2330
Anal. Chem. 1989, 61, 2330-2332
alyte during the drying time (18) was, most likely, responsible for the unfavorable precision observed.
CONCLUSION TlLS was used as a matrix modifier for Whatman No. 1 filter paper. This new substrate was used successfully for nanogram detection of several PAHs. The analytical figures of merit compare favorably with those in literature reports (9,18-22),which reveal a variety of filter papers and matrix modifiers utilized for such molecules. This study suggests that Whatman No. 1 filter paper pretreated with TlLS can be used as a general method for trace analysis of different kinds of PAHs. The substrate was stable over a long period of time, meaning it could be prepared ahead of time and stored for future use. The reproducibility of the substrates' background was less than 7%. As previously reported, the Tl(1) cation and the analyte were protected from source irradiation damage by the long alkyl chains of the anion (17). An additional advantage of the nonpolar environment was the protection of the analyte against quenching during the measurement (17). All of the analyte signals, with the exception of that of naphthalene, reached a steady state in less than 3 min as long as a N2 purge was used. Although PABA and carbazole were chosen because of their polarity differences with respect to the other PAHs, these species were just as well behaved as the nonpolar PAHs. ACKNOWLEDGMENT A. D. Campiglia thanks the Coordenacao de Aperfeicoamento de Nivel Superior-Capes for the grant that is making possible his stay in Gainesville and the University of Brasilia (Brazil) for permission to leave. Registry No. PABA, 150-13-0;1,2,3,4-DBA,215-58-7; TlLS,
72925-49-6; naphthalene, 91-20-3; carbazole, 86-74-8; pyrene, 129-00-0,
LITERATURE CITED Vo-Dinh, T. Room Temperature Phosphorhetry for Chemical Analysis; Elving, P. M., Winefordner, J. D., Eds.: John Wiiey 8 Sons: New York, 1984. Roth, M. J. Chromatogr. 1987, 30, 276-278. Schulman. E. M.; Wailing. C. J. Phys. Chem. 1973, 7 7 , 902-905. Schulman. E. M.; Walling, C. Sclence 1972, 778, 53-54. Hurtubise, R. J. Sola Surface Luminescene Analysis: Theory Instrumentation, Applkations; Marcel Dekker: New York, 1981. Dakerio, R. A.; Hurtubise, R. J. Anal. Chem. 1982, 5 4 , 224-228. Burrell. G. J.; Hurtubise, R. J. Anal. Chem. 1988, 60, 564-568. von Wandruszka, R. M. A.; Hurtubise, R. J. Anal, Chem. 1977, 4 9 , 2164-2 169. Lue-Yen Bower, E.; Winefordner, J. D. Anal. Chlm. Acta 1978, 702, 1-18. Vo-Dinh, T.; Gammage, R. B.; Martinez, P. R. Anal. Chem. 1981. 53, 253-258. SeyboM, P. G.; White, W. Anal. Chem. 1975, 4 7 , 1199, 1200. Vo-Dinh, T.; Lue-Yen, E.; Winefordner, J. D. Anal. Chem. 1978, 4 8 , 1186-1188. Suter, G. W.; Kallir, A. J.; Wild, U. P. Anal. Chem. 1987, 5 9 , 1644-1 646. Karnew, H. T.; Schulman, S. G.; Winefordner, J. D. Anal. Chim. Acta 1984, 764, 257-262. Alak, A. M.; Vo-Dlnh, T. Anal. Chem. 1988, 60. 596-600. Parker, R. T.; Freelander, R. S.; Schulman, E. M.; Dunlap, R. B. Anal. Chem. 1979, 57, 1921-1926. Perry, L. M.; Campiglia, A. D.; Wlnefordner, J. D. Anal. Chkn. Acta, in press. Ramis Ramos. G.; Khasawneh, I. M.; Garcia AlvarezCoque, M. C.; Winefordner, J. D. Takmta, 1988. 35, 41-48. VoDinh, T.; Gammage, R. 8. Anal. Chlm. Acta 1979, 707, 261-271. Vc-Dinh. T.; Hooyman, J. R. Anal. Chem. 1979, 57, 1915-1921. Ramis Ramos, G.; Garcia AlvarezCoque, M. C.; O'Reiily, A. M.; Khasawneh, I . M.; Winefordner, J. D. Anal. Chem. 1988, 60. 418-420. VeDin. T.; Lue-Yen, E.; Winefordner, J. D. Talenta 1977, 2 4 , 146-1 48.
RECEIVED for review March 24,1989. Accepted June 19,1989. This research was supported by NIH-GM11373-27.
