- Email: [email protected]
Simultaneous polarographic chemometric determination of lead, copper, vanadium, cadmium and nickel
Chemometrics and Intelligent Laboratory Systems 45 Ž1999. 105–111
Simultaneous polarographic chemometric determination of lead, copper, vanadium, cadmium and nickel Yongnian Ni ) , Ling Jin Department of Chemistry, Nanchang UniÕersity, Nanchang 330047, China
Abstract Chemometric approaches, such as classical least squares ŽCLS., principal component regression ŽPCR., partial least squares ŽPLS. and iterative target transformation factor analysis ŽITTFA., were applied to the simultaneous determination of mixtures of lead, copper, vanadium, cadmium and nickel by differential pulse polarography ŽDPP.. The conventional and first-derivative polarograms of the mixtures were used to perform the optimization of the calibration procedure by chemometric models. The proposed method was applied satisfactorily to the determination of a set of synthetic mixtures of metal in Britton–Robinson buffer ŽpH 2.87. and potassium thiocyanate and acceptable results were obtained. The results obtained by the application of the different chemometric approaches are discussed and compared. It was found that factor analysis methods generally give better results than CLS and no significant advantages were found with the application of derivative technique, except for ITTFA in this polarographic work. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Chemometrics; Polarography; Metals
Contents . . . . . . 3. 3.1. . 3.2. Effect of pulse amplitude and scan rate on the peak currents . 3.3. Prediction of synthetic mixtures of metals . . . . . . . . . . . 3.4. The application of derivative techniques . . . . . . . . . . . 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
)
... . . . .
. . . . . Results and discussion .
. . . . . . The choice of medium .
1.
Introduction
2.
Experimental . . 2.1. Apparatus . 2.2. Reagents . 2.3. Procedure .
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Corresponding author. E-mail: [email protected]
0169-7439r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 7 4 3 9 Ž 9 8 . 0 0 0 9 4 - X
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106 106 106 107 107 107 107 108 108 110 111 111 111
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Y. Ni, L. Jin r Chemometrics and Intelligent Laboratory Systems 45 (1999) 105–111
1. Introduction Polarographic and voltammetric methods, such as differential pulse polarography, stripping voltammetry and adsorptive voltammetry, generally have high sensitivity and are widely used in most areas of analytical chemistry,though their applicabilities to the determination of mixtures of several components are rather limited when they display strongly partially overlapped polarograms or voltammograms. Several chemometric methods have been proposed to overcome this limitation and resolve the overlapped polarograms or voltammograms. Turnes et al. w1x have applied multiple linear regression ŽMLR. to resolve the overlapped peaks of the binary, ternary and quaternary mixtures of lead, thallium, indium and cadmium produced by differential pulse polarography data. Garcia et al. w2x have also applied MLR to treat the highly overlapped peaks obtained from a mixture of cadmium, copper and nickel by differential pulse polarography. Brown and Brown w3x demonstrated the application of the Kalman filter to the resolution of overlapped multicomponent linear sweep voltammograms of a cadmium, indium and lead system. Ni et al. w4x applied the Kalman filter to resolve the overlapped polarograms of pyrazines, and it was found that the baseline obtained from calculation was different from the experimental one. It must be noted that, in contrast to spectroscopic systems, in electroanalytical techniques there is generally non-linearity between intensity and concentration, and the additivity of the signals is not satisfied for all the potential values selected for calculation. Therefore, multicomponent analysis by multiple linear regression is not completely suitable for this case, and only less satisfactory results can be obtained. Partial least squares ŽPLS. and principal component regression ŽPCR., however, can be used to describe the non-linearity by incorporating a larger number of latent variables than would be required for a linear system or using the non-linear or quadratic version of the algorithms w5,6x. Henrion et al. w7x applied PLS to resolve quantitatively overlapped response obtained from differential pulse anodic stripping voltammetry. PLS and PCR were also used to resolve the overlapping polarograms or voltammograms of organic compounds, such as pyrazine and its methyl derivatives w8x, nitrofuran derivatives w9,10x and food col-
