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A batch injection analysis system with square-wave voltammetric detection for fast and simultaneous determination of naphazoline and zinc
Talanta 152 (2016) 308–313
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A batch injection analysis system with square-wave voltammetric detection for fast and simultaneous determination of naphazoline and zinc Thiago da Costa Oliveira, Jhonys Machado Freitas, Rodrigo Alejandro Abarza Munoz, Eduardo Mathias Richter n Instituto de Química, Universidade Federal de Uberlândia, Av. João Naves de Ávila, 2121 Uberlândia, MG, Brazil
art ic l e i nf o
a b s t r a c t
Article history: Received 4 January 2016 Received in revised form 11 February 2016 Accepted 12 February 2016 Available online 15 February 2016
In this work, a batch-injection analysis system with square-wave voltammetric (BIA–SWV) detection was applied for the first time to the simultaneous determination of inorganic (zinc) and organic (naphazoline) species. Both compounds were detected in a single run (70 injections h 1) with a small injection volume (∼100 mL). The calibration curves exhibited linear response range between 3.0 and 21.0 μmol L 1 (r ¼0.999) for naphazoline and between 10.0 and 60.0 μmol L 1 (r ¼0.992) for zinc. The detection limits were 0.13 and 0.04 μmol L 1 for zinc and naphazoline, respectively. Good reproducibility was achieved for multiple measurements of a solution containing both species (RSDo 1.0%; n ¼ 20). The results obtained with the BIA–SWV method for the simultaneous determination of naphazoline and zinc were compared to those obtained by HPLC (naphazoline) and by FAAS (zinc); no statistically significant differences were observed (95% confidence level). & 2016 Elsevier B.V. All rights reserved.
Keywords: Simultaneous analysis Inorganic and organic species Fast analysis Boron doped diamond Portable system
1. Introduction Naphazoline (NPZ) or [2-(1-naphtylmethyl)-2-imidazoline] is a sympathomimetic drug used in over-the-counter eye and nasal preparations [1]. The compound has vasoconstrictive and decongestive properties [2,3] and acts on β-adrenergic receptors in the arterioles of the conjunctiva to induce vasoconstriction, resulting in decreased conjunctival congestion. NPZ is often combined with zinc salts in pharmaceutical formulations. According to the literature, the astringent properties of zinc increase the rate at which the therapeutic effect of naphazoline is achieved without increasing its concentration (synergistic effect) [4]. The quantification of NPZ in different samples has been addressed in some reports. Various analytical techniques including potentiometry [5], voltammetry [6], photometry [7], thin layer chromatography [8], gas chromatography [9], high performance liquid chromatography [10,11], capillary electrophoresis [12–14], atomic absorption/emission spectrometry [15], fluorimetry [16] and phosphorimetry [17,18] have been reported for the determination of NPZ. In addition, a significant number of analytical techniques are suitable for the determination of Zn(II), such as spectrophotometry [19–22], atomic absorption spectroscopy [23– n
Corresponding author. E-mail address: [email protected] (E.M. Richter).
http://dx.doi.org/10.1016/j.talanta.2016.02.031 0039-9140/& 2016 Elsevier B.V. All rights reserved.
26], atomic emission spectroscopy (ICP-AES) [27–31], titrimetry [32,33], capillary electrophoresis [34], polarography [35], and neutron activation analysis [36]. Ideally, the quality control of pharmaceutical formulations containing more than one active ingredient should be carried out using only one analytical technique and in a single analytical run. However, to the best of our knowledge, there has been no previous work that reports the possibility of the simultaneous determination of NPZ and Zn. In recent years, boron-doped diamond (BDD) electrodes have attracted much interest due to their attractive properties, including low background currents, a wide working potential window, favorable electron-transfer kinetics, and surface inertness, which result in high resistance to deactivation. The electrode itself is resistant to fouling and can be reused many times [37,38]. As previously demonstrated by other research groups, Zn can be determined by stripping voltammetry using BDD as the working electrode [37,39,40]. Batch injection analysis (BIA) with electrochemical detection represents an alternative way of performing rapid assays even in on-site applications (portable characteristics) [41]. This system involves the injection of a sample plug through a micropipette tip directly onto the working electrode surface (wall–jet configuration) which is immersed in a large-volume blank solution [42,43]. The association of BIA with electrochemical techniques provides additional precision and rapidness for the development of
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electroanalytical methods. Moreover, BIA eliminates the need for a reproducible stirring condition in stripping analysis since the deposition step occurs simultaneously with sample injection by the micropipette, and then facilitates the development of portable analytical methods. Methods showing the association of BIA with stripping analysis employing conventional or electronic micropipettes have been described in the literature [44–49]. In this work, we demonstrate, for the first time, that a BIA system with square-wave voltammetric (BIA–SWV) detection can be used for the simultaneous determination of NPZ and Zn in pharmaceutical formulations. This unique method, with a single run, allowed for this analysis, which to date has only been possible using two different methods.
