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Material Science Research India Vol. 5(2), 383-390 (2008)
Voltammetric determination of doxepin
using chemically modified electrodes
C. NARASIMHA RAO¹, M. SUMAN¹, C. NARASIMHA RAO² and P. VENKATESWARLU¹*
¹Department of Chemistry, Sri Venkateswara University, Tirupati - 517 502 (India).
²Department of Zoology, Sri Venkateswara University, Tirupati - 517 502 (India).
(Received: September 06, 2008; Accepted: October 09, 2008) The voltammetric reduction behaviour of Doxepin (DXP) is utilized for its determination in pharmaceutical formulations and biological samples. The sepeolite modified carbon paste electrode(SMCPE) exhibited a significant increase in sensitivity and selectivity for DXP compared to bare carbonpaste electrode (CPE). The effect of accumulation time, accumulation potential and pH of buffer solutionon peak currents were studied. The peak currents were found to be linear over the concentration rangeof 4 x 10-8 to 5 x 10-7M and 3.6 x 10-9 to 4.2 x 10-8 M for CPE and SMCPE respectively. Differential pulsevoltammetric method (DPV) and cyclic voltammetric method (CV) were developed for the determinationof DXP in pharmaceutical formulations and biological samples.
Key words: Doxepin, Carbon paste electrode, sepeolite, differential pulse voltammetry,
cyclic voltammetry, pharmaceutical formulation, biological samples.
phar macopoeic methods18-20 recommend thedetermination of doxepin hydrochloride by titration in non-aqueous solvents or by ylidene)-N,N-dimethylpropan- 1-amine (Fig. 1) is a tr icyclic antidepressant. The tr icyclicantidepressants are the most widely used drugs In this work carbon paste electrode (CPE) for the treatment of depression. Like many other and sepeolite modified carbon paste electrode tricyclic antidepressants, doxepin provides effective (SMCPE) are used as working electrodes. The treatment for depression and it also has anti-anxiety construction of the SMCPE offers the advantages and anti-histamine properties2,3.
of being inexpensive, simple and rapid. There areno considerable differences in the repeatability and Gas liquid chromatography, high reproducibility after several days of experiments with performance liquid chromatography, thin-layer SMCPE. In the present study, the SMCPE is applied chromatography, and capillary electrophoresis1-15 for the determination of Doxepen in spiked biological have been applied to determination of doxepin in fluid samples and pharmaceutical formulations by biological fluids. Gas chromatography and high differential pulse voltammetr y. The optimum performance liquid chromatography have been also experimental conditions such as preconcentration used for separation or quantify low concentrations time, accumulation potential and pH of the buffer of doxepin geometric isomers16-17. The are thoroughly examined.
Rao et al., Mat. Sci. Res. India., Vol. 5(2), 383-390 (2008)
their low residual current, less noise and becausethey are ver y cheap, easy to prepare and replaceable22-24, These electrodes have a wide range Voltammograms were recorded with Metrohm 757 of anodic and cathodic applications.
VA computrace (Herisau, Switzerland), with a threeelectrode system consisting of Droppimg mercury (DME) and Hanging mercur y drop electrode Fig. 2 illustrates the cyclic voltammograms (HMDE) were used as working electrodes, Ag/AgCl recorded for 3.5 x 10-8 M Doxepin at carbon paste and platinum wire electrodes were used as a (CPE) and sepeolite modified carbon paste reference and auxiliary electrodes respectively.
electrode (SMCPE). On scanning from 0.0 to -0.7 These electrodes were assembled in a cell.
V (vs Ag/AgCl) towards a negative potential on a Electrolysis of controlled potential has been carried bare carbon paste electrode, only a much smaller out with a model PS 603 Techno potentiostat. A cathodic peak at -0.48 V is observed. When SMCPE model Metrohm 632-pH meter was used to carry is used a large increase in the peak currents is out the pH measurements.
