Application of orthogonal functions to differential pulse voltammetric analysis

Analytica Chimica Acta 401 (1999) 173–183

Application of orthogonal functions to differential pulse voltammetric analysis Simultaneous determination of tin and lead in soft drinks Suzy M. Sabry, Abdel-Aziz M. Wahbi ∗ Faculty of Pharmacy, University of Alexandria, Alexandria, Egypt Received 26 January 1999; received in revised form 12 May 1999; accepted 15 May 1999

Abstract The application of two-component analysis methods, differentiation of signals and orthogonal function, to the resolution of partially overlapping differential pulse voltammetric (DPV) peaks is demonstrated. The study was extended to differential pulse cathodic stripping voltammetry (DPCSV). A binary system of tin(II) and lead(II), having 72 mV of DPV peaks separation in 0.04 M acetic/o-phosphoric/boric acids’ mixture, was used as a model throughout the work. The stripping voltammetric analysis data processed by orthogonal functions and the first-derivative (1 D) methods, were successfully applied to the simultaneous determination of both metals in canned soft drinks. Moreover, the applicability of the methods were demonstrated by the recovery of lead in a drinking water sample. ©1999 Elsevier Science B.V. All rights reserved.

1. Introduction Selectivity in voltammetric analysis is due to the fact that different electroactive species undergo reduction (or oxidation) at different electrode potentials, but many analytes might interfere if they have close peak potentials or if they are present in large concentrations in excess over the other component(s) present in the sample. The oldest strategy for resolving overlapping polarographic waves was to use chemical reagents for complexing one component in the mixture, in order to shift further apart the peak potentials or to mask one component completely [1]. ∗ Corresponding author. Tel.: +20-34833810; fax: +20-34833273 E-mail address: [email protected] (A.-A.M. Wahbi)

Berzas and Rodriguez [2] have examined the possibilities of applying differential pulse polarographic (DPP) signals to the simultaneous determination of binary mixtures of inorganic ions, cadmium(II)/indium(III) and thallium(I)/lead(II). These mixtures were resolved by the first-derivative of the differential pulse polarograms and measurement of the analytical signal, taking into account the zero-crossing technique. Mixtures of these inorganic ions have also been the subject of a multi-component analysis method based on multiple regression procedures to resolve the highly overlapping peaks obtained by DPP [3]. Other related methods that have been reported in voltammetric analysis, include Kalman filter [4], Fourier transformation in the least-square [5] and partial least squares (PLS) [6,7]. Glenn’s method of orthogonal functions [8] has been extensively used to eliminate interferences in

0003-2670/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 4 7 3 - 0

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spectrophotometric analysis [9–12]. In this context, attention was directed to studying the applicability of the orthogonal functions method to multi-component voltammetric analysis. The work here demonstrates the applicability of the method to resolve overlapping voltammograms of a model mixture of metals, tin(II) and lead(II), ‘Sn(II)/Pb(II)’ in DPV and DPCSV. A comparative study was considered. For this purpose, differentiation of voltammetric data was suggested. Well-resolved first-derivative curves with mutual zero-crossing working potential were obtained. The main source for tin uptake by man is food, with the exception of some polluted industrial areas, where tin concentrations in water and air are high. In acidic canned fruit juice values of as much as 2 g l−1 have been found [13]. The maximum tolerance level of tin in foods defined by the WHO is 250 mg kg−1 [14]. The tin contents of environmental and biological samples cover a wide range, reaching from ␮g g1−1 quantities in canned foods down to ng g−1 and even pg g−1 quantities in water [15]. Lead is a potentially toxic metal. Lead poisoning may be due to inorganic or organic lead. Provisional maximum tolerable weekly intake of lead is, for adults, 3 mg and for infants, 25 ␮g per kg body weight. The limit of lead in food is defined as a maximum of 1 mg l−1 [16]. Simultaneous determination of tin and lead by polarography or ASV was made difficult by overlapping peaks [17]. The addition of methanol [18] or ammonium iodide [19] to 1 or 0.5 M hydrochloric acid, respectively, provided a peak separation of ca. 70 mV in a.c. polarography. The polarographic behaviour of tin and lead in the presence of the surfactant Hyamine-2389 has been studied. The peak potentials of these ions were well separated and the method was applied to the determination of tin in canned grape juices [20]. Simultaneous determination of tin, lead and molybdenum by DPP using 0.05 M EDTA and 0.02 M sodium acetate buffer of pH 3.5 as supporting electrolyte has been described [21]. Tropolone-modified carbon paste electrodes for trace measurements of tin using DPP have been reported [22]. Determination of tin in the presence of lead by stripping voltammetry with collection at a rotating mercury film disc-ring electrode has been performed [23] in glassy-carbon methanolic hydrobromic acid solution.

