electrochemical behaviour and voltammetric determination of rosuvastatin calcium in pharmaceutical...

AnalyticalMethods

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Hacettepe University, Faculty of Pharmac

Turkey. E-mail: [email protected];

305 14 99

Cite this: DOI: 10.1039/c3ay40863a

Received 23rd May 2013Accepted 15th August 2013

DOI: 10.1039/c3ay40863a

www.rsc.org/methods

This journal is ª The Royal Society of

Electrochemical behaviour and voltammetricdetermination of rosuvastatin calcium inpharmaceutical preparations using a square-wavevoltammetric method

Sacide Altınoz* and Banu Uyar

In this study, the electrochemical behaviour of rosuvastatin calcium,which is a hydroxymethyl glutaryl Co-A

inhibitor (a member of the statin group), used for the treatment of hypercholesterolemia and dyslipidemia

was investigated using cyclic voltammetry (CV) and chronoamperometry (CA) methods. According to these

studies it is assumed that the reaction is a diffusion-controlled process and irreversible. The results from the

CA were calculated using Cottrell's equation and the diffusion coefficient was found to be 5.79 � 10�5 �0.22 � 10�5 cm2 s�1. It was calculated that 2 electrons were transferred. For the determination of

rosuvastatin calcium from the pharmaceutical preparations, a square wave voltammetry (SWV) method

was selected and developed because it is more sensitive and faster than the other voltammetric

methods. Rosuvastatin calcium's reduction peak was seen at �1184 mV in pH 5 acetate buffer with a

hanging mercury drop electrode (HMDE) used as the working electrode, an Ag/AgCl with saturated 3 M

KCl reference electrode and a platinum wire counter electrode. 70 Hz frequency, 4 mV scan increment

and 25 mV pulse amplitude were chosen as optimum parameters. This method was validated according

to the ICH guidelines on analytical method validation processes. Linearity for rosuvastatin calcium was

found between 0.20 and 10.00 mg mL�1. While the limit of detection for rosuvastatin calcium was 0.07

mg mL�1, the limit of quantitation was 0.20 mg mL�1. As a result of these validation studies, the

selective, accurate and precise square wave voltammetric method, which gives sensitive and repeatable

results, was applied to the determination of rosuvastatin calcium from pharmaceutical preparations. The

results obtained from the developed method were compared with a spectrophotometric method and a

capillary electrophoresis method reported in the literature and no significant difference was found

statistically.

1. Introduction

Rosuvastatin calcium (RC) is a synthetic lipid lowering agentthat is widely used to treat hypercholesterolemia and hyperlip-idemia. In clinical trials, rosuvastatin achieved a markedreduction in serum levels of LDL cholesterol, accompanied bymodest increases in HDL cholesterol and a reduction intriglycerides.1–3 RC is a selective and competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase.4,5

The chemical name of RC is bis((E)-7-[4-(4-uorophenyl)-6-iso-propyl-2-[methyl (methylsulfonyl)amino)pyrimidin-5yl] (3R, 5S)-3,5-dihydroxyhept-6-enoic acid] calcium salt (Fig. 1).

In the literature, for the time being, few methods have beenreported for quantitation of this compound in pharmaceuticalformulations and biological samples. These methods include

y, Department of Analytical Chemistry,

Fax: +90 312 305 40 15; Tel: +90 312

Chemistry 2013

spectrophotometry,6–8 HPLC,9–24 TLC25 and capillary electro-phoresis.26 These published methods are time consuming andmight have some limitations such as expensive instrumentalset-up and rigid experimental conditions. For example, aspecic column needs to be used to obtain identical retentiontimes for HPLC, and reproducible analysis is hard to performusing capillary electrophoresis. Although voltammetric

Fig. 1 Chemical structure of rosuvastatin calcium.

Anal. Methods

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Analytical Methods Paper

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methods require experienced analysts, they are suitable toinvestigate the redox properties of drugs and for the determi-nation of the pharmaceuticals in samples. They have advan-tages like high sensitivity, low cost, simplicity and relativelyshort analysis time.

Voltammetric techniques are important methods for the tracequantitation of many organic and inorganic substances. Althoughthere are some analytical techniques described for determinationof RC, neither electrochemical behavior nor electroanalyticaldetermination of RC in pharmaceutical formulations has beenpublished.