Apparatus for the Fabrication of Poly(chlorotrifluoroethy1ene) Composite Electrodes Jeffrey E. Anderson,* Dale Hopkins, John W. Shadrick, and Yee Ren Department of Chemistry, Murray State University, Murray, Kentucky 42071-3306
INTRODUCTION Since the introduction of Kel-F graphite (Kelgraf)electrodes (1, 2), a number of papers have appeared dealing with the application (3-7) of these electrodes, as well as the characterization of their behavior ( 4 9 ) . The application of Kelgraf electrodes has primarily been as voltammetric detectors in liquid chromatography and flow injection analysis (2-5). Characterization studies indicate that they exhibit behavior typical of microelectrode ensembles and as such have the advantage of enhanced current densities leading to a higher signal to noise ratio than observed for other carbon electrodes such as glassy carbon. Kelgraf electrodes that have been reported previously have been fabricated from Kel-F 81 brand plastic resin from 3M. The recommended fabrication techniques from the manufacturer (10) include compression molding of sheets and injection molding. In both cases, the physical properties of the resulting material are affected by the time and temperature of heating and cooling. For example, the degree of crystallinity is a function of the thermal history of the polymer. Rapid cooling of the Kel-F from above its crystalline melting point (212 "C) to below 150 "C yields a more amorphous material that is less dense, more elastic, more transparent, and tougher than its crystalline counterpart. The more amorphous material is also favored by minimizing the time that the plastic 0003-2700/89/0361-233080 1.50/0
is exposed to temperatures above 212 "C. This minimizes the thermal degradation of the polymer, which would lead to a lower molecular weight and more crystalline product. The denser translucent crystalline product is favored by slow cooling of the melt. It should be emphasized that under no conditions is the product purely crystalline or amorphous, which implies that the fabrication conditions must be strictly controlled to ensure a consistent product. An additional fabrication parameter that should be mentioned is the pressure. In the compression molding used to produce Kelgraf electrodes, it is important to maintain a pressure of a t least 1000 psi during the cooling phase of the fabrication. If the pressure is not maintained, shrink voids may develop caused by shrinkage of the plastic during cooling. Given the above considerations, an apparatus was designed to provide rapid heating of the Kel-F graphite mixture to above its melting point to decrease the fabrication time and minimize the time above 212 "C. In addition, provisions were made to simplify and provide control over the cooling time in an attempt to ensure a consistent product. Note that in the past, cooling was provided by squirting water on the fabrication die, a rather messy nonreproducible affair. This apparatus should be of interest given the more recent introduction of composite electrodes based on mixtures of Kel-F and silver (11)and anticipated extension of this work to other 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 20, OCTOBER 15, 1989
2331
Section A-A
Section C-C
t 3
T
5
a 4-3
Section 8-5
Slices of the apparatus illustrating the path of the water through the cooling channels. Also shown is a cross section of the stainless steel die and heater holes. Key: 1, aluminum block; 2, stainless steel die; 3, cooling channel cap: 4, cooling water inlet: 5, heater hole. Figure 2.
Flguro 1. Illustration of apparatus for fabrlcation of Kelgraf electrodes. All unks are in millimeters. Note that slices of the apparatus, A, B, and C, are shown In Figure 2. Key: 1, 500-W heater hole (10 mm); 2, stainless steel die; 3, aluminum block; 4, temperature probe hole; 5, cooling water inlet; 6, cooling water outlet: 7, die hole (6 mm); 8,
cooling channel cap.
precious metals. In addition the apparatus should prove useful in the fabrication of electrodes using other inert binders such as Teflon (12,13).
EXPERIMENTAL SECTION Reagents and Materials. All chemicals used were of reagent grade. The Kel-F 81 resin was obtained from the 3M Commercial Chemicals Division, St. Paul, MN. The powdered graphite (UCP-2-325)was obtained from Ultra Carbon Corp. Bay City, MI. The Kel-F rod used in the electrode fabrication was from Plastic hofdes, Inc., East Hanover, NJ. The 120 V-500 W heaters were obtained from O.T.E., Inc., Henderson, KY. The temperature probe used (bimetallicdial thermometer, Model G)was from Omega Engineering Inc., Stanford, CT. Instrumentation and Procedures. The potential step experiments were done on a solution that was 2 mM ferricyanide in 1.0 M KCI (pH 2.7). The potential was stepped from 0.6 to -0.2 V relative to an Ag/AgCl reference electrode (4 M KCl) using a Pt auxiliary electrode. The potentiostat used was homemade. The experiments were performed by using an Apple IIe computer equipped with a TM-AD213 (12-bitanalog-to-digitalconverter) board and a TM-DAlOl(12-bitdigital-banalog converter) board, both from TecMar, Inc. The software used was written in Applesoft basic and machine language. This system has been described previously (14). The electrode fabrication procedure and the apparatus are described in the following section. RESULTS AND DISCUSSION Figures 1 and 2 provide a detailed illustration of the described apparatus. It consists of a block of aluminum into which water cooling channels were drilled. Figure 2 illustrates the route of the water through the aluminum block, A stainless steel sleeve (die) is centered in the middle of the block. Note that the die is screwed into the block to hold it in place when the resulting Kelgraf electrode is pressed out of the die. Placed symmetrically about the die in the aluminum block are four 120-V, 500-W heaters. These heaters are powered in parallel with a variable transformer. Finally, an additional hole was drilled for a temperature probe. Obviously this hole must be drilled such that it does not pass through a cooling channel yet comes as close as possible to the stainless steel die to ensure that the appropriate temperature is being measured.