orants w11x. Recently, iterative target transformation factor analysis ŽITTFA., an alternative factor analysis based chemometric approach, was introduced in electroanalytical chemistry. It has been applied to simultaneous polarographic determination of mixtures of pyrazines w12x and simultaneous voltammetric determination of mixtures of colorants w13x. In Britton–Robinson buffer with the presence of potassium thiocyanate, ions, of metals such as lead, copper, vanadium, cadmium and nickel are reducible at the drop mercury electrode ŽDME. and give sensitive polarographic waves. But they cannot usually be determined individually in a mixture without a prior separation because they are reduced at similar potentials and only overlapping peaks will be obtained. The aim of this work is to apply the multivariate calibration approaches of CLS, PCR, PLS and ITTFA to such waves obtained from differential pulse polarography ŽDPP.. Recently, the combination of derivative techniques with multivariate analysis methods has been proposed and the convenience of such an approach has been evaluated by several spectrophotometric works w14–17x. In this paper, the derivative technique was applied to polarography. Generally, the influence of baseline Žresidual current and background. could be eliminated or reduced with the use of this technique. 2. Experimental 2.1. Apparatus The polarograms were obtained with an electroanalyzer ŽBAS 100A. equipped with an electrolytic cell stand ŽPARC 303A.. A three-electrode cell, containing a mercury drop electrode Žthe function of a static mercury drop electrode ŽSMDE. was used in this work. as the working electrode, an Ag–AgCl Ž3.0 mol ly1 KCl. electrode as the reference electrode and a platinum wire as the auxiliary electrode, was used. The polarographic spectra were plotted by using a plotter ŽDMP 40, Houston Instrument., and the current data were recorded by a computer connected to BAS 100A. The pH of the solution was measured by the use of a pH meter ŽORION SA720. equipped with a combined glass–calomel electrode. All experiments were performed at 258C.
Y. Ni, L. Jin r Chemometrics and Intelligent Laboratory Systems 45 (1999) 105–111
2.2. Reagents All reagents used were of analytical reagent grade. Stock solutions of copper, lead and nickel Ž10.0 mg ly1 . and solutions of cadmium and vanadium Ž30.0 mg ly1 . were prepared from their nitrate or chloride salts according to the classical method. Solution of potassium thiocyanate Ž4.0 mol ly1 . was prepared by dissolving 388.72 g crystal in water and diluting to 11. Britton–Robinson buffer solution ŽpH 2.87. was prepared from phosphoric acid, acetic acid, boric acid and sodium hydroxide w18x. 2.3. Procedure Analysis was performed by pipetting a suitable amount of mixture of metals, together with 3.0 ml of the Britton–Robinson buffer solution ŽpH 2.87. and 1.0 ml of 4.0 mol ly1 solution of potassium thiocyanate into a cell and made up with deionized water to 10.0 ml. The solution was purged with purified nitrogen gas for 480 s. After a 10 s quiet time, the potential was scanned from y250 to y850 mV vs. Ag–AgCl reference electrode using differential pulse polarography ŽDPP. at a static mercury drop electrode ŽSMDE.. The pulse amplitude potential was y50 mV, the drop time interval was 2000 ms, and the scan rate was 2 mV sy1 . After sweeping, the instrument plotted the polarograms and printed the response current data automatically. The data were sampled at 200 points by a computer, in the range of y300 to y700 mV with 2 mV intervals, and all these data were used in calibration analysis because this range contained maximum analytical information. The first derivative polarograms were obtained with a D E s 4 mV and smoothed through the use of seven experimental points w19–21x. Calculations were made on a 486DX personal computer. The programs for CLS, PCR and PLS calculation algorithm were written in Q BASIC according to the literature w5x, and the ITTFA was written partially according to the literature w22,23x. The measured data are never noise-free and some of the small components in the PCR and PLS calibration procedure only describe noise. It is common to neglect the small components because they tend to introduce problems of collinearity. The cross-validation w24x was used to evaluate the number of signifi-
107
cant dimensions Ži.e., components. and to stop the computation procedures.
3. Results and discussion 3.1. The choice of medium From Figs. 1 and 2, it can be seen that the polarographic peak potentials for all of the metals become slightly more negative with increasing concentration of potassium thiocyanate, but almost do not change with increasing pH. However, the peak current vs. the pH and the concentration of potassium thiocyanate behavior indicates a variation in sensitivity over the 1.5–7.0 pH range and potassium thiocyanate of 0.20–2.00 mol ly1 concentration range, respectively Žsee Figs. 3 and 4.. In this work, the Britton–Robinson buffer solution of pH 2.78 and the potassium thiocyanate of 0.4 mol ly1 were selected as a suitable analytical medium because the relative sensitivities for most metals were high and overlap of the polarograms of the metals would be minimal ŽFig. 5.. Despite these measures, peaks for the metals show considerable overlap, which would preclude their determination without the aid of chemometric procedures.