In this study, a new voltammetric method for determinationof RC in pharmaceutical preparations was developed, and thismethod was validated and proposed for the direct determina-tion of RC in bulk form and in pharmaceutical formulations.The diffusion coefficient and number of electrons transferredwere determined by cyclic voltammetry (CV) and chro-noamperometry (CA). The proposed method did not utilize anyextraction step for recovering the drug from the formulationmatrix. The developed electroanalytical method decreased thedegree of error, time in estimation of the drugs and the overallcost of the analysis.

2. Experimental2.1. Reagents

RC was supplied from Dr Reddy's Lab and Crestor� tablets weresupplied from AstraZeneca. RC was tested for purity bymeasuring its melting-point, taking its UV and IR spectra andno impurities were found. All other chemicals were of analyticalreagent grade (Merck and Sigma). Triply distilled mercury wasused throughout. Since RC is slightly soluble in water andhighly soluble in methanol, RC stock standard solution (1000.0mg mL�1) was prepared by dissolving 25.0 mg of standard RC in25.0 mL MeOH. The stock standard solution was kept at 4 �Cand shielded from daylight. Working standard solutions wereprepared by appropriate dilution of the stock standard solutionin supporting electrolyte.

2.2. Tablet solutions

Ten tablets of RC (Crestor� 10 mg) were accurately weighed andpowdered. The amount equivalent to one tablet was weighed andtransferred to a 50 mL of volumetric ask and 30 mL MeOH wasadded. It was treated in an ultrasonic bath for 15min at 25 �C andthe remaining volume was made up with MeOH. Aer shaking,part of the ask content was centrifuged at 3500 rpm for 15 min.Appropriate solutions were prepared by taking suitable aliquotsof the clear supernatant into supporting electrolyte.

2.3. Synthetic tablet preparations

Pharmaceutical preparations of RC contain 10 mg of standardRC and the following inactive ingredients: microcrystallinecellulose, lactose monohydrate, tribasic calcium phosphate,crospovidone, magnesium stearate, hypromellose, triacetin,titanium dioxide, yellow ferric oxide, and red ferric oxide. Toprepare the synthetic tablet these inactive ingredients and

Anal. Methods

standard RC of an equivalent amount to ten tablets wereweighed. An amount of this mixture equivalent to one tablet wastransferred to a 50 mL of volumetric ask and the volume wascompleted to 50 mL with MeOH as described in Section 2.2.

2.4. Apparatus and experimental conditions

BAS 100 B/W model electrochemical workstation was used. Thereference electrode was Ag/AgCl, a platinum wire was used asthe auxiliary electrode and a hanging mercury drop electrode(HMDE) was used as the working electrode. Rosuvastatin cal-cium's reduction peak was seen at �1184 mV in pH 5 acetatebuffer with a hanging mercury drop electrode (HMDE) used asthe working electrode, an Ag/AgCl with saturated 3 M KCl as areference electrode and a platinum wire used as a counterelectrode. 70 Hz frequency, 4 mV scan increment and 25 mVpulse amplitude were chosen as optimum parameters. For pHmeasurements, a pH meter (Mettler Toledo MA 235, Switzer-land) was used.

2.5. Procedure

2.0 mL volume of the supporting electrolyte (acetate bufferpH ¼ 5.0) was deoxygenated with pre-puried nitrogen for 12min. Aer the voltammogram of this solution had been recor-ded, the RC standard solution was added by micropipette.Nitrogen was passed through the solution for 1 min to mix thesolution. The voltammogram was recorded again. This proce-dure was repeated until the peak height no longer increased.

3. Results and discussion

Many electroanalytical methods employ adsorptive accumula-tion at the HMDE combined with different stripping voltam-metric techniques. One of the most sensitive techniques amongthese is square-wave voltammetry. It has benecial usage inanalytical applications and in fundamental studies of electrodemechanisms. Square-wave voltammetry has several advantagessuch as its excellent sensitivity and the rejection of backgroundcurrents. It is also a rapid technique in comparison to the otherelectroanalytical techniques. In this study, SWV method wasused because it is adequate and sensitive for measurements.