The fabrication process begins by placing approximately 2 g of the Kel-F-graphite mixture into the die. Preparation of the mixture has been described previously (1-3). We have found that manual mixing of the Kel-F and graphite is sufficient for obtaining a homogeneous mixture. In addition, we sieve the Kel-F obtained from the manufacturer and use resin with particle sizes less than 450 Mm. It should also be noted that the graphite used in this work had particle sizes less than 45 pm as opposed to the less than 1Mm particle size graphite that has been used previously. Once the mixture is in the die, a pressure of approximately 2000 psi is applied. The temperature is then increased rapidly (100% power) to 200 "C. The power is then adjusted to approximately 45%, providing an equilibrium temperature of 250 OC. At this point, the pressure is again adjusted to 2000 psi and the system is maintained at this pressure and temperature for approximately 5 min. Cooling is then initiated by passing a slow stream of water through the cooling channel. Because of the steam produced, it is important that cooling line fittings be secure and copper tubing is recommended for the water input and output lines. The system is cooled from 250 to below 150 O C in about 30-60 s. Faster cooling results in shrink voids even with the pressure applied. Once the system is at room temperature, the Kel-F-graphite pellet may be pressed from the die. This often requires considerable pressure, and therefore, to minimize frustration and possible damage to the apparatus, we frequently treat the die with a parting agent. This amounts to coating the walls of the die with high-temperature silicon grease and heat treating the die at 300 "C. It is important that the die then be wiped clean of excess grease before subsequent use of the apparatus. The resulting Kel-F-graphite pellet can be easily machined to the desired diameter (3 mm in these studies). For testing of the product, we have found it convenient to press fit a 5 mm long pellet into a hole in a pure Kel-F rod. This eliminates the encapsulation procedure that has been used in the past, which can result in distortion of the Kel-F-graphite pellet. We have seen no evidence of seepage of the solution between the pure Kel-F and Kel-F-graphite mixture on the millisecond time scale provided the pellet is 1to 2 thousandths of an inch bigger than the hole in the Kel-F rod. Electrical connection was made with a brass rod pushed through a hole in the opposite end of the rod. Polishing of the surface is accomplished by using progressively finer abrasives down to 1-pm alumina. Before the resulting electrode is used, the resistance
Anal. Chem. 1989, 61. 2332-2336
2332
-4
~
.4 .6 .B 1 Tlme (SI Figure 3. Potential step data (+) obtained after background subtraction when the potential of a 15% (w/w) graphite electrode was stepped from 4-03 V to -0.2 V vs Ag/AgCI (4.0 M KCI). The data shown is the average of four experiments on a single electrode with no pdlshing between runs. The solution contained 2 mM ferricyanide (1.0M KCI, pH 2.8). Data collection was initiated at 10 ms after the step, with a total of 100 current measurements being made at 1 h s intervals. The solid line is the theoretical fit to the data.
