- Email: [email protected]
A New Screen-printed Ion Selective Electrode for Determination of Citalopram Hydrobromide in Pharmaceutical Formulation
CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 42, Issue 4, April 2014 Online English edition of the Chinese language journal
Cite this article as: Chin J Anal Chem, 2014, 42(4), 565–572.
RESEARCH PAPER
A New Screen-printed Ion Selective Electrode for Determination of Citalopram Hydrobromide in Pharmaceutical Formulation Tamer Awad Ali1,*, Gehad G. Mohamed2, A. M. Al-Sabagh1, M. A. Migahed1 1 2
Egyptian Petroleum Research Institute (EPRI), Cairo, 11727, Egypt Chemistry Department, Faculty of Science, Cairo University, Giza, 12613, Egypt
Abstract: Novel citalopram screen-printed ion selective electrodes were fabricated, characterized and used for the determination of citalopram in pharmaceutical formulations. The proposed sensors incorporated potassium tetrakis(p-chlorophenyl) borate (KTpClPB) ionophore (electrode V) and citalopram-phosphotungstate (CP-PT) ion pair complex (electrode X) as electroactive materials in screen-printed electrodes and tricresylphosphate (TCP) as solvent mediator. The fabricated electrodes demonstrated near Nernstain response over wide linear range of 4.9 × 10–7–1.0 × 10–2 M and 1.0 × 10–6–1.0 × 10–2 M citalopram with lower limit of detection of 4.9 × 10–7 M and 1.0 × 10–6 M and slope of (60.47 ± 0.80) mV decade–1 and (59.93 ± 1.45) mV decade–1 for electrode (V) and (X), respectively. The results showed that the proposed sensors had the characteristics such as fast and stable response, good reproducibility, long term stability (5 and 4 months) and applicability over a wide pH range of 2–9 and 2–8 for electrodes (V) and (X). The sensors displayed good selectivity for citalopram with respect to number of common foreign inorganic, organic species, excipients and the fillers added to the pharmaceutical preparation. The sensors were successfully applied for the determination of citalopram in tablet, urine and serum. Key Words:
1
Citalopram ion-selective electrode; Screen-printed electrode; Pharmaceutical preparation; Potentiometric determination
Introduction
Citalopram (Fig.1), 1-(3-dimethylaminopropyl)-1-(4fluorophenyl)-5-phthalan carbonitrile, is an antidepressant drug used to treat depression associated with mood disorders. It is also used on occasion for the treatment of body dysmorphic disorder and anxiety. Citalopram belongs to a class of drugs known as selective serotonin reuptake inhibitors (SSRIs). It is primarily used to treat the symptoms of depression and can also be prescribed for social anxiety disorder, panic disorder, obsessive-compulsive disorder, the Huntington’s disease, and premenstrual dysphoric disorder[1‒4]. Citalopram affects neurotransmitters, the chemical transmitters within the brain. Neurotransmitters manufactured and released by nerves attached to adjacent nerves and alter their activities. Thus, neurotransmitters are considered the communication system of the brain. Many experts believe that
an imbalance among neurotransmitters is the cause of depression. Citalopram works by preventing the uptake of serotonin by nerve cells after it has been released. Such uptake is an important mechanism for removing released neurotransmitters and terminating their actions on adjacent nerves. The reduced uptake caused by citalopram results in stimulation of the nerve cells by the free serotonin in the brain[5–7].
Fig.1 Chemical structure of citalopram hydrobromide (CPB)
Received 23 March 2013; accepted 7 May 2013 * Corresponding author. Email: [email protected], Tel: +2 010 06890640 Copyright © 2014, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(13)60725-2
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
An ion-selective electrode (ISE) is capable of measuring selectivity and activity of a given ion regardless of other ions present in the solution[8]. It was used for determination of many drugs[9–12]. Compared with other analytical techniques, ISE has an impressive list of advantages such as being portable, suitable for either direct determination or using as a sensor in titrations besides, these membrane electrode don’t affect the studied solutions[13–16]. In this work, the construction of plasticized screen-printed type citalopram ion selective electrodes and their application in pharmaceutical analysis were described. It was based on using ion-associate species, formed by drug cation and phosphotungstic acid (PTA) as counter ion or incorporating KTpClPB ionophore in the electrode matrix. The fabricated potentiometric sensors were applied for the determination of CPB in pure and in tablets, urine and serum.
2
Experimental
2.1
Apparatus and reagents
Laboratory potential measurements were performed using HANNA 211 pH meter. An Ag/AgCl double-junction reference electrode (Metrohm 6.0726.100) was used in conjugation with different ion selective electrodes. Digital burette was used for the field measurement of drugs under investigation. Elemental analysis for carbon, hydrogen, nitrogen, and sulphur were carried out at the Microanalytical centers, Cairo University, using a Perkin-Elmer CHN 2400 Elemental Analyzer. All chemicals used were of analytical reagent grade unless otherwise stated, and doubly distilled water was used. Citalopram hydrobromide (CPB) was purchased from Western pharmaceutical industries, Egypt. Depaway 40 (sample 1), cipram® 20 (sample 2) were purchased from Memphis, Egypt and multi pharma/lundbeek, Denmark. Sodium tetraphenylborate (NaTPB) and phosphotungstic acid (PTA) were commercially available from Sigma-Aldrich and Fluka, respectively. Tricresylphosphate (TCP) from Alfa Aesar was used for the preparation of the sensors. Other types of plasticizers, namely dioctylphthalate (DOP), dibutylphthalate (DBP) and dioctylsebacate (DOS) were purchased from Sigma, Merck and Merck, respectively. Relative high molecular weight polyvinyl-chloride (PVC) (Aldrich), potassium tetrakis (p-chlorophenyl) borate (KTpClPB) (Aldrich) and graphite powder (synthetic 1–2 µm, Aldrich) were used for the fabrication of different electrodes. 2.2 2.2.1
Procedures Preparation of citalopram-modified screen-printed electrodes
Modified SPEs were printed in arrays of six couples
consisting of the working and the reference electrodes (each 5 mm × 35 mm) following the procedures previously described[17–20]. A polyvinyl chloride flexible sheet (0.2 mm) was used as a substrate which was not affected by the curing temperature or the ink solvent and easily cutted by scissors. A pseudo Ag/AgCl electrode was firstly printed using a home-made polyvinyl chloride ink containing silver-silver chloride (65:35, V/V) which was cured at 60 ºC for 30 min. The working electrodes were prepared depending on the method of fabrication. The working electrode was printed using homemade carbon ink which was prepared by mixing 2.5–15 mg potassium tetrakis (p-chlorophenyl) borate (KTpClPB) ionophore or CP-PT ion pair, 450 mg TCP, 1.25 g of polyvinyl chloride (8%, w/V) and 0.75 g carbon powder. They were printed using homemade carbon ink and cured at 50 ºC for 30 min. A layer of an insulator was then placed onto the printed electrodes, leaving a defined rectangular shaped (5 mm × 5 mm) working area and a similar area (for the electrical contact) on the other side. Fabricated electrodes were stored at 4 ºC and used directly in the potentiometric measurements[17–21]. 2.2.2
Sensor calibration
SPE sensors in conjunction with Ag/AgCl reference electrode was immersed in standard CPB solutions with concentration of 5.0 × 10–7–1.0 × 10–2 M in a 50-mL beaker. The solutions were stirred and the potential was recorded after stabilization and plotted as a function of CPB concentration, and the graph was used for the subsequent determination of unknown concentration of CPB. The potential readings were recorded after stabilization and the emf were plotted as a function of logarithm citalopram concentration. The lower detection limit was taken at the point of intersection of the extrapolated linear segments of the citalopram calibration curve. 2.2.3
Preparation of ion-pair compound
Ion-pair of citalopram-phosphotungstate (CP-PT): About 20 mL of 0.01 M CPB solution was mixed with 65 mL of 0.01 M phosphotungstate under stirring. The resulting precipitates were filtered off, washed with water, dried at 60 °C, washed with petroleum ether to remove any residual moisture, then ground to fine powder and kept dry in desiccator. Small sample portions were sent for elemental analysis. 2.3
Interference effects
The response of the electrodes was also examined in the presence of a number of organic and inorganic ions. The potentiometric selectivity coefficients Kpotdrug, j were used to evaluate the degree of interference[17,22,23].
