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Anal. Chem. 1984, 56, 156-159
directly measure the concentration of free surfactant from the emf at the CMC,,,. This also allows us to determine the true CMC at the CMC,, which was found to be less than the CMC without PVP. The studies with @-lactoglobulinillustrate that comparable results are obtained for the binding of decyl sulfate to @lactoglobulin by equilibrium dialysis or using surfactant selective electrodes. Our results show that a reference titration without protein must be used to determine the amount of binding due to the likely binding of surfactant at low surfactant concentrations. This level of binding must be quite small since it does not produce large changes in the conformation of (3-lactoblobulin as detected by circular dichroism studies. The major limitation to the use of ion-selective electrodes in the presence of polymers (including proteins) is their effect on the electrode response per se. This leads to altered values of Eo and N a n d may preclude quantitative use of the electrodes except for determining break points in the titration. The latter may be useful if their significance can be established by other types of measurements. The use of surfactant electrodes to monitor the surfactant concentration in the outer fluid during dialysis experiments appears to be without cornplications.
ACKNOWLEDGMENT We are indebted to Kalidas M. Kale for assistance in
preparing our first surfactant ion selective electrodes. Registry No. Poly(vinylpyrrolidone),9003-39-8; sodium octyl sulfate, 142-31-4;sodium decyl sulfate, 142-87-0.
LITERATURE CITED (1) Blrch, B. J.; Clarke, D. E. Anal. Chim. Acta 1973, 6 7 , 387-393. (2) Birch, 8. J.; Clarke, D. E.; Lee, R. S.; Oakes, J. Anal. Chim. Acta 1974, 7 0 , 417-423. (3) Kale, K. M.; Cussler, E. L.; Evans, D. F. J . Phys. Chem. 1980, 54, 593-598. (4) Kale, K. M.; Kresheck, G. C.; Erman, J. I n "Solution Behavior of Surfactants"; Mittal, K. L., Fendler, E. J., Ed.; Plenum: New York, 1982; Vol. 1, pp 665-676. (5) Kresheck, G. C.; Kale, K.; Erman, J. I n "Solution Behavior of Surfactants"; Mittal, K. L., Fendler, E. J., Eds.; Plenum: New York, 1982; VOi. 1, pp 677-692. (6) Birch, B. J.; Clarke, D. E. Anal. Chim. Acta 1972, 67,159-162. (7) Mukerjee, P.; Mysels, K. J. "Critlcal Micelle Concentrations of Aqueous
Surfactant Systems"; US. Superintendent of Documents: Washington, DC, 1971. (8) Molyneux, P. I n "Water: A Comprehensive Treatise"; Franks, F., Ed.; Plenum: New York, 1975; Vol. 4, pp 569-757. (9) Kresheck, G. C.; Hargraves, W. A. J . Colloidlnterface Sci. 1981, 8 3 ,
1-8. (10) Damon, A. J. H.; Kresheck, G. C. Biopo/ymers 1982, 27, 895-908. (11) DeLlsl, R.; Perron, G.; Paquette, J. Can. J . Chem. 1981, 59, 1865-1871. (12) Nikolov, A,; Martynov, G.; Ekserova, D. J . Collokj Interface Scl. 1981. 87,116-124. (13) Aral, H.; Murata, M.; Shlnoda, K. J . Colloid Interface Sci. 1971, 37, 223-227. (14) Shlrahama, K.; Tsujji, K.; Takagi, T. J . Biochem. 1974, 75,309-319.
RECEIVED for review May 25,1983. Accepted October 24,1983.
Subtractive Anodic Stripping Voltammetry with Flow Injection Analysis Joseph Wang* and Howard D. Dewald Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
A new approach to subtractive anodic strlpplng voltammetry, utilirlng the manifold of a flow Injection system, is descrlbed. For thls purpose the carrier solution stripping voltammogram Is subtracted from the sample one, yleidlng a purely analytlcai response. The effectlve correctlon for both Faradaic and non-Faradaic background current contributions allows a detectlon limit for cadmlum of about 6 X lo-' M (0.14 ng) with l-mln deposlon. Nondeaerated samples can be used due to the effective correctlon of the oxygen reduction current. The system permits simultaneous measurement of several trace metals in the part-per-billion concentration level, using a 2 0 0 - ~ Lsample volume and an injection rate of 24 samples per hour. Good precision and linear cailbration plots are obtained. Appllcabillty to various real samples Is illustrated.