A method for the simultaneous and direct determination of lead and tin (in fruit juices and soft drinks) based on potentiometric stripping analysis (PSA), using methanol–water (18 + 2 v/v) at pH 1 as supporting electrolyte has been described by Mannino [24]. The application of continuous flow potentiometric stripping analysis (PSA) to samples of fruit juices and soft drinks has also been reported [25]. Recently, resolution of overlapping polarographic waves of lead and tin using on-line Kalman filtering has been carried out [26]. In the present work, the validity of the proposed method is assessed through the simultaneous determination of both metals in canned soft drinks and the recovery of lead in a drinking water sample. 2. Orthogonal function method in voltammetric analysis With reference to Glenn’s method of orthogonal functions in spectrophotometry, the analytical data in voltammetric analysis have to be processed in a similar way. Accordingly, the voltammogram, f(I), (the current ‘I in nA’ measured over the potential range ‘E’), can be expanded in terms of orthogonal functions as follows: f (I ) = p0 P0 + p1 P1 + p2 P2 + p3 P3 + · · · + pj Pj (1) where f(I) is the current ‘I in nA’ measured at the potential that belongs to a set of (n + 1) equally spaced potentials. pj are the coefficients of the orthogonal polynomial, Pj [27]. p0 P0 is known as the constant component, p1 P1 is the linear component, p2 P2 is the quadratic component, p3 P3 is the cubic component etc. of the voltammogram. In view of the orthogonality of Eq. (1), any coefficient, pj , can be obtained from n + 1 analytical readings (‘I in nA’ measured at equally spaced potentials) as follows: pj =

n X Ii Pj i /Nj

(2)

i=0

where j stands for the order the polynomial (0, 1, 2, 3, . . . , n), i stands for the number of point and Nj is the corresponding normalizing factor [27] and is calculated according to the next equation:

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Nj =

n X i=0

Pj2i

(3)

The coefficient, pj , of the orthogonal polynomial, Pj , is proportional to the concentration of analyte, (x), if there is no interference, through the equation: pj = αj Cx

(4)

where α j is the coefficient, analogous to extinction coefficient in spectrophotometric analysis of the pure compound, X; Cx is the concentration. In the presence of irrelevant currents, each observed coefficient is the sum of two terms; thus, pj = αj Cx + pj (z)

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with a maximum or a minimum in the convoluted curve.

2.2. Convoluted curves For the voltammetric data collected, the comparative coefficients, qj , are computed at different potential ranges and intervals. The values of qj , for the specified number of points, are plotted versus the mean potential, Em , [where Em = (Ei + Ef )/2] to get the convoluted curves.

(5)

where z denotes contribution from irrelevant currents. Eq. (4) contains two unknowns, Cx and pj (z) and can only be used to evaluate Cx from pj when there are good grounds for supposing pj (z) to be negligible relative to α j Cx . To minimize pj (z) to a negligible value, great care must be taken in choosing the polynomial, potential range, number of points and the mean potential, all of these choices being made with reference to the irrelevant current. 2.1. Choice of optimum conditions [28] To obtain good results using the orthogonal function method, great care must be taken in the selection of the polynomials, the number of points and the potential range and intervals. With regard to the choice of polynomial (it is considered for the compound to be estimated), P0 and P1 , the constant and linear polynomials, respectively, are ignored because the voltammogram of the irrelevant interferent, whatever its shape, contributes significantly to these polynomials. The quadratic polynomial, P2 , is suitable as it contributes greatly to the voltammogram of the estimated compound. The set of potentials associated with a given coefficient is defined by the number of points, intervals and the mean potential (Em ). The number of points are selected to suit the need for drawing maximum information from the continuous part of the voltammogram. The optimum potential range is selected to maximize q2 of the estimated compound whilst the other coexisting interferent shows no contribution. The mean potential of the optimal range is preferably corresponds

