ORIGINAL PAPER Square-wave adsorptive stripping

Chemical Papers 62 (4) 339–344 (2008) DOI: 10.2478/s11696-008-0041-z

ORIGINAL PAPER

Square-wave adsorptive stripping voltammetric determination of an antihistamine drug astemizole Ahmad H. Alghamdi* Department of Chemistry, College of Science, King Saud University, P.O Box 2455, 11451 Riyadh, Saudi Arabia Received 1 July 2007; Revised 19 November 2007; Accepted 20 November 2007

The square-wave voltammetric technique was used to explore the adsorption properties of the astemizole drug. The analytical methodology used was based on the adsorptive preconcentration of the drug on a hanging mercury drop electrode (HMDE), followed by the electrochemical reduction process which yielded a well-defined cathodic peak at −1.184 V (vs. the Ag/AgCl electrode). To achieve high sensitivity, various experimental and instrumental variables were investigated such as the supporting electrolyte, pH, accumulation time and potential, drug concentration, scan rate, SW frequency, pulse amplitude, convection rate, and the working electrode area. Under the optimized conditions, the AdSV peak current was proportional over the analyte concentration range of 5 × 10−7 to 2.5 × 10−6 mol L−1 (r = 0.998) with the detection limit of 1.4 × 10−8 mol L−1 (6.4 ng mL−1 ). The precision of the proposed method in terms of RSD was 2.4 %, whereas the method accuracy was indicated by the mean recovery of 100.1 %. Possible interferences of several substances usually present in the pharmaceutical tablets and formulations were also evaluated. The applicability of this electroanalytic approach was illustrated by the determination of astemizole in tablets and biological fluids. c 2008 Institute of Chemistry, Slovak Academy of Sciences Keywords: adsorptive stripping voltammetry, astemizole, tablet, biological fluids

Introduction Over the last two decades, adsorptive stripping voltammetry (AdSV) has assumed increased popularity and a recognized place in the instrumental arsenal for the determination of a wide variety of organic compounds of pharmaceutical and biomedical significance. Adsorptive voltammetric approach is very suitable for studying clinical and pharmaceutical samples owing to its remarkable sensitivity and unique simplicity. In addition, adsorptive stripping is considered a universal and highly useful voltammetric approach due to its adequate selectivity, multi-elements applicability, low costs, and stability to on-line measurements. Adsorption of numerous species on the surface of mercury and other metal and modified electrodes leads to the preconcentration of these analytes, a process ensuring outstanding sensitivity in electroanalytical determinations (Wang, 1985; Bard & Faulkner, 1980;

Kissinger & Heineman, 1996; Dewald, 1996). Applicability of the adsorptive accumulation procedure in the determination of pharmaceutical drugs in biological fluids and drug dosage forms has been reviewed elsewhere (Vire et al., 1998; Yarnitzky & Smyth, 1991; Alghamdi, 2002; Wang, 1988). Astemizole is a relatively new antihistamine with a long duration of action used to treat symptoms of allergic disorders. It joins a new class of drugs with little sedative effect (Morton, 1991). Astemizole competitively binds to histamine H1-receptor sites in the gastrointestinal tract, uterus, blood vessels, and bronchial muscles. This suppresses the formation of edema and pruritus thereby producing adverse effects. Astemizole has recently been found to be a potent treatment for malaria. It has a mechanism of action similar to chloroquine but is active even in chloroquine resistant parasites (Chong et al., 2006). Due to such medicinal importance of astemizole,

*Corresponding author, e-mail: [email protected]

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A. H. Alghamdi/Chemical Papers 62 (4) 339–344 (2008)

there are several instrumental techniques available for its determination. Among them are the chromatographic separation methods, particularly HPLC (Suryanarayana et al., 1993; Woestenborghs et al., 1983; El-Ealily et al., 1995), thin layer chromatography (Mangalan et al., 1991; Al-Deeb et al., 1992), and capillary zone electrophoresis (Lamparczyk & Kowalski, 1996). In addition, this antihistamine drug has been also analyzed by spectrophotometry (Gungor & Onur, 2001; Kelani et al., 1999; Sastry & Naidu, 1997, 1998; Alwarthan & Alobaid, 1996; Qureshi & Khan, 1996), fluorimetry (Karam et al., 1999), and radioimmunoassay methods (Woestenborghs et al., 1986). However, to the best of my knowledge, the adsorptive stripping behavior of astemizole and its AdSV determination have not been performed and reported so far. Consequently, the aim of this research work was to develop a more sensitive, reliable, and simple AdSV procedure for the determination of astemizole in complex biological media and pharmaceutical formulations.