Anodic stripping voltammetry (ASV) is a powerful electroanalytical technique for trace metal measurements (1, 2). Various approaches have been proposed for further improving its sensitivity, aimed mainly a t correcting the non-Faradaic charging background current. These include the application of potential-time wave forms such as differential pulse (3), phase selective ac ( 4 ) ,or staircase (5) to replace the conventional linear scan during the stripping step. Despite the improved sensitivity offered by these potential-time excita-
tions, the remaining Faradaic background current contributions (e.g., oxidation of mercury, reduction of hydrogen ions or oxygen) limit the detectability. By use of a subtractive stripping mode, it is possible to correct for both Faradaic and non-Faradaic background current contributions and to shorten the deposition time. Subtractive ASV is based upon subtracting a background voltammogram from the analytical one. Different approaches have been suggested for generating the background curve. These include the use of twin working electrodes placed in two cells (sample and electrolyte) ( 6 ) , different deposition times or convection rates on twin electrodes immersed in the sample solution ( 7 - I O ) , or different deposition times on a single electrode (11, 12). The present paper describes a new and convenient approach to generate the background curve for subtractive ASV, utilizing small sample volumes and a rapid sampling rate. The method is based upon the incorporation of ASV with the manifold configuration of a flow injection system. In flow injection analysis (FIA) small volumes of sample solution are injected into a carrier stream that transports the sample toward the detector (13,14). Conventional voltammetric and potentiometric stripping analyses have been incorporated recently as sensitive detection modes for FIA (15, 16). The unique nature of the FIA manifold can be exploited toward achieving the desired subtractive ASV response. For this purpose the "analytical" and "background stripping voltam-
0 1984 American Chemical Society 0003-2700/84/0356-0156$01.50/0
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
157
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mograms are recorded while the sample and carrier (electrolyte) solutions flow through the detector, respectively (Figure 1). The carrier contribution is then subtracted from that of the sample plug, yielding a purely analytical response
where the subscripts s and c represent the sample and carrier solutions, respectively, k is a constant (depending on the geometry, mass-transport conditions, and the dispersion), t d is the deposition period, and C, is the metal concentration in the sample plug. Since the need for a dual electrodes system is eliminated, problems associated with matching two working electrodes (or maintaining them in an identical state) no longer exist. In addition, the electrode identity is maintained between the "analytical" and "background" cycles, because the same potential sequence is applied in both measurements. Successful implementation of the FIA/subtractive ASV approach (i.e., effective background correction and accurate measurement of the metals) requires careful matching of the electrolytic content and pH of the two solutions and the minimization of trace metal impurities in the carrier solution. The "background" curve can be recorded periodically, e.g., following three sample injections (as in most of this study) so that the speed of the FIA/ASV system is not compromised. In fact, the effective background correction obtained permits the use of significantly shorter deposition times; combining this with the elimination of the deaeration step makes the method rapid and convenient. The characteristics and advantages of the method are discussed in this study.