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
A 9.0-mL aliquot of distilled water was placed in a 50-mL beaker where the electrodes and the double junction Ag/AgCl electrode were immersed. The potential response upon addition of 1.0 mL aliquot of 1.0 mM solution of the interferent was recorded and compared with that of 1.0 mM pure citalopram hydrobromide solution. The selectivity coefficients were calculated using Eisenman-Nicolsky equation: lgKpotCPB, j = ((E1 – E2)/S) + (1 + (Z1/Z2))lga) (1) where, E1 is the potential measured in 1.0 × 10−3 M septonex, E2 is the potential measured in 1.0 × 10−3 M of the interfering compound, Z1 and Z2 are the charges of the citalopram and interfering species j, and S is slope of the electrode calibration plot. The selectivity coefficients were also measured by the match method according to the equation[24]: lgKpotCPB, j = (a’D – aD)/aj (2) 2.4
Analytical method for pharmaceutical formulation
Ten citalopram hydrobromide tablets were weighed accurately, crushed and mixed in a mortar, and dissolved in 30 mL of distilled water under stirring condition. The solution was transferred into a 50-mL volumetric flask, and then completed to the mark with the same solvent. 5 mL of this solution was taken and transferred into another 50-mL volumetric flask, completed to volume with the distilled water. The citalopram selective screen-printed electrodes and the reference electrode were immersed into the test solution. The content of CP in the tablets was determined by using the direct potentiometric method. 2.5
Procedure for determination of citalopram in human urine
Urine samples containing different citalopram concentrations were prepared by adding known amounts of citalopram to 25 mL aliquots of blank urine samples of four volunteers, the citalopram-selective and reference electrodes were immersed and the citalopram concentration was determined by direct potentiometry using the standard addition technique.
2.6
Procedure for determination of citalopram in serum
The electrodes were placed into 100 µL of serum under constant stirring with a stirring bar. The serum samples were prepared by adding citalopram hydrobromide to human serum. All measurements were carried out at 25 °C.
3
Results and discussion
Phosphotungsate was used as an ion-pairing agent for the preparation of an electroactive ion-association complex for the determination of citalopram. The elemental analysis result of the sparingly soluble complex of CPB/PTA showed that the composition of the complex was 1:3 [Citalopram]: [Phosphotungsate] ion pair (% found (calculated): C = 7.22 (7.50), H = 0.61 (0.69) and N = 0.79 (0.87). The dry powder of the formed ion-pair was used for the preparation of the new screen printed ion selective electrode for CPB determination. 3.1
Effects of ionophore and ion pair content
For this purpose, six electrodes with different KTpClPB ionophore and CP-PT ion pair compositions were prepared and the data obtained are summarized in Table 1. The proportions of KTpClPB ionophore and CP-PT ion pair in these six electrodes were 2.5, 5, 7.5, 10.0, 12.5 and 15.0 mg. The slopes and correlation coefficients of the above electrodes were found to be 42.30 (0.985), 46.26 (0.987), 51.45 (0.988), 58.64 (0.995), 60.47 (0.999), 55.97 (0.992) and 43.66 (0.962), 47.47 (0.984), 52.87 (0.992), 59.93 (0.998), 56.49 (0.989), 45.32 (96.95) for the six electrodes with KTpClPB ionophore and CP-PT, respectively. Figure 2 and Table 1 show that the electrodes have good response to the CPB within the concentration ranges of 4.9 × 10–7–1.0 × 10–2 M and 1.0 × 10‒6–1.0 × 10–2 M CPB for SPE modified with KTpClPB ionophore (electrode V) and CP-PT (electrode X), respectively. According to these results, the optimum amount of the ionophore was 12.5 and 10 mg for electrode (V) and electrode (X), respectively. In this optimum proportion, the slopes of the electrodes were Nernstian.
Table 1 Effect of CP-PT ion pair and KTpClPB ionophore content on the electrode performance of modified using SPE electrodes at 25 ºC Type of electrodes
KTpClPB ionophore
CP-PT ion pair
No. of electrodes I II III IV V VI VII VIII IX X XI
Ion pair content (mg) 2.5 5 7.5 10 12.5 15 2.5 5 7.5 10 12.5
Concentration rang (M) 4.9 × 10‒7‒1.0 × 10‒2 4.9 × 10‒7‒1.0 × 10‒2 4.9 × 10‒7‒1.0 × 10‒2 4.9 × 10‒7‒1.0 × 10‒2 4.9 × 10–7‒1.0 × 10‒2 4.9 × 10-7‒1.0 × 10‒2 1.0 × 10‒5‒1.0 × 10‒2 1.0 × 10-6‒1.0 × 10‒2 1.0 × 10‒6‒1.0 × 10‒2 1.0 × 10–6–1.0 × 10–2 1.0 × 10–6–1.0 × 10–2
Slope (mV decade‒1) 42.30 ± 1.23 46.26 ± 1.52 51.45 ± 0.96 58.64 ± 0.73 60.47 ± 0.52 55.97 ± 0.60 43.66 ± 1.40 47.47 ± 1.62 52.87 ± 0.98 59.93 ± 0.40 56.49 ± 0.85
Recovery (%) 98.55 98.70 98.82 99.52 99.90 99.20 96.22 98.40 99.25 99.80 98.90
Total potential change (mV) 213 229 255 290 299 279 198 219 241 269 258
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
Fig.2 Effect of ionophore content on calibration of modified SPEs, (a) electrode (V) and (b) electrode (X) at 25 ºC
3.2
Effect of plasticizer
Four different plasticizers viz. TCP, DBP, DOP and DOS were employed to study their effect on the electrochemical behaviour of the membrane (Fig.3). Generally, the use of plasticizers improved certain characteristics of the membranes, and in some cases, the slopes get affected adversely. Here, the slopes in the case of the DBP and DOP were super-Nernstian and DOS was sub-Nernstian. It was found that TCP gave a near Nernstian slope (Fig.3). The potentiometric response characteristics of the CPB sensors based on the use of KTpClPB ionophore (electrode (V)) and CP-PT ion pair (electrode (X)) as the electroactive material and TCP as a plasticizer in screen-printed were examined. 3.3
Effect of soaking
The performance characteristics of the CP-PT SPE were studied as a function of soaking time. For this purpose, the electrodes were soaked in 0.01 M solution of CPB and the calibration graphs (pCPB versus Eelec (mV)) were plotted after 5, 10, 15, 30 min, 1, 2, 5, 10, 12 and 24 h. The optimum soaking time was found to be 10 min to 1 h, where the slopes
of the calibration curves were 57.0–59.93 mV per pCPB decade, at 25 °C. Soaking the electrodes for longer time than 24 h was not recommended. This was to avoid leaching of, although very little, the electroactive species into the bathing solution. The electrodes should be kept dry in an opaque closed vessel and stored in a refrigerator while not in use. The reproducibility of repeated measurements on the same solutions was ±1 mV. 3.4
Performance of electrodes
Citalopram cation reacts with phosphotungstate anion to form water insoluble ion association complex. The prepared complex was identified and examined as ion exchanger site in screen-printed sensor responsive for citalopram cation. The electrochemical performance characteristics of the sensors were evaluated according to IUPAC recommendation, as summed in Table 2. The proposed sensors showed Nernstian response for citalopram concentrations range of 4.9 × 10‒7‒1.0 × 10‒2 M and 1.0 × 10‒6–1.0 × 10‒2 M with calibration slopes of (60.47 ± 0.52) mV decade‒1 and (59.93 ± 0.40) mV decade‒1, and the limit of detection was 4.9 × 10‒7 M and 1.0 × 10‒6 M for electrode (V) and electrode (X), respectively.
Fig.3 Effect of plasticizer on calibration of modified SPEs (a) electrode (V) and (b) electrode (X) at 25 ºC
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
Table 2 Response characteristics of KTpClPB ionophore (Electrode V) and CP-PT ion pair (Electrode X) screen-printed electrodes at 25 ºC Value
Parameter Slope (mV decade–1) # Correlation coefficient (r) Lower detection limit (M) Response time (s) Working pH range Usable range (M) SD of slope (mV decade− 1) Intercept (mV) # Life time (months) Accuracy (%) Precision (%)
Electrode (V)
Electrode (X)
60.47 ± 0.80 0.999 4.9 × 10‒7 7‒12 2‒9 4.9 × 10‒7‒1.0 × 10‒2 0.32 305.23 ± 0.49 5 99.87 0.92
59.93 ± 1.45 0.997 1.0 × 10‒6 9‒13 2‒8 1.0 × 10‒6–1.0 × 10‒2 0.43 321.13 ± 0.31 4 99.00 1.02
* Average of four determination. # y = 60.47x + 305.23 (for Electrode (V)); y = 59.93x + 321.13 (for Electrode (X)).