2.3. Orthogonal function coefficients and their comparative coefficients In view of the fact that the magnitude of the orthogonal coefficients depends upon the number of points used in the calculation and does not reflect the precision associated with each magnitude, the comparative coefficients ‘qj ’ have been proposed [11]. In this regard, Glenn’s theory of comparative coefficients has to be considered [29]. If the magnitude of the comparative coefficient, for the set of analytical readings used in the calculation, exceeds 0.14, this means that the relative standard deviation of this particular coefficient is less than 1%. The comparative coefficient ‘qj ’ is related to the orthogonal function coefficient ‘pj ’ through the following equation: 1/2

qj = pj · Nj

(6)

where Nj is the normalizing factor. The calculated values of qj will not change whether the true polynomials or multiples of the polynomials are used for the calculation of the coefficients [30]. Therefore it is preferable to convolute the voltammogram using the comparative coefficients. The convolution using the comparative coefficients is called transformation. Accordingly, the comparative coefficient, qj , is used throughout the present work. To obtain precise estimate of the analyte’s concentration, the magnitude of qj must be taken into consideration. The optimal working qj preferably corresponds with a maximum or a minimum in the polynomial’s convoluted curve.

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2.4. Two-component analysis In the case of two components, X and Y, the observed coefficient is the sum of two terms; thus, pj = αj Cx + βj Cy

(7)

Based on the zero-crossing technique, selective determination of X and Y compounds could be possible. When β j Cy is negligibly small or even zero relative to α j Cx , pj is directly proportional to Cx . By analogy, pj is correlated to Cy at zero contribution of the term α j Cx . 3. Experimental 3.1. Apparatus The voltammograms were obtained with a Metrohm 693 VA Processor. A Metrohm 694 VA Stand was used in the hanging mercury drop electrode (HMDE) mode. The three-electrode system was completed by means of a Ag/AgCl (3 M KCl) reference electrode and a Pt auxiliary electrode. 3.2. Reagents and standard solutions All the reagents used were of analytical reagent grade. De-ionized water was used throughout. Stock standard lead nitrate solution (1.0 mg ml−1 ) was prepared in water. Suitable dilutions were made with water. Stock standard tin(II) chloride solution (1.0 mg ml−1 ) was prepared in 0.1 M HCl. Suitable dilutions were made with water. 3.3. Sample preparation Canned soft drinks (cola products, apple drink and soda drink) and two drinking water samples were analysed, without any sample pre-treatment, after acidification to 0.04 M acetic/o-phosphoric/boric acids. 3.4. Voltammetric measurements A 20 ml aliquot of standard (or sample) solution, previously acidified to 0.04 M acetic/o-phosphoric/boric

Fig. 1. DPV peaks of (1) 0.84 ␮g ml−1 Sn(II) and (2) 1.6 ␮g ml−1 Pb(II) in 0.04 M acetic/o-phosphoric/boric acids’ mixture.

acids, was transferred into the voltammetric cell. The solution was de-aerated by bubbling nitrogen for 5 min. The differential pulse voltammograms were obtained between −0.25 and −0.55 V with a −100 mV pulse amplitude, a 30 ms modulation time, a 4 mV s−1 scan rate and a drop size of ca. 0.60 mm2 drop area. The differential pulse cathodic stripping voltammograms were obtained between −0.25 and −0.55 V with a −100 mV pulse amplitude, a 30 ms modulation time, a 4 mV s−1 scan rate, a −0.1 V working accumulation potential (Eacc ) and 60 s accumulation time (tacc ). For Pb(II) determination, the Eacc was adjusted at −0.5 V Quantitative results were obtained by the standard additions method. Appropriate aliquots of the standard solutions of the two metals were added and the concentration was evaluated.

4. Results and discussion 4.1. Differential pulse voltammetry 4.1.1. Application of orthogonal function method: ‘qj method’ To validate the proposed method the model overlapping system Sn(II)/Pb(II) was investigated by DPV at ␮g ml−1 concentration level. The differential pulse voltammograms of Sn(II) and Pb(II) in the range of −0.25 to −0.55 V potential region are shown in Fig. 1. It can be seen that the reduction potentials of both metals are reasonably similar (−0.438 and −0.368 V, respectively). This fact shows the impossibility of their simultaneous determination by DPV directly in a mixture.

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Fig. 2. (A) q2 convoluted curves (calculated for 10-point, 12 mV intervals), (1) and (2), derived from DPV in Fig. 1, (1) and (2), respectively. (B) as (A) except that q2 are calculated for 12 points at 8 mV intervals.