EXPERIMENTAL SECTION Apparatus. The flow injection system consisted of carrier and sample reservoirs (400-mL Nalgene beakers), a FMI lab pump RP-SY (Fluid Metering, Inc.), a Rheodyne Model 7010 sample injection valve fitted with a 200-fiL sample loop, and the electrochemical detector. Interconnections were Teflon tubing (0.5 mm id.) and polypropylene tube end fittings (Pierce Chemical Co.). An 8 cm long tubing connected the valve to the detector. The electrochemical detector (17)consisted of a 1.8-mm solution inlet drilled through a Plexiglas body, widened to 8.3 mm to accept the working electrode shaft. The Ag/AgC1(3 M NaC1) reference electrode perpendicularly joined the working electrode compartment close to the face of the 3-mm glassy carbon disk working electrode. The carbon rod auxiliary electrode was dipped into the solution outlet channel that had been drilled perpendicular to the working electrode compartment. Measurements were made with a Sargent-Welch Model 4001 polarograph. Reagents. Chemical and reagents used were described in detail previously (15), except as noted. The preparation of the multivitamin with minerals tablet solutions was described elsewhere
(18). Supporting electrolytes were 0.1 M potassium nitrate, 1 M or 3 x M nitric acid, and 0.1 M acetate buffer (pH 4.5). Procedure. The glassy carbon disk electrode was polished with a 0.05-wm alumina slurry until a mirrorlike surface was obtained. The electrode was filmed as the carrier solution (containing 5 X 10" M Hg2+in the supporting electrolyte) passed through the detector at a rate of 0.2 mL/min for 20 min while the potential was kept at -0.8 V. This was followed by a cleaning step for 2 min at 0.0 V. Sample and carrier measurements were obtained as shown in Figure 1. Nondeaerated sample and carrier solutions were used. Sample loop filling was accomplished under gravity flow. The deposition potential was applied (5 s after sample injection) for 1min. The flow was then stopped; a 15-s rest period was observed, and a 2 V/min differential pulse scan was commenced. Upon scan completion, the potential was held at 0.0 V and solution flow was resumed. A 45-s wash period was followed by measurement on another sample injection or on the carrier solution using the same deposition and stripping conditions. In most cases, the carrier response was recorded following three samples injections. The subtractive voltammograms were obtained by use of a Tektronix 4010 terminal of PROPHET (a time-sharing computer operated by the National Institutes of Health), as was described previously (19).
RESULTS AND DISCUSSION Figure 2 illustrates typical voltammograms obtained by the subtractive and conventional ASV approaches (curves c and a, respectively) following the injection of a nondeaerated sample containing 2 X lo-' M lead and cadmium. The carrier stripping response, used for obtaining the subtractive response, is shown in curve b. In the conventional ASV approach, the large background current (associated mainly with the charging of the double layer and the reduction of oxygen) masks almost completely the two peaks of interest. Following subtraction of the carrier response, a flat base line is achieved and the two peaks are quantified easily. As a result, 1-min deposition is sufficient for measurements at the parts-per-billion concentration level. The inconvenience associated with the deaeration of small sample volumes (20) is eliminated by the effective compensation of the oxygen background. Nondeaerated sample and carrier solutions were used throughout this study. The ionic strength of the two solutions was adjusted, by adding the supporting electrolyte, to maintain similar oxygen solubility. (A previous ASV system, based on the medium exchange nature of the FIA manifold (15),offered the advantage of nondeaerated samples when coupled with an oxygen-free carrier; the present procedure eliminates the need for deaerating the carrier solution.) As in other FIA systems, controlled dispersion and reproducible timing, based on the sequence of Figure 1, are essential for successful operation.
158
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E,V Figure 2. Linear scan stripping measurements of 2 X lo-’ M Pb2+ and Cd2+: (a) “analytical”(sample)curve; (b) “background”(carrier) curve; (c)subtractive response, (a) - (b). One minute deposition was at -1.1 V, scan rate was 2 V/min, supporting electrolyte and carrier were 0.1 M KNO,, and flow rate was 0.2 mL/min. In order to demonstrate the ability of the method to correct for various Faradaic background current contributions, a differential pulse ramp was employed in the stripping step. Figure 3 compares subtractive and conventional differential pulse stripping voltammograms (curves c and a, respectively) for 5 X M copper, 2.5 X lo-’ M lead and cadmium, and 2 X lo4 M zinc. While the differential pulse stripping response is free of the charging current contributions, Faradaic background contributions limit its detectability. The mercury oxidation and hydrogen evolution background currents obscure the copper and zinc peaks of interest (curves a). By use of the subtractive mode (curve c), these background contributions are compensated and the two peaks are well-shaped and easily measurable. The pH of the sample and carrier solutions should be matched in order to achieve effective compensation of the hydrogen evolution background. In addition, quantitation of the four ions is improved with respect to the oxygen reduction background interference. Zinc was measured in the absence of the other ions, to prevent complication caused by the Cu-Zn intermetallic formation. Notice the lead and copper peaks in the carrier response (curve b). As in most ultratrace measurements, these are the result of metallic impurities present in the salt used as supporting electrolyte and the water. Since the same salt is used in the preparation of the carrier and sample solutions, the subtractive mode corrects for errors originating from this source of contamination. A similar correction is obtained for errors originating from impurities present in the water used for dissolving or diluting the sample. In contrast, if the sample and carrier solutions are of a different nature, impurities in the carrier solution would lead to errors in the subtractive response; in these situations further purification of the carrier solution is required. Overall, since this subtractive approach is based upon matching two solutions (rather than two electrodes) judicious precautions must be taken. These include matching of the temperature or viscosity, in addition to the pH or electrolytic content discussed earlier. The flat base line, obtained throughout this work, indicates the accuracy of the background correction.