3.4.1
Response time
The static response time of the electrodes was measured after successive immersion of the electrodes in a series of CPB solutions, in each of which the CPB concentration increased 10-fold, from 4.9 × 10‒7 M to 1.0 × 10‒2 M. The static response time obtained was less than 7 and 9 s for 1.0 × 10‒2 M and 1.0 × 10‒3 M CPB concentration. At lower concentrations, however, the response time was little longer and reached 12 and 13 s for electrode (V) and electrode (X), respectively. The actual potential versus time traces is shown in Fig.4. The potentials remained constant for approximately 3 min, after which a very slow change within the resolution of the meter was recorded. The sensing behavior of the membrane electrode did not depend on whether the potentials were recorded from low to high concentrations or vice versa. The electrodes worked well over a period of 30 d without observing any significant change in the working concentration range, slope or response time. 3.4.2
of 1.0 × 10–7–1.0 × 10–2 M CPB solutions were determined at 10, 20, 30, 40, 50 and 60 °C and the calibration graphs were constructed. The slope, response time, usable concentration range and the standard electrode potentials (E0elec) (obtained from the calibration plots as the intercepts at pCP = 0) corresponding to each temperature are listed in Table 3. It is obvious that the electrode gave a good Nernstian response in the temperature range of 10–60 °C. For the determination of the isothermal coefficient (dE°/dT) of the electrode, the standard electrode potential (E0elec) at different temperatures was plotted vs. (t – 25), where t is the temperature of the test solution. A straight-line plot was obtained according to the following equation[31]: E0 = E0(25) + (dE0/dt)(t ‒ 25) (3) The slope of the straight line obtained [E0 = –305.23 + 3.805 (t – 25)] represent the isothermal coefficient of the electrode (amounting to 0.0038 and 0.0049 V/°C) for electrode (V) and electrode (X), respectively, indicating a good thermal stability of the electrode within the permitted temperature range.
Influence of pH value 3.4.4
The effect of pH value on the electrode potential at various citalopram concentrations in the range 1.0 × 10–4 –1.0 × 10–3 M was studied. The pH value was varied by adding HCl or NaOH. As shown in Fig.5, the electrodes potential were little influenced by pH value in the range 2–9 and 2–8 for citalopram concentrations between 1.0 × 10–3–1.0 × 10–4 M for electrode (V) and electrode (X), respectively. At higher pH values, the potential decreased due to the gradual increase in the concentration of the deprotonated form of the CP (pK1 = 8.9). In working with ion-selective electrodes for cationic drugs, some problems of precipitation of the drug were observed at higher pH values. A pH of 4.7 adjusted by 0.2 M acetic/acetate buffer was used for further studies. 3.4.3
Lifetime of electrode
The lifetimes of electrodes were investigated by performing the calibration periodically with standard solutions and
Effect of temperature Fig.4
To study the effect of temperature, the electrode potentials
Dynamic response time of modified SPEs Sensors, (a) electrode (V) and (b) electrode (X)
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
Fig.5 Effect of pH of the test solution on the potential readings of using SPEs, (a) electrode (V) and (b) electrode (X) at 25 ºC Table 3 Performance characteristics of citalopram screen-printed electrode at different temperatures at 10–60 ºC Temperature (°C)
Slope (mV decade–1) Electrode (V)
Slope (mV decade-1) Electrode (X)
Usable concentration range (M) Electrode (V)
Usable concentration range (M) Electrode (X)
E°elec. (mV) Electrode (V)
E°elec. (mV) Electrode (X)
10 20 30 40 50 60
57.60 60.10 60.67 61.23 63.00 63.86
56.25 59.85 59.98 60.35 62.20 62.90
1.0 × 10–6–1.0 × 10–2 4.9 × 10–7–1.0 × 10–2 4.9 × 10–7–1.0 × 10–2 4.9 × 10–7–1.0 × 10–2 4.9 × 10–7–1.0 × 10–2 1.0 × 10–6–1.0 × 10–2
5.0 × 10–6–1.0 × 10–2 1.0 × 10–6–1.0 × 10–2 1.0 × 10–6 –1.0 × 10–2 1.0 × 10–6–1.0 × 10–2 1.0 × 10–6–1.0 × 10–2 5.0 × 10–6–1.0 × 10–2
–320 –270 –245 –215 –202 –190
–290 –259 –223 –197 –181 –165
calculating the response slopes. The results indicated that the electrode could be used continuously for about 4 or 5 months without considerable decrease in its slope values. Firstly, a slight gradual decrease was observed in the slopes (from 60.47 to 52.20 and 59.93 to 47.86 mV decade–1) and, secondly, an increase was obtained in the detection limit of 4.9 × 10–7–2.5 × 10–6 M and 1.0 × 10–6–1.0 × 10–5 M for electrode (V) and electrode (X), respectively. These kinds of electrodes did not require any preconditioning in the solutions of corresponded drugs or maintenance before use. The surface of the electrodes was washed with water after each application and stored in a desicator under atmospheric condition and kept far from the light. 3.4.5
Reproducibility and stability of electrodes
The reproducibility and stability of the screen-printed electrodes were evaluated by repeated calibration of the electrodes in CPB solutions. The repeated monitoring of potentials and calibration, using the same electrodes, over several days gave good slope reproducibility; almost over a period of five weeks, with standard deviation (SD) of slope ≤ 0.5 mV decade‒1. For 10 successive replicate measurements at 1.0 × 10–2 M CPB concentrations, the SD was 0.92 mV. The specifications and general characteristic performance of the proposed CPB sensors are given in Table 2. 3.5
Selectivity of electrodes
The interference effect of different organic and inorganic cations on the electrodes response was evaluated. The
interference of these compounds was assessed by measuring the selectivity coefficient Kpotdrug, j using the separate solutions and matched potential methods[22–24] with 1.0 mM concentration of both the standard CPB and the interference. The results obtained are listed in Table 4. The selectivity of the electrodes was observed in presence of some interferents. In most cases, no significant influence on the electrode performance was observed. 3.6
Quantification, accuracy and precision
The proposed potentiometric method was used for direct determination of the investigated drug where its concentration was calculated from calibration graph. The results were compared with the official method[26]. The data reported in Table 5 indicated that results obtained by the two reported methods were in good agreement; however, the proposed method was more selective, rapid, simple and less time consuming. In addition, the proposed methods were used for determination of the studied drug in pharmaceutical preparations (Table 5). 3.7
Application to urine and human serum
Citalopram can be determined in urine and human serum by using potentiometric determinations and the results obtained are summed in Table 6. The accuracy of the proposed potentiometric method was confirmed by the determination of CPB in spiked citalopram samples prepared from serial concentrations of CPB reference standards. Therefore, the proposed method could be applied to the determination of
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
Table 4 Potentiometric selectivity coefficients of some interfering ions using the citalopram SPE sensors at 25 ºC Ionophore Interferent, J
Glucose Lactose Maltose Glycine Fructose Starch Urea Picric acid Na+ K+ Ni2+ Co2+ Ca2+ Mg2+ Fe3+ Cl‒ Br‒ SO42‒
Electrode (V)
Electrode (X)
KpotCPB,J
KpotCPB,J
SSM
MPM
SSM
MPM
----------------3.4 × 10‒5 2.7 × 10‒4 3.7 × 10‒4 4.1 × 10‒4 1.1 × 10‒3 3.5 × 10‒3 4.6 × 10‒3 7.0 × 10‒3 4.1 × 10‒3 5.5 × 10‒2
4.2 × 10‒5 1.6 × 10-4 2.2 × 10-6 2.6 × 10‒5 3.0 × 10‒4 8.3 × 10‒5 4.6 × 10‒4 1.5 × 10‒4 ---------------------
----------------3.1 × 10‒5 2.4 × 10‒4 3.8 × 10‒2 4.3 × 10-4 1.0 × 10‒3 3.2 × 10‒3 5.0 × 10‒3 6.6 × 10‒3 3.8 × 10‒3 5.2 × 10‒2
3.1 × 10‒5 0.9 × 10–4 1.8 × 10‒6 2.4 × 10‒5 3.2 × 10‒4 6.9 × 10‒5 3.7 × 10‒4 1.8 × 10‒4 ---------------------
Table 5 Determination citalopram in pure solutions and pharmaceutical preparations using ionophore and IP SPE sensors Sample
Pure CPB Sample 1* Sample 2* a
Electrode (V)
[CPB] Taken (mg mL–1)
Found (mg mL‒1)
2.00 5.00 1.50 5.32 2.50 6.05
2.00 4.92 1.54 5.29 2.48 6.02
Electrode (X)
British Pharmacopiea
Recovery (%)
RSDa (%)
Found (mg mL–1)
Recovery (%)
RSDa (%)
Found (mg mL–1)
Recovery (%)
RSDa (%)
100.00
0.33
1.99
99.50
1.33
1.92
96.00
2.45
100.02
1.65
1.48
99.33
1.12
1.45
97.00
2.68
100.00
1.23
2.49
99.60
0.85
2.43
97.20
1.85
Number of replicates is 4. Sample 1 = Depaway (40 mg/tablet), Sample 2 = Cipram® (20 mg/tablet).