Fig. 1. The plot of the peak potential of each metal vs. the concentration of potassium of thiocyanate. The concentration of each metal is as in Fig. 5.
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Y. Ni, L. Jin r Chemometrics and Intelligent Laboratory Systems 45 (1999) 105–111
Fig. 2. The plot of the peak potential of each metal vs. pH. The concentration of each metal is as in Fig. 5.
3.2. Effect of pulse amplitude and scan rate on the peak currents The relationships between peak current of each metal and pulse amplitude potential was investigated Žsee Fig. 6.. It shows a rectilinearity within a wide potential range. In this work, y50 mV of the pulse amplitude was selected for sufficient sensitivity and good shape waves for all metal can be obtained. From Fig. 7 it can be seen that the sensitivities Žpeak currents. for most metals decrease with the increase of
Fig. 3. The peak current of each metal vs. pH. The concentration of each metal is as in Fig. 5.
Fig. 4. The peak current of each metal vs. the concentration of potassium of thiocyanate. The concentration of each metal is as in Fig. 5.
scan rate, so 2.0 mV sy1 of the scan rate was selected. 3.3. Prediction of synthetic mixtures of metals In contrast to spectrophotometric analysis, in polarography the property of additivity of peak current is somewhat less good and deviations from linearity
Fig. 5. The DPP polarograms of lead Ž1.6 mg ly1 ., copper Ž0.8 mg ly1 ., vanadium Ž2.5 mg ly1 ., cadmium Ž2.0 mg ly1 ., nickel Ž0.8 mg ly1 . and their mixture solutions in pH 2.87 Britton–Robinson buffer and 0.4 mol ly1 of potassium thiocyanate.
Y. Ni, L. Jin r Chemometrics and Intelligent Laboratory Systems 45 (1999) 105–111
109
Table 1 The composition of the calibration samples
Fig. 6. The influence of pulse amplitude on the peak current of each metal. The experimental condition is as in Fig. 5.
occur due to invalidity of Beer’s law or background and residual current changes. In this work the quantitative analysis of mixtures of lead, copper, vanadium, cadmium and nickel by the use of CLS, PCR and PLS was investigated. The calibration set for multivariate calibration was prepared according to an orthogonal array design w25,26x. A four-level orthogonal array design w27,28x, denoted by OA 16 Ž4 5 ., was selected in this experiment in order to obtain maximum information on each metal from the calibration procedure and its composition is given in
Sample number
Concentration Žmg ly1 . Lead Copper Cadmium
Nickel
Vanadium
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
0.700 0.700 0.700 0.700 1.800 1.800 1.800 1.800 4.200 4.200 4.200 4.200 3.500 3.500 3.500 3.500
0.800 0.300 1.800 1.200 1.800 1.200 0.800 0.300 1.200 1.800 0.300 0.800 0.300 0.800 1.200 1.800
1.000 2.500 3.500 5.500 5.500 3.500 2.500 1.000 2.500 1.000 5.500 3.500 3.500 5.500 1.000 2.500
1.800 1.400 0.800 0.300 1.800 1.400 0.800 0.300 1.800 1.400 0.800 0.300 1.800 1.400 0.800 0.300
0.800 4.500 3.000 1.500 4.500 0.800 1.500 3.000 3.000 1.500 0.800 4.500 1.500 3.000 4.500 0.800
Table 1. This design is a 16 = 5 matrix, where 16 is the number of rows, which corresponds to the experimental trials, and 5 is the number of columns, which corresponds to the factors Žthe statistician’s terminology for independent parameters.. The intersections between the rows and columns indicate the level setting that apply to that factor for the experimental trials. From this matrix, it can be noted that each of the Table 2 The composition of the test samples
Fig. 7. The influence of scan rate on the peak current of each metal. The experimental condition is as in Fig. 5.