N N

N O

N F

N

Fig. 1. Chemical structure of astemizole.

ric cell, and the required standard solutions of astemizole were added. The test solutions were purged with nitrogen for 5 min while the solution was stirred. The accumulation potential of 0.0 V vs. Ag/AgCl was applied to a new mercury drop while the solution was stirred for 60 s (unless otherwise stated). Following the preconcentration period, the stripping was stopped, and after 20 s, cathodic scans were carried out over the range of 0.0 to −1.4 V. All measurements were made at room temperature.

Results and discussion Experimental All adsorptive stripping measurements were carried out using a 757 VA computrace (Metrohm, Herisau, Switzerland) in connection with a Dell computer and controlled by the (VA computrace 2.0) control software. A conventional three-electrode system was used in the hanging mercury drop electrode (HMDE) mode. The pH values were measured using a Metrohm 744 pH meter (Herisau, Switzerland). Oxford adjustable micropipette (Ireland) was used to measure microliter volumes of the standard solutions. All chemicals used were of analytical reagent grade and were used without further purification. Astemizole (Sigma, UK) stock solution of 1 × 10−2 mol L−1 was prepared by dissolving the appropriate amount of this drug in distilled water in a 25 mL volumetric flask. This stock solution was stored in dark and under refrigeration in order to minimize the decomposition. Standard solutions of astemizole with lower concentration were prepared daily by diluting the stock solutions with distilled water. Britton-Robinson supporting buffer (pH ≈ 2; 0.04 mol L−1 in each constituent) was prepared by dissolving 2.47 g of boric acid (Winlab, UK) in 500 mL of distilled water containing 2.3 mL of glacial acetic acid (BDH, UK), adding 2.7 mL of ortho-phosphoric acid (BDH, UK), and diluting to 1 L with distilled water. The carbonate buffer concentration was 0.1 mol L−1 in both sodium hydrogen carbonate and disodium carbonate, while the phosphate buffer was prepared from 0.1 mol L−1 in both phosphoric acid and sodium phosphate (Winlab, UK). The general procedure adopted to obtain adsorptive stripping voltammograms was as follows: a 20 mL aliquot of the carbonate supporting buffer at the desired pH was pipetted into a clean and dry voltammet-

Electrochemical and SW-AdSV behavior of astemizole When the reductive determination of 1 × 10−4 mol L−1 astemizole was performed in a pH 8 B-R buffer using differential pulse polarography (DDP), a single broad reduction peak at Ep = −1041 mV was observed. This obtained electrochemical signal is probably related to the reduction of the fluorine atom on the aromatic halogen site (Fig. 1) according to the following electrochemical reduction process RX + 2e− + H+ → RH + X−

(1)

Aromatic halogen-containing organic compounds can be reduced in a two electrons process involving a loss of the halogen atom with the reduction potentials being markedly affected by the nature and number of substituents in the aromatic ring (Smyth, 1992). Due to the nature of this electro active group present in the chemical structure of this pharmaceutical drug, the expected reduction mechanism involved an irreversible reduction process, an assumption confirmed by cyclic voltammetric measurement of astemizole in a pH 8 B-R buffer at the scan rate of 50 mV s−1 . As can be seen from Fig. 2, no anodic peaks were observed on the measured cyclic voltammogram. The interfacial accumulation of this drug was designated from repetitive cyclic voltammograms for astemizole recorded after 60 s accumulation time at 0.0 V prior to the first scan which produced a high cathode peak. However, a considerable decrease of the monitored electrochemical signal was observed in the subsequent repetitive scans, such behavior was probable to occur since the resulting product of the reduction process is more predom-


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I/nA

I/nA


U/V U/V Fig. 2. Cyclic voltammogram of 1 × 10−4 mol L−1 astemizole in B-R buffer, pH 8 at 50 mV s−1 scan rate, accumulation time of 0 s, and accumulation potential of 0.0 V.