For the selective determination of Sn(II), the quadratic comparative coefficients, q2 , were computed at different potential ranges and intervals using the software set by Wahbi et al [29]. Fig. 2(a) shows a plot of q2 computed at 12 mV intervals for 10 points against Em , the mean of the set of potentials, which gives the convoluted curves of the voltammograms of Sn (Curve 1) and Pb (Curve 2). The optimum working potential was found to occur at Em = −0.420 V for Sn (zero-crossing Em of Pb). The selected q2 (at Em = −0.420 V, indicted by a solid vertical line) lies in the proximity of a maximum in the convoluted curve of Sn(II). Similarly, Fig. 2(b) shows a plot of q2 computed at 8 mV intervals for 12 points against Em which gives the convoluted curves of the voltammograms of Sn (Curve 1) and Pb (Curve 2). The optimal working potential was found to occur at Em = −0.382 V for Pb (zero-crossing Em of Sn). The selected q2 (at Em = −0.382 V, indicated by a solid vertical line) lies at a maximum in the convoluted curve of Pb(II) and can be used for quantitation of Pb(II) without interference from Sn(II). The optimal analytical parameters concerned with qj method are presented in Table 1. In order to demonstrate the validity of the proposed orthogonal function method for simultaneous determination of Sn(II) and Pb(II) in combination, several synthetic mixtures of both metals were prepared in different ratios. The voltammetric measurements were taken for all solutions and the q2 were calculated at

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Fig. 3. First-derivative DPV peaks of (1) 0.84 ␮g ml−1 Sn(II) and (2) 1.6 ␮g ml−1 Pb(II) in 0.04 M acetic/o-phosphoric/boric acids’ mixture.

the optimized analytical parameters (Table 1). It can be seen that using 10-point orthogonal polynomials over the potential range −0.366 to −0.474 V at 12 mV intervals, the q2 calculated for a mixture of the two metals is independent of the Pb(II) concentration and gives precise estimates of Sn(II) with RSD value (calculated for three determinations) less than 2% (Table 2). The mean percentage recoveries were found to be between 98 and 102% for Sn(II). Considering Pb(II) determination, the results also show good accuracy and precision of the method (Table 2). 4.1.2. Application of the first-derivative technique In order to achieve a resolution in the determination of overlapping peaks, the 1 D curves were computed (1E = 8 mV ) for the differential pulse voltammograms data of both metals. Fig. 3 shows the first-derivative voltammograms of Sn(II) and Pb(II). It can be seen that the zero-crossing method is the most appropriate for resolving mixtures of these compounds and it was used in this work with satisfactory results. Experimental work showed that the intensity of the 1 D at −0.310 V [nil contribution of Sn(II)] was proportional to the Pb(II) concentration. A linear calibration graph with a very small intercept was obtained and the variation of 1 D−0.310 was not affected by the presence of Sn(II) for any ratio of Pb(II) to Sn(II) over the full range of concentration investigated. The 1 D intensity at −0.370 V [zero-crossing potential of Pb(II)] was independent of the amount of Pb(II) and was proportional to the Sn(II) concentration.

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Table 1 Optimal conditions for the determination of tin and lead in combination using the orthogonal function method in differential pulse voltammetric analysis and the adopted parameters for stripping voltammetry Metal

Number of points

Differential pulse voltammetry Tin 10 Lead 12 Differential pulse stripping voltammetry Tin 14 Lead 10 Data for leada 12 a

Potential range (mV)

Interval (mV)

Em

qj

−366 to −474 −338 to −426

12 8

−420 −382

q2 q2

−346 to −502 −298 to −370 −306 to −438

12 8 12

−424 −334 −372

q2 q2 q2

Eacc =−0.5 V.

Table 2 Precision and accuracy for the determination of tin and lead by the orthogonal function and the first-derivative methods applied to differential pulse voltammetry

␮g ml−1 added

Tin Meana % recovery (±SD)

Lead Meana % recovery (±SD)

Tin

Lead

2q −0.420

1D

2.10 2.10 2.10 1.68 1.68 1.26 0.84 0.84 0.84 0.42

1.60 4.80 9.60 3.20 6.40 8.00 4.80 0.80 3.20 1.00

99.7(±0.20) 99.6(±1.20) 99.8(±0.78) 98.4(±1.03) 98.1(±0.82) 98.1(±0.81) 98.9(±0.28) 100.1(±0.26) 100.2(±0.09) 101.6(±1.81)