Sample number
Concentration Žmg ly1 . Lead Copper Cadmium
Nickel
Vanadium
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
4.000 0.800 1.600 4.000 3.000 3.000 3.000 0.800 1.600 1.600 0.800 3.000 0.800 4.000 4.000 1.600
0.800 0.400 1.200 0.400 1.200 0.800 0.400 1.200 0.400 1.600 0.800 1.600 1.600 1.200 1.600 0.800
1.000 2.500 3.500 3.500 2.500 3.500 1.000 1.000 5.000 1.000 5.000 5.000 3.500 5.000 2.500 2.500
0.400 0.400 0.400 0.800 0.800 1.200 1.600 1.200 1.200 0.800 0.800 0.400 1.600 1.600 1.200 1.600
1.000 2.000 3.000 4.000 1.000 2.000 3.000 4.000 1.000 2.000 3.000 4.000 1.000 2.000 3.000 4.000
Y. Ni, L. Jin r Chemometrics and Intelligent Laboratory Systems 45 (1999) 105–111
110
Table 3 Relative prediction error for mixture of metals Component in mixture
CLS method Normala
1st-der a
PCR method Normal
1st-der
PLS method Normal
1st-der
ITTFA method Normal
1st-der
Lead Copper Cadmium Nickel Vanadium RPE total
10.22 9.29 17.00 10.08 8.73 11.90
10.65 12.71 20.12 9.38 17.24 16.28
8.51 9.01 14.39 8.95 8.80 10.54Ž6. b
11.95 13.29 9.91 8.90 8.30 10.02Ž7.
8.79 8.33 15.01 9.68 9.29 10.99Ž5.
12.07 12.72 12.60 10.71 9.05 11.10Ž7.
10.58 8.51 13.36 10.70 10.30 11.22Ž8.
9.59 14.68 11.03 9.44 7.73 9.61Ž9.
a b
Normal and 1st-der represent the polarogram and first-derivative polarogram data, respectively. Values in parentheses are the optimum number of factors used for prediction.
five columns is varied over four level settings and each level setting repeats four times. There, a total of 4 = 4 s 16 experimental trials is necessary for each column. Furthermore, in any two columns, the horizontal combination of any two level values appears the same number of times. The above features of the OA 16 Ž4 5 . matrix provide the orthogonality among all the five columns. Sixteen mixtures of metals Žtheir compositions are listed in Table 2. were analyzed by the proposed polarographic method and multivariate calibration methods. The analytical results are summarized in Table 3. The predictive ability of each method is described in terms of the relative predictive error ŽRPE. w29x. The RPE for total analytes can be written as: m
RPE total s 100 =
3.4. The application of deriÕatiÕe techniques Derivative techniques have been widely used over the last few years in the UVrvisible spectrophotometric analysis of multicomponent mixtures. Recently, the multivariate statistical approaches, such as CLS and PLS, have been applied to derivative data with good results w17,30x. In this work, the application of the first derivative technique to polarographic data for simultaneous determination of mixed metals with CLS, PCR, PLS and ITTFA was investigated. Fig. 8 shows the first-derivative polarographic peaks
n
Ý Ý Ž Cfound y Cadded . 2 ts1 js1
m
0.5
n
Ý Ý Ž Cadded .
2
ts1 js1
and for each analyte: n
RPE single s 100 =
Ý Ž Cfound y Cadded . 2 js1 0.5
n
Ý Ž Cadded .
2
js1
where Cfound is the concentration calculated, Cadded is the concentration present in the mixture, m is the number of analytes, and n is the number of samples. It can be seen that the factor analysis based approaches, include PCR, PLS and ITTFA, generally give better results than the classical CLS.
Fig. 8. The first-derivative DPP polarograms of lead Ž1.6 mg ly1 ., copper Ž0.8 mg ly1 ., vanadium Ž2.5 mg ly1 ., cadmium Ž2.0 mg ly1 ., nickel Ž0.8 mg ly1 . and their mixture solutions in pH 2.87 Britton–Robinson buffer and 0.4 mol ly 1 of potassium thiocyanate.
Y. Ni, L. Jin r Chemometrics and Intelligent Laboratory Systems 45 (1999) 105–111
of each metal. The prediction results are also listed in Table 3. It is shown that, as compared to the results obtained using the normal polarographic data, the first-derivative data give better precision than the normal one when ITTFA was used and no significant advantages are found for PCR and PLS when the first-derivative technique was applied. In our work, it is found that one or two more factors than the components to be determined should be chosen when the first-derivative polarography is used to evaluate the components in mixtures. It is possible that the effect of the interaction is enhanced in the derivative calculation procedure.
4. Conclusion Chemometric approaches were applied to resolve the overlapping polarographic peaks of five-component combination mixtures of metal ions in Britton– Robinson buffer ŽpH 2.87. with the presence of potassium thiocyanate and acceptable results were obtained. This work indicates that, like spectroscopic analysis, much electrochemical work could benefit from the application of chemometric techniques.