3.1. Type of supporting electrolyte and pH

It is well known that the type of supporting electrolyte and pH ofthe media are very important for electrochemical studies.Various buffers such as borate (pH ¼ 7–10), acetate (pH ¼ 5–7),phosphate (pH¼ 5–9), Britton Robinson (pH¼ 5–12), Mcilvainecitrate (pH ¼ 5–9) were examined as supporting electrolytes inthe presence of 0.25–10 mg mL�1 of RC. The solution conditionssuch as the pH and the concentration of RC affect the peakcurrent signicantly. According to the initial experiments, it wasseen that dened peaks were only obtained by using BrittonRobinson and acetate buffers. The effect of the pH of acetatebuffer on peak currents of RC and peak potential was studiedover the pH range 5–7. The peaks are not symmetric and welldened below pH 5 and above pH 7. The results showed that RCin 0.01 M acetate buffer (pH ¼ 5.0) gave the optimum signal

This journal is ª The Royal Society of Chemistry 2013


Fig. 2 Cyclic voltammogram of (a) 10.0 mg mL�1 of RC on HMDE and (b) sup-porting electrolyte: acetate buffer (pH ¼ 5.0).

Fig. 3 Cyclic voltammograms of rosuvastatin calcium (2.91 mg mL�1) withincreasing scan rate using the optimum conditions; supporting electrolyte:acetate (pH ¼ 5.0), (a) 10 mV s�1, (b) 25 mV s�1, (c) 75 mV s�1, (d) 100 mV s�1.

Paper Analytical Methods

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response. Due to the fact that RCmolecular weight is high and itincludes uorine atom in the molecular structure, the effect oftetra alkyl ammonium salts (e.g. tetrabutyl ammonium iodate)addition to the supporting electrolyte was investigated. Sincethere was no signicant difference between the added one andthe non-added one, it was decided not to add ammonium saltsinto the supporting electrolyte.

3.2. Optimization of the instrument parameters

SWV method was used to optimize a rapid and sensitive elec-troanalytical method for determination of RC and this methodwas applied to the determination of the drug in pharmaceuticaldosage forms. The square-wave voltammetric responses are adirect response to the instrumental conditions. To obtain amuch more sensitive peak current, the optimum instrumentalconditions (for example pulse amplitude, frequency and scanincrement) were studied for 2.91 mg mL�1 RC. The frequencywas varied from 15 to 300 Hz. Although the signal responseincreased with frequency, above 70 Hz the peak shape wasdeformed. When the pulse amplitude was varied in the range5–45 mV, the peak current increased with rising pulse ampli-tude, but above 25 mV peak broadening was observed. The scanincrement was varied from 2 to 10 mV. When peak height andthe peak shape were taken into consideration, a 4 mV scanincrement was selected for the application.

3.3. Type of reduction current

The type of reduction current was investigated by CA and CVmethods. In the CV technique, the cyclic voltammograms wererecorded at scan rates (v) between 10 and 1000 mV s�1. A plot oflog ip versus log v gave a straight line ( y ¼ 0.488x � 1.92), thecorrelation coefficient was 0.9782. The slope, which was nearly0.5, supports that the reduction peak current of RC was adiffusion controlled electrode process.27 The characteristic ofthe limiting current was also investigated using CV technique. Aplot of peak current (ip) versus scan rate (v1/2) gave a straight line( y¼ 68.79x + 130.97) and the correlation coefficient was 0.9952.The graph of peak current was found to be linear, and showedthat the reduction peak current of RC was controlled by diffu-sion. In the investigation of limiting current by CV, it was seenthat the peak potential of RC was changed by changing the scanrate when CVs of 10 mg mL�1 RC in acetate buffer (pH ¼ 5) wererecorded. As is seen in Fig. 2, there was no peak observed beforethe main peak, which showed that the reduction product wasnot adsorbed on the electrode surface. In addition, that therewas no peak observed aer the main peak showed that theanalysed RC was not adsorbed on the electrode surface28 (Fig. 2).

3.4. Reversibility

In SWV method, the peak potential shied to a negative valuewhen the concentration of RC was increased. The reversibility ofRC reduction was also studied using CV method. In the CV,there was no anodic peak on the reverse scan (Fig. 2). A plot ofpeak current versus square root of the scan rate did not give astraight line. The peak potential shied toward negative valueson increasing scan rate. The loss of the anodic peak on the


reverse scan showed that the reduction reaction was notreversible (Fig. 2).29 The results conrmed that the reductionreaction was irreversible. Moreover, the peak potential shiedto a more negative value with increasing scan rate, conrmingthe irreversible nature of the reaction (Fig. 3).