100.1(±0.72) 98.6(±1.03) 98.6(±1.66) 99.4(±1.06) 99.6(±0.94) 98.9(±0.24) 98.7(±1.42) 98.9(±0.64) 99.0(±0.83) 102.0(±1.45)

a

−0.370

2q

−0.382

100.5(±0.56) 99.7(±0.66) 99.0(±1.62) 101.6(±1.35) 98.3(±1.40) 101.1(±0.88) 100.2(±0.09) 101.9(±0.87) 100.9(±0.56) 98.3(±1.40)

1D −0.310

100.1(±0.72) 99.5(±0.90) 99.2(±0.40) 102.1(±0.60) 99.6(±0.83) 99.6(±0.12) 99.1(±0.70) 101.7(±1.80) 99.6(±0.58) 99.1(±0.81)

Average of three determinations.

4.2. Differential pulse cathodic stripping voltammetry At first, a study of the factors and different instrumental parameters that may influence both the accumulation process and the voltammetric response was carried out. 4.2.1. Factors influencing the accumulation step The effect of the Eacc on the stripping peak current of Sn(II) was evaluated over the range from 0 to −0.8 V (Fig. 4, plot 1). Larger peaks were obtained over the range from 0 to −0.2 V; the peak decreased at higher potentials (−0.2 to −0.5 V) with a sudden enhancement at potentials over than −0.5 V. Depending on this behaviour, two working accumulation potentials were suggested, −0.6 and −0.1 V. However, the study was extended to check whether both potentials

Fig. 4. Effect of accumulation potential on the voltammetric stripping response of (1) 210 ng ml−1 Sn(II) and (2) 400 ng ml−1 Pb(II). tacc = 40 s.

are suitable for the quantitative work or not. In this regard, the peak current obtained for 84 ng ml −1 Sn (at Eacc −0.6 V) over a tacc range of 0−120 s was followed. As can be seen from Fig. 5 (Curve 1) that the

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Fig. 5. Effect of accumulation time on the peak current of (1) 84 ng ml−1 Sn(II) at Eacc =−0.6 V, (2) 84 ng ml−1 Sn(II) at Eacc =−0.1 V and (3) 400 ng ml−1 Pb(II) at Eacc =−0.5 V.

plot obtained displays the resulting peak current versus pre-concentration time for 84 ng ml−1 Sn(II), and shows no linear pattern even at very short adsorption time. From such a finding, it may be possible to say that the current is not diffusion-controlled at an accumulation potential of −0.6 V. Additionally, over the concentration range of 80–350 ng ml−1 of Sn (at Eacc −0.6 V and tacc 60 s), the signals obtained show no linear correlation to the concentration. To summarize, an accumulation potential of −0.6 V does not suit the stripping voltammetric analysis of Sn(II). The spontaneous accumulation of Sn(II) was studied (the Eacc was adjusted at −0.1 V) for effective pre-concentration prior to the voltammetric scan. Fig. 5(Curve 2) displays the resulting peak current versus pre-concentration time plot for 84 ng ml−1 Sn(II). The rapid increase of the current observed at short pre-concentration time is followed by a levelling-off for longer periods. A linear plot up to 60 s is observed. Hence to maximize sensitivity, 60 s accumulation time was generally used for subsequent quantitative determinations. An adsorption potential of −0.1 V was adopted for the determination of Sn(II). The effect of the accumulation potential on the adsorptive stripping peak current of Pb(II) was evaluated over the range from 0 to −0.8 V (Fig. 4, plot 2). The plot shows that the Pb(II) signal follows a normal pattern with a larger peak current at the working accumulation voltage −0.5 to −0.8 V. Lower signals were obtained at the Eacc = 0 to −0.3 V. The spontaneous accumulation of Pb(II) was studied (at Eacc =−0.5 V) for effective pre-concentration prior to the voltammetric scan. Fig. 5 (Curve 3)