Reagents and Chemicals
Doxepen were purchased from Sigma sepeolite powder (l-2 mm particle size), paraffin oilfrom Aldrich India Ltd., Bangalore. All chemicalsused for the preparation of buffers and supportingelectrolytes are reagent grade.
The stock solutions of 1 x 10-3 M Doxepin are prepared in methanol and kept at dark place.
More dilute solutions are prepared daily with triple Fig. 1: Structure of doxepin
distilled deionized water just before use. The BRbuffers are prepared using 0.04 M ortho phosphoric,acetic and boric acids. pH is adjusted by the additionof 0.2 M NaOH solution.
Process of Analysis
An appropr iate amount of Doxepin, working standard solution is placed in the electrolyticcell which contained Britton Robinson buffer of pH3.0. Subsequently a steam of oxygen-free nitrogengas is passed into the solution for 10 min. Thelaboratory made working electrode is placed in thecell, during the deposition of the test species thesolution is stirred for 150 s at -0.4 V. The stirring(2000rpm) is stopped and after 30s of equilibrationtime, the cathodic sweep is carried out from 0.0 to-0.7 V. All the measurements are made at 21± 2°C.
RESULTS AND DISCUSSION
Fig 2: Typical CV of 3.5xl0-8 M doxepin at
(a) bare CPE; (b) SMCPE in BR buffer of
Modified electrodes acquire greater pH 3.0. Accumulation time 300sec at bare
importance in the field of electrochemistry due to CPE and 150sec atCDMCPE. accumulation
potential -0.4 V, rest time 30see, stirring rate
2000rpm and scan rate 1 OmV/sec.
Rao et al., Mat. Sci. Res. India., Vol. 5(2), 383-390 (2008)
observed; it is observed that the peaks achieved is a nucleation loop and it is due to the strong during the scan from 0.0 to -0.7 V towards negative adsorption of the drug on the surface of the potentials are broad at bare CPE, while at the electrode, moreover SMCPE has a large SMCPE their response are considerably improved, background current.
with better and sharper peaks. From the Fig. 2 thebare CPE (a) and SMCPE (b) for concentration 3.5 Differential pulse voltammetry
x 10-8 M of Doxepin gives reduction peak at -0.48 V.
Fig.3 illustrates differential pulse The peak currents at SMCPE are nearly twice to voltammograms obtained at bare carbon paste that of the bare CPE. No peaks are observed in the electrode and sepeolite modified carbon paste anodic sweep indicating that the reduction of DXP electrodes of Doxepin in BR buffer of pH 3.0. From under investigation is of irreversible process. Owing the obtained results the peak current obtained at to the reduction of the C=C group in two electron sepeolite modified carbon paste electrode almost process according to Sreedhar20. The cross over twice than compared with that of carbon paste point in the reverse sweep of cyclic voltammograms electrode of concentration 3.5x10-8 M on the basis Fig 3: Typical DPV of 3.5x10-8 M doxepen (a) bare CPE; (b) CDMCPE in BR buffer of pH 3.0.
Accumulation time 300sec at bare CPE and 150sec at SMCPE, accumulation potential -0.4 V,
rest time 30sec, stirring rate 2000rpm, pulse amplitude 50mV and scan rate 10 mV/sec
Rao et al., Mat. Sci. Res. India., Vol. 5(2), 383-390 (2008)
Table 1: Chosen experimental conditions
of the above results, the following mechanism wasproposed.
Chosen Value
Effect of supporting electrolytes
The peak shape and peak currents are Buffer volume (ml) closely related to pH of the buffer solution. Therefore the selection of buffer solution is very important.
Because in electroanalysis of organic compounds, Accumulation potential (V) the proton is always involved in the electrode Accumulation time (s) reaction, so a buffer solution is often used as the supporting electrolyte. The cyclic voltammograms Stirring rate (rpm) are taken in different supporting electrolytes such Scan rate (mVs-1) as acetate, phosphate, HCl, borate and BR buffers.