2. Material and methods 2.1. Reagents and samples Highly-pure deionized water (R4 18 MΩ cm) obtained from a Millipore Direct-Q3 water purification system (Bedford, MA, USA) was used to prepare all aqueous solutions. Acetic, phosphoric, hydrochloric and sulfuric acids, sodium hydroxide and metallic zinc were purchased from Synth (Diadema – Brazil), and naphazoline hydrochloride (NPZ) from Sigma-Aldrich (St. Louis, United States). All reagents were used without further purification. The Britton–Robinson 0.12 mol L 1 buffer solutions (pH 2.0–10.0) were prepared using 0.04 mol L 1 acetic, boric, and phosphoric acid solutions; the adjustment of pH was carried out using a 2.0 mol L 1 sodium hydroxide solution. The stock solution of Zn was prepared by dissolving 100 mg of metallic zinc in a mixture of 1.0 mL of water and 2.0 mL of HCl in a 100 mL calibrated flask. The solution was diluted to volume with water and the concentration of the standard solution was 1000 mg L 1. A stock solution of NPZ was freshly prepared just before the experiments by dissolution in water. Standard solutions were prepared by the dilution of stock solutions in the supporting electrolyte (0.05 mol L 1 sodium acetate buffer; pH 4.7). Two different pharmaceutical formulations (eye drops) containing NPZ and Zn (0.15 and 0.30 mg mL 1, respectively) were purchased from a local drugstore. An adequate amount of the solution was dissolved (under sonication for 3 min) in buffer solution (supporting electrolyte) for subsequent injection in the BIA system.
Fig. 1. Cyclic voltammograms of 1 mmol L 1 of NPZ obtained in: (A) different supporting electrolytes: (1) 0.1 mol L 1 H2SO4; (2) 0.05 mol L 1 acetate buffer solution (pH 4.7); (3) phosphate buffer solution (pH 2.0) and (4) phosphate buffer solution (pH 7.0); (B) 0.12 mol L 1 BR buffer solutions of different pH values (2.0– 10). Inset in (B) is the plot of Ep vs. pH. Working electrode: BDD; scan rate: 50 mV s 1; step potential: 5 mV.
2.2. Instrumentation and apparatus Electrochemical measurements using cyclic voltammetry and square-wave voltammetry were performed using a m-Autolab type III potentiostat (Metrohm Autolab B.V., the Netherlands) interfaced to a microcomputer and controlled by GPES 4.9.007 software. A mini Ag/AgCl (saturated KCl) [50] and platinum wire were employed as the reference and auxiliary electrodes, respectively. Boron-doped diamond (BDD) from Neocast SA, La Chaux-de-Fonts, Switzerland was used as the working electrode. The working electrode consisted of a thin film of BDD ( 1.2 μm thickness; boron doping level of 8000 ppm) deposited on a polycrystalline silicon wafer (7 7 mm) with a thickness of 1.0 mm. Prior to use for the first use (new electrode), the BDD electrode was anodically pre-treated by applying þ0.01 A for 1000 s in 0.12 mol L 1 Britton-Robinson buffer solution and then cathodically pretreated by applying 0.01 A for 1000 s in a 0.1 mol L 1 H2SO4 solution. These pretreatments are similar to that used in previously published studies [51,52]. After the first pretreatment, the BDD electrode was pretreated only cathodically once at the beginning of the workday. All results presented in the proposed work were performed with
Fig. 2. Proposed electrochemical oxidation mechanism of NPZ (pH¼ 4.7).
