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Improved On-Line Stripping Voltammetry Using a Large Volume Wall-Jet Detector Joseph Wang* and Bassam A. Freiha Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003 Stripping analysis is a sensitive, precise, and economical electroanalytical technique for measuring heavy metals (1,2). As a result of its inherent sensitivity, the technique is widely used for direct measurements in environmental, industrial, and clinical samples. The need for rapid monitoring of trace metals has led to the development of various flow systems for stripping voltammetry (3-7). Such on-line systems offer various advantages for many practical situations. For example, flow measurements, effected in the natural environment, offer the option of continuous surveillance ( 3 ) . Automated flow systems, based on the flow injection or automated analyzer principles ( 4 , 5 ) ,are especially suitable for clinical or quality control applications. In this paper, we describe a number of advantages obtained by using a large volume wall-jet detector in on-line stripping measurements. Various electrode geometries have been used in anodic stripping flow systems. These include mainly cells in which the solution flows through a tubular electrode (3) or a thin-layer channel (5)or onto a wall-jet electrode (6, 7). Such low dead volume designs are used in other electroanalytical applications (e.g., liquid chromatographic detection) and are available commercially. In the wall-jet design the inlet solution is introduced via a nozzle and impinges normally on the planar disk electrode. (In common practice, the wall-jet design is actually a radial-flow thin-layer cell (8).) Recent studies by Gunasingham et al. (9, 10) have demonstrated that a large volume wall-jet detector results in a similar performance, because the jet remains intact up to quite large nozzle-electrode separations. Such design offered various benefits for amperometric detection for normal-phase liquid chromatography (10). We will show that for stripping flow systems such design eliminates the need of deaerating the sample or adding supporting electrolyte. As a result of minimizing the sample manipulation, on-line measurements of trace metals are significantly improved with respect to speed, elimination of contamination from added reagents, use of resistive organic solvents, and simplicity. These performance benefits are illustrated using both flow injection and continuous flow stripping systems, as applied to a variety of real samples.
EXPERIMENTAL SECTION Apparatus. The cell body was made of a 100-mL glass beaker (5 cm diameter, 7 cm height). The working electrode, the Ag/AgCl reference electrode (Model RE-1, Bioanalytical Systems), the graphite rod auxiliary electrode, and the nitrogen delivery tube, were placed in the cell through holes in its Plexiglas cover, with 1-cm separation from each other. A 3 mm diameter glassy carbon disk (BioanalyLicalSystems) served as the support for the mercury film working electrode. Solution inflow was maintained through a vertical glass tube sealed to the cell bottom. The tube was narrowed (to 0.2 mm i.d.), toward its upper end, to form the jet nozzle. The nozzle was kept 0.2 mm away from the center of the working electrode. Solution outflow was maintained through a 4 mm i.d. glass tube, branching from the cell wall at a right angle (about 6 cm above the bottom). The flow injection system was constructed of 400-mL sample and carrier reservoirs, fitted with covers. The sample injection valve was a Rheodyne Model 5020 with a 500-r(L loop. All interconnections were made with 1 mm i.d. PTFE tubing. Flow of the carrier solution was maintained by gravity. For continuous flow measurements, a similar system-with one reservoir-was employed. Measurements were made with a Princeton Applied Research Model 174 polarographic analyzer and a Houston Omniscribe strip-chart recorder,
Reagents. Deionized water was used to prepare all solutions. Supporting electrolyte (in the detector compartment) was 0.1 M acetate buffer (pH 4.5). Metal ion stock solutions, lo9 M, were prepared (by dissolving pure metal) and stored in polyethylene bottles. The preparation of the multivitamin with minerals tablet solutions was described elsewhere (11). River (Rio Grande) and tap water samples were collected in Las Cruces (New Mexico). Acetonitrile was LC grade (Aldrich)and methanol was synthetic grade (Sargent). Procedure. Continuous flow and flow injection measuremenb were used to demonstrate the features of the large volume wall-jet detector. In the flow injection experiment the glassy carbon electrode was coated with a mercury film as the carrier solution, containing 5 X 10" M Hg(II), flowed at a rate of 0.7 mL/min for 10 min, with a potential of -0.65 V imposed on the working electrode. Continuous flow measurements utilized similar electrode coating, with mercuric ions present in the flowing sample solution; in situ mercury plating proceeded in the subsequent measurement cycles. Following the electrode conditioning, the potential was changed to 0.0 V for 1 min to remove metallic contaminants which might have codeposited with the mercury. Following this, the system was ready for the analytical cycle. Nondeaerated sample and carrier solutions and a deaerated detector solution were used throughout. In flow injection, the deposition potential was applied (15 s after sample injection) for 90 s. The flow was then stopped; a 15-s rest period was