Table 6 Determination of citalopram in spiked urine and human serum using ionophore and IP SPE sensors Sample
Urine
Serum
Statistical parameters
Direct method
Electrode (V)
Mean recovery (%) N Variance RSD (%) Mean recovery (%) N Variance RSD (%)
98.95 5 0.94 0.58 98.99 5 0.67 0.75
Calibration graphs
Standard addition method
98.23 5 0.90 0.46 99.00 5 0.77 0.67
CPB alone and in pharmaceutical preparations or in biological fluids without fear of interferences caused by the excipients expected to be present in tablets or the constituents of body fluids. As shown in Table 7, the inter- and intra-day accuracy of the potentiometric method for CPB were ranged from 99.0% to 100.6%, 98.0% to 98.6% (SD = 0.12–0.45 and 0.12–0.74; and RSD from 0.9%–1.4 and 1.1%–2.4% for electrode (V) and electrode (X), respectively. The recovery of the corresponding amounts of CPB was obtained at concentration levels (1.00 ‒ 2.50 mg mL–1) in tablets and found to be ranged
99.00 5 0.83 0.79 99.45 5 0.59 0.62
Direct method
Calibration graphs
98.45 5 0.65 0.46 97.55 5 0.85 0.85
98.73 5 0.77 0.39 98.26 5 0.68 0.98
from 98.0% to 101.33% and 97.0% to 101.0% (SD = 0.19 to 0.65 and 0.06–0.69; and RSD from 0.79% to 1.38%, and 0.87 to 2.46 for electrode (V) and electrode (X), respectively.
4
Conclusions
Citalopram-screen printed electrodes on the basis of the KTpClPB ionophore, CP-PT ion pair and TCP as plasticizer was developed. Its linear range, slope and limit of detection were 4.9 × 10–7–1.0 × 10–2 M and 1.0 × 10–6–1.0 × 10–2 M, 60.47 and 59.93 mV decade–1, 4.9 × 10–7 M and 1 × 10–6 M for
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
Table 7
Intra- and Inter-days precision of the determination of CPB using the two types of electrodes with determination of pure and pharmaceutical tablet Electrodes type
Drug
Electrode (V) Pure form Electrode (X)
Depaway (40 mg/tablet)
Electrode (V) Electrode (X)
Taken (mg mL–1)
Intra day Found (mg mL–1)
Recovery (%)
1.98 3.52 4.47 1.96 3.45 4.43 0.98 1.52 1.98 1.01 1.47 1.94
99.00 100.57 99.33 98.00 98.59 98.44 98.00 101.33 99.00 101.00 98.00 97.00
2.00 3.50 4.50 2.00 3.50 4.50 1.00 1.50 2.00 1.00 1.50 2.00
SPE sensors (electrode (V) and electrode (X), respectively). The effect of pH on the potential response indicated that larger influence of pH occurred when pH of the solution was in the range of 2–9 and 2–8 for electrode (V) and electrode (X), respectively. The proposed electrodes were successfully applied to the determination of citalopram hydrobromide in pharmaceutical preparation, urine and human serum. This analytical method proved to be a simple, rapid and accurate method.
Inter day SD
RSD (%)
Found (mg mL–1)
Recovery (%)
SD
RSD (%)
0.12 0.45 0.26 0.52 0.74 0.14 0.19 0.65 0.37 0.06 0.32 0.69
1.02 0.89 1.34 1.08 1.45 2.42 1.38 0.97 1.17 0.87 1.79 2.46
1.97 3.49 4.51 1.94 3.42 4.45 1.02 1.48 1.96 0.97 1.51 1.95
98.50 99.71 100.22 97.00 97.77 98.88 102.00 98.66 98.00 97.00 100.66 97.50
0.23 0.15 0.09 0.67 1.12 0.39 0.07 0.33 0.91 1.21 0.05 1.17
0.75 1.12 0.93 1.26 2.96 0.75 1.83 1.49 2.36 2.27 0.88 2.33
[10] Aboul-Enein H Y, Sun X X, Sun C J. Sensors, 2002, 2: 424–431 [11] Farhadi K, Bahram M, Shokatynia D, Salehiyan F. Talanta, 2008, 76: 320–326 [12] Drozd J, Hopkala H. Desalination, 2004, 163: 119–125 [13] Salem A A, Barsoum B N, Izake E L. Anal. Chim. Acta, 2003, 498: 79–91 [14] García M S, Ortuño J A, Albero M I, Abuherba M S. Sensors, 2009, 9: 4309–4322 [15] Abdel-Fattah L, El-Kosasy A, Abdel-Aziz L, Gaied M. J. Am.
References
Sci., 2010, 6: 1115–1121 [16] Lenik J, Wardak C, Marczewska B. Acta Pol. Pharm., 2006,
[1]
Redmond A M, Harkin A, Kelly J P, Leonard B E. Eur. Neuropsychopharm., 1999, 9: 165–170
[2]
Raman B, Sharma B A, Ghugare P D, Karmuse P P, Kumar A. J. Pharmaceut. Biomed., 2010, 90: 377–383
[3]
Ma Z, Strecker R E, McKenna J T, Thakkar M M, McCarley R W, Tao R. Neuroscience, 2005, 135: 949–958
[4]
Smolders I, Clinckers R, Meurs A, Bundel D D, Portelli J, Ebinger G, Michotte Y. Neuropharmacology, 2008, 54: 1017–1028
[5]
Nguyen K Q, Tohyama Y, Watanabe A, Hasegawa S, Skelin I, Diksic M. Neurochem. Int., 2009, 54: 161–171
[6]
Weikop P, Yoshitake T, Kehr J. Eur. Neuropsychopharm., 2007, 17: 658–671
[7] [8]
[17] Mohamed G G, Ali T A, El-Shahat M F, Al-Sabagh A M, Migahed M A, Khaled E. Anal. Chim. Acta, 2010, 673: 79–87 [18] Khaled E, Mohamed G G, Awad T. Sensors and Actuators B, 2008, 135: 74–80 [19] Mohamed G G, Ali Tamer Awad, El-Shahat M F, Al-Sabagh A M, Migahed M A. Electroanal., 2010, 22: 2587–2599 [20] Mohamed G G, El-Shahat M F, Al-Sabagh A M, Migahed M A. Analyst, 2011, 136: 1488–1495 [21] Frag E Y Z, Mohamed G G, F.A. Nour El-Dien, Mohamed M E. Analyst, 2011, 136: 332–339 [22] Bakker E, Pretsch E, Buhlmann P. Anal. Chem., 2000, 72: 1127–1133
Williams S M, Bryan-Lluka L J, Pow D V. Brain. Res., 2005,
[23] Abbas M N, Zahran E. J. Electroanal. Chem., 2005, 576: 205–213
1042: 224–232
[24] Frag E Y Z, Ali T A, Mohamed G G, Awad Y H H. Int. J.
Dimeski G, Badrick T, John A S. Clin. Chim. Acta, 2010, 411: 309–317
[9]
63: 239–244
Faridbod F, Ganjali M R, Dinarvand R, Riahi S, Norouzi P, Olia M B A. J. Food Drug Anal., 2009, 17: 264–273
Electrochem. Sci., 2012, 7: 4443 – 4464 [25] Antropov L I. Theoretical Electrochemistry, Mir Publisher, Moscow, 1977 [26] Vytras K. Mikrochimica Acta, 1984(III): 139–148
Cite this article as: Chin J Anal Chem, 2014, 42(4), 565–572.