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displays the resulting peak current versus preconcentration time plot for 400 ng ml−1 Pb(II). A linear response was observed up to ∼5 min. Accordingly, an accumulation potential of −0.5 V was proposed to be optimal for the stripping voltammetric analysis of Pb(II). The voltammetric behaviour of Pb, 400 ng ml−1 , at the accumulation potential −0.1 V (proposed working accumulation potential of Sn) was studied over a pre-concentration time range of 0–120 s. Constant peak current was observed indicating that Pb exhibits no accumulation character at the HMDE at the working potential −0.1 V. However, at such a condition, the Pb signal shows a linear response over the concentration range 0.2–10 ␮g ml−1 . From the previous study, the accumulation working potential, −0.1 V, was suggested for simultaneous determination of Sn(II) and Pb(II). This is suitable for samples rich in Pb, down to the 200 ng ml−1 level. However, to maximize sensitivity towards the determination of Pb(II), an accumulated working voltage of −0.5 V was proposed. 4.2.2. Supporting electrolyte Optimization of acids’ mixture (acetic/o-phosphoric/ boric acids) concentration was done by variation of the concentration from 0.04 to 0.25 M. It was found that the acids’ mixture concentration has no appreciable effect on peak current of Pb(II) in the range of 0.04–0.25 M. On the contrary, Sn(II) shows ∼50% decrease in peak current upon changing the acids’

Fig. 6. (A) DP cathodic stripping voltammogram of a binary mixture of (1) 50 ng ml−1 Sn(II) and (2) 500 ng ml−1 Pb(II) in 0.04 M acetic/o-phosphoric/boric acids’ mixture. Eacc =−0.1 V, tacc = 60 s. (B) first-derivative DPCSV peaks derived thereafter.

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mixture concentration from 0.04 to 0.25 M. Accordingly, 0.04 M acids’ mixture was used as the most appropriate. 4.2.3. Simultaneous determination of tin and lead Following the optimum instrumental parameters, Fig. 6(A) presents the stripping voltammograms obtained for Sn(II), Curve 1, and Pb(II), Curve 2 at Eacc =−0.1 V and tacc =60 s. A slight shift in peak potential of Sn(II), relative to that of DPV in Fig. 1, is seen. Peak separation of 78 mV is attained. However, mutual interference at the respective peak hindered selective direct measurement. Standard solutions of Sn(II) and Pb(II) gave a linear stripping voltammetric response over the concentration range of 10–120 ng ml−1 and 0.20–10 ␮g ml−1 , respectively.

4.2.4. Determination of lead in the presence of tin Measurement of a low concentration of Pb(II) down to 30 ng ml−1 was possible through its spontaneous accumulation at HMDE at −0.5 V. A standard Pb(II) solution exhibits a linear stripping voltammetric response over the range of 30–1000 ng ml−1 (peak potential at −0.346 V). The interference of Sn(II) at the Pb(II) peak (−0.346 V) could be corrected by q2 as well as 1 D methods. The 1 D−0.306 values (1E = 8 mV) and q2 (at Em =−0.372 V) calculated from the stripping voltammograms of binary mixtures of Pb(II) and Sn(II) were independent on Sn(II) concentration (in the range of 0–300 ng ml−1 ). Accordingly, the recovery data show good accuracy and precision for Pb(II) determination (Table 3). 4.3. Calibration and regression analysis

4.2.3.1. Orthogonal function method: ‘qj method’. Depending on the general rules previously discussed in Section 2, adopted analytical parameters were set (Table 1). To prove the selectivity of the chosen q2 at Em =−0.424 V (working zero-crossing Em of Pb) and q2 at Em =−0.334 V (working zero-crossing Em of Sn) for the determination of Sn(II) and Pb(II), respectively, synthetic mixtures of both metals prepared at different ratios were analysed. The results obtained are given in Table 3. The mean percentage recoveries ±SD data were satisfactory and validate the applicability of the proposed qj method for the simultaneous determination of Sn(II) and Pb(II) by differential pulse cathodic stripping voltammetry (DPCSV).

4.2.3.2. First-derivative technique. The shift in peak potential of Sn(II) in the DPCSV was concerned. The first-derivative curves were derived at 1E = 8 mV for the differential pulse stripping voltammograms data of both metals. Fig. 6(B) shows the first-derivative voltammograms of Sn(II), Curve 1, and Pb(II), Curve 2. The 1 D−0.490 and 1 D−0.314 values of the derivative voltammogram of a binary mixture show nil contribution of either of Pb(II) or Sn(II), respectively. The results presented in Table 3 prove that the 1 D−0.490 and 1 D−0.314 provide accurate and precise estimates of Sn(II) and Pb(II), respectively.