observed and a 5 mV/s differential pulse scan was commenced. Upon scan completion, the potential was held at 0.0 V for 30 s and solution flow was resumed. Continuous flow measurements were performed with the deposition potential applied for a selected time, followed by a rest, stripping, and cleaning steps, as in the flow injection experiment. The mercury film was removed at the end of the experiment by wiping with a soft tissue wetted with 1 M nitric acid. Specific details of the stripping procedures are given in the following section. RESULTS AND DISCUSSION Stripping voltammetry is usually performed in the absence of oxygen. The deaeration step-accomplished by bubbling an inert gas-is a major time-consuming step, thus reducing the effectiveness of on-line stripping warning systems. In addition, in flow injection systems, deaeration of small sample volumes is inconvenient and partial reaeration may occur through the Teflon connecting tubing. Various approaches for continuous removal of oxygen from flowing streams have been suggested, including the use of continuous deaeration devices (12) or electrochemical scrubbing (13, 14). The use of the large-volume wall-jet detector eliminates the need of deaerating the sample, thus increasing substantially the efficiency and speed of on-line stripping measurements. For this purpose, nitrogen is continuously bubbled into the solution present in the detector compartment, thus providing the desired oxygen-free atmosphere during the stripping step. Figure 1 shows stripping voltammograms, obtained under continuous flow (A) and flow injection (B) conditions, for nondeaerated river water and pharmaceutical tablet samples, respectively. Nitrogen bubbling to the detector compartment (curves a) yields well-defined stripping voltammograms, similar to those obtained by deaerating both the sample and detector solutions (no shown). In contrast, quantification is not feasible without deaerating the detector solution, as the large oxygen background current obscures the lead and copper stripping peaks (curves b). Notice the significant change in the background (in the river water experiment) from the different current scales used. The continuous deaeration of
0003-2700/85/0357-1778$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985 8
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-0.6 -0.2 E, V Flgure 1. Differential pulse anodic stripping voltammograms for nondeaerated river water (acidified to pH 2) (A) and multivitamin and minerals (B) samples, continuous flow (A) and flow injection (B) mea-
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surements. The pharmaceutical (One-A-Day) tablet was dissolved in 1 M nitric acid and diluted 1:400 in deionized water: detector solution, 0.1 M acetate buffer with (a)and without (b) nitrogen purging; deposition for 5 (A) and 1.5 (B) min at -1.1 V (A) and -0.8V (B); differential pulse conditions, 50 mV amplitude, 5 mV/s scan rate. the detector solution assures that oxygen introduced by the inflowing sample is removed (during the rest period). This, and the use of acidic solutions, avoids chemical interference by oxygen. Similar advantages were obtained in continuous flow measurements of copper in a nondeaerated tap water sample (not shown) and in additional experiments described later. To minimize resistive and migration effects, an inert supporting electrolyte is usually added to the sample solution (at the 0.05-0.5 M level) prior to the stripping measurement. Copeland et al. (15) demonstrated that stripping measurements in aqueous solutions containing low electrolyte concentrations exhibit significant peak current diminutions. Analysis of resistive nonaqueous solutions requires the use of tetraalkylammonium salts, and a satisfactory reference electrode placed closed to the face of the workifig electrode. Being a trace analytical technique, detection limits are often determined by contaminant contributions from the added electrolyte. Even high-purity reagents often require further electrolytic purification. Stripping measurements in the absence of added electrolyte are attractive to eliminate such impurities, as well as for minimizing sample manipulation and alteration of the equilibrium properties (as desirable for effective on-line and speciation measurements, respectively). By the use of a large-volume wall-jet detector a conductive medium is maintained in the detector, and stripping measurements can be performed with resistive samples (with no added electrolyte). Figure 2A shows an anodic stripping voltammogram for an injected methanol solution containing 1 X 10-7M Pb(I1) and Cd(I1) and 3 x M Cu(I1). Welldefined and sharp stripping peaks are observed; in contrast, the corresponding response with methanol present in the detector compartment (dotted line) can provide no useful quantification at all. Application for measurements of metallic impurities in acetonitrile is illustrated in Figure 2B. The
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Flgure 2. Differential pulse anodic strippifig voltammograms in electrolyte-free nondeaerated organic solvents: (A) methanol splked with M Cu(I1); (B) acetonitrile. 1X M Pb(I1) and Cd(I1) and 3 X Flow Injection (A) and continuous flow (8) measurements with 1.5 (A) and 3 (8)min deposition at -1.1 V. Flow rate, 0.7 mL/min. Differential pulse conditions and detector solution are given in Figure 1. The dotted lines represent the response with the organic solvent in the detector compartment.
