Digital Pulse ac Voltammetry for the Simultaneous Analysis of

A digital two-step and three-step pulse potential ac volta- mmetry system was proposed and applied for the simul- taneous analysis of electrosorptive ...
0 downloads 0 Views 142KB Size
Download PDF
Anal. Chem. 2003, 75, 6368-6373

Digital Pulse ac Voltammetry for the Simultaneous Analysis of Electroactive and Electrosorptive Species in Flow Systems Chur-Min Chang† and Hsuan-Jung Huang*,‡

Department of Chemistry, National Sun Yat-sen University, Kaohsiung, 80424 Taiwan, and Department of Medical Chemistry, Chia Nan University of Pharmacy and Science, Tainan, 71712 Taiwan

A digital two-step and three-step pulse potential ac voltammetry system was proposed and applied for the simultaneous analysis of electrosorptive and electroactive species in flow systems. To perform the ac polarography function, a PC was interfaced to a potentiostat to mimic all the necessary hardware functions of an analog ac polarograph. From the measurement of the change of phase-selective charging current and the zero-order current, I3-, Br- specifically adsorbed and Cd2+, Pb2+ reduced at a hanging mercury drop electrode can be determined simultaneously in a FIA and IC system. With the digital pulse ac voltammetry-coupled IC, detection limits as low as 5.0 µM and linear dynamic ranges from 5.0 to 100 or 200 µM with linear correlation coefficients better than 0.9990 were found for the analysis of I3-, Br-, and S2O32-. Although amperometric detection has become one of the most sensitive and frequently used methods for electrochemical analysis in FIA and liquid chromatography, its applications are limited to the analysis of electroactive species. Different from the characteristics of amperometric analysis for electroactive compounds, electroactive or electroinactive species that adsorbed specifically on an electrode surface can be determined by the ac polarographic method. The ac polarographic method is thus a useful method complementary to the very sensitive amperometric method for analyzing electroinactive species. From the change of differential capacitance or charging current, detection of electrosorptive species has been determined successfully with the conventional analog ac polarographic method.1-8 However, instruments consisting of a small-amplitude ac signal source, a high-pass filter or * To whom correspondence should be addressed. E-mail: hjhuang@ mail.nsysu.edu.tw. Tel: 886-7-5252000 ext. 3919. Fax: 886-7-5253919. † Chia Nan University of Pharmacy and Science. ‡ National Sun Yat-sen University. (1) Ramstad, T.; Weaver, M. J. Anal. Chim. Acta 1988, 204, 95. (2) Ramstad, T.; Weaver, M. J. J. Chromatogr. 1988, 456, 287. (3) Ramstad, T.; Weaver, M. J. J. Chromatogr. 1988, 456, 307. (4) Bond, A. M.; Jone, R. D. Anal. Chim. Acta 1983, 152, 13. (5) Dejong, H. G.; Voogt, W. H.; Bos, P.; Frei, R. W.; Lig, J. J. Chromatogr. 1983, 6, 1745. (6) Ramstad, T.; Milner, D. Anal. Instrum. 1989, 18, 147. (7) Muller, E.; Muller, K.; Dorfler, H.-D. Colloid Interface Sci. 1987, 3, 342. (8) Bednarkiewicz, E.; Donten, M.; Kublik, Z. Anal. Chim. Acta 1985, 176, 133.