Acknowledgements The financial support for this project was provided by the Jiangxi Province Natural Science Foundation and the National Natural Science Foundation of China.
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Simultaneous polarographic chemometric determination of lead, copper, vanadium, cadmium and nickel Yongnian Ni ) , Ling Jin Department of Chemistry, Nanchang UniÕersity, Nanchang 330047, China
Abstract Chemometric approaches, such as classical least squares ŽCLS., principal component regression ŽPCR., partial least squares ŽPLS. and iterative target transformation factor analysis ŽITTFA., were applied to the simultaneous determination of mixtures of lead, copper, vanadium, cadmium and nickel by differential pulse polarography ŽDPP.. The conventional and first-derivative polarograms of the mixtures were used to perform the optimization of the calibration procedure by chemometric models. The proposed method was applied satisfactorily to the determination of a set of synthetic mixtures of metal in Britton–Robinson buffer ŽpH 2.87. and potassium thiocyanate and acceptable results were obtained. The results obtained by the application of the different chemometric approaches are discussed and compared. It was found that factor analysis methods generally give better results than CLS and no significant advantages were found with the application of derivative technique, except for ITTFA in this polarographic work. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Chemometrics; Polarography; Metals
Contents . . . . . . 3. 3.1. . 3.2. Effect of pulse amplitude and scan rate on the peak currents . 3.3. Prediction of synthetic mixtures of metals . . . . . . . . . . . 3.4. The application of derivative techniques . . . . . . . . . . . 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
)
... . . . .
. . . . . Results and discussion .
. . . . . . The choice of medium .
1.
Introduction
2.
Experimental . . 2.1. Apparatus . 2.2. Reagents . 2.3. Procedure .
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Corresponding author. E-mail: [email protected]
0169-7439r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 7 4 3 9 Ž 9 8 . 0 0 0 9 4 - X
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106
Y. Ni, L. Jin r Chemometrics and Intelligent Laboratory Systems 45 (1999) 105–111
1. Introduction Polarographic and voltammetric methods, such as differential pulse polarography, stripping voltammetry and adsorptive voltammetry, generally have high sensitivity and are widely used in most areas of analytical chemistry,though their applicabilities to the determination of mixtures of several components are rather limited when they display strongly partially overlapped polarograms or voltammograms. Several chemometric methods have been proposed to overcome this limitation and resolve the overlapped polarograms or voltammograms. Turnes et al. w1x have applied multiple linear regression ŽMLR. to resolve the overlapped peaks of the binary, ternary and quaternary mixtures of lead, thallium, indium and cadmium produced by differential pulse polarography data. Garcia et al. w2x have also applied MLR to treat the highly overlapped peaks obtained from a mixture of cadmium, copper and nickel by differential pulse polarography. Brown and Brown w3x demonstrated the application of the Kalman filter to the resolution of overlapped multicomponent linear sweep voltammograms of a cadmium, indium and lead system. Ni et al. w4x applied the Kalman filter to resolve the overlapped polarograms of pyrazines, and it was found that the baseline obtained from calculation was different from the experimental one. It must be noted that, in contrast to spectroscopic systems, in electroanalytical techniques there is generally non-linearity between intensity and concentration, and the additivity of the signals is not satisfied for all the potential values selected for calculation. Therefore, multicomponent analysis by multiple linear regression is not completely suitable for this case, and only less satisfactory results can be obtained. Partial least squares ŽPLS. and principal component regression ŽPCR., however, can be used to describe the non-linearity by incorporating a larger number of latent variables than would be required for a linear system or using the non-linear or quadratic version of the algorithms w5,6x. Henrion et al. w7x applied PLS to resolve quantitatively overlapped response obtained from differential pulse anodic stripping voltammetry. PLS and PCR were also used to resolve the overlapping polarograms or voltammograms of organic compounds, such as pyrazine and its methyl derivatives w8x, nitrofuran derivatives w9,10x and food col-