the same BDD electrode and without removal of the dissolved oxygen. The HPLC measurements (NPZ) were performed using a Shimadzu LC-10 VP equipped with an UV–vis detector (SPD-10AV), a LC column (Phenomenex 110 A° Gemini-C18, 250 mm 4.6 mm, 5 mm), a manual injector (20 mL) and a pump (LC-10AD-VP). The mobile phase consisted of a mixture (32:68, v/v) of methanol and 0.01 mol L 1 H3PO4 (pH adjusted to 2.8 by adding triethylamine). The detector was fixed at 280 nm. The flow rate was 1.0 mL min 1. A Varian SpectrAA 220 atomic absorption spectrometer was used for the zinc measurements. The following analytical parameters were used: slit width: 0.7 nm; wavelength: 213.9 nm; lamp
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Fig. 3. Scheme of the BIA–SWASV procedure used for the simultaneous determination of Zn and NPZ: (a) reference electrode, (b) auxiliary electrode, (c) pipette tip, (d) micro DC motor (for solution stirring). The stages of the measurement are: (A) conditioning step (the solution in the BIA cell stirred using the micro DC motor); (B) deposition step (the solution in the BIA cell not stirred and injection of 100 mL of the standard or sample solution is performed using an electronic pipette); (C) SWASV scanning (the solution in the BIA cell not stirred); and (D) the BIA system is ready for another injection (the solution inside the cell is stirred, and the injected aliquot is diluted by 1500 times).
Table 1 Optimized parameters for simultaneous determination of NPZ and Zn by BIA– SWASV. Electrochemical parameters Conditioning step Conditioning potential (V) Conditioning time (s)
Studied range
Optimized value
0.0–1.0
1.0
5–60
30
Deposition step
Deposition potential (V) Deposition time (s) Dispensing rate (mL s 1)
1.4 to −1.6 1.5 5–30 5 28–100 28
SWASV scanning
Initial potential (V) End potential (V)
1.4 to −1.6 1.5 þ1.6 to þ1.8 þ1.9 1–10 6 10–70 60 10–140 100
Step potential (mV) Amplitude (mV) Frequency (Hz)
current 10 mA; hollow cathode lamp (HCL): L1788-30NE; flame temperature: about 1700 K; flame type: C2H2/air – 1.25/ 5.25 L min 1 (adapted from [53]).
3. Results and discussion 3.1. Electrochemical behavior of NPZ In the initial studies, the electrochemical behavior of NPZ was investigated by cyclic voltammetry in different electrolyte solutions: 0.1 mol L 1 H2SO4 (pH 1.0), 0.05 mol L 1 acetate buffer (pH 4.7), and 0.1 mol L 1 phosphate buffers (pH 2.1 and 7.2). Fig. 1 shows the effects of the pH value and electrolyte composition on the electrochemical oxidation of NPZ. Voltammograms obtained at the BDD electrode presented an irreversible electrochemical behavior for NPZ in different electrolyte solutions (Fig. 1A), with an oxidation peak around þ 1.5 V (vs Ag/AgCl). The best results (peak shape and current intensity) were obtained using acetate buffer and H2SO4 as the supporting
Fig. 4. Square-wave voltammograms obtained for different concentrations of (A) Zn (10.0–60.0 mmol L 1) in the presence of 25.0 mmol L 1 NPZ and of (B) NPZ (5.0–25.0 mmol L 1) in the presence of 60.0 mmol L 1 Zn in acetate buffer solution (pH 4.7). Insets: (A) Calibration curve for Zn (r¼ 0.997); (B) Calibration curve for NPZ (r ¼ 0.998). Step potential: 6 mV; amplitude: 60 mV; frequency: 100 Hz; deposition potential: 1.5 V; deposition time: 5 s; dispensing rate: 28 mL s 1.
