1578
Anal. Chem. lQ86, 58,1578-1580
LITERATURE CITED (1) Murray, R. W., I n "Electroanalytical Chemistry"; Bard, A. J., Ed.; Marcei Dekker: New York, 1964, Vol. 13. (2) Denisevich, P.; Abruna, H. D.; Leidner, C. R.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1082, 2 1 , 2153. (3) Calvert, J. M.; Schmehl, R. H.; Sullivan, B. P.; Facci, S.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1083, 22, 2151. (4) Ellis, C. D.; Meyer, T. J. Inorg. Chem. 1984, 2 3 , 1748. (5) Mattson, J. S.;Smith, C. A. Anal. Chem. 1975, 4 7 , 1122. (6) Mattson, J. S.;Jones, T. T. Anal. Chem. 1878, 48, 2164. (7) Laser, D.; Ariel, M. J. Nectroanal. Chem. 1973, 4 1 , 381. (8) Hupp, J. T.; Otruba, J. P.; Parus, S. J.; Meyer, T. J. J. Electroanal. Chem. 1985, 190, 287.
(9) Hupp, J. T.; Meyer, T. J., in preparation.
(IO) Morse, D. L.; Wrighton, M. S . J. Am. Chem. SOC. 1974, 9 6 , 998. ( I l ) Surridge, N . A., unpublished results. (12) Hupp, J. T., unpublished results.
RECEIVED for review September 3,1985. Accepted December 12,1985. Acknowledgments for funding of this work are made to the Army Research office-Durham L d e r Grant DAAG29-82-K-0111.
Automated Mercury Film Electrode for Flow Injection Analysis and High-Performance Liquid Chromatography Detection Hari Gunasingham,* B. T. Tay, a n d K. P. Ang
Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 0511 The mercury electrode, in the form of the dropping mercury electrode or hanging mercury drop electrode, has not gained widespread use in flowing stream analysis because of its inherent instability and due to practical difficulties. Reductive detection a t solid electrodes is however limited by their cathodic potential range, particularly a t low pH. Moreover, reactions at solid electrodes are generally found to be sluggish compared to those a t the mercury electrode. This is particularly true for glassy carbon electrodes (GCE). Many reactions, such as the reduction of nitrobenzene (l),are more irreversible a t glassy carbon than say platinum or gold. However, glassy carbon is usually preferred, especially in continuous monitoring applications, on account of its relative immunity to passivation and poisoning. The use of the mercury film electrode (MFE) as an alternative to the mercury electrode has been well applied to anodic stripping voltammetry (ASV) in both discrete and flowing stream analysis. Its application to organic analysis is however relatively unexplored. In one of the few works on this aspect, Kublik showed by cyclic voltammetric studies that the MFE actually has a similar response to the HMDE (2). This work was, however, applied to static analysis, and the mercury film was prepared in situ. In this paper, we show the practical application of an automated MFE for reductive detection in flow injection analysis (FIA) and HPLC. The approach is to prepare the mercury film before each analysis, thereby presenting a fresh surface to the analyte. The use of microprocessor control in conjunction with the wall-jet electrode (WJE) enables the reproducible control of the film plating and stripping and the actual analyses. We also show that, compared with a bare glassy carbon electrode, the MFE is superior in regard to the reversibility of electrode reactions and the cathodic working range. The theory of the WJE was first described by Yamada and Matsuda (3). It has since been thoroughly investigated by several groups in its application to continuous-flow monitoring (3-11). The unique feature of the WJ configuration is the requirement for a large geometric cell volume. Unlike other configurations, the effective dead volume is only of the order of the hydrodynamic boundary layer, which is unaffected by the geometric cell volume. Also, there is no cross-contamination of the active solution by the bulk volume (12).High sample throughput is thereby feasible. The exploitation of
these features in continuous monitoring ASV at a mercury film WJE was recently described (13). EXPERIMENTAL SECTION Electrochemical System. The large-volume WJ cell used in this work is identical to the one used in a previous work (6). The geometric cell volume is 20 mL. The working electrode was a 5.4-mm-diameter glassy carbon disk (Tokai, Tokyo, Japan), which was press fitted into a poly(tetrafluoroethy1ene) (PTFE) casing. A copper lead afforded contact between the GCE and the polarographic analyzer. Similarly a 3-mm-diameter platinum disk fitted into a PTFE casing, served as the counter electrode. The reference system used was a AglAgCl electrode with saturated KC1. All potentials quoted in this paper are with respect to this reference electrode. The 0.6-mm inlet