Fig. 3. SW-AdSV of astemizole in B-R buffer at pH 8. Experimental conditions: accumulation time of 90 s, accumulation potential of 0.0 V, scan rate of 300 mV s−1 , and drug concentrations of 2 × 10−6 mol L−1 (B) and 3 × 10−6 mol L−1 (A).

Factors affecting adsorptive stripping response inant and adsorbed on the HMDE surface; this obviously promotes the inhibition of the observed electrochemical signal. The voltammetric cycles carried out to increase the scan rate values over the range of 50– 400 mV s−1 gave rise to an electrochemical response with increased peak current intensities. The plot of log ip versus log ν gave a straight line with the slope value of 1.26, which is close to the theoretical value of 1.0 expected for an adsorption-controlled process (Laviron, 1980). This indicates the interfacial adsorptive character of astemizole. In addition, the observed peak potential shift, by 60 mV, to more negative values on the increase of the scan rate confirmed the irreversible nature of the suggested cathodic reduction process. The adsorption phenomena of astemizole can be used as an effective preconcentration step prior to voltammetric measurements. Hence, HMDE was used to study the adsorptive properties of astemizole in order to obtain a voltammetric response with higher sensitivity. Fig. 3 shows the square-wave adsorptive stripping voltammogram for the samples containing 2 × 10−6 mol L−1 and 3 × 10−6 mol L−1 of astemizole, recorded after the accumulation process at 0.0 V for 90 seconds in a pH 8 B-R buffer. This pharmaceutical drug yielded a sizeable and well-defined cathode peak at −1184 mV versus the Ag/AgCl reference electrode. Obviously, such electrochemical signal with enhanced sensitivity illustrates the superiority and advantage of the stripping voltammetric approach over the conventional polarographic method which suffers from poor peak development.

The choice of a suitable medium is an important parameter of the adsorptive stripping determination of organic compounds of clinical or pharmaceutical significance. Thus, 1 × 10−6 mol L−1 solutions of astemizole were studied by the SW-AdSV method in Britton–Robinson (pH 4, 6.5, and 8.5), carbonate (pH 8.5), acetate (pH 4), and phosphate (pH 4) buffers after the preconcentration time of 120 s at the accumulation potential of 0.0 V. The ideal adsorptive stripping response in terms of the peak shape and current and the smoothness of the baseline was observed when the carbonate buffer, selected as optimal for subsequent experiments, was used. In general, the peak height of the obtained AdSV signal reached its maximum values in mild alkaline media. The influence of pH variation over the range of 7.0–11.5 on the peak height and potential of the analyzed drug signal was investigated further and is presented in Fig. 4, where is the stripping voltammetric peak of the 1 × 10−6 mol L−1 astemizole solution plotted as a function of pH. After an initial voltammetric peak enhancement over the pH range of 7.0–10.0, buffer solutions with pH values higher than pH 10 gradually weakened and the monitored AdSV peak current was reduced. For analytical purposes, the optimum pH value for the determination of this pharmaceutical molecule seems to lie around pH 10. The peak potential of the electro active group was found to be dependent on pH of the buffer solution. A gradual shift by 150 mV to a more negative potential was observed when increasing the


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60 50

40

Peak current/nA

Peak current/nA

50

30 20 10

40 30 20 10 0

0 7

7.5 8

8.5 9

0

9.5 10 10.5 11

12

Peak current/nA

600

800

Scan rate/(mV s )

Fig. 4. Effect of pH on the electrochemical signal of astemizole. Experimental conditions: supporting electrolyte in carbonate buffer, accumulation time of 90 s, accumulation potential of 0.0 V, scan rate of 300 mV s−1 , and drug concentration of 1 × 10−6 mol L−1 .