3.5. Number of electrons transferred

In an irreversible system, one of the most important criteria isnot observing the anodic peak on the reverse scan. The othercriteria are provided when they are Ipcav

1/2 and [Ep � Ep/2] ¼48/(acna) mV. The CV method was applied in order to nd thenumber of transferred electrons (n ¼ 6). The acna value wasfound to be 1.16 according to the formula given above. If thenumber of transferred electrons was found to be 2 according tothe formula, a (the rate of mass transport value) is found to be0.6. In an irreversible reaction, the a value is less than 1. Thus,it was decided that the electrochemical reaction was irrevers-ible for RC and there were 2 electrons transferred for thisreaction.30

Anal. Methods



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3.6. Diffusion coefficient (D)

In the CA method, potential E1 and E2 were applied to theworking electrode. E1 is chosen because no reduction reactionof RC occurs. Then the potential is suddenly changed to a newvalue E2, where the reduction of RC is diffusion controlled. IfFick's 2nd Law can be solved with the appropriate boundaryconditions, the following equation can be expressed as theCottrell equation:32

i ¼ (nFAD1/2C0)/(p1/2t1/2)

The diffusion coefficient was calculated from Cottrell'sequation where i is the current (nA), n is the number of elec-trons transferred per molecule, F is the Faraday constant(96.485 C per eq.), A is the electrode area (cm2), D is the diffu-sion coefficient (cm2 s�1), C is the concentration (mol cm�3) andt is the time (s).

That is, if the current is diffusion controlled, the current fallswith t as shown.31,32 Chronoamperometric voltammograms ofRC are given in Fig. 4. These results also conrmed that the RCreduction current was diffusion controlled.

The experimental Cottrell's slope was determined from thechronoamperometric ip versus t

�1/2 plot. The constant potentialapplied was slightly more cathodic than the cyclic voltammetricEp from �1400 to �1200 mV. An HMDE with a surface area of0.0199 cm2 was employed. The diffusion coefficient was calcu-lated as 5.79 � 10�5 � 0.22 � 10�5 cm2 s�1 (n ¼ 6).

3.7. Proposed mechanism

The reduction mechanism of RC was investigated. RC has apyrimidine ring. In CV and SWV methods, the peak potentialwas shied to more negative values when increasing the scanrate. This behavior indicates that hydrogen ions are partici-pating in the electrode processes. The reduction step isexpressed with a heterocyclic ring33 or a C–S–N moiety at theC–SO2–N group, which serves as the electroactive center forelectron uptake.34 It is proposed that the transfer of 2 electronsrelated to the reduction of RC observed at �1184 mV occurredon the nitrogen–carbon double bond of the pyrimidine ring orin the single bonds of the C–S–N moiety. Both C–SO2–Ar and

Fig. 4 Chronoamperometric voltammogram of rosuvastatin calcium. (a) Sup-porting electrolyte: acetate (pH ¼ 5.0), (b) 2.91 mg mL�1 rosuvastatin calcium(E1 ¼ �950 mV, E2 ¼ �1350 mV).

Anal. Methods

C–SO2–N groups include C–S bonds that are reduced; theirelectrochemical behavior is identical. Thus, the electrochemicalreduction might be expressed with the breaking of the singlebond with the methyl and SO2 at the methylsulphonylaminogroup.35

3.8. Validation of the proposed method

Analytical method validation is required during drug develop-ment and manufacturing. The validation process would includethe validation of production processes, cleaning procedures,the analytical method, in process control test procedures andcomputerised systems. Validation of the proposed method forthe quantitative determination of RC was examined via evalu-ation of linear range, sensitivity [limit of detection (LOD), limitof quantitation (LOQ)], repeatability, precision, accuracy,selectivity/specicity, recovery, robustness, ruggedness andstability.36–38