Differential pulse measurements at the HMDE gave a linear relationship between peak current and metal concentration over the ranges stated in Table 4. The q2 and the 1 D values calculated for Sn(II) and Pb(II) voltammograms were correlated to concentration and the regression data obtained were listed in Table 4. 4.4. Detection limits According to IUPAC [31], the detection limit DL = 3 s/k, where s is the standard deviation of replicate determination values under the same conditions as for sample analysis in the absence of the analyte and k is the sensitivity, namely, the slope of the calibration graph. The DL values were calculated as 0.349 and 0.774 ng ml−1 for Sn(II) (at Eacc −0.1 V, peak potential at −0.446 V) and Pb(II) (at Eacc −0.5 V, peak potential at −0.346 V), respectively. 4.5. Interferences of other elements For studying interferences voltammograms were recorded in the range from −0.2 to −0.8 V. Metal ions tested at the 1.0 ␮g ml−1 level included Ca, Mg, Cu, Fe, Al, Zn, Cd, Mn, Co and Ni. The recoveries of 50 ng ml−1 Sn(II) (Eacc =−0.1 V) after the addition of different elements were ∼98–101%. Within the potential range of −0.2 to –0.8 V, only Cd

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Table 3 Precision and accuracy for the determination of tin and lead by the orthogonal function and first-derivative methods applied to DPCSV Tin Meana % recovery (±SD)

Lead Meana % recovery (±SD)

Simultaneous determination, accumulation potential −0.1 V Tin, added (ng ml−1 )

Lead, added (ng ml−1 )

2q

1D

80 60 80 120 120

240 200 200 260 300

99.9(±1.41) 99.6(±0.65) 100.6(±0.43) 99.7(±1.50) 100.3(±1.98)

100.5(±1.07) 99.3(±0.66) 99.6(±1.12) 100.2(±1.16) 101.1(±1.25)

−0.424

−0.490

Lead determination, accumulation potential −0.5 V Tin, added (ng ml−1 ) Lead, added (ng ml−1 ) 80 300 200 300 a

40 100 400 600

2q

−0.334

1D −0.314

99.9(±0.36) 100.0(±1.18) 101.1(±0.21) 99.4(±1.14) 99.1(±1.19)

99.8(±0.81) 99.6(±0.40) 100.7(±1.19) 99.1(±1.13) 99.7(±1.34)

2q

1D −0.306

−0.372

100.2 100.1 101.0 100.9

(±0.77) (±0.28) (±1.63) (±1.19)

100.1(±0.21) 100.4(±0.91) 99.8(±1.45) 100.6(±0.86)

Average of three determinations.

Table 4 Analytical data of the calibration graphs of tin and lead Metal

Measurement

Differential pulse voltammetry Tin

1D

Linearity range

0.21–2.1 ␮g ml−1

−0.370

2q

−0.420

1D

Lead

0.20–10 ␮g ml−1

−0.310

2q

−0.382

Differential pulse stripping voltammetry 1D −0.490 Tin

10–120 ng ml−1

2q

−0.424

1D

Lead

0.2–10 ␮g ml−1

−0.314

2q

−0.334

Data for lead at Eacc =−0.5 V

1D

30–1000 ng ml−1

−0.306

2q

−0.372

Table 5 Determination of tin and lead in soft drinks Sample

Tin concentrationa ±SD, ␮g ml−1 2q

1D

2.11±0.0360 0.030±0.0005 0.122±0.0028

2.08±0.0510 0.031±0.0007 0.126±0.0021

−0.424

Canned colab Canned sodac Canned apple drinkc a

Average of three determinations. Lead cannot be detected. c Lead was not detected. b

−0.490

Regression data a (Intercept, nA)

b (Slope, nA ml ng−1 )

r

−0.0102 −0.4517 0.0015 −0.1830

1.7650 59.9920 0.9625 19.9727

0.9998 0.9997 0.9999 0.9996

−0.0085 −0.7215 −0.0009 0.1710 −0.0157 −0.1348

0.01446 0.6336 1.1916 15.7794 0.0078 0.2452

0.9997 0.9995 0.9996 0.9990 0.9998 0.9990

exhibits a neighbouring peak at −0.560 V but without interference with the Sn(II) signal. A similar study was carried out for Pb(II) 100 ng ml−1 , (Eacc =−0.5 V) and good recoveries (∼97–99%) were obtained. Some of the materials which are sometimes present in soft drinks are caffeine, citric and ascorbic acids. Possible interference of either of these additives with the voltammetric measurement of Sn(II) and Pb(II) was examined. It was found that caffeine, citric and ascorbic acids have no effect on the peak