RESEARCH PAPER
A New Screen-printed Ion Selective Electrode for Determination of Citalopram Hydrobromide in Pharmaceutical Formulation Tamer Awad Ali1,*, Gehad G. Mohamed2, A. M. Al-Sabagh1, M. A. Migahed1 1 2
Egyptian Petroleum Research Institute (EPRI), Cairo, 11727, Egypt Chemistry Department, Faculty of Science, Cairo University, Giza, 12613, Egypt
Abstract: Novel citalopram screen-printed ion selective electrodes were fabricated, characterized and used for the determination of citalopram in pharmaceutical formulations. The proposed sensors incorporated potassium tetrakis(p-chlorophenyl) borate (KTpClPB) ionophore (electrode V) and citalopram-phosphotungstate (CP-PT) ion pair complex (electrode X) as electroactive materials in screen-printed electrodes and tricresylphosphate (TCP) as solvent mediator. The fabricated electrodes demonstrated near Nernstain response over wide linear range of 4.9 × 10–7–1.0 × 10–2 M and 1.0 × 10–6–1.0 × 10–2 M citalopram with lower limit of detection of 4.9 × 10–7 M and 1.0 × 10–6 M and slope of (60.47 ± 0.80) mV decade–1 and (59.93 ± 1.45) mV decade–1 for electrode (V) and (X), respectively. The results showed that the proposed sensors had the characteristics such as fast and stable response, good reproducibility, long term stability (5 and 4 months) and applicability over a wide pH range of 2–9 and 2–8 for electrodes (V) and (X). The sensors displayed good selectivity for citalopram with respect to number of common foreign inorganic, organic species, excipients and the fillers added to the pharmaceutical preparation. The sensors were successfully applied for the determination of citalopram in tablet, urine and serum. Key Words:
1
Citalopram ion-selective electrode; Screen-printed electrode; Pharmaceutical preparation; Potentiometric determination
Introduction
Citalopram (Fig.1), 1-(3-dimethylaminopropyl)-1-(4fluorophenyl)-5-phthalan carbonitrile, is an antidepressant drug used to treat depression associated with mood disorders. It is also used on occasion for the treatment of body dysmorphic disorder and anxiety. Citalopram belongs to a class of drugs known as selective serotonin reuptake inhibitors (SSRIs). It is primarily used to treat the symptoms of depression and can also be prescribed for social anxiety disorder, panic disorder, obsessive-compulsive disorder, the Huntington’s disease, and premenstrual dysphoric disorder[1‒4]. Citalopram affects neurotransmitters, the chemical transmitters within the brain. Neurotransmitters manufactured and released by nerves attached to adjacent nerves and alter their activities. Thus, neurotransmitters are considered the communication system of the brain. Many experts believe that
an imbalance among neurotransmitters is the cause of depression. Citalopram works by preventing the uptake of serotonin by nerve cells after it has been released. Such uptake is an important mechanism for removing released neurotransmitters and terminating their actions on adjacent nerves. The reduced uptake caused by citalopram results in stimulation of the nerve cells by the free serotonin in the brain[5–7].
Fig.1 Chemical structure of citalopram hydrobromide (CPB)
Received 23 March 2013; accepted 7 May 2013 * Corresponding author. Email: [email protected], Tel: +2 010 06890640 Copyright © 2014, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(13)60725-2
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
An ion-selective electrode (ISE) is capable of measuring selectivity and activity of a given ion regardless of other ions present in the solution[8]. It was used for determination of many drugs[9–12]. Compared with other analytical techniques, ISE has an impressive list of advantages such as being portable, suitable for either direct determination or using as a sensor in titrations besides, these membrane electrode don’t affect the studied solutions[13–16]. In this work, the construction of plasticized screen-printed type citalopram ion selective electrodes and their application in pharmaceutical analysis were described. It was based on using ion-associate species, formed by drug cation and phosphotungstic acid (PTA) as counter ion or incorporating KTpClPB ionophore in the electrode matrix. The fabricated potentiometric sensors were applied for the determination of CPB in pure and in tablets, urine and serum.
2
Experimental
2.1
Apparatus and reagents
Laboratory potential measurements were performed using HANNA 211 pH meter. An Ag/AgCl double-junction reference electrode (Metrohm 6.0726.100) was used in conjugation with different ion selective electrodes. Digital burette was used for the field measurement of drugs under investigation. Elemental analysis for carbon, hydrogen, nitrogen, and sulphur were carried out at the Microanalytical centers, Cairo University, using a Perkin-Elmer CHN 2400 Elemental Analyzer. All chemicals used were of analytical reagent grade unless otherwise stated, and doubly distilled water was used. Citalopram hydrobromide (CPB) was purchased from Western pharmaceutical industries, Egypt. Depaway 40 (sample 1), cipram® 20 (sample 2) were purchased from Memphis, Egypt and multi pharma/lundbeek, Denmark. Sodium tetraphenylborate (NaTPB) and phosphotungstic acid (PTA) were commercially available from Sigma-Aldrich and Fluka, respectively. Tricresylphosphate (TCP) from Alfa Aesar was used for the preparation of the sensors. Other types of plasticizers, namely dioctylphthalate (DOP), dibutylphthalate (DBP) and dioctylsebacate (DOS) were purchased from Sigma, Merck and Merck, respectively. Relative high molecular weight polyvinyl-chloride (PVC) (Aldrich), potassium tetrakis (p-chlorophenyl) borate (KTpClPB) (Aldrich) and graphite powder (synthetic 1–2 µm, Aldrich) were used for the fabrication of different electrodes. 2.2 2.2.1
Procedures Preparation of citalopram-modified screen-printed electrodes
Modified SPEs were printed in arrays of six couples
consisting of the working and the reference electrodes (each 5 mm × 35 mm) following the procedures previously described[17–20]. A polyvinyl chloride flexible sheet (0.2 mm) was used as a substrate which was not affected by the curing temperature or the ink solvent and easily cutted by scissors. A pseudo Ag/AgCl electrode was firstly printed using a home-made polyvinyl chloride ink containing silver-silver chloride (65:35, V/V) which was cured at 60 ºC for 30 min. The working electrodes were prepared depending on the method of fabrication. The working electrode was printed using homemade carbon ink which was prepared by mixing 2.5–15 mg potassium tetrakis (p-chlorophenyl) borate (KTpClPB) ionophore or CP-PT ion pair, 450 mg TCP, 1.25 g of polyvinyl chloride (8%, w/V) and 0.75 g carbon powder. They were printed using homemade carbon ink and cured at 50 ºC for 30 min. A layer of an insulator was then placed onto the printed electrodes, leaving a defined rectangular shaped (5 mm × 5 mm) working area and a similar area (for the electrical contact) on the other side. Fabricated electrodes were stored at 4 ºC and used directly in the potentiometric measurements[17–21]. 2.2.2
Sensor calibration
SPE sensors in conjunction with Ag/AgCl reference electrode was immersed in standard CPB solutions with concentration of 5.0 × 10–7–1.0 × 10–2 M in a 50-mL beaker. The solutions were stirred and the potential was recorded after stabilization and plotted as a function of CPB concentration, and the graph was used for the subsequent determination of unknown concentration of CPB. The potential readings were recorded after stabilization and the emf were plotted as a function of logarithm citalopram concentration. The lower detection limit was taken at the point of intersection of the extrapolated linear segments of the citalopram calibration curve. 2.2.3
Preparation of ion-pair compound
Ion-pair of citalopram-phosphotungstate (CP-PT): About 20 mL of 0.01 M CPB solution was mixed with 65 mL of 0.01 M phosphotungstate under stirring. The resulting precipitates were filtered off, washed with water, dried at 60 °C, washed with petroleum ether to remove any residual moisture, then ground to fine powder and kept dry in desiccator. Small sample portions were sent for elemental analysis. 2.3
Interference effects
The response of the electrodes was also examined in the presence of a number of organic and inorganic ions. The potentiometric selectivity coefficients Kpotdrug, j were used to evaluate the degree of interference[17,22,23].