6368 Analytical Chemistry, Vol. 75, No. 22, November 15, 2003

turned amplifier input, a lock-in amplifier, a phase shifting circuitry, and a low-pass filter output are needed for performing the analog ac polarographic measurement.9-11 To circumvent the demand of so many sophisticated instruments, a versatile digital ac polarographic method that can generate the fundamental and second harmonic ac polarograms in phase-sensitive or total current versions was developed by Bond et al.12-14 Currently, digital ac polarography has become one of the standard functions included in instruments of electrochemical analysis. But the functions of digital ac polarography embedded in those commercially available sophisticated instruments are often inflexible and limited. They are not applicable for the electrochemical analysis in a flow system. In this study, based on the digital ac polarographic techniques reported in the literature,12-14 a digital sine waves superimposed two-step and three-step pulse potential ac voltammetry system was proposed and applied for the simultaneous analysis of electrosorptive and electroactive species in flow systems. To form the digital ac voltammetric system, a PC was interfaced to a potentiostat to mimic all the necessary hardware functions provided by the conventional analog ac polarography instruments. Solutions containing ions of I3-, Br-, S2O32-, and SCN- known to be adsorbed electrochemically on Hg electrode surface and Cd2+ and Pb2+ were analyzed in systems of FIA and IC to demonstrate the applicability of the developed method. EXPERIMENTAL SECTION The functions of the digital ac voltammetry was developed by interfacing a PC to a potentiostat. The control system for performing the digital ac experiments consists of a PCL-818 highperformance data acquisition card with programmable gain (from BQC Microsystem, Sunnyvale, CA), a PCLD-889 amplifier/ multiplexer board (from BQC Microsystem), and a personal computer. Explanation of the interfacing of PCL-818 highperformance data acquisition card with PC was reported in our previous work.15 Besides the control system, three circuitries (9) Smith, D. E. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1966; Vol. 1, Chapter 1. (10) Bond, A. M. Modern Polargraphic Methods in Analytical Chemistry; Marcel Dekker: New York, 1980; Chapter 7. (11) Smith, D. E. CRC Crit. Rev. Anal. Chem. 1971, 2, 247. (12) Anderson, J. E.; Bond, A. M. Anal. Chem. 1981 53, 1394. (13) Bond, A. M.; Heritage, I. D. J. Electroanal. Chem. 1987, 222, 35. (14) Anderson, J. E.; Bond, A. M. Anal. Chem. 1982, 54, 1575. 10.1021/ac034491s CCC: $25.00

© 2003 American Chemical Society Published on Web 09/30/2003

Figure 1. Scheme of the waveforms for sine waves superimposed (a) staircase, (b) two-step pulse, and (c) three-step pulse potential.

designated for the waveform synthesizing (summing circuitry), potential scale modulating (modulating circuitry), and background current offsetting (offsetting circuitry) and a potentiostat (amperometric detector, BAS LC-4B) were included to form the digital ac voltammetry detection system. The FIA system consists of a peristaltic pump (Gilson Minipulse 2), a pulse damper, and an injection valve (Rheodyne 50) with a 100-µL sample loop. A hanging mercury drop electrode (EG&G PARC model 303A) in a wall-jet configuration, a piece of Pt wire, and an Ag/AgCl (saturated KCl) electrode were used respectively as the working, auxiliary, and reference electrodes in the flow system. For IC analysis, a LC pump (SSI, model 300) with a pulse damper single piston, a Dionex HPIC CS5 cation column, and an injection valve (Rheodyne 7125) with a sample loop of 50 µL were used. Solutions of 0.05 and 0.15 M Mg(NO3)2 were used as the eluent for separation in IC. Flow rate used for IC separation was 0.6 mL/min. (Hg is a hazardous and toxic material and should be handled carefully.) Programs for the functional control, data acquisition, storage, display, and processing were written in turbo C. Generation of the Sine Wave Superimposed Staircase, Two-Step, and Three-Step Pulse Potential. The waveforms applied to the system were composed of the superposition of sine waves on a staircase, two-step pulse, or three-step pulse potential. Figure 1 shows the waveforms of the sine wave superimposed staircase (a), two-step pulse (b), and three-step pulse (c) potential scheme. For each step of ramp or pulse potential, a delay time of 50 ms was allowed to elapse and 20 cycles of sine wave (with a 10-mV peak-to-peak amplitude) were then superimposed. Sine waves of various steps were generated by the look-up table method. As the ac current was sampled at the end of each step (corresponding to different phase angles), the resolution of phase (15) Chao, M. H.; Huang, H. J. Anal. Chem. 1997, 69, 463.