8

4

0 50

400

-1

pH

0

200

100

Accumulation time/s

Fig. 5. Effect of accumulation time on the monitored electrochemical signal. Experimental conditions: concentration of 1 × 10−6 mol L−1 of astemizole in pH 10 carbonate buffer, accumulation potential of 0.0 V, and scan rate of 300 mV s−1 .

pH value over the range of 7.0–11.0, due to the consumption of hydrogen ions in the electrode reaction. Interfacial accumulation of astemizole onto the HMDE surface depends on some operational factors, worthy of additional investigations in order to ensure highly sensitive determinations of this drug. Therefore, the effect of accumulation time on the efficiency of accumulation of 1 × 10−6 mol L−1 astemizole onto the working electrode was evaluated by prolonging the accumulation time over the range of 0–120 s. Fig. 5 shows the resulting peak current to accumulation time profile exhibiting a steady enhancement in the peak current over the range of 0–60 s and thereafter, the peak intensity decreased gradually probably due to the hydrolysis process of the drug occurring under the alkaline conditions applied, especially the long accumulation time. Hence, accumulation time of 60 s was selected for all future experiments. Furthermore, variation of the accumulation potential over the range of +0.2 V to −0.8 V at the accumulation time of 60 s revealed that the preconcentration potential of 0.0 V was the ideal choice for optimal sensitivity. The developed AdSV procedure should exhibit lin-

Fig. 6. Effect of scan rate on the monitored electrochemical signal. Experimental conditions: concentration of 1 × 10−6 mol L−1 of astemizole in a pH 10 carbonate buffer, accumulation time of 60 s, and accumulation potential of 0.0 V.

ear concentration dependence to fulfill significant analytical utility. As a result, the voltammetric peak current to drug concentration curve was sketched and evaluated. When the variation of the peak current against the astemizole concentration was drawn at constant experimental conditions, it was found that the AdSV peak current varied linearly with the drug concentration over the range of 5.5 × 10−7 mol L−1 to 3.0 × 10−6 mol L−1 . However, beyond the linearity point, the measured peak current reached a plateau due to the saturation of the HMDE surface. The observed stripping voltammetric signal can be further maximized by adjusting the potential scanning method. The relationship between the measured peak intensity and the scan rate was found to be directly proportional over the scan rate of 50–300 mV s−1 . However, when employing scan rates faster than 300 mV s−1 , the peak current decreased slightly, as can be seen from Fig. 6. The 300 mV−1 scan rate value was adopted as optimal for further investigations. Moreover, varying the value of square wave frequency also plays an important role in the SW-AdSV approach signal measuring. Varying this parameter over the range of 10–100 Hz resulted in a substantial enhancement of the voltammetric peak current particularly at high frequency rates. Consequently, for future work, the 80 Hz SW frequency value was adopted. The monitored AdSV peak currents can be further enhanced by optimizing other instrumental parameters affecting the adsorption-based accumulation. The influence of the mercury drop working electrode surface size on the adsorptive stripping responses was evaluated. The relationship between the measured peak currents and the surface area of the drop was found to be linear for the surface areas of 0.15–0.60 mm2 . In addition, the effect of the convection rate was also studied by increasing the stirring rate from 400 min−1 to 3000 min−1 . The investigated adsorptive peak heights were significantly increased when high stirring rates or large drop sizes were employed. For


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optimal sensitivity, the drop size of 8 mm (0.60 mm2 ) and the stirring speed of 2000 min−1 were chosen for further investigations.

tives prior to its determination in real pharmaceutical tablets. Application of the developed method

Method Validation Before a procedure provides useful analytical information, it is necessary to demonstrate that it is capable of providing acceptable results. Hence, the process of validation of the developed electrochemical procedure was carried out. Under the optimum experimental conditions, a linear correlation (r = 0.998, n = 6) was obtained between the astemizole analytical response and its concentration in the range of 5 × 10−7 mol L−1 to 2.5 × 10−6 mol L−1 . Parameters of the dye concentration to the current straight line were calculated by the least-squares method. The respective regression equation of the calibration line is given by the following equation ip = 69 + 3.4 × 109 C