diameter nozzle was positioned 5 mm away from the back wall of the cell so as not to interfere with the flow of the hydrodynamic boundary layer (6, 7). The WJ cell was controlled by a PAR Model 174A polarographic analyzer (Princeton Applied Research, NJ). The cyclic voltammograms and amperometric peaks were recorded on a Graphtec Model WX 2400 x-y recorder (Watanabe Instruments, Japan). Preparation of the GCE Surface. The GCE was polished to a mirror finish with a fine slurry of alumina that was prepared by grinding 1-pm particles with a pestle and mortar. The electrode was then polished further with a slurry of 0.05-pm alumina (BAS, Inc., IN) to produce a scratch-free surface. Instrumentation. A schematic diagram of the instrumentation is shown in Figure 1. An Apple IIe microcomputer was used to automate the mercury film plating and stripping sequence at the MFWJE. In the plating step the computer controls the plating potential through a digital-analog converter connected to the external input of the PAR 174A polarographic analyzer (Princeton Applied Research, Princeton, NJ). The plating step commences when the computer causes the pneumatically actuated four-way valve (Rheodyne Model 5010) to switch from the buffer stream to the mercury stream. Once the plating is completed, the valve switches to the buffer or sample stream. On completion of a sample run, the mercury film is stripped off. This sequence may be repetitively employed for as many analyses as required. Figure 2 gives the flow chart of the control program for the sequencing operation. The user inputs the various parameters for the analysis sequence into a menu. When no more analyses have to be performed the program returns to the menu. The input-output and timing control is implemented in machine code, whereas the menu driver is written in Applesoft BASIC. A peristaltic pump (Eyela Model MP-3) and a syringe pump (Sage Instruments, MA) were used to deliver the blank buffer
0003-2700/68/0358-1578$01.50/00 1988 American Chemical Society
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Flgure 3. Cathodic cyclic voltammograms of 0.5 mM 9,lOphenanthrenequinone in 0.1 M sodium acetate buffer, pH 4.0, with 30% ethanol at the following scan rates (mV s-’): (a) 100, (b) 50, and (c) 20. Sample flow rate is 0.7 mL/mln. (Tokyo Kasei Kogyo, Tokyo, Japan), and nitrobenzene (BDH, Poole, England). Stock solutions of the above compounds were dissolved in either pH 4.0,O.l M sodium acetate buffer or in a mixture of ethanol (Merck, Darmstadt, FRG) and the huffer. The water used in preparing the buffers was deionized and triply distilled. The mercury stock solution was prepared by dissolving triply distilled mercury in 0.5 M nitric acid that was diluted to the required concentration using the buffer. RESULTS AND DISCUSSION Background Scans a n d Working Potential Range. Kublik found that the anodic and cathodic limits of the MFE electrode were similar to those of the HMDE (2). On the anodic side the potential range is limited by the oxidation of mercury and on the cathodic side by the hydrogen evolution reaction. We found that for acidic solutions the cathodic limit of the MFE was extended by 150 mV in comparison with the GCE. At higher pH (>5) however, the improvement for the MFE becomes less significant. It should be noted that there is a practical problem in achieving the actual potential limits when working a t high sensitivities (sub-nanoampere) because of difficulties of removing trace oxygen. This is particularly a problem in HPLC detection where we are dealing with large eluent volumes. However, with proper precautions (e.g., avoiding tubing made of Teflon and careful deaeration of the eluent), good results may be obtained. Cyclic Voltammetry Studies. The ability to perform cyclic voltammetry in continuous-flowing systems is a promising area of work that was described by Therrgeson et al. (14). The WJE with its well-defined electrode geometry and hydrodynamic characteristics is suited for this application. When performing cyclic voltammetry in continuous-flow systems, the flow rate and the scan rate must be carefully chosen. Generally, at lower flow rates and higher scan rates voltammograms are more like cyclic voltammograms in stationary solution, whereas at higher flow rates and lower scan rates they are like hydrodynamic voltammograms. Following Thargeson (14), the ratio of the difference between the cathodic peak current and the current at the cathodic switching potential to the cathodic peak current, r, is indicative of the classification of the voltammogram. The higher the value of r the better the signal-to-noise ratio and the more it is like a cyclic voltammogram in stationary solution. Due to the limitations of our sample pump, the lowest flow rate possible was at 0.7 mL min-l. By varying the scan rates, the maximum r value was found to differ for each compound a t the GCE. Figure 3 shows the cyclic voltammograms of 9,lO-phenanthrenequinonea t various scan rates. Similar variations with scan rate were observed for the other quinone
