degree of anerobicity, reasonable solution residence times, and long cell lifetime.
ACKNOWLEDGMENT The authors wish to thank L. A. Bottomley, P. C. Minor, and R. Wilkins for helpful suggestions during the evolution of this cell design.
LITERATURE CITED Murray, R. W.; Heineman, W. R.; O'Dom, G. W. Anal. Chem. 1967, 39. 1966. Heheman, W. R.; Burnett, J. N.; Murray, R. W. Anal. Chem. 1968, 40, 1970. Helneman, W. R.; Burnett, J. N.; Murray, R. W. Anal. Chem. 1968, 40, 1974. Kreishman, G. P.; Anderson, C. W.; Su, C. H.; Haisali, H. B.; Heineman, W. R. Bloelectrochem. Bioenerg. 1978, 5 , 196. Hawkridge, F. M.; Ke, B. Anal. Biochem. 1977, 78, 76. Meyer, M. L.; DeAngelis, T. P.; Heineman, W. R. Anal. Chem. 1977, 49, 602. Yildiz, A.; Kissinger, P. T.; Reilley, C. N. Anal. Chem. 1988, 40, 1018. NONeli, V. E.; Mamantov, G. Anal. Chem. 1977, 49, 1970. Norrls, B. J.; Meckstroth, M. L.; Heineman, W. R. Anal. Chem. 1976, 48, 630. Tom, G. M.; Hubbard, A. T. Anal. Chem. 1971, 43, 671. Pinkerton, T. C.; Hajizadeh, H.; Deutsch, E.; Heineman, W. R. Anal. Chem. 1980, 52, 1542. Petek, M.; Neal. T. E.;Murray, R. W. Anal. Chem. 1971, 43, 1069. Kenyhercz, T. M.; DeAngelis, T. P.; Norris, B. J.; Heineman, W. R.; Mark, H. B. J. Am. Chem. Sac. 1978, 98, 2469. Stargardt, J. F.; Hawkridge, F. M.; Landrum, H. L. Anal. Chem. 1978, 50, 930. Rohrbach, D. F.; Heineman, W.R.; Deutsch, E. Inorg. Chem. 1979, 18, 2536. Pezchal-Helling, G.; Wilson. G. S. Anal. Chem. 1971, 43, 550. Neal, T. E.; Murray, R. W. Anal. Chem. 1970, 42, 1654. Piljac, I.; Tkalcec, M.; Grabani, B. Anal. Chem. 1975, 47, 1369. Piljac, I.; Murray, R. W. J. Electrochem. SOC. 1971, 718, 1758. Rhodes, R. K.; Kadish, K. M., unpublished work, University of Houston, 1979-1980. Summerviile, D. A.; Cohen 1. A. J. Am. Chem. SOC.1976, 98, 1747. Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. "Purification of Laboratory Chemicals", 2nd ed.; Permagon Press: Oxford, 1980. pp 204, 224,-229, 402. Bottomley, L. A.; Kadish, K. M. Inorg. Chem. 1981, 20, 1346. Brault, D.; Rougee, M. Blochemistfy 1974, 13, 4591.
Anal. Chem. 1981, 53, 1541-1543 (25) Kadish, K. M. "The Chemical Physics of Biologically Important Inorganic Chromophores"; VOl. 1, Lever, A. B. p., @aY, H. 8.3 Ed% in press. (26) Kadish, K. M.; Rhodes, R. K. Inorg. Chem., in press.
RECEIVED for review February 27,1981. Accepted April 17,
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1981. The authors wish to gratefully acknowledge the suppofi of this research in part by the National Science Foundation (Grant CHE-7921536), the National Institutes of Health (Grant GM 2517-02), and the Robert A. Welch Foundation (Grant E-680).
Analysis of Fish and Sediment for Volatile Priority Pollutants Michael H. Hiatt U S .Environmental Protection Agency, Environmental Monitoring Systems Laboratmy, P.O. Box 75027, Las Vegas, Nevada 89 7 74 The U.S. Environmental Protection Agency's (EPA's) water quality monitoring and hazardous waste monitoring programs require analysis of a variety of samples for volatile organic compounds. The samples analyzed are primarily sediments, soils, water, and fish. Determination of the volatile priority pollutants in water samples is routinely accomplished with acceptable results in accordance with the EPA-recommended Method 624 (1). However, the EPA-recommended procedures for the determination of volatile priority pollutants in sediment and fish (2-4) produce unacceptable results, as evidenced in other studies ( 5 , 6 ) by low spike recoveries and high detection limits. An improved method was required for the Agency's monitoring programs. The vaporization of volatile organic compounds from a sample under vacuum and subsequent condensation in a super-cooled trap seemed an ideal approach when compared to other methods available (5-7). Cryogenic concentration has been used successfully for the determination of tritiated methane and the radioisotopes of krypton and xenon (8) and appeared to be applicable to the determination of volatile organic compounds in hiolid matrices. Another advantage of vacuum extraction was that it did not require elevated temperatures or the addition of reagents which could produce unwanted byproducts due to sample degradation. In this method volatile organic compounds are vaporized from the fish or sediment matrix under vacuum and are condensed in a purging trap cooled by liquid nitrogen. The purging trap is transferred to a conventional purge and trap device where the concentrate is treated as a water sample and is analyzed as described in Method 624 ( I ) . With this method, the average recovery of volatile organic compounds from samples spiked a t the 25 pg/kg level was found to be 94% for sediments and 74% for fish tissue.