angle for current measurement and the maximum frequency of applicable sine wave were related to the number of steps used for composing the sine wave. A digital sine wave consisting of 360 steps will result in a phase angle resolution of 1° and a frequency of 112.5 Hz in this experiment. To eliminate the transient response resulting from the change of applied staircase potential, only the ac currents resulting from the last 15 cycles of the superimposed sine wave were sampled and processed. Digital Processing of ac Voltammetry Current. The current response measured from the ac voltammetry includes ac and dc components of an electrochemical reaction and is called total current (It). From the literature, the phase-selective ac components can be extracted digitally by processing the total current at a specified phase angle with a simulated lock-in amplifier.12,13 The extraction process involves multiplication of the total current at a specified phase angle by a square wave of the same frequency with an amplitude of 1.0. The phase angle of the fundamental harmonic current was determined by the counterclockwise shift of square wave with respect to the steps of sine wave. The phase-selective fundamental harmonic currents were obtained by averaging the currents extracted from the 15 superimposed sine waves. Pure dc component can be obtained by passing the total current to an ac filter in an analog ac voltammetric system. Extraction of the dc component was obtained by averaging the total current at all phase angles of sine waves. This procedure is based on the fact that, for current measured at each phase angle, there is a 180° out-of-phase equivalent ac current located at another phase angle in the ac voltammogram.13 Summation of the ac current at all the steps of a sine wave would result in the cancellation of the ac component from the total current. The extracted dc current is called the zero-order current in this experiment. Chemicals and Reagents. All chemicals and solvents used were of analytical grade and were used as received. The deionized water prepared from a Milli-Q system (Millipore) with a R of 18 MΩ‚cm was used for the solution preparation. RESULTS AND DISCUSSION For a reversible electrochemical process, the phase relationships for faradaic current, If, and charging current, Ic, with other parameters can be expressed by the following equations,10

If ) n2F2AC(ωD)1/2 ∆E sin(ωt + 45°)/4RT cosh2(j/2) (1) Ic ) ACdl ∆Eω cos ωt

(2)

where ω is the angular frequency, ∆E is the amplitude of applied ac potential, j is equal to nF(Edc - E1/2)/RT, Edc is the dc potential applied, E1/2 is the reversible half-wave potential, and Cdl is the differential capacity of the double layer. The other symbols have their conventional definitions. From the phase relationship shown in eqs 1 and 2, pure If can be obtained at the phase angles of 90° and 270°, where the component of charging current is zero, and pure Ic can be obtained at the phase angles of 135° and 315°, where the component of faradaic current is zero. Analytical Chemistry, Vol. 75, No. 22, November 15, 2003

6369

Figure 2. Digital 3-D fundamental harmonic current phase angle voltammograms obtained with injection of solutions of (a) 1.0 × 10-4 M Cd2+ in 1.0 M KCl and (b) 1.0 M KCl. Sine waves of 112.5 Hz (with a peak-to-peak amplitude of 10 mV) were superimposed on the staircase potential covering a range of -0.4 to -0.8 V with a ramp step of 2 mV. Digital 3-D fundamental harmonic current phase angle chronoamperograms obtained by injection of solutions of 1.78 × 10-4 M Cd2+ + 5.0 × 10-4 M I3- in 0.1 M NaNO3 with pulse potential at (c) -0.8 and (d) -0.5 V, respectively.