orants w11x. Recently, iterative target transformation factor analysis ŽITTFA., an alternative factor analysis based chemometric approach, was introduced in electroanalytical chemistry. It has been applied to simultaneous polarographic determination of mixtures of pyrazines w12x and simultaneous voltammetric determination of mixtures of colorants w13x. In Britton–Robinson buffer with the presence of potassium thiocyanate, ions, of metals such as lead, copper, vanadium, cadmium and nickel are reducible at the drop mercury electrode ŽDME. and give sensitive polarographic waves. But they cannot usually be determined individually in a mixture without a prior separation because they are reduced at similar potentials and only overlapping peaks will be obtained. The aim of this work is to apply the multivariate calibration approaches of CLS, PCR, PLS and ITTFA to such waves obtained from differential pulse polarography ŽDPP.. Recently, the combination of derivative techniques with multivariate analysis methods has been proposed and the convenience of such an approach has been evaluated by several spectrophotometric works w14–17x. In this paper, the derivative technique was applied to polarography. Generally, the influence of baseline Žresidual current and background. could be eliminated or reduced with the use of this technique. 2. Experimental 2.1. Apparatus The polarograms were obtained with an electroanalyzer ŽBAS 100A. equipped with an electrolytic cell stand ŽPARC 303A.. A three-electrode cell, containing a mercury drop electrode Žthe function of a static mercury drop electrode ŽSMDE. was used in this work. as the working electrode, an Ag–AgCl Ž3.0 mol ly1 KCl. electrode as the reference electrode and a platinum wire as the auxiliary electrode, was used. The polarographic spectra were plotted by using a plotter ŽDMP 40, Houston Instrument., and the current data were recorded by a computer connected to BAS 100A. The pH of the solution was measured by the use of a pH meter ŽORION SA720. equipped with a combined glass–calomel electrode. All experiments were performed at 258C.
Y. Ni, L. Jin r Chemometrics and Intelligent Laboratory Systems 45 (1999) 105–111
2.2. Reagents All reagents used were of analytical reagent grade. Stock solutions of copper, lead and nickel Ž10.0 mg ly1 . and solutions of cadmium and vanadium Ž30.0 mg ly1 . were prepared from their nitrate or chloride salts according to the classical method. Solution of potassium thiocyanate Ž4.0 mol ly1 . was prepared by dissolving 388.72 g crystal in water and diluting to 11. Britton–Robinson buffer solution ŽpH 2.87. was prepared from phosphoric acid, acetic acid, boric acid and sodium hydroxide w18x. 2.3. Procedure Analysis was performed by pipetting a suitable amount of mixture of metals, together with 3.0 ml of the Britton–Robinson buffer solution ŽpH 2.87. and 1.0 ml of 4.0 mol ly1 solution of potassium thiocyanate into a cell and made up with deionized water to 10.0 ml. The solution was purged with purified nitrogen gas for 480 s. After a 10 s quiet time, the potential was scanned from y250 to y850 mV vs. Ag–AgCl reference electrode using differential pulse polarography ŽDPP. at a static mercury drop electrode ŽSMDE.. The pulse amplitude potential was y50 mV, the drop time interval was 2000 ms, and the scan rate was 2 mV sy1 . After sweeping, the instrument plotted the polarograms and printed the response current data automatically. The data were sampled at 200 points by a computer, in the range of y300 to y700 mV with 2 mV intervals, and all these data were used in calibration analysis because this range contained maximum analytical information. The first derivative polarograms were obtained with a D E s 4 mV and smoothed through the use of seven experimental points w19–21x. Calculations were made on a 486DX personal computer. The programs for CLS, PCR and PLS calculation algorithm were written in Q BASIC according to the literature w5x, and the ITTFA was written partially according to the literature w22,23x. The measured data are never noise-free and some of the small components in the PCR and PLS calibration procedure only describe noise. It is common to neglect the small components because they tend to introduce problems of collinearity. The cross-validation w24x was used to evaluate the number of signifi-
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cant dimensions Ži.e., components. and to stop the computation procedures.
3. Results and discussion 3.1. The choice of medium From Figs. 1 and 2, it can be seen that the polarographic peak potentials for all of the metals become slightly more negative with increasing concentration of potassium thiocyanate, but almost do not change with increasing pH. However, the peak current vs. the pH and the concentration of potassium thiocyanate behavior indicates a variation in sensitivity over the 1.5–7.0 pH range and potassium thiocyanate of 0.20–2.00 mol ly1 concentration range, respectively Žsee Figs. 3 and 4.. In this work, the Britton–Robinson buffer solution of pH 2.78 and the potassium thiocyanate of 0.4 mol ly1 were selected as a suitable analytical medium because the relative sensitivities for most metals were high and overlap of the polarograms of the metals would be minimal ŽFig. 5.. Despite these measures, peaks for the metals show considerable overlap, which would preclude their determination without the aid of chemometric procedures.