(2)

where ip /nA is the SW-AdSV peak current and C/(mol L−1 ) is the astemizole concentration. The detection limit, defined as the signal-to-noise ratio (S/N = 3), for monitoring this drug was 1.4 × 10−8 mol L−1 (6.4 ng mL−1 ). High sensitivity of the adsorptive voltammetry is accompanied by good reproducibility. This analytical performance measure was evaluated from 8 repeated experiments of the electrochemical signal of a 5 × 107 mol L−1 astemizole solution. The precision of the developed electrochemical method, in terms of the relative standard deviation (RSD), was 2.4 %. The accuracy of the proposed method was checked by calculating the recovery of a known amount of astemizole (1.25 × 10−7 mol L−1 ) added to the carbonate buffer solution and analyzed via an optimized stripping voltammetric procedure. The value of recovery obtained by the standard addition method was 100.1 %. Taking into account the method of the drug preconcentration on the working electrode prior to the actual voltammetric measurements, the major interference effect may be caused by surface-active materials competing with astemizole for the HMDE adsorption sites. The competitive co-adsorption interference was evaluated in the presence of various substances usually occurring in the pharmaceutical tablets and formulations. In these investigations, interfering species were added at different concentrations; twofold, 10fold, and 50-fold higher than the concentration of astemizole (1 × 10−6 mol L−1 ). Additions of the filling materials (sucrose, lactose, and cellulose), disintegrating agent (starch), and lubricants (magnesium stearate) exhibited no significant effects on the SWAdSV response of astemizole. Hence, this drug need not be extracted from these tablet ingredients or addi-

Following the procedure described above, usefulness of the developed electrochemical method for the analysis of astemizole in pharmaceutical formulations and biological fluids was checked. Without any sample pretreatment but only with an adequate dilution of this drug, AdSV was applied successfully to the assay of astemizole in five aliquot solutions of the dissolved tablet. The analyzed 50 mg tablet contained 8 mg of astemizole (active material), 26 mg of lactose, 10 mg of starch, 5 mg of cellulose, and 2 mg of magnesium stearate. The results obtained in these studies gave a recovery mean of 98.9 % with the standard deviation of ± 1.99 %. The accuracy of these analytical results was checked via the Student t-test. Since, at the confidence level of 95 %, the calculated t value (1.6) was less than its critical value (2.78), there is no statistical difference between the added and the recovered values (Miller & Miller, 1994). Acknowledgements. The author would like to thank Mr. Monther Abdlkareem for his technical assistance in applying the method and the King Saud University for financial support.

References Al-Deeb, O. A., Abdel-Moety, E. M., Bayomi, S. M., & Khattab, N. A. (1992). Spectophotometric quantification of astemizole and its demethylated metabolite in urine after TLC separation. European Journal of Drug Metabolism and Pharmacokinetics, 17, 251–255. Alghamdi, A. H. (2002). Application of adsorptive stripping voltammetry in pharmaceutical and clinical analysis. Journal of Saudi Chemical Society, 6, 185–198. Alwarthan, A. A., & Alobaid, A. M. (1996). Colorimetric determination of astemizole in bulk and in its pharmaceutical dosage forms using flow injection. Journal of Pharmaceutical and Biomedical Analysis, 14, 579–582. DOI: 10.1016/07317085(95)01653-8. Bard, A. J., & Faulkner, L. R. (1980). Electrochemical method: Fundamentals and applications. New York: John Wiley & Sons. Chong, C. R., Chen, X., Shi, L., Liu, J. O., & Sullivan, D. J. (2006). A clinical drug library identifies astemizole as an antimalarial agent. Nature Chemical Biology, 2, 415–416. DOI: 10.1038/nchembio806. Dewald, H. D. (1996). Stripping Analysis. In J. D. Winefordner (Ed.), Modern techniques in electroanalysis. New York: John Wiley & Sons. El-Ealily, A. M., El-Gindy, A., & Wahbi, A. M. (1995). Determination of astemizole using iodine charge-transfer complexation and HPLC. Journal of Pharmaceutical Sciences, 5, 403–408. Gungor, S., & Onur, F. (2001). Determination of astemizole in pharmaceutical preparations using spectrophtometric methods. Journal of Pharmaceutical and Biomedical Analysis, 25, 511–521. DOI: 10.1016/S0731-7085(00)00600-2. Karam, H., El-Kousy, N., & Towakkol, M. (1999). Colorimetric and fluorimetric methods for the determination of some anti-