0
.2
is measured to ensure that it is below 100 fl. Chronoamperometric experiments have been used to characterize Kelgraf electrodes. In these experiments, the active surface area and the size of the active sites on the surface have been determined by fitting existing theory for microelectrode arrays to the experimental data. Weisshaar and Tallman (8)used this approach with the theory developed by Gueshi et al. (15). However, only after modification of the theory to include a distribution of two sizes of active sites were satisfactory results obtained. It was concluded that although the information obtained is useful, the model used to develop the theory is oversimplified with respect to the actual distribution and size of sites on the Kelgraf electrode surface. We have performed similar experiments with electrodes fabricated as described above. However, the theory of Shoup and Szabo (16)was used in the simplex optimization for the determination of the active area and site radii. This theoretical expression for the current has been shown to be more accurate than that used previously although it uses the same model for the distribution of active sites on the surface. Figure 3 shows the experimental potential step data obtained for the reduction of 2 mM ferricyanide at a 15% (w/w) graphite electrode after background subtraction. The experimental conditions are given in the caption. Also shown in Figure 3 is the theoretical fit (solid line), which is much better than that obtained in the study mentioned above when only one size of active site was considered. For one 15% (w/w) graphite electrode used in this study, the surface was found to be 18.0% active with a standard deviation of 0.15%. The active site radii determined were 25.0 pm (standard deviation 1.5). These
results are from four sets of experiments as described in the caption to Figure 3, with polishing between sets. This active site radius is in agreement with values determined previously by using chronoamperometry and scanning electron microscopy (8). In terms of reproducibility, results from another 15% electrode fabricated by a different individual yielded a percent active area and site radii of 21% and 30 Mm, respectively. This variation between electrodes undoubtedly results from differences in the mixture preparation rather than from the compression molding step. It is this later step that we have attempted to refiie, and its reproducibility is excellent with respect to the success rate and physical characteristics of the product. There are various reasons why our theoretical fit appears to be better than the results reported previously. Obviously the theory used was different. In addition, the fabrication technique is somewhat different as is the particle size of the graphite used to prepare the electrodes. Even though our results are rewarding, we agree with the previous conclusions that the model used to derive the theory is undoubtedly much simpler than the conditions that exist at the electrode surface. However, the percent active area and active site radii determined here are still useful in that they are important in determining the enhancement effects associated with microelectrode arrays in stationary solutions and flowing streams (17).
LITERATURE CITED Anderson, J. E.; Tallman. D. E.; Chesney, D. J.; Anderson, J. L. Anal. Chem. 1978, 50, 1051-1056. Anderson, J. E. Ph.D. Dissertation, North Dakota State University, 1979. Chesney, D. J.; Anderson, J. L.; Weisshaar, D. E.; Tallman, D. E. Anal. Chem. 1981, 724, 321-331. Welsshaar, D. E.; Tallman, D. E.; Anderson, J. L. Anal. Chem. 1981, 5 3 , 1809-1813. Tallman, D. E.: Weisshaar, D. E. J . Liq. Chromatogr. 1983, 6 , 2157-2172. Anderson, J. L.; Wh%en,K. K.; Brewster, J. D.; Ou,T. Y.: Nonldez, W. K. Anal. Chem. 1985, 5 7 , 1366-1373. Cox, J. A.; Kulkarni, K. R. Talanfa 1988, 33. 911-913. Weisshaar, D. E.: Tallman. D. E. Anal. Chem. 1983, 55, 1146-1151. Petersen, S.L.; Weisshaar, D.E.; Tallman, D. E.; Schulze. R. K.; Evans. J. F.; DesJarlais, S. E.; Engstrom, R. C. Anal. Chem. 1988, 6 0 , 2385-2392. Commercial Chemicals Division 3M, Technical Information, Y-IKCM (28.1) R1. Petersen, S. L.; Tallman, D. E. Anal. Chem. 1988, 60, 82-86. Klatt, L. N.; Connell, D. R.; Adams, R. E.; Honlgberg, I. L.; Prlce, J. C. Anal. Chem. 1975, 4 7 , 2470-2472. Shah, M. H.; Honlgberg, I. L. Anal. Let?. 1983, 76(A15), 1149-1163. Anderson, J. E. Am. Lab. News 1988, December, 54, 55. Gueshi, T.; Tokuda, K.; Matsuda, H. J . Nectroanal. Chem. Interfacial Electrochem. 1978, 89, 247-260. Shoup, D.; Szabo, A. J . Electroanal. Chem. Interfacial Electrochem. 1984, 160, 19-26. Cope, D. K.; Tallman, D. E. J . Electroanal. Chem. Inte/faclalElectrochem. 1988, 205, 101-123.
RECEIVED for review May 9, 1989. Accepted July 18, 1989. The authors acknowledge financial support of this work by the Committee for Institutional Studies and Research at Murray State University.
Hollow Fiber Membrane Probes for the in Situ Mass Spectrometric Monitoring of Nitrogen Trichloride Formation during Wastewater Treatment P. J. Savickas,* M. A. LaPack, and J. C. Tou The Dow Chemical Company, Analytical Sciences, 1897 Building, Midland, Michigan 48667
The utility of mass spectrometry in the broad field of process analytical chemistry and in situ analysis has grown steadily over the past few years ( 1 , 2) with concurrent re-
finements in mass spectrometers and their inlet systems. In our case, these mass spectrometers are used for a variety of analyses ( 3 , 4 ) . The characteristic role is to apply its excellent
0003-2700/89/0361-2332$01.50/0C 1989 American Chemical Society