Fig. 1. The plot of the peak potential of each metal vs. the concentration of potassium of thiocyanate. The concentration of each metal is as in Fig. 5.
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Y. Ni, L. Jin r Chemometrics and Intelligent Laboratory Systems 45 (1999) 105–111
Fig. 2. The plot of the peak potential of each metal vs. pH. The concentration of each metal is as in Fig. 5.
3.2. Effect of pulse amplitude and scan rate on the peak currents The relationships between peak current of each metal and pulse amplitude potential was investigated Žsee Fig. 6.. It shows a rectilinearity within a wide potential range. In this work, y50 mV of the pulse amplitude was selected for sufficient sensitivity and good shape waves for all metal can be obtained. From Fig. 7 it can be seen that the sensitivities Žpeak currents. for most metals decrease with the increase of
Fig. 3. The peak current of each metal vs. pH. The concentration of each metal is as in Fig. 5.
Fig. 4. The peak current of each metal vs. the concentration of potassium of thiocyanate. The concentration of each metal is as in Fig. 5.
scan rate, so 2.0 mV sy1 of the scan rate was selected. 3.3. Prediction of synthetic mixtures of metals In contrast to spectrophotometric analysis, in polarography the property of additivity of peak current is somewhat less good and deviations from linearity
Fig. 5. The DPP polarograms of lead Ž1.6 mg ly1 ., copper Ž0.8 mg ly1 ., vanadium Ž2.5 mg ly1 ., cadmium Ž2.0 mg ly1 ., nickel Ž0.8 mg ly1 . and their mixture solutions in pH 2.87 Britton–Robinson buffer and 0.4 mol ly1 of potassium thiocyanate.
Y. Ni, L. Jin r Chemometrics and Intelligent Laboratory Systems 45 (1999) 105–111
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Table 1 The composition of the calibration samples
Fig. 6. The influence of pulse amplitude on the peak current of each metal. The experimental condition is as in Fig. 5.
occur due to invalidity of Beer’s law or background and residual current changes. In this work the quantitative analysis of mixtures of lead, copper, vanadium, cadmium and nickel by the use of CLS, PCR and PLS was investigated. The calibration set for multivariate calibration was prepared according to an orthogonal array design w25,26x. A four-level orthogonal array design w27,28x, denoted by OA 16 Ž4 5 ., was selected in this experiment in order to obtain maximum information on each metal from the calibration procedure and its composition is given in
Sample number
Concentration Žmg ly1 . Lead Copper Cadmium
Nickel
Vanadium
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
0.700 0.700 0.700 0.700 1.800 1.800 1.800 1.800 4.200 4.200 4.200 4.200 3.500 3.500 3.500 3.500
0.800 0.300 1.800 1.200 1.800 1.200 0.800 0.300 1.200 1.800 0.300 0.800 0.300 0.800 1.200 1.800
1.000 2.500 3.500 5.500 5.500 3.500 2.500 1.000 2.500 1.000 5.500 3.500 3.500 5.500 1.000 2.500
1.800 1.400 0.800 0.300 1.800 1.400 0.800 0.300 1.800 1.400 0.800 0.300 1.800 1.400 0.800 0.300
0.800 4.500 3.000 1.500 4.500 0.800 1.500 3.000 3.000 1.500 0.800 4.500 1.500 3.000 4.500 0.800
Table 1. This design is a 16 = 5 matrix, where 16 is the number of rows, which corresponds to the experimental trials, and 5 is the number of columns, which corresponds to the factors Žthe statistician’s terminology for independent parameters.. The intersections between the rows and columns indicate the level setting that apply to that factor for the experimental trials. From this matrix, it can be noted that each of the Table 2 The composition of the test samples
Fig. 7. The influence of scan rate on the peak current of each metal. The experimental condition is as in Fig. 5.