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Figure 3. Stripping measurements for 5 X lo-’ M Cu2+, 2.5 X lo-’ M Pb2+ and Cd2+, and 2 X lo-’ M Zn2+: (a) “analytical” curve; (b) “background” curve; (c) subtractive response. Differential pulse conditions are as follows: amplitude, 50 mV; repetition time, 0.5 s; scan rate, 2 V/mln. Deposition time was 1 min; deposition potential was -1.1 V (mixture)and -1.4 V (Zn2+). Supporting electrolyte and carrier were 0.1 M KNO, (mixture)and 0.1 M acetate buffer (pH 4.5) (Zn2+). Flow rate was 0.2 mL/min. Since the differential pulse stripping mode offers the advantage of peak current enhancement (due to the metal replating effect ( I ) ) , this mode was used in conjunction with the rest of the subtractive measurements. Relatively fast (2 V/min) scan rates, as compared to previous differential pulse studies, were employed to maintain the advantage of a rapid assay rate. For the same reason, the carrier curve was recorded after a given number of injections (usually three) and not after each injection. We found that the carrier curve remains stable even when various real samples are used (vida infra). The frequency of this “background recording may depend on the nature of the sample; with samples that tend to foul the electrode, more frequent recording may be required. Overall, the system allows simultaneous quantitation of metals present at the part-per-billion concentration level at a rate of 24 samples per hour. Our previous FIA/ASV system allowed an injection of 10 samples per hour (15);the increased rate is due mainly to the gain in sensitivity offered by the subtractive mode, which permits the use of shorter deposition periods. In addition, the present system eliminates the need for deaeration. Quantitative evaluation is based on the linear correlation between peak currents and concentration. Figure 4 compares conventional and subtractive differential pulse stripping voltammograms obtained after successive injections of a lead solution of increasing concentration ((1.0-5.0) X M). The subtractive response yielded a linear calibration plot (slope 3.59 pA/pM; correlation coefficient, 0.995; intercept, 0.02 PA). A similar FIA/subtractive ASV experiment for cadmium (five injections from 0.5 x M to 2.5 x IO-’ M) yielded a linear calibration plot (slope, 4.27 pA/pM; correlation coefficient, 0.998; intercept, -0.04 pA (not shown)). On the basis of the signal-to-noise ( S I N = 2) characteristics of the response, the detection limit for cadmium (using a 1-min deposition) would be 6 X M, corresponding to 0.14 ng for the sample volume injected. Still lower detectability is obtainable by using longer
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
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Flgure 4. Comparison of conventional (a-e) and subtractive (f-j) stripping response of Pb2+ solutions of increasing concentrations: (a, f) 1 X M, (b, g) 2 X lo-' M, (c, h) 3 X M, (d, i) 4 X lo-' M. Conditions are the same as for the mixture M, and (e, j) 5 X of Figure 3, except that the deposition potential was -1.0 V.
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with mineral tablets are used as representative samples. The quantitation of copper and zinc in such tablets was reported recently (18). Comparison of the conventional and subtractive curves (a vs. c) demonstrates again the advantages offered by the subtractive mode, i.e., effective compensation of various Faradaic background contributions (hydrogen evolution, oxygen reduction, mercury oxidation). As a result, peaks occurring near the limits of the working potential range are well-defined. These data (coupled with subsequent standard additions) yielded values of 26.6 and 0.5 mg of zinc and copper in the original tablets; the labeled values are 23.9 and 2 mg, respectively. In a similar FIA/subtractive ASV experiment copper was determined in a local tap water sample (not shown). Effective discrimination against the mercury oxidation and oxygen reduction background currents was obtained, and a 1.4 X lo-' M concentration was calculated. In conclusion, FIA, originally designed to automate and speed up routine analyses, allows the convenient implementation of some novel approaches, such as subtractive ASV. The resulting FIA/subtractive ASV system permits sensitive measurements of trace metals in small volumes of nondeaerated samples a t a relatively high injection rate. The accuracy of background correction is good as evidenced from the flat base line of the subtractive response. This is due to the use of a single electrode, that overcomes matching problems associated with dual electrodes subtractive ASV. Successful implementation of the method requires matching of the sample and carrier solutions. Matrix interferences (e.g., oxygen) can be eliminated when the approximate concentration of the interference is present or added to carrier solution. A similar approach could be applied for other preconcentration schemes, e.g., spontaneous adsorption or anodic deposition (as in cathodic stripping voltammetry). Registry No. Cadmium, 7440-43-9; lead, 7439-92-1; copper, 7440-50-8; zinc, 7440-66-6; water, 7732-18-5.