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Table 6 Recovery experiments Sample

Canned cola Canned soda Canned apple drink a

(ng ml−1 ) added

40 60 100

Tin Meana % recovery (±SD) 2q −0.424

1D

99.2 (±0.67) 99.8 (±0.61) 98.0 (±1.16)

98.4 (±0.11) 99.6 (±1.21) 99.5 (±1.23)

(ng ml−1 ) added

−0.490

Lead Meana % recovery (±SD) 2q

1D

99.9(±1.27) 98.5(±0.87) 101.4(±0.55)

101.2(±0.68) 98.6(±1.19) 101.3(±0.70)

−0.372

400 100 150

−0.306

Average of three determinations.

current of both metals and exhibit no signal in the vicinity of Sn(II) and Pb(II) signals. Fortunately, the supporting electrolyte used in this work, 0.04 M acetic/o-phosphoric/boric acids, eliminates the effect of citric acid on the tin signal. It has been reported [32] that in the ammonium acetate solution, at pH 4.6, citric acid shows a great masking effect on the tin signal due to the formation of a tin–citrate complex with a high conditional stability constant. 4.6. Determination of tin and lead in canned soft drinks The validity of the method was demonstrated by the determination of Sn(II) and Pb(II) (if present) in canned soft drinks, with no sample pre-treatment. The acidity of soft drinks stored in cans causes dissolution of tin from the can. If required, samples containing high concentrations of metal were diluted with de-ionized water and a volume was taken for analysis after acidity adjustment. The concentrations of Sn(II) and Pb(II) were determined in canned products using the proposed orthogonal function method and the results were compared with those obtained by 1 D measurements. The average results of triplicate analyses are reported in Table 5. For all drinks examined in cans, the difference between the two methods was not greater than 3% for the levels of Sn(II) found. The analyses indicated that the level of Pb(II) was not detectable in the samples examined with the exception of the cola drink. In the cola drink, the high content of Sn(II) necessitates a 20-fold dilution step before voltammetric measurement leading to a concentration level of Pb(II) that cannot be detected. 4.6.1. Recovery of added metal To determine the recovery of Sn(II) and Pb(II), appropriate volumes of the standard solutions were

added to all samples examined. The results listed in Table 6 show recoveries ranging from 98 to 102% for both metals. 4.7. Determination of lead in drinking water The applicability of the proposed method was further assessed through examining the Pb(II) level in a drinking water sample. As it was found that the water contained an undetectable quantity of Pb(II), the sample was spiked with an aliquot of Pb(II) of known concentration. The voltammetric measurement of the spiked sample was taken. The calculated 1 D−0.306 and 2 q−0.372 were compared with those of a standard. For a sample with an added Pb(II) concentration of 200 ng ml−1 , the analysed concentration was 197.10±2.6 ng ml−1 for three determinations. References [1] L. Meites, Polarographic Techniques, Interscience, New York, 1965, pp. 341–348, p. 379. [2] J.J. Berzas, J. Rodriguez, Fresenius J. Anal. Chem. 342 (1992) 273. [3] G. Turnes, A. Cladera, E. Gomez, J.M. Estela, V. Cerda, Electroanal. Chem. 338 (1992) 49. [4] T.F. Brown, S.D. Brown, Anal. Chem. 53 (1981) 1410. [5] D.P. Binkley, R.E. Dessy, Anal. Chem. 52 (1980) 1335. [6] A. Henrion, R. Henrion, G. Henrion, F. Scholz, Electroanalysis 2 (1990) 309. [7] D. Jagner, L. Renman, S.H. Stefansdottir, Anal. Chim. Acta 281 (1992) 315. [8] A.L. Glenn, J. Pharm. Pharmac. 15 (1963) 123T. [9] A.M. Wahbi, Die Pharmazie 26 (1971) 291. [10] A.M. Wahbi, H. Abdine, J. Pharm. Pharmacol. 25 (1973) 69. [11] A.M. Wahbi, S. Ebel, J. Pharm. Pharmacol. 26 (1974) 317. [12] M.M. Amer, A.M. Wahbi, S. Hassan, Ind. J. Technology 13 (1975) 564. [13] S. Warurton, H.W. Udler, R.M. Ewert, W.S. Hayes, Publ. Health 77 (1962) 798. [14] WHO Trace elements in human nutrition, WHO Technical Reports Series 532, 1973, p 38.

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