resulting stripping peaks correspond to 3 X M Cd(I1) and Pb(I1) and 1x lo-' M Cu(I1). Independent of the conductivity of the sample solution, a small ohmic drop is sustained between the working and reference electrodes (as determined by the conductive detector solution). No special effort has been made for a close placement of the working and reference electrodes. The data of Figure 2 clearly demonstrate that sensitive stripping measurements are feasible in electrolytefree organic solvents. A similar advantage of the large-volume wall-jet detector was reported for normal-phase HPLC, using a nonconducting eluent (9). The entire data reported in this study were obtained in the presence of oxygen, thus combining the two features of the detector (use of electrolyte-free and nondeaerated samples) in the same stripping operation. A pronounced improvement is observed also when electrolyte-free aqueous samples are tested (not shown). A tap water sample, not containing any deliberately added supporting electrolyte yielded a well-defined and sharp copper peak. In contrast, a significantly lower and broader peak was obtained in the corresponding experiment with deionized water as the detector solution (conditions: flow injections measurements, with 0.7 mL/min flow rate and -0.8 V deposition potential; other conditions, as in Figure l). Similar results were observed for a "synthetic" water sample, spiked with 5 X M Cd(II), Pb(II), and Cu(I1) (same flow injection conditions, except for a -1.1 V deposition potential). The change of the detector solution resistance resulted in a -0.03 V shift in the peak potentials. The sensitivity and stability of the response were evaluated by using electrolyte-free nondeaerated deionized water solutions. A series of six successive standard additions of 1x lo-' M Pb(I1) yielded a linear calibration plot. A least-squares treatment of the standard additions data yielded a slope of 8.55 0.07 nA/nM, with a correlation coefficient of 0.999 (continuous flow measurements with 2 min deposition at -1.0 V and 0.7 mL/min flow rate; stripping conditions, as in Figure
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Anal. Chem. 1985, 57, 7778-7779
1). A series of 28 repetitive measurements of 2.5 X lo-’ M Pb(I1) over an unbroken period of 3 h yielded highly reproducible peaks with a relative standard deviation of 1.8%, a mean current of 1.83 FA, and a range of 1.75-1.87 pA (conditions, as in the standard additions experiment). The above data illustrate that the electrolyte presence in the detector compartment swamps the effect of variable amounts of electrolyte inadvertently present in the sample. For example, each standard addition of Pb(I1) increased the nitric acid content of the sample by approximately 10“ M (the latter being used for preparing the metal ion stock solution). Similarly, the highly stable response demonstrates that small changes in the composition of the detector solution due to the continuous sample inflow (e.g., increased content of Hg(I1) and associated nitric acid or some dilution) do not affect the performance. The renewable surface of the in situ plated mercury f i i electrode makes it especially suitable for long-run monitoring. In conclusion, the use of a large volume wall-jet detector--with a deaerated electrolytic solution at the detector compartment-permits trace metal analyses of electrolyte-free nondeaerated samples. This would facilitate analyses of nonaqueous matrices and result in increased speed and simplicity of on-line stripping measurements, as well as reduced contamination risks. Such operation is not feasible with the commonly used low-dead volume detectors. It has been shown recently that microelectrodes can be used to obtain undistorted cyclic voltammograms in highly resistive solutions (16). We are presently evaluating this approach for trace metal stripping measurements in such solutions. The present wall-jet detector possesses the additional distinct advantage of using
nondeaerated samples. Unlike the medium-exchange procedure (7, 17), where the stripping step is performed after transferring a more favorable solution to the cell, the favorable detector solution-in the present design-is “active” (with respect to the cell conductivity) during both the deposition and stripping steps. This is important for measurements in electrolyte-free solutions, as conductive medium is essential in both steps.