3.8.1. Linearity range. The linearity of an analytical proce-dure is its ability (within a given range) to obtain test resultswhich are directly proportional to the concentration (amount)of analyte in the sample. The linear range of an analyticalprocedure is the interval between the upper and lowerconcentration (amounts) of analyte in the sample (includingthese concentrations) for which it has been demonstrated thatthe analytical procedure has a suitable level of precision,accuracy and linearity. The applicability of the proposedmethod as an analytical method for the determination of RCwas examined by measuring square peak current as a functionof concentration of the bulk form at least three times under theoptimized conditions. The calibration graphs of the peakcurrent versus concentration were found to be linear over therange of 0.20–10.0 mg mL�1 for SWV. The linearity was checkedby preparing standard solutions at 12 different concentrationsfor SWV method (Fig. 5). Calibration graphs were constructedusing data from these measurements and least-squares wereevaluated using the linear regression method. The results aregiven in Table 1.

3.8.2. Sensitivity. The detection limit (LOD) of an indi-vidual analytical procedure is the lowest amount of analyte in asample which can be detected but not necessarily quantied asan exact value. The calculated LOD values of RC at a S/N ratio of3 was 0.07 mg mL�1 (RSD% ¼ 0.968; n ¼ 6) for SWV underoptimum conditions. The limit of quantitation (LOQ) of an



Fig. 5 The effect of concentration on the peak current of rosuvastatin calciumusing SWV: (a) supporting electrolyte; (b) 0.49; (c) 0.96; (d) 2.06; (e) 2.91; (f) 3.57;(g) 4.41 mg mL�1 of RC (frequency of 70 Hz, pulse amplitude of 25 mV, scanincrement of 4 mV).

Table 2 Precision and accuracy of the proposed method (n ¼ 6)

Intra-day

Added(mg mL�1)

Founda

(mg mL�1)PrecisionRSD%

Accuracyb

(bias%)

0.96 0. 96 � 0.001 0.45 0.412.91 2.90 � 0.004 0.35 0.274.45 4.45 � 0.003 0.14 0.12

Inter-day

Added(mg mL�1)

Founda

(mg mL�1)PrecisionRSD%

Accuracyb

(bias%)

0.96 0.96 � 0.002 0.63 0.552.91 2.91 � 0.001 0.12 0.094.45 4.45 � 0.001 0.19 0.09

a Found ¼ �x ¼ mean � standard error (S.E.), RSD% ¼ relative standarddeviation. b Accuracy ¼ [(found � added)/added] � 100.


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individual analytical procedure is the lowest amount of analytein a sample which can be quantitatively determined with suit-able precision and accuracy. The quantitation limit is aparameter of quantitative assays for low levels of compounds insample matrices, and is used particularly for the determinationof impurities and/or degradation products. The calculated LOQvalue of RC at a S/N ratio of 10 was found to be 0.2 mg mL�1

(RSD ¼ 0.9534; n ¼ 6) (Table 2).35,36

3.8.3. Repeatability. The repeatability of the method wasevaluated by performing 10 repeat measurements for 2.91mg mL�1 of RC in SWV method under the optimum condi-tions.34 The amount of RC was found to be 2.91 � 0.002 and thepercentage recovery was found to be 99.99 � 0.08 with RSD% of0.26. These values indicate that the proposed method has highrepeatability and precision for RC analysis.

3.8.4. Accuracy and precision. The accuracy of an analyticalprocedure expresses the closeness of agreement between thevalue which is accepted either as a conventional true value or anaccepted reference value and the value found. The accuracy ofthe analysis was determined by calculating the percentagerelative error between the measured mean concentrations andadded concentrations (bias%). The precision of an analyticalprocedure expresses the closeness of agreement (degree of

Table 1 Analytical characteristics of the developed SWV method (n ¼ 6)

Regression equation of calibration curvemethoda

Standard error of slopeStandard error of interceptCorrelation coefficient (r)Regression equation of standard additionmethoda