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
A 9.0-mL aliquot of distilled water was placed in a 50-mL beaker where the electrodes and the double junction Ag/AgCl electrode were immersed. The potential response upon addition of 1.0 mL aliquot of 1.0 mM solution of the interferent was recorded and compared with that of 1.0 mM pure citalopram hydrobromide solution. The selectivity coefficients were calculated using Eisenman-Nicolsky equation: lgKpotCPB, j = ((E1 – E2)/S) + (1 + (Z1/Z2))lga) (1) where, E1 is the potential measured in 1.0 × 10−3 M septonex, E2 is the potential measured in 1.0 × 10−3 M of the interfering compound, Z1 and Z2 are the charges of the citalopram and interfering species j, and S is slope of the electrode calibration plot. The selectivity coefficients were also measured by the match method according to the equation[24]: lgKpotCPB, j = (a’D – aD)/aj (2) 2.4
Analytical method for pharmaceutical formulation
Ten citalopram hydrobromide tablets were weighed accurately, crushed and mixed in a mortar, and dissolved in 30 mL of distilled water under stirring condition. The solution was transferred into a 50-mL volumetric flask, and then completed to the mark with the same solvent. 5 mL of this solution was taken and transferred into another 50-mL volumetric flask, completed to volume with the distilled water. The citalopram selective screen-printed electrodes and the reference electrode were immersed into the test solution. The content of CP in the tablets was determined by using the direct potentiometric method. 2.5
Procedure for determination of citalopram in human urine
Urine samples containing different citalopram concentrations were prepared by adding known amounts of citalopram to 25 mL aliquots of blank urine samples of four volunteers, the citalopram-selective and reference electrodes were immersed and the citalopram concentration was determined by direct potentiometry using the standard addition technique.
2.6
Procedure for determination of citalopram in serum
The electrodes were placed into 100 µL of serum under constant stirring with a stirring bar. The serum samples were prepared by adding citalopram hydrobromide to human serum. All measurements were carried out at 25 °C.
3
Results and discussion
Phosphotungsate was used as an ion-pairing agent for the preparation of an electroactive ion-association complex for the determination of citalopram. The elemental analysis result of the sparingly soluble complex of CPB/PTA showed that the composition of the complex was 1:3 [Citalopram]: [Phosphotungsate] ion pair (% found (calculated): C = 7.22 (7.50), H = 0.61 (0.69) and N = 0.79 (0.87). The dry powder of the formed ion-pair was used for the preparation of the new screen printed ion selective electrode for CPB determination. 3.1
Effects of ionophore and ion pair content
For this purpose, six electrodes with different KTpClPB ionophore and CP-PT ion pair compositions were prepared and the data obtained are summarized in Table 1. The proportions of KTpClPB ionophore and CP-PT ion pair in these six electrodes were 2.5, 5, 7.5, 10.0, 12.5 and 15.0 mg. The slopes and correlation coefficients of the above electrodes were found to be 42.30 (0.985), 46.26 (0.987), 51.45 (0.988), 58.64 (0.995), 60.47 (0.999), 55.97 (0.992) and 43.66 (0.962), 47.47 (0.984), 52.87 (0.992), 59.93 (0.998), 56.49 (0.989), 45.32 (96.95) for the six electrodes with KTpClPB ionophore and CP-PT, respectively. Figure 2 and Table 1 show that the electrodes have good response to the CPB within the concentration ranges of 4.9 × 10–7–1.0 × 10–2 M and 1.0 × 10‒6–1.0 × 10–2 M CPB for SPE modified with KTpClPB ionophore (electrode V) and CP-PT (electrode X), respectively. According to these results, the optimum amount of the ionophore was 12.5 and 10 mg for electrode (V) and electrode (X), respectively. In this optimum proportion, the slopes of the electrodes were Nernstian.
Table 1 Effect of CP-PT ion pair and KTpClPB ionophore content on the electrode performance of modified using SPE electrodes at 25 ºC Type of electrodes
KTpClPB ionophore
CP-PT ion pair
No. of electrodes I II III IV V VI VII VIII IX X XI
Ion pair content (mg) 2.5 5 7.5 10 12.5 15 2.5 5 7.5 10 12.5
Concentration rang (M) 4.9 × 10‒7‒1.0 × 10‒2 4.9 × 10‒7‒1.0 × 10‒2 4.9 × 10‒7‒1.0 × 10‒2 4.9 × 10‒7‒1.0 × 10‒2 4.9 × 10–7‒1.0 × 10‒2 4.9 × 10-7‒1.0 × 10‒2 1.0 × 10‒5‒1.0 × 10‒2 1.0 × 10-6‒1.0 × 10‒2 1.0 × 10‒6‒1.0 × 10‒2 1.0 × 10–6–1.0 × 10–2 1.0 × 10–6–1.0 × 10–2
Slope (mV decade‒1) 42.30 ± 1.23 46.26 ± 1.52 51.45 ± 0.96 58.64 ± 0.73 60.47 ± 0.52 55.97 ± 0.60 43.66 ± 1.40 47.47 ± 1.62 52.87 ± 0.98 59.93 ± 0.40 56.49 ± 0.85
Recovery (%) 98.55 98.70 98.82 99.52 99.90 99.20 96.22 98.40 99.25 99.80 98.90
Total potential change (mV) 213 229 255 290 299 279 198 219 241 269 258
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
Fig.2 Effect of ionophore content on calibration of modified SPEs, (a) electrode (V) and (b) electrode (X) at 25 ºC
3.2
Effect of plasticizer
Four different plasticizers viz. TCP, DBP, DOP and DOS were employed to study their effect on the electrochemical behaviour of the membrane (Fig.3). Generally, the use of plasticizers improved certain characteristics of the membranes, and in some cases, the slopes get affected adversely. Here, the slopes in the case of the DBP and DOP were super-Nernstian and DOS was sub-Nernstian. It was found that TCP gave a near Nernstian slope (Fig.3). The potentiometric response characteristics of the CPB sensors based on the use of KTpClPB ionophore (electrode (V)) and CP-PT ion pair (electrode (X)) as the electroactive material and TCP as a plasticizer in screen-printed were examined. 3.3
Effect of soaking
The performance characteristics of the CP-PT SPE were studied as a function of soaking time. For this purpose, the electrodes were soaked in 0.01 M solution of CPB and the calibration graphs (pCPB versus Eelec (mV)) were plotted after 5, 10, 15, 30 min, 1, 2, 5, 10, 12 and 24 h. The optimum soaking time was found to be 10 min to 1 h, where the slopes
of the calibration curves were 57.0–59.93 mV per pCPB decade, at 25 °C. Soaking the electrodes for longer time than 24 h was not recommended. This was to avoid leaching of, although very little, the electroactive species into the bathing solution. The electrodes should be kept dry in an opaque closed vessel and stored in a refrigerator while not in use. The reproducibility of repeated measurements on the same solutions was ±1 mV. 3.4
Performance of electrodes
Citalopram cation reacts with phosphotungstate anion to form water insoluble ion association complex. The prepared complex was identified and examined as ion exchanger site in screen-printed sensor responsive for citalopram cation. The electrochemical performance characteristics of the sensors were evaluated according to IUPAC recommendation, as summed in Table 2. The proposed sensors showed Nernstian response for citalopram concentrations range of 4.9 × 10‒7‒1.0 × 10‒2 M and 1.0 × 10‒6–1.0 × 10‒2 M with calibration slopes of (60.47 ± 0.52) mV decade‒1 and (59.93 ± 0.40) mV decade‒1, and the limit of detection was 4.9 × 10‒7 M and 1.0 × 10‒6 M for electrode (V) and electrode (X), respectively.
Fig.3 Effect of plasticizer on calibration of modified SPEs (a) electrode (V) and (b) electrode (X) at 25 ºC
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
Table 2 Response characteristics of KTpClPB ionophore (Electrode V) and CP-PT ion pair (Electrode X) screen-printed electrodes at 25 ºC Value
Parameter Slope (mV decade–1) # Correlation coefficient (r) Lower detection limit (M) Response time (s) Working pH range Usable range (M) SD of slope (mV decade− 1) Intercept (mV) # Life time (months) Accuracy (%) Precision (%)
Electrode (V)
Electrode (X)
60.47 ± 0.80 0.999 4.9 × 10‒7 7‒12 2‒9 4.9 × 10‒7‒1.0 × 10‒2 0.32 305.23 ± 0.49 5 99.87 0.92
59.93 ± 1.45 0.997 1.0 × 10‒6 9‒13 2‒8 1.0 × 10‒6–1.0 × 10‒2 0.43 321.13 ± 0.31 4 99.00 1.02
* Average of four determination. # y = 60.47x + 305.23 (for Electrode (V)); y = 59.93x + 321.13 (for Electrode (X)).