344


histaminics using dyes and charge transfer techniques. Analytical Letters, 32, 79–96. DOI: 10.1080/00032719908542600. Kelani, K., Bedawy, L. I., & Abdel-Fattah, L. (1999). Determination of astemizole, terfenadine, and flunarizine hydrochloride by ternary complex formation with eosin and lead (II). Journal of Pharmaceutical and Biomedical Analysis, 18, 985–992. DOI: 10.1016/S0731-7085(98)00107-1. Kissinger, P. T., & Heineman W. R. (1996). Laboratory techniques in electroanalytical chemistry, (2nd ed.). New York: Marcel Decker Inc. Lamparcyzk, H., & Kowalski, P. (1996). Determination of astemizole in pharmaceutical formulation by capillary electrophoresis. European Journal of Pharmaceutical Sciences, 4, 165. DOI: 10.1016/S0928-0987(97)86499-8. Laviron, E. (1980). A multilayer model for the study of space distributed redox modified electrodes. Journal of Electroanalytical Chemistry, 122, 1–9. DOI: 10.1016/S00220728(80)80002-7. Mangalan, S., Patel, R. B., Gandhi, T. P., & Chakavarthy, B. K. (1991). Detection and determination of free and plasma protein bound astemizole by thin-layer chromatography. Journal of Chromatography, 567, 498–503. DOI: 10.1016/03784347(91)80158-9. Miller, J. C., & Miller, J. N. (1994). Statistics for analytical chemistry (3rd ed.). New York: Ellis Horwood. Morton, K. M. (1991). Medicines – the comprehensive guide. London: Bloomsbury. Qureshi, S. Z., & Khan, M. A. (1996). Spectrophtometric determination of astemizole by charge-transfer complex formation of chloranilic acid. Analusis, 24, 190–192. Sastry, C. S., & Naidu, P. Y. (1998). Spectrophtometric determination of astemizole. Talanta, 45, 795–799. DOI: 10.1016/S0039-9140(97)00160-4.

Sastry, C. S. P., & Naidu, P. Y., (1997). Spectrophtometric determination of astemizole in pure and pharmaceutical formulations. Indian Drugs, 34, 140–142. Smyth, W. F. (1992). Voltammetric determination of molecules of biological significance. New York: John Wiley & Sons. Suryanarayana, M. V., Venkataraman, S., Reddy, M. S., Reddy, B. P., Sastry, C. S. P., & Krupadanam, G. L. (1993). Evaluation of astemizole purity by HPLC. Talanta, 40, 1357–1360. DOI: 10.1016/0039-9140(93)8040-1. Vire, J. C., Kauffmann, J. M., & Patriache, G. J. (1989). Adsorptive stripping voltammetry applied to drug analysis: powerful tool. Journal of Pharmaceutical and Biomedical Analysis, 7, 1323–1335. DOI: 10.1016/0731-7085(89)801384. Wang, J. (1985). Stripping analysis: Principle, instrumentation and application. Florida: VCH Publishers Inc. Wang, J. (1988). Electroanalytical techniques in clinical chemistry and laboratory medicine. New York: VCH Publishers Inc. Woestenborghs, R., Embrechts, L., & Heykants, J. (1983). Simultaneous determination of astemizole in animal plasma and tissues by HPLC. Journal of Chromatography, 278, 359– 366. Woestenborghs, R., Geuens, I., Michiels, M., Hendriks, R., & Heykants, J. (1986). Radioimmunoassay procedures for astemizole and metabolites in plasma. Drug Developing Research, 8, 63–69. DOI: 10.1002/ddr.430080108. Yarnitzky, C., & Smyth, W. F. (1991). Square wave polarographic and voltammetric analysis of selected electroreducible drugs. International Journal of Pharmaceutics, 75, 161–169. DOI: 10.1016/0378-5173(91)90190-Y.