Linearity range (mg mL�1)Number of data pointsLOD (mg mL�1)LOQ (mg mL�1)

a y ¼ bx + a; x ¼ concentration (mg mL�1), y ¼ peak current (nA), a ¼ inte


scatter) between a series of measurements obtained frommultiple sampling of the same homogeneous sample under theprescribed conditions. Precision may be considered at threelevels: repeatability, intermediate precision and reproducibility.Repeatability is also termed intra-assay precision. Intermediateprecision expresses within-laboratory variation: different days,different analysts, different equipment, etc. Reproducibilityexpresses the precision between laboratories (collaborativestudies, usually applied to standardization of methodology).The precision of an analytical procedure is usually expressed asthe variance, standard deviation or coefficient of variation of aseries of measurements. The precision of the analysis wasdetermined by calculating the relative standard deviation (RSD%). The accuracy and precision of the proposed methods wereinvestigated by intra-day and inter-day determination of RC atthree different concentrations of RC solution three times (0.96,2.91 and 4.45 mg mL�1) for the SWV method in the linear range.The intra-day studies were performed in one day; inter-daystudies were performed on six days over a period of one weekand the accuracy and precision of RC obtained by SWV method

SWV Method

y ¼ 336.6x � 11.601

0.0910.1170.9997y ¼ 337.82x + 996.05

0.20–10.00120.070.20

rcept, b ¼ slope, LOD ¼ limit of detection, LOQ ¼ limit of quantitation.

Anal. Methods



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(n ¼ 6). It was seen that RSD% and bias values were found to beless than 1%. The results obtained for intra-day and inter-dayaccuracy and precision indicate high accuracy and precision ofthe proposed method (Table 2).

3.8.5. Selectivity/specicity and recovery. Specicity is theability to assess unequivocally the analyte in the presence ofcomponents which may be expected to be present. Typically,these might include impurities, degradation products, matrix,etc. The selectivity of the optimized methods for the determi-nation of RC was examined in the presence of inactive ingre-dients: microcrystalline cellulose, lactose monohydrate, tribasiccalcium phosphate, crospovidone, magnesium stearate, hypro-mellose, triacetin, titanium dioxide, yellow ferric oxide, and redferric oxide added in the same ratio as in pharmaceuticalformulations. The amounts of 10 mg of RC were found to be10.02 � 0.02, the mean percentage recoveries in this syntheticmixture of RC were found to be 100.25 � 0.22 (n ¼ 6) with RSDof 0.545 for SWV method. These values showed no signicantexcipient interference; thus, the procedures were able todetermine the amount of RC in the presence of excipients.Comparison of the SWV voltammograms of RC standard,synthetic and tablet solutions showed that the peak potentialand peak current of RC did not change. On the basis of theseresults, the proposed method can be considered selective. As itis seen in Fig. 6, the RC peak was observed at �1184 mV andthere is no interference from matrix components. In order toevaluate the effect of the presence of the excipients on theproposed method, the standard addition method was applied.For this reason, the appropriate volume of Crestor� tabletsolution was added to the supporting electrolyte. Aer the vol-tammogram was recorded, known amounts of standard solu-tions of RC were added and voltammograms were recorded. Theregression equation of the standard additionmethod was found

Fig. 6 SW voltammograms of RC (2.91 mg mL�1) using the optimum conditionson HMDE: (a) placebo solution (b) standard solution (c) Crestor� tablet solution(d) synthetic tablet solution.

Anal. Methods

to be y ¼ 337.82C + 996.05, r ¼ 0.9996 and for SWV method.There was no difference between the slopes of the two methodsusing a calibration curve or standard addition method. Thesedata showed that there was no interaction of excipients in theanalysis of RC in pharmaceutical formulations by the proposedmethod. Therefore, the calibration curve method, which iseasier and quicker than the standard addition method, wasused fort the quantitative analysis of RC.

3.8.6. Robustness. The robustness of an analytical proce-dure is a measure of its capacity to remain unaffected by small,but deliberate variations in method parameters and provides anindication of its reliability during normal usage. Robustnesstest were performed with deliberate small changes of buffer pH(pH 4.90 and pH 5.10), and initial potential (�1.100 V and�0.90 V). Each deliberate small change was analyzed for 6independent series containing 2.91 mg mL�1 RC in SWVmethod.34,37 These results were compared using the Wilcoxontest and there was no signicant difference between the resultsunder changed conditions ( p > 0.05). None of these variablessignicantly affected the assay of RC and the proposed methodcould be considered robust.