LITERATURE CITED Vydra, F.; Stulik, K.; Julakova, E. “Electrochemical Strlpping Analysls”; Wlley: New York, 1976. Wang, J. “Stripping Analysis: Prlnclples, Instrumentation and Applicatlons”; VCH Publishers: Deerfield Beach, FL, 1985. Zirino, A.; Lieberman, S. H.; Clavell, C. Envlron. Scl. Techno/. 1978, 12, 73. Wang, J. Am. Lab. (Fairfhld, Conn.) 1983, 7 , 14. Anderson, L.; Jagner, D.; Jsefson, M. Anal. Chem. 1982, 5 4 , 1371. Wang, J.; Dewaid, H. D.; Greene, B. Anal. Chim. Acta 1983, 146, 45. Hu, A.; Dessy, R. E.; Granell, A. Anal. Chem. 1983, 55, 320. Elbicki. J. M.; Morgan, D. M.; Weber, S. G. Anal. Chem. 1984, 56, 978. Gunaslngham, H.; Fleet, B. Anal. Chem. 1983, 55, 1409. Gunaslngham, H.; Tay, 0. T.; Ang, K. P. Anal. Chem. 1984, 56, 2422. Wang, J.; Dewald. H. D. Anal. Len. 1983, 16, 925. Wang, J.; Ariel, M. Anal. Chim. Acta 1978, 99, 89. Hanekamp, H. B.; Voogt, W. H.; Bos, P.; Frei, R. W. Anal. Chlm. Acta 1980, 118, 81. Wang, J.; Dewald, H. D. Anal. Chem. 1983, 55, 933. Copeland, T. R.; Christie, J. H.; Skogerboe, R. K.; Osteryoung, R. Anal. Chem. 1973, 4 5 , 995. Bond, A. M.; Fleischmann, M.; Robinson, J. J. Nectroanal. Chem. 1984, 168, 299. Wang, J.; Greene, B. Water Res. W83, 17, 1635.
RECEIVED for review January 28, 1985. Accepted March 25, 1985. This work was supported in part by the National Institutes of Health, Grant No. GM30913-01A1.
Fast in Vivo Monitoring of Dopamine Release in the Rat Brain with Differential Pulse Amperometry Franpoise Marcenac and Franpois Gonon*
Inserm U 171, H6pital Ste. EugBnie, Pauillon 4 H, 1, Avenue Georges ClBmenceau, 69230 St. Genis Laual, France In an attempt to study dopaminergic neurotransmission in the brain, electrochemical techniques have already been applied to monitor in vivo spontaneous as well as evoked dopamine (DA) release from dopaminergic nerve terminals (1-7). Wightman and his group combined chronoamperometry (5) or fast cyclic voltammetry (3) with untreated carbon fiber microelectrodes to monitor evoked DA release after the electrical stimulation of the dopaminergic neuronal pathway. With these techniques the extracellular DA concentration was monitored every 0.25 to every 6 s (2,3). However, due to their low sensitivity (detection limit around 5 pM) their application was, up to now, restricted to the measurement of DA release, provided that it was strongly stimulated by high-frequency electrical stimulations. In order to improve the selectivity of chronoamperometry, Marsden et al. (4) suggested the use of double-step chronoamperometry. However, no applications of this technique have already been reported. In our group we recently combined a new technique, differential normal pulse voltammetry (DNPV) with electrochemically treated carbon fiber electrodes and we were able to detect from the striatum of pargyline-treated rats a signal due to the spontaneous DA release (1,6). Moreover, DNPV allowed us to record the effect of low-frequency electrical stimulations (1). Unfortunately, with that technique, DNP voltammograms were recorded every 1 win. Thus, in an attempt to monitor the kinetics of rapid phenomena, we developed a new technique that is as selective and sensitive as
DNPV but much faster: differential pulse amperometry (DPA). DPA was directly derived from DNPV. It consisted of an unlimited series of dual pulses identical with that used in DNPV but at a constant potential (Figure 1). Like with DNPV, the oxidation current was differentiated during a measuring pulse. The parameter values were chosen like those of DNPV, except for the final potential, that was set at a value slightly under the oxidation potential of DA (+80 mV vs. the Ag/AgCl reference electrode (6)). With DPA, we were able to detect very low variations of DA concentrations, to measure these concentrations every 0.4 s and thus to monitor kinetics of electrically evoked DA release under “physiological” conditions from the striatum of pargyline-treated and anaesthetized rats.
EXPERIMENTAL SECTION Reagents. Dopamine hydrochloride (Sigma) and ascorbic acid (AA) (Merck) were dissolved in a phosphate buffered saline (PBS) solution (KC1,0.2 g/L; NaC1,0.8 g/L; NazHPO4-2Hz0,1.44 g/L; KH2P04,0.2 g/L; pH 7.4). In order to prevent spontaneous oxidation of DA, AA was always added into the solutions before the addition of DA. However, the DA response did not depend on the AA concentration (between 50 and 300 pM). Electrochemical Procedures. The carbon fiber electrodes were prepared as previously described (7). Before each experiment, they were electrochemically treated ( I , 6). Electrochemical treatments, as well as DPA measurements, were performed by
0003-2700/65/0357-1778$01.50/00 1985 American Chemlcal Society