Sample number
Concentration Žmg ly1 . Lead Copper Cadmium
Nickel
Vanadium
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
4.000 0.800 1.600 4.000 3.000 3.000 3.000 0.800 1.600 1.600 0.800 3.000 0.800 4.000 4.000 1.600
0.800 0.400 1.200 0.400 1.200 0.800 0.400 1.200 0.400 1.600 0.800 1.600 1.600 1.200 1.600 0.800
1.000 2.500 3.500 3.500 2.500 3.500 1.000 1.000 5.000 1.000 5.000 5.000 3.500 5.000 2.500 2.500
0.400 0.400 0.400 0.800 0.800 1.200 1.600 1.200 1.200 0.800 0.800 0.400 1.600 1.600 1.200 1.600
1.000 2.000 3.000 4.000 1.000 2.000 3.000 4.000 1.000 2.000 3.000 4.000 1.000 2.000 3.000 4.000
Y. Ni, L. Jin r Chemometrics and Intelligent Laboratory Systems 45 (1999) 105–111
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Table 3 Relative prediction error for mixture of metals Component in mixture
CLS method Normala
1st-der a
PCR method Normal
1st-der
PLS method Normal
1st-der
ITTFA method Normal
1st-der
Lead Copper Cadmium Nickel Vanadium RPE total
10.22 9.29 17.00 10.08 8.73 11.90
10.65 12.71 20.12 9.38 17.24 16.28
8.51 9.01 14.39 8.95 8.80 10.54Ž6. b
11.95 13.29 9.91 8.90 8.30 10.02Ž7.
8.79 8.33 15.01 9.68 9.29 10.99Ž5.
12.07 12.72 12.60 10.71 9.05 11.10Ž7.
10.58 8.51 13.36 10.70 10.30 11.22Ž8.
9.59 14.68 11.03 9.44 7.73 9.61Ž9.
a b
Normal and 1st-der represent the polarogram and first-derivative polarogram data, respectively. Values in parentheses are the optimum number of factors used for prediction.
five columns is varied over four level settings and each level setting repeats four times. There, a total of 4 = 4 s 16 experimental trials is necessary for each column. Furthermore, in any two columns, the horizontal combination of any two level values appears the same number of times. The above features of the OA 16 Ž4 5 . matrix provide the orthogonality among all the five columns. Sixteen mixtures of metals Žtheir compositions are listed in Table 2. were analyzed by the proposed polarographic method and multivariate calibration methods. The analytical results are summarized in Table 3. The predictive ability of each method is described in terms of the relative predictive error ŽRPE. w29x. The RPE for total analytes can be written as: m
RPE total s 100 =
3.4. The application of deriÕatiÕe techniques Derivative techniques have been widely used over the last few years in the UVrvisible spectrophotometric analysis of multicomponent mixtures. Recently, the multivariate statistical approaches, such as CLS and PLS, have been applied to derivative data with good results w17,30x. In this work, the application of the first derivative technique to polarographic data for simultaneous determination of mixed metals with CLS, PCR, PLS and ITTFA was investigated. Fig. 8 shows the first-derivative polarographic peaks
n
Ý Ý Ž Cfound y Cadded . 2 ts1 js1
m
0.5
n
Ý Ý Ž Cadded .
2
ts1 js1
and for each analyte: n
RPE single s 100 =
Ý Ž Cfound y Cadded . 2 js1 0.5
n
Ý Ž Cadded .
2
js1
where Cfound is the concentration calculated, Cadded is the concentration present in the mixture, m is the number of analytes, and n is the number of samples. It can be seen that the factor analysis based approaches, include PCR, PLS and ITTFA, generally give better results than the classical CLS.
Fig. 8. The first-derivative DPP polarograms of lead Ž1.6 mg ly1 ., copper Ž0.8 mg ly1 ., vanadium Ž2.5 mg ly1 ., cadmium Ž2.0 mg ly1 ., nickel Ž0.8 mg ly1 . and their mixture solutions in pH 2.87 Britton–Robinson buffer and 0.4 mol ly 1 of potassium thiocyanate.
Y. Ni, L. Jin r Chemometrics and Intelligent Laboratory Systems 45 (1999) 105–111
of each metal. The prediction results are also listed in Table 3. It is shown that, as compared to the results obtained using the normal polarographic data, the first-derivative data give better precision than the normal one when ITTFA was used and no significant advantages are found for PCR and PLS when the first-derivative technique was applied. In our work, it is found that one or two more factors than the components to be determined should be chosen when the first-derivative polarography is used to evaluate the components in mixtures. It is possible that the effect of the interaction is enhanced in the derivative calculation procedure.
4. Conclusion Chemometric approaches were applied to resolve the overlapping polarographic peaks of five-component combination mixtures of metal ions in Britton– Robinson buffer ŽpH 2.87. with the presence of potassium thiocyanate and acceptable results were obtained. This work indicates that, like spectroscopic analysis, much electrochemical work could benefit from the application of chemometric techniques.
Acknowledgements The financial support for this project was provided by the Jiangxi Province Natural Science Foundation and the National Natural Science Foundation of China.
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