3.4.1
Response time
The static response time of the electrodes was measured after successive immersion of the electrodes in a series of CPB solutions, in each of which the CPB concentration increased 10-fold, from 4.9 × 10‒7 M to 1.0 × 10‒2 M. The static response time obtained was less than 7 and 9 s for 1.0 × 10‒2 M and 1.0 × 10‒3 M CPB concentration. At lower concentrations, however, the response time was little longer and reached 12 and 13 s for electrode (V) and electrode (X), respectively. The actual potential versus time traces is shown in Fig.4. The potentials remained constant for approximately 3 min, after which a very slow change within the resolution of the meter was recorded. The sensing behavior of the membrane electrode did not depend on whether the potentials were recorded from low to high concentrations or vice versa. The electrodes worked well over a period of 30 d without observing any significant change in the working concentration range, slope or response time. 3.4.2
of 1.0 × 10–7–1.0 × 10–2 M CPB solutions were determined at 10, 20, 30, 40, 50 and 60 °C and the calibration graphs were constructed. The slope, response time, usable concentration range and the standard electrode potentials (E0elec) (obtained from the calibration plots as the intercepts at pCP = 0) corresponding to each temperature are listed in Table 3. It is obvious that the electrode gave a good Nernstian response in the temperature range of 10–60 °C. For the determination of the isothermal coefficient (dE°/dT) of the electrode, the standard electrode potential (E0elec) at different temperatures was plotted vs. (t – 25), where t is the temperature of the test solution. A straight-line plot was obtained according to the following equation[31]: E0 = E0(25) + (dE0/dt)(t ‒ 25) (3) The slope of the straight line obtained [E0 = –305.23 + 3.805 (t – 25)] represent the isothermal coefficient of the electrode (amounting to 0.0038 and 0.0049 V/°C) for electrode (V) and electrode (X), respectively, indicating a good thermal stability of the electrode within the permitted temperature range.
Influence of pH value 3.4.4
The effect of pH value on the electrode potential at various citalopram concentrations in the range 1.0 × 10–4 –1.0 × 10–3 M was studied. The pH value was varied by adding HCl or NaOH. As shown in Fig.5, the electrodes potential were little influenced by pH value in the range 2–9 and 2–8 for citalopram concentrations between 1.0 × 10–3–1.0 × 10–4 M for electrode (V) and electrode (X), respectively. At higher pH values, the potential decreased due to the gradual increase in the concentration of the deprotonated form of the CP (pK1 = 8.9). In working with ion-selective electrodes for cationic drugs, some problems of precipitation of the drug were observed at higher pH values. A pH of 4.7 adjusted by 0.2 M acetic/acetate buffer was used for further studies. 3.4.3
Lifetime of electrode
The lifetimes of electrodes were investigated by performing the calibration periodically with standard solutions and
Effect of temperature Fig.4
To study the effect of temperature, the electrode potentials
Dynamic response time of modified SPEs Sensors, (a) electrode (V) and (b) electrode (X)
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
Fig.5 Effect of pH of the test solution on the potential readings of using SPEs, (a) electrode (V) and (b) electrode (X) at 25 ºC Table 3 Performance characteristics of citalopram screen-printed electrode at different temperatures at 10–60 ºC Temperature (°C)
Slope (mV decade–1) Electrode (V)
Slope (mV decade-1) Electrode (X)
Usable concentration range (M) Electrode (V)
Usable concentration range (M) Electrode (X)
E°elec. (mV) Electrode (V)
E°elec. (mV) Electrode (X)
10 20 30 40 50 60
57.60 60.10 60.67 61.23 63.00 63.86
56.25 59.85 59.98 60.35 62.20 62.90
1.0 × 10–6–1.0 × 10–2 4.9 × 10–7–1.0 × 10–2 4.9 × 10–7–1.0 × 10–2 4.9 × 10–7–1.0 × 10–2 4.9 × 10–7–1.0 × 10–2 1.0 × 10–6–1.0 × 10–2
5.0 × 10–6–1.0 × 10–2 1.0 × 10–6–1.0 × 10–2 1.0 × 10–6 –1.0 × 10–2 1.0 × 10–6–1.0 × 10–2 1.0 × 10–6–1.0 × 10–2 5.0 × 10–6–1.0 × 10–2
–320 –270 –245 –215 –202 –190
–290 –259 –223 –197 –181 –165
calculating the response slopes. The results indicated that the electrode could be used continuously for about 4 or 5 months without considerable decrease in its slope values. Firstly, a slight gradual decrease was observed in the slopes (from 60.47 to 52.20 and 59.93 to 47.86 mV decade–1) and, secondly, an increase was obtained in the detection limit of 4.9 × 10–7–2.5 × 10–6 M and 1.0 × 10–6–1.0 × 10–5 M for electrode (V) and electrode (X), respectively. These kinds of electrodes did not require any preconditioning in the solutions of corresponded drugs or maintenance before use. The surface of the electrodes was washed with water after each application and stored in a desicator under atmospheric condition and kept far from the light. 3.4.5
Reproducibility and stability of electrodes
The reproducibility and stability of the screen-printed electrodes were evaluated by repeated calibration of the electrodes in CPB solutions. The repeated monitoring of potentials and calibration, using the same electrodes, over several days gave good slope reproducibility; almost over a period of five weeks, with standard deviation (SD) of slope ≤ 0.5 mV decade‒1. For 10 successive replicate measurements at 1.0 × 10–2 M CPB concentrations, the SD was 0.92 mV. The specifications and general characteristic performance of the proposed CPB sensors are given in Table 2. 3.5
Selectivity of electrodes
The interference effect of different organic and inorganic cations on the electrodes response was evaluated. The
interference of these compounds was assessed by measuring the selectivity coefficient Kpotdrug, j using the separate solutions and matched potential methods[22–24] with 1.0 mM concentration of both the standard CPB and the interference. The results obtained are listed in Table 4. The selectivity of the electrodes was observed in presence of some interferents. In most cases, no significant influence on the electrode performance was observed. 3.6
Quantification, accuracy and precision
The proposed potentiometric method was used for direct determination of the investigated drug where its concentration was calculated from calibration graph. The results were compared with the official method[26]. The data reported in Table 5 indicated that results obtained by the two reported methods were in good agreement; however, the proposed method was more selective, rapid, simple and less time consuming. In addition, the proposed methods were used for determination of the studied drug in pharmaceutical preparations (Table 5). 3.7
Application to urine and human serum
Citalopram can be determined in urine and human serum by using potentiometric determinations and the results obtained are summed in Table 6. The accuracy of the proposed potentiometric method was confirmed by the determination of CPB in spiked citalopram samples prepared from serial concentrations of CPB reference standards. Therefore, the proposed method could be applied to the determination of
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
Table 4 Potentiometric selectivity coefficients of some interfering ions using the citalopram SPE sensors at 25 ºC Ionophore Interferent, J
Glucose Lactose Maltose Glycine Fructose Starch Urea Picric acid Na+ K+ Ni2+ Co2+ Ca2+ Mg2+ Fe3+ Cl‒ Br‒ SO42‒
Electrode (V)
Electrode (X)
KpotCPB,J
KpotCPB,J
SSM
MPM
SSM
MPM
----------------3.4 × 10‒5 2.7 × 10‒4 3.7 × 10‒4 4.1 × 10‒4 1.1 × 10‒3 3.5 × 10‒3 4.6 × 10‒3 7.0 × 10‒3 4.1 × 10‒3 5.5 × 10‒2
4.2 × 10‒5 1.6 × 10-4 2.2 × 10-6 2.6 × 10‒5 3.0 × 10‒4 8.3 × 10‒5 4.6 × 10‒4 1.5 × 10‒4 ---------------------
----------------3.1 × 10‒5 2.4 × 10‒4 3.8 × 10‒2 4.3 × 10-4 1.0 × 10‒3 3.2 × 10‒3 5.0 × 10‒3 6.6 × 10‒3 3.8 × 10‒3 5.2 × 10‒2
3.1 × 10‒5 0.9 × 10–4 1.8 × 10‒6 2.4 × 10‒5 3.2 × 10‒4 6.9 × 10‒5 3.7 × 10‒4 1.8 × 10‒4 ---------------------
Table 5 Determination citalopram in pure solutions and pharmaceutical preparations using ionophore and IP SPE sensors Sample
Pure CPB Sample 1* Sample 2* a
Electrode (V)
[CPB] Taken (mg mL–1)
Found (mg mL‒1)
2.00 5.00 1.50 5.32 2.50 6.05
2.00 4.92 1.54 5.29 2.48 6.02
Electrode (X)
British Pharmacopiea
Recovery (%)
RSDa (%)
Found (mg mL–1)
Recovery (%)
RSDa (%)
Found (mg mL–1)
Recovery (%)
RSDa (%)
100.00
0.33
1.99
99.50
1.33
1.92
96.00
2.45
100.02
1.65
1.48
99.33
1.12
1.45
97.00
2.68
100.00
1.23
2.49
99.60
0.85
2.43
97.20
1.85
Number of replicates is 4. Sample 1 = Depaway (40 mg/tablet), Sample 2 = Cipram® (20 mg/tablet).