EXPERIMENTAL SECTION Samples. Sediments and fish samples previously found to contain less than detectable levels of volatile priority pollutants were used as matrices for this study. Spiked samples were prepared by placing 10 g of isediment or ground fish into a 125-mL septum vial and adding the compounds to be studied (Tables 1-111). Two separate additions were necessary to add the organic compounds. The first addition was 10 pL of a water solution that contained acrolein and acrylonitrile (1pg/pL), which are more stable in water than methanol. The second injection was 5 pL of a methanol solution that contained 250 ng (50 ng/pL) of the remaining volatile organic compounds. The spiked samples were sealed with a Teflon-lined septum cap and sonicated for 5 min. The sample vials were then sealed in cans containing activated charcoal and stored overnight in a freezer. Apparatus. A diagram of the apparatus for vacuum extraction and cryogenic concentration is shown in Figure 1. The vacuum extractor can be assembled from materials normally available in the laboratory. The low pressure necessary for extraction is supplied by a vacuum pump capable of producing a torr
vacuum and a flow rate of 25 L/min. The concentrator traps (25 mL Tekmar purging tubes or equivalent) are used for condensing the volatile vapors and transferring the extract to the purge and trap device. The concentrator trap is connected to the transfer lines of the vacuum extractor with l/a in. compression fittings and graphite ferrules. The transfer lines are made of glass-lined 1/4 in 0.d. stainless steel tubing. Gastight valves (Vl-V4), Nupro B-4BKT, are connected with compression fittings and graphite ferrules. The 125-mL septum vial containing the sample aliquot is connected to the system with a one-hole rubber stopper pierced with the 1/4 in 0.d. tubing. A liquid nitrogen cold trap is placed between the vacuum pump and concentrator trap in order to prevent condensation of pump oil vapors in the concentrator trap. The helium line and pressure gauge are connected at a junction in the transfer line between the sample and V2 and are used primarily to test the apparatus for leaks. The helium used is 99.999% pure, and normally because of the small quantities used, it is not necessary to purify further. If it is desired, however, a gas filter trap can be added to the helium line to ensure purity. An ultrasonic cleaner, Branisonic 12, is used to agitate the sample during extraction. Procedure. The vacuum extractor must be airtight and free of moisture before an extraction can be started. A clean 125-mL septum vial is connected, the vacuum pump started, and V2 to V4 are opened to evacuate the apparatus. The elimination of line condensation is accomplished by warming the transfer lines while evacuating the system. Heating tape is effective in creating even transfer line temperatures and can be used continuously during extraction. The vacuum extractor is pressurized with helium by closing V3 and opening V1. The apparatus is then leak-tested by applying soapy water on all connections and making the appropriate adjustments when leaks are located. When the apparatus is airtight, close V1 and open V3. Heat the transfer lines and concentrator trap for 5 min to eliminate any contamination from previous extraction at parts-per-billionconcentration. The system is now ready for sample extraction. To begin the extraction process close V2 (V3 and V4 remain open), cool the concentrator trap with a liquid nitrogen bath, and replace the empty 125-mL vial with the sample vial. Disconnect the vacuum source by closing V3. Open V2 to permit vapors from the sample vial to reach the concentrator trap. The sample vial is then immersed in the ultrasonic water bath. The equilibrium temperature generated in the ultrasonic bath is 50 "C.Therefore the bath is initially fiiled with 50 O C water and that temperature maintained by continuous ultrasonic operation. After 5 min of ultrasonic agitation the vacuum source is connected by opening V3. The lower pressure hastens the transfer of volatile compounds from the sample to the super-cooled concentrator trap. After 15 min of vacuum close V3 and open V1 to fill the system with helium until atmospheric pressure is obtained. Close V1 and V2 to isolate the concentrate. The sample extraction is now complete and the concentrate is ready for transfer to a purge and trap device. The concentrate can be held in the liquid nitrogen bath for up to an hour prior to analysis. Disconnect the sample concentrator trap from the vacuum extractor and connect it to the purge and trap device. Some outgassing is observed when the sample extract is melted;
This article not subject to U.S. Copyright. Published 1981 by the American Chemical Society