From the literature,16 the change of double layer capacitance, ∆Cdl, is proportional to the surface excess, Γi of adsorbed species when the adsorption isotherm follows Henry’s law. As the surface excess Γi varies proportionally to the concentration of adsorbed species in solution, the change of charging current and differential capacitance are therefore proportional to the concentration of adsorbed species in solutions and can be used for the determination of electrosorptive species in solutions.1-8 From the digital 3-D ac voltammograms, pure If and Ic at the selected phase angle can be obtained readily. The concentration of electroactive and electrosorptive species in solutions can thus be determined simultaneously by extracting the pure If, Ic from the obtained 3-D ac voltammograms. Characteristics of the Digital ac Voltammetry System. To demonstrate the function of the developed system, solutions of 1.0 × 10-4 M Cd2+ in 0.10 M KCl and 0.10 M KCl were run respectively with the digital ac voltammetry system in an electrolytic cell. Parts a and b of Figure 2 show the digital 3-D fundamental harmonic current phase angle voltammogram (θI-V) obtained. The staircase potential applied was from -0.4 to -0.8 V with a ramp step of 2 mV. From Figure 2a and b, maximum (16) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 2001; Chapter 10.

6370 Analytical Chemistry, Vol. 75, No. 22, November 15, 2003

If and Ic occurred at 135°, 315° and 90°, 270°, respectively. As there is a 90° shift in phase angle for phase-selective fundamental harmonic ac current compared with that of total ac current,12,14 the phase angles for maximum faradaic current and charging currents found in Figure 2a and b agree very well with that predicted in eqs 1 and 2. Although the faradaic and charging currents can be monitored by the digital sine wave superimposed staircase voltammetry shown above, there are difficulties in running the same analysis in a flow system. First, it takes quite a long time (∼45 s for the analytical scheme used above) to go through one cycle of staircase potential (from -0.4 to -0.8 V). It is impossible to monitor the sample plug flowing through the detector with such a potential scheme. Second, as so many data will be collected from the flowing process, it would take a long time to extract the useful current of If and Ic from the collected data. To circumvent these difficulties, superimposing sine waves on a simpler dc potential scheme was proposed. Instead of applying the staircase potential of numerous ramp steps, a two-step pulse potential (e.g., -0.5 and -0.8 V) and a three-step pulse potential scheme were chosen for the sine wave to superimpose. Solutions of 1.78 × 10-4 M Cd2+ and 5.0 × 10-4 M I3- in 0.1 M NaNO3 were injected into a FIA system and analyzed with the

two-step pulse potential ac voltammetry. Parts c and d of Figure 2 show the 3-D harmonic current phase angle chronoamperogram (θ-I-t) obtained from two successive injections. No peak current was found from the phase angle chronoamperogram obtained at -0.8 V (Figure 2c). The absence of faradaic current is due to the deviation of the applied potential from the reversible half-wave potential, E1/2 (-0.63 V) of Cd2+. According to the theory of ac voltammetry, a reversible electrochemical reaction is characterized by a bell-shaped voltammogram with a half-width of 90/n mV.10,16 The magnitude of faradaic current falls off rapidly as the applied potential is ∼(0.1 V deviated from the reversible half-wave potential. From Figure 2d, two maximums of charging current at the phase angles of 35° and 215° were found from the 3-D phase angle chronoamperograms obtained at -0.5 V. This should be attributed to the adsorption of I3- ions on the electrode surface. There are differences among the phase angles of 35° and 215° obtained with the theoretical values of 45° and 225° (90° phase shift from 315° and 135°). The 10° shift of phase angle for maximum charging current should be attributed to the change of double layer capacitance (Cdl) resulting from the adsorption of I3- ions on electrode surface and the existence of uncompensated solution resistance (RΩ) in solution. Due to the special characteristics of ac voltammetry, no faradaic current could be found if the deviation of applied dc potential to the half-wave potential of analyte is larger than (0.1 V. Its application for flow injection analysis of an electroactive species becomes difficult. A different approach for solving this problem is to transfer the obtained fundamental harmonic currents into the zero-order currents mentioned above. Figure 3 shows the current responses of zero-order current, Izero, and the phaseselective charging current, Ic (at 35° phase angle), extracted from the two-step pulse potential (at -0.8 and -0.5 V) ac voltammograms. Two successive injections of solutions of (a) 1.78 × 10-4 M Cd2+ (b) 5.00 × 10-4 M I3-, and (c) 1.78 × 10-4 M Cd2+ + 5.00 × 10-4 M I3- were made. From Figure 3, two peak currents characterizing the reduction of Cd2+ were found for the injection of solution a at a step potential of -0.8 V and two fundamental harmonic charging currents (at phase angle of 35°) characterizing the adsorption of I3- were found for the injection of solution b at a step potential of -0.5 V and current responses for both the reduction of Cd2+ and adsorption of I3- were found for injection of solution c. For comparison, change of differential capacitance resulting from the injection of above solutions were measured according to the following equation.1,2