3.8.7. Ruggedness. The ruggedness of the proposedmethods was evaluated by applying the developed procedures toassay 2.91 mg mL�1 of RC in SWV method using the sameinstrument by two different analysts under the same optimizedconditions on different days.34,37 These results were comparedusing the Wilcoxon test and there was no signicant differencebetween by the two analysts ( p > 0.05). So the developedmethodcould be said to be rugged.

3.8.8. Stability. The stability of RC was evaluated throughlong-term and short-term periods. Under the optimum condi-tions, the stability of 1000 mg mL�1 of RC solution prepared inmethanol was evaluated by SWVmethod. For long-term stabilitystudies, RC samples were prepared and shielded from daylightat 4 �C for 2 months. During this period, the sample wasanalyzed every week. For short-term stability studies, sampleswere analyzed with 2.91 mg mL�1 RC in the supporting elec-trolyte every 2 hours for 1 day. There was no signicant differ-ence found between freshly prepared RC samples and thesamples prepared 2 months ago. No changes were observed inthe peak potential and peak current of RC over a period of 2months.

3.9. Analysis of Crestor� tablets

The optimized voltammetric methods were applied to thedetermination of RC in Crestor� tablets. The amount of RC intablets was calculated using the calibration curve method. Theobtained percentage recoveries and the relative standard devi-ations based on the average of six replicate measurements werefound. The obtained results were compared to those obtainedby the reported spectrophotometric method and the capillaryelectrophoresis method reported in the literature.7,26 Thestatistical comparison of SWV analysis results with spectro-photometric analysis results and CE analysis results were doneusing the Wilcoxon paired test and the results are shown inTable 3. In addition, all three method analysis results were



Table 3 Comparison of the results obtained by SWV and UV specrophometric methods and capillary electrophoresis for Cerestor� tablets containing 10 mg RC (n ¼6)a

SWV method Compared method:7 UV spectrophotometric method Compared method:26 capillary electrophoresis methodFound (mg) Found (mg) Found (mg)

10.02 10.12 10.249.95 10.06 10.189.92 10.11 10.0410.14 9.97 10.0110.07 10.03 9.9910.04 9.89 10.00�x: 10.02 � 0.03 �x: 10.03 � 0.04 �x: 10.08 � 0.04RSD%: 0.79 RSD%: 0.88 RSD%: 1.06Comparison of SWV with UV spectrophotometric method tc: 10.0 > tT: 0 ( p > 0.05)Comparison of SWV with CE method tc: 7.0 > tT: 0 ( p > 0.05)Comparison of SWV, UV spectrophotometric with CE method tc: 0.59 > tT: 19.43 ( p > 0.05)

a �x: mean � S.E. RSD: relative standard deviation, (tc: calculated and, tT: tabulated t values).


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compared with each other using one way variance analysis. Theexperimental values (tc) did not exceed the theoretical ones (tT),indicating a good agreement ( p > 0.05) with the comparisonmethod.

Conclusions

Both the electrochemical behavior and the determination inpharmaceutical dosage form of RC were examined in this study.The electrochemical behavior of RC at an HMDE electrode wasinvestigated in detail so that it could be used for analyticalpurposes. A new, simple, selective, accurate and precise square-wave voltammetric procedure was optimized for the determina-tion of RC in bulk form and in pharmaceutical formulations. Thiswork shows that RC can be determined using voltammetrictechniques on the basis of its reduction process at the HMDE.This voltammetric technique was applied directly to the analysisof pharmaceutical dosage forms without the need for separationor complex sample preparation such as time-consuming extrac-tion steps prior to the drug analysis. Only centrifugal separationof excipients was used to precipitate the excipients from tabletformulations. The described method was rapid, requiring lessthan 2 min to perform. Moreover, the proposed method hasbetter sensitivity than HPLC and spectrophotometric methodsreported in the literature. Comparison of the developed methodto UV and CE methods shows its advantages such as simplicity,low cost, short analysis time and selectivity. The proposed elec-trochemical method may be preferred to HPLC methods for thedetermination of RC in pharmaceutical formulations. Thismethod is the only electrochemical method for the determinationof RC in the literature and it could be easily used in qualitycontrol laboratories for the analysis of RC.

Acknowledgements

The authors thank Dr Reddy's Laboratories for their kind supplyof pure rosuvastatin calcium. The authors also thank AstraZe-neca A.Sx. for providing the Cerestor� tablets.


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