Pulse amplitude (mV) The experimental results show that the shapes of Table 2: Experimental data of Doxepin
Linearity range (M) 4×10-6 to 5×10-5 3.6×10-7 to 4.2×10-6 Calibration curve equation Y(µA) = 0.2980X + 0.03156 Y = 0.2985 X + 0.0305 Correlation coefficient peack currents%RSD)Repeatability of Peack potentias
%RSD)
Reproducibility of peak currents
%RSD)Reproducibility of potentials %RSD)Numbers of assays curves and peak current intensity (i ) are better in buffering capacity. The peak potential (E ) moved BR buffer, so it is used for subsequent experiments.
towards less negative values as the pH of the bufferis increased.
Effect of pH
The influence of pH on the peak currents Effect of accumulation time
of 3.5 x 10-8 M Doxepin at SMCPE is examined.
The effect of accumulation time period on The i vs pH plot (Fig.4), shows that peak current is the peak currents of the concentration 3.5 x 10-8 M the maximum at the pH 3.0. The results from the Doxepin at pH 3.0 is studied at bare CPE and overall the experiment stands that shapes of curves SMCPE and the results are shown in (Fig.5). Sharp are nearly same in all cases, however, the current increasing peak currents arc obtained up to intensity in BR buffer is higher than acetate and accumulation time 150 s of doxepen at CPE and phosphate buffers. So, 0.04 M concentrations of SMCPE. For longer accumulation times above 300 the buffer pH 3.0 are selected to seize an adequate and 150 s the peak currents practically level off.
Rao et al., Mat. Sci. Res. India., Vol. 5(2), 383-390 (2008)
The uptake of the drug at SMCPE is faster (nearly the deposition time gradually level off after 150 s at half of preconcentration time to that of bare CPE).
SMCPE. This is because the active sites of the Moreover the maximum peak currents are electrode surface are fully saturated by the analytes.
observed, compared with that of bare CPE. The So in the further studies, a preconcentration time elevation of the peak current height according to of 150 s for Doxepin is preferred as effective criteria.
Table 3: Determination of DXP in Pharmaceutical formulations
by DPAdSV in BRB of pH 3.0 at SMCPE
Name of the
* Each value is an average of three determinations Table 4: Determination of DXP in 6 spiked human serum
samples by DPAdSV in BRB of pH 3.0 at SMCPE
Name of the
* Each value is an average of three determinations.
Table 5: Determination of DXP in spiked human urine
samples by DPAdSV in BRB of pH 3.0 at SMCPE
Name of the
* Each value is an average of three determinations.
Rao et al., Mat. Sci. Res. India., Vol. 5(2), 383-390 (2008)
Effect of accumulation Potential
pH 3.0, shows a linear relationship from 3.6x10-9 M The effect of the accumulation potential to 4.2x10-8 M Y(mA) = 0.2985 X+0.0305 at SMCPE.
) on the peak current (i ) at concentration The LOD and LOQ are calculated using the 3.5x10- M for Doxepin with an accumulation time of equations, LOD=3 S.D./m, LOQ=10 S.D./m. Here 150 s over the range -0.1 to -0.7 V (vs SCE) is ‘S.D.' is the standard deviation of the peak currents evaluated. The results show that the peak currents and ‘m' is the slope of the calibration curve.
of Doxepin increase as the accumulation potentialincreases from -0.1 V and reaches maximum at - The precision of the method is evaluated 0.4 V; as the E increases further, the i values by repeating six experiments on the same day in started decreasing. The maximum peak currents the same standard solution (repeatability) and over at an accumulation potential of -0.4 V at pH 3.0 10 days from the different standard solutions and observed is due to increased accumulation rate, different electrodes of same composition which may be attributed to more favorable alignment (reproducibility) repeating the experiments for 12 of the molecules by the electric field at the electrode times. To study these experiments, the selected solution interface. Thus, an optimal accumulation concentration of the stock solution is 3.5 x 10-8 M.
potential of-0.4 V is used for further studies.