LITERATURE CITED
A
-0.4 -0.6 -0.8
159
-Id
Flgure 5. Response for injection of multivitamin plus minerals tablet solutions, Cu from "One-A-Day'' tablet and Zn from K-Mart "Stress Formula": (a) "analytical" curve; (b) "background" curve; (c) subtractive response (200X dilution of Cu containing solution to 1 M nltric acid supporting electrolyte, 6500X dilution of Zn containing solution to 0.1 M acetate buffer (pH 4.5) supporting electrolyte). 1 X M Ga3+was added in Zn measurements to avoid formation of a Cu-Zn intermetallic compound. Conditions are the same as in Figure 3, except that the Cu deposition was for 2 min at -0.8 V.
deposition periods (in conjunction with larger sample volumes and lower injection rates). The precision was estimated from 15 successive injections of a 2.5 X M cadmium solution. The mean peak current found was 1.06 PA with a range of 1.00-1.11 PA and a relative standard deviation of 3.2% (conditions, as in Figure 3). Figure 5 demonstrates the applicability of the method to measurements of trace metals in real samples. Multivitamin
(1) Copeland, T. R.; Skogerboe, R. K. Anal. Chem. 1974, 4 6 , 1257A. (2) Wang, J. Envlron. Scl. Technol. 1982, 16, 104A. (3) Copeland, T. R.; Christie, J. H.; Osteryoung, R. A.; Skogerboe, R. K. Anal. Chem. 1973, 4 5 , 2171. (4) Underkofler, W. L.; Shah, I. Anal. Chem. 1965, 37, 218. (5) Eisner, U.; Turner, J. A.; Osteryoung, R. A. Anal. Chem. 1976, 4 8 , 1608. (6) Steeman, E.; Temmerman, E.; Verbinnen, R. Anal. Chlm. Acta 1978, 96, 177. (7) Kernula, W. Pure Appl. Chem. 1967, 15,283. (8) Zirino, A.; Healy, M. L. Environ. Sci. Technol. 1972, 6 , 243. (9) Wang, J.; Ariel, M. J. Electroanal. Chem. 1977, 85,289. (IO) Wang, J.; Ariel, M. Anal. Chim. Acta 1981, 128, 147. (11) Brown, S.D.; Kowalski, 6. R. Anal. Chim. Acta 1979, 107, 13. (12) Wang, J.; Greene, B. Anal. Chlm. Acta 1982, 144,137. (13) Betteridge, D. Anal. Chem. 1978, 50, 832A. (14) RuiiEka, J.; Hansen, E. H. "Flow Injection Analysls"; Wiiey: New
York, 1981. (15) Wang, J.; Dewald, H. D.; Greene, B. Anal. Chim. Acta 1983, 146,45. (16) Hu, A.; Dessy, R. E.; Graneli, A. Anal. Chem. 1983, 55,320. (17) Wang, J.; Greene, B. J. Elecfroanal. Chem. 1983, 154,261. (18) Wang, J.; Dewald, H. D. Anal. Lett. 1983, 16,925. (19) Wang, J.; Freiha, A. B. Talanfa 1963, 30, 837. (20) Lewis, J. Y.; Zodda, J. P.; Deutch, E.; Heinernan, W. R. Anal. Chem. 1983, 55, 708.
RECEIVED for review July 25,1983. Accepted October 21,1983. This work was supported by a grant from the US. Department of the Interior, through the New Mexico Water Resources Research Institute.