Cd ) (ω∆E)-1 [(Iout2 + Iin2)/Iout2]

(3)

where Iout and Iin are the magnitudea of the out-of-phase and inphase components of ac current. Two capacitance peaks characterizing the adsorption of I3- were found for the injection of solutions b and c, respectively. From the responses shown in Figure 3, the applicability of the proposed two-step pulse ac voltammetry for FIA was confirmed. The reliability of the proposed two-step pulse ac voltammetry for FIA was further tested by repetitive injection (n ) 7) of solution c into the flow system. Satisfactory RSD of 1.03, 0.31, and 0.89% for Izero, Ic, and Cd were obtained, respectively.

Figure 3. Current responses for change of zero-order current, Izero, phase-selective fundamental charging current (at 35° phase angle), Ic, and differential capacitance, Cd, obtained by the application of sine wave superimposed two-step (at -0.8 and -0.5 V) pulse ac voltammetry with two successive injections of solutions of (a) 1.78 × 10-4 M Cd2+, (b) 5.0 × 10-4 M I3-, and (c) 1.78 × 10-4 M Cd2+ + 5.0 × 10-4 M I3-.

As the developed analytical scheme has no selectivity for both the electroactive and electrosorptive analytes, a preseparation procedure is thus needed for its practical applications. To demonstrate its applicability as a useful electrochemical detection scheme, solutions containing (a) 4.45 × 10-4 M Cd2+ + 5.00 × 10-4 M I3- and (b) 2.67 × 10-4 M Cd2+ + 4.34 × 10-4 M Pb2+ + 3.00 × 10-4 M Br- were run respectively with the sine waves superimposed two-step and three-step pulse potential ac voltammetry coupled IC. Figure 4 shows the chromatograms obtained. The step potentials used were -0.80, -0.40 V and -0.80, -0.20, -0.10 V, respectively, for monitoring the electroactive and electrosorptive species. Peaks of Izero for Cd2+ and Ic for I3- were found at retention times of 246 and 155 s, respectively, from Figure 4a while peaks of Izero for Cd2+, Pb2+ and Ic for Br- were found at retention times of 149, 238, and 129 s, respectively, from Figure 4b. The dependence of Ic on step potential for the adsorption of Br- on an electrode surface was evidenced by the different magnitudes of Ic found at step potentials of -0.10 and -0.20 V. The chromatograms obtained from Figure 4 confirm the feasibility of the developed system for simultaneous analysis of electrosorptive and electroactive analytes in a LC system. For quantitative analysis, calibration graphs, linear dynamic ranges, and detection limits for electrosorptive species such as I3-, Br-, S2O32-, and SCN- were studied and listed in Table 1. Satisfactory linear correlation coefficients (better than 0.9990) for the calibration graphs with ∼2 orders of linear dynamic range were obtained. The detection limits for the studied species are close to values of 0.2-0.5 mg/L reported by Ramstad and Weaver, using the analog ac voltammetry coupled IC method.1 Due to the difference in detection mechanism, the detection limits for the electrosorptive Analytical Chemistry, Vol. 75, No. 22, November 15, 2003