1580
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voltammograms of 2 mM Cnltroacetophenone in 0.1 M sodium acetate buffer, pH 4.0, with 30% ethanol at (a) glassy carbon electrode, (b) 10 A, and (c) 154 A. Sample flow rate is 0.7 mUmin. as well as the nitro compounds. In general, it was found that peak potentials at the MFE were more positive and the redox couples more reversible than a t the GCE. Also, as the thickness of the mercury film was increased, the trend was toward more positive reduction potentials and greater reversibility. Interestingly, the response was unaffected when the film thickness exceeded 154 A. This may possibly be because, at this point, the GCE surface is well-covered with mercury. Previous studies of thin mercury films a t GCEs have indicated that such films are actually droplets (15). One could then imagine that as the quantity of mercury plated increases the drop size increases, and the film eventually thus coalesces. Figure 4 shows the effect of film thickness on the cyclic voltammograms of 4-nitroacetophenone. Although electrode reactions are more reversible for the thicker MFE, peak heights generally remain unchanged. HPLC Detection. As mentioned earlier, the removal of atmospheric oxygen from HPLC eluents is one of the main limitations of using reductive detection. Approaches to the problem have included the use of heating (16)and an on-line zinc scrubber column (17). In our work we have used helium as a means of removing oxygen from the eluent. Figure 5 shows the trace for successive injections of the test compounds and the corresponding control sequence for the automation of the mercury film plating and stripping. The film is stripped here by scanning anodically. Whereas this approach is satisfactory for thin films, for thicker films it is necessary to hold the electrode at a positive potential (+1.2 V) for about a
minute to ensure complete stripping of the film. It is interesting that once the mercury film has been stripped, the bare GCE shows no memory effects in that ita response is identical with that of a freshly polished GCE. The sample throughput is thus limited only by the time taken to plate and strip the merucry film. For a thin film (of the order of a monolayer) the time required is about 40 s. For the thicker films only 2 min, at the most, would be required. The average relative standard deviation for HPLC detection of the nitro and quinone compounds at the MFE was found to be less than *3%, and, with adequate deoxygenation of the injected sample and eluent, detection limits at the subnanogram level are feasible. Registry No. Hg, 7439-97-6; p-benzoquinone, 106-51-4; 4nitroacetophenone, 100-19-6;9,lO-phenanthrenequinone, 84-11-7: 1-nitroanthraquinone, 82-34-8; 2-nitrofluorenone, 3096-52-4.
LITERATURE CITED (1) Rubinsteln, I . J. Elecfroanal. Chem. 1985, 783, 379-386. (2) StoJek, S.; Stepnik, B.; Kubllk, 2 . J. Electroanal. Chem. 1976, 7 4 , 277-295. (3) Yamada, J.; Matsuda, H. J. Electroanal. Chem. 1973, 4 4 , 189. (4) Gunaslngham, H.; Tay, B. T.; Ang, K. P.; Koh, L. L. J. Chromatogr. 1984, 265, 103-114. (5) Gunasingham, H.; Tay, B. T.; Ang, K. P. Anal. Chem. 1984, 5 6 , 2422-2426 (6) Gunasingham, H.; Tay, B. T.; Ang, K. P. J. Chromafogr. 1985, 347, 27 1-278. (7) Gunaslngham, H.; Fleet, B. Anal. Chem. 1983, 55, 1409-1414. (8) Gunasingham, H.;Fleet, B. J. Chromatogr. 1983, 267, 43-53. (9) Roston. D. A.; Kissinger, P. T. Anal. Chem. 1982, 5 4 , 1798-1802. (10) Lunte, C. E.; Kissinger, P. T. Anal. Chem. 1983. 55, 1458-1462. (11) Elblckl, J. M; Morgan, D. M.; Weber, S. G. Anal. Chem. 1984, 56, 978-985. (12) Albery, W. J.; Brett, C. M. A. J. J. Electroanal. Chem. 1983, 748, 201. (13) Gunasingham, H.; Ang, K. P.; Fleet, B.;Ngo, C. C.; Thlak, P. C. J. Electroanal. Chem. 1985, 786, 51. (14) Thargesen, N.; Janata, J.; RBiiEka, J. Anal. Chem. 1983, 55, 1988-1 990. (15) Vydra, F.; Stullk, K.; Julakova, E. "Electrochemical Stripping Analysis"; Ellis Horwood: Chichester, 1976. (16) Rappaport, S. M.; Jln, 2 . L.; Xu, X. B. J. Chromatogr. 1982, 240, 145-154. (17) McCrehan, W. A,; May, W. E. Anal. Chem. 1984, 5 6 , 625-628.
RECEIVED for review September 16, 1985. Resubmitted December 30, 1985. Accepted December 30, 1985.