Table 6 Determination of citalopram in spiked urine and human serum using ionophore and IP SPE sensors Sample
Urine
Serum
Statistical parameters
Direct method
Electrode (V)
Mean recovery (%) N Variance RSD (%) Mean recovery (%) N Variance RSD (%)
98.95 5 0.94 0.58 98.99 5 0.67 0.75
Calibration graphs
Standard addition method
98.23 5 0.90 0.46 99.00 5 0.77 0.67
CPB alone and in pharmaceutical preparations or in biological fluids without fear of interferences caused by the excipients expected to be present in tablets or the constituents of body fluids. As shown in Table 7, the inter- and intra-day accuracy of the potentiometric method for CPB were ranged from 99.0% to 100.6%, 98.0% to 98.6% (SD = 0.12–0.45 and 0.12–0.74; and RSD from 0.9%–1.4 and 1.1%–2.4% for electrode (V) and electrode (X), respectively. The recovery of the corresponding amounts of CPB was obtained at concentration levels (1.00 ‒ 2.50 mg mL–1) in tablets and found to be ranged
99.00 5 0.83 0.79 99.45 5 0.59 0.62
Direct method
Calibration graphs
98.45 5 0.65 0.46 97.55 5 0.85 0.85
98.73 5 0.77 0.39 98.26 5 0.68 0.98
from 98.0% to 101.33% and 97.0% to 101.0% (SD = 0.19 to 0.65 and 0.06–0.69; and RSD from 0.79% to 1.38%, and 0.87 to 2.46 for electrode (V) and electrode (X), respectively.
4
Conclusions
Citalopram-screen printed electrodes on the basis of the KTpClPB ionophore, CP-PT ion pair and TCP as plasticizer was developed. Its linear range, slope and limit of detection were 4.9 × 10–7–1.0 × 10–2 M and 1.0 × 10–6–1.0 × 10–2 M, 60.47 and 59.93 mV decade–1, 4.9 × 10–7 M and 1 × 10–6 M for
Tamer Awad Ali et al. / Chinese Journal of Analytical Chemistry, 2014, 42(4): 565–572
Table 7
Intra- and Inter-days precision of the determination of CPB using the two types of electrodes with determination of pure and pharmaceutical tablet Electrodes type
Drug
Electrode (V) Pure form Electrode (X)
Depaway (40 mg/tablet)
Electrode (V) Electrode (X)
Taken (mg mL–1)
Intra day Found (mg mL–1)
Recovery (%)
1.98 3.52 4.47 1.96 3.45 4.43 0.98 1.52 1.98 1.01 1.47 1.94
99.00 100.57 99.33 98.00 98.59 98.44 98.00 101.33 99.00 101.00 98.00 97.00
2.00 3.50 4.50 2.00 3.50 4.50 1.00 1.50 2.00 1.00 1.50 2.00
SPE sensors (electrode (V) and electrode (X), respectively). The effect of pH on the potential response indicated that larger influence of pH occurred when pH of the solution was in the range of 2–9 and 2–8 for electrode (V) and electrode (X), respectively. The proposed electrodes were successfully applied to the determination of citalopram hydrobromide in pharmaceutical preparation, urine and human serum. This analytical method proved to be a simple, rapid and accurate method.
Inter day SD
RSD (%)
Found (mg mL–1)
Recovery (%)
SD
RSD (%)
0.12 0.45 0.26 0.52 0.74 0.14 0.19 0.65 0.37 0.06 0.32 0.69
1.02 0.89 1.34 1.08 1.45 2.42 1.38 0.97 1.17 0.87 1.79 2.46
1.97 3.49 4.51 1.94 3.42 4.45 1.02 1.48 1.96 0.97 1.51 1.95
98.50 99.71 100.22 97.00 97.77 98.88 102.00 98.66 98.00 97.00 100.66 97.50
0.23 0.15 0.09 0.67 1.12 0.39 0.07 0.33 0.91 1.21 0.05 1.17
0.75 1.12 0.93 1.26 2.96 0.75 1.83 1.49 2.36 2.27 0.88 2.33
[10] Aboul-Enein H Y, Sun X X, Sun C J. Sensors, 2002, 2: 424–431 [11] Farhadi K, Bahram M, Shokatynia D, Salehiyan F. Talanta, 2008, 76: 320–326 [12] Drozd J, Hopkala H. Desalination, 2004, 163: 119–125 [13] Salem A A, Barsoum B N, Izake E L. Anal. Chim. Acta, 2003, 498: 79–91 [14] García M S, Ortuño J A, Albero M I, Abuherba M S. Sensors, 2009, 9: 4309–4322 [15] Abdel-Fattah L, El-Kosasy A, Abdel-Aziz L, Gaied M. J. Am.
References
Sci., 2010, 6: 1115–1121 [16] Lenik J, Wardak C, Marczewska B. Acta Pol. Pharm., 2006,
[1]
Redmond A M, Harkin A, Kelly J P, Leonard B E. Eur. Neuropsychopharm., 1999, 9: 165–170
[2]
Raman B, Sharma B A, Ghugare P D, Karmuse P P, Kumar A. J. Pharmaceut. Biomed., 2010, 90: 377–383
[3]
Ma Z, Strecker R E, McKenna J T, Thakkar M M, McCarley R W, Tao R. Neuroscience, 2005, 135: 949–958
[4]
Smolders I, Clinckers R, Meurs A, Bundel D D, Portelli J, Ebinger G, Michotte Y. Neuropharmacology, 2008, 54: 1017–1028
[5]
Nguyen K Q, Tohyama Y, Watanabe A, Hasegawa S, Skelin I, Diksic M. Neurochem. Int., 2009, 54: 161–171
[6]
Weikop P, Yoshitake T, Kehr J. Eur. Neuropsychopharm., 2007, 17: 658–671
[7] [8]
[17] Mohamed G G, Ali T A, El-Shahat M F, Al-Sabagh A M, Migahed M A, Khaled E. Anal. Chim. Acta, 2010, 673: 79–87 [18] Khaled E, Mohamed G G, Awad T. Sensors and Actuators B, 2008, 135: 74–80 [19] Mohamed G G, Ali Tamer Awad, El-Shahat M F, Al-Sabagh A M, Migahed M A. Electroanal., 2010, 22: 2587–2599 [20] Mohamed G G, El-Shahat M F, Al-Sabagh A M, Migahed M A. Analyst, 2011, 136: 1488–1495 [21] Frag E Y Z, Mohamed G G, F.A. Nour El-Dien, Mohamed M E. Analyst, 2011, 136: 332–339 [22] Bakker E, Pretsch E, Buhlmann P. Anal. Chem., 2000, 72: 1127–1133
Williams S M, Bryan-Lluka L J, Pow D V. Brain. Res., 2005,
[23] Abbas M N, Zahran E. J. Electroanal. Chem., 2005, 576: 205–213
1042: 224–232
[24] Frag E Y Z, Ali T A, Mohamed G G, Awad Y H H. Int. J.
Dimeski G, Badrick T, John A S. Clin. Chim. Acta, 2010, 411: 309–317
[9]
63: 239–244
Faridbod F, Ganjali M R, Dinarvand R, Riahi S, Norouzi P, Olia M B A. J. Food Drug Anal., 2009, 17: 264–273
Electrochem. Sci., 2012, 7: 4443 – 4464 [25] Antropov L I. Theoretical Electrochemistry, Mir Publisher, Moscow, 1977 [26] Vytras K. Mikrochimica Acta, 1984(III): 139–148