6371

Figure 4. Chromatograms obtained by running solutions of (a) 4.45 × 10-4 M Cd2+ + 5.0 × 10-4 M I3- and (b) 2.67 × 10-4 M Cd2+ + 4.34 × 10-4 M Pb2+ + 3.00 × 10-4 M Br-. The two-step pulse potential (-0.4 and -0.8 V) and three-step pulse potential (-0.10, -0.20, and -0.80 V) ac voltammetry were applied, respectively. A Dionex HPIC CS5 cation column and an injection valve (Rheodyne 7125) with a sample loop of 50 µL were used for IC analysis. Solutions of 0.05 and 0.15 M Mg(NO3)2 were used respectively as the eluent for running solutions a and b.

Table 1. Characteristics of the Digital ac Voltammetry for Analysis of Electrosorptive Species

b

species studied

step potential applied (V)

dynamic rangea (µM)

detection limit (with S/N ) 3) (µM)

sensitivityb (µA/mM)

I3BrS2O32SCN-

-0.300 -0.100 -0.300 -0.300

5.0-100 5.0-200 5.0-200 30-1000

5.0 5.0 5.0 30

6.86 4.80 2.36 0.48

a Linear correlation coefficients better than 0.9990 were obtained. Sensitivity was represented by the slope of the calibration curve.

species obtained are ∼3 orders of magnitude higher than that obtained by similar IC with isotope dilution mass spectrometry17 or amperometric detection18 but are in the same order as that obtained by using conductivity detector coupled IC.17 Although mass spectrometry detection provides a much lower detection limit than that by digital ac voltammetry, the costs of the sophisticated mass spectrometer and its routine operation are much higher than that of the microcomputer-based ac voltammetric method. Based on the measurement of differential capacitances or charging currents, the sensitivity of digital ac pulse (17) Reifenhauser, C.; Heumann, K. G. Fresenius J. Anal. Chem. 1990, 336, 559. (18) Application Update No. 37, Dionex, Sunnyvale, CA, p 1.

6372 Analytical Chemistry, Vol. 75, No. 22, November 15, 2003

voltammetry is inherently worse than that of amperometry, but its capability for analyzing electroinactive species should make it a useful complement to amperometry in flow analysis. CONCLUSIONS The proposed digital ac pulsed voltammetry has extended the functions of conventional analog ac voltammetry effectively by coupling to a flow system. With the proposed sine waves superimposed two-step or three-step pulse potential scheme, the zero-order faradaic current and the phase-selective fundamental charging current at a specified phase angle can be obtained in a single run and used for the simultaneous analysis of electroactive and electrosorptive species in a FIA or LC system. For those anions, cations, unsaturated alcohols, or other neutral organic molecules known to be electroinactive but which can be adsorbed electrochemically on Hg or other electrodes, the proposed method should provide a favorable and practical alternative for simultaneous analysis of these species with other electroactive species present in sample solutions. The versatile and efficient digital ac pulse voltammetry should thus provide an effective and useful procedure for analyzing constituents in complicated samples, e.g., electroplating solutions in which salt of various ions, surfactants, (19) Sawada, S.; Torii, H.; Osakai, T.; Kimoto, T. Anal. Chem. 1998, 70, 4286. (20) Augelli, M. A.; Nascimento, V. B.; Pedrotti, J. J.; Gutz, I. G. R.; Angnes, L. Analysis 1997, 122, 843.

wetting agents, organic brighteners, and ions of the plating elements are included. As the pulse potential scheme of the developed method is similar to that used by the pulse amperometry detection method,19,20 superposition of the sine waves to the detection step in pulse amperometry should result in an ac information-rich versatile electrochemical detection scheme.

ACKNOWLEDGMENT The authors thank the National Science Council of R.O.C. for financial support of this work (Project NSC 90-2113-M-110-027). Received for review May 8, 2003. Accepted August 15, 2003. AC034491S

Analytical Chemistry, Vol. 75, No. 22, November 15, 2003

6373