The statistical parameters are shown in Table 2.
Stock solutions of DXP show same peak current values even after a month without any appreciable The obtained peak current in DPV change; which confirms the stability of the solutions.
depends on different instrumental parameters, suchas scan rate, stirring rate and pulse amplitude. It is Determination of DXP in pharmaceutical
found that these parameters have interrelated effects on the peak current response and little effect Five tablets of each for mulation of on the peak potential. Table 1 shows the selected compounds are finely powdered by pestle in a working conditions.
mortar. All these samples are accurately weighed;the desired amount is transferred into 10ml calibrated flask. It is dissolved in methanol and The dependence of the DPV peak current diluted up to the mark with triple distilled deionized on Doxepin (DXP), concentration for an water. The contents of the flasks are shaken for accumulation time of 150s at -0.4V in BR buffer, 20min and then allowed to settle. The contents are Fig. 4: Effect of pH on the analytical signal of DXP at CPE. Measurements taken in a 3.5 x10-8 M
of concentration solution (DXP) in BR buffer at pH 3.0. Accumulation time 150 s, accumulation
potential -0.4, stirring rate 2000 rpm, pulse amplitude 50 mV and scan rate 10 mV/s
Rao et al., Mat. Sci. Res. India., Vol. 5(2), 383-390 (2008)
filtered. From the filtrate 2 to 8 mg/L Doxepin are an aliquot volume of serum sample is spiked with taken for the analysis, by using the standard addition different concentrations of steroid sample varied method. The differential pulse voltammograms are between 4 to 8 mg/L are taken for analysis. The recorded, recovery percentages were calculated.
differential pulse voltammograms are recorded,recovery percentages were calculated Determination of DXP in spiked serum samples
Blood serum samples are collected from Determination of DXP in spiked human urine
the healthy individuals (after having obtained their written consent) and stored frozen until assay, and Blank urine samples are collected from then treated with 1 ml of acetonitrile as serum healthy individuals for 24hr. filtered through a denaturing and precipitating agent. The samples are cellulose acetate filter paper (0.2 mm per size) and vortexed for 10 min and then centrifuged for 5 min are added to the voltammetric cell containing BR at 2000rpm to remove protein residues. The buffer of pH 3.0. The voltammograms are recorded supernatant of the sample is taken carefully and for the blank urine sample, then 2-6 mg/L of DXP is Fig. 5: Effect of accumulation time on the analytical signal of DXP at SMCPE. Measurements
taken in a 3.5 x 10-8 M of concentration solution (DXP) solution in BR buffer at pH 3.0.
Accumulation time 150s, accumulation potential -0.4, stirring rate 2000
rpm, pulse amplitude 50 mV and scan rate 10 mV/s.
spiked each time and voltammograms are recorded increase in the peak currents at SMCPE is due to after each addition followed by the above mentioned the formation of complex between sepeolite and olefinic group present in the drug. Absence of anodicpeak in the reverse sweep in CV studies for the compound, indicate the irreversibility nature of theelectrode process.
From the experimental results, the compound yielded a single cathodic peak at bare A differential pulse voltammetric method CPE and SMCPE. The peak is due to the reduction has been employed for the determination of DXP of olefinic group present in the compound; the in pharmaceutical formulations and biological fluid Rao et al., Mat. Sci. Res. India., Vol. 5(2), 383-390 (2008)
samples using a SMCPE. The described analytical and spiked biological fluid samples without any procedure in this paper is a sensitive, selective, preliminary treatment, by DPV with a SMCPE is a inexpensive method for the determination of suitable method. Further, due to SMCPE, stability, Doxepin. Hence the present method certainly is accuracy and lesser cost, it offers a good possibility used as an alter native to the colorimetr ic, as a substitute for the previous approaches used in spectrophotometric and chromatographic methods.
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