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less positive potential values with an increase in pH (Fig. 1B). The plot of peak potential vs. pH values presented a slope of 40 mV per pH unit (inset of Fig. 1B), which indicates that the same number of protons and electrons were involved in the electro-oxidation process for NPZ. The mechanism of the electrode reaction for NPZ was assigned to oxidation of imidazole group, consuming one electron and yielding a cation radical which, after losing a further proton, can undergo dimerization or polymerization [55,56]. Fig. 2 shows the proposed mechanism for the oxidation of NPZ. 3.2. Simultaneous determination of Zn and NPZ using BIA–SWASV
Fig. 5. Square-wave voltammetric curves obtained after the injection (n ¼3) of five standard solutions containing simultaneously increasing concentrations of Zn (10.0–50.0 mmol L 1) and NPZ (3.5–17.5 mmol L 1) in 0.05 mol L 1 acetate buffer solution (pH 4.7). Inset: (A) Zn and (B) NPZ analytical curves. Table 2 Analytical characteristics of the proposed BIA–SWASV method for simultaneous determination of NPZ and Zn (confidence interval ¼ 95%). Characteristic
Zn
NPZ
r Sensibility Intercept LOD (mmol L 1) LOQ (mmol L 1) AF (injections h 1) Intra-day RSD (n¼ 20) Inter-day RSD (n¼ 3)
0.992 0.1457 0.009 0.2047 0.030 0.126 0.420 70 r 1.0% 3.1%
0.999 0.0677 0.001 0.1397 0.012 0.040 0.132 70 r 1.0% 4.1%
r: correlation coefficient; LOD: limit of detection; AN: analytical frequency; RSD: relative standard deviation.
electrolytes. The effect of the potential scan rate on the oxidation peak current of NPZ was also evaluated by cyclic voltammetry using acetate buffer (pH 4.7) as the supporting electrolyte. The results show that the logarithm of the peak current was proportional to the logarithm of the scan rate (ν) over the 10 to 220 mV s 1 range, which indicated that electrode process was diffusion-controlled. Moreover, the value of αn could be calculated from the slope of Ep vs. log ν. In this system, the slope was 0.106, and the αn value was found to be 0.559. Generally, α is assumed to be 0.5 for organic molecules with irreversible electrode processes [54]. Thus, the number of electron transferred (n) in the electrooxidation of NPZ was calculated to be 1. The effect of pH on the voltammetric oxidation of NPZ was also investigated in 0.12 mol L 1 Britton-Robinson buffer solutions (in pH range from 2.0 to 10). The results show that the pH of the solution affects the peak potential (Ep) of NPZ, with the Ep shifting linearly toward
Recently, Honório et al. [40] developed a method for the determination of Zn by differential pulse anodic stripping voltammetry (DPASV) using BDD as the working electrode. In this work, the best performance for Zn determination was achieved when acetate buffer (pH 4.7) was used as the supporting electrolyte. As the oxidation of NPZ also presented good results in acetate buffer solution (Fig. 1A), this electrolyte was used in subsequent studies. Initially, attempts were made to perform the simultaneous determination of NPZ and Zn by employing a conventional batch type electrochemical cell (E10 mL) with square wave voltammetric detection and using the standard addition method. Zn was determined by stripping voltammetry (after a pre concentration step) and NPZ was determined by direct electrooxidation. Under these conditions, satisfactory results were obtained for NPZ determination; however, the results obtained for Zn were unstable and presented low accuracy. It is likely that the preconditioning step (usually required in stripping analysis to achieve reproducible responses) was not effective when NPZ was also present in the solution (standard addition method). To overcome this limitation, studies were performed using a BIA–SWASV system. Fig. 3 shows an illustration of the BIA–SWASV system and the scheme proposed for the simultaneous determination of NPZ and Zn. More details on the construction of the BIA cell can be found in a previous study [57]. In the procedure shown in Fig. 3, an aliquot of 100 mL of the standard or sample solution was used for the analysis. Basically, for each determination, the following steps are required: (1) first, a conditioning potential was applied for 30 s. During this step, the solution in the BIA cell is stirred; (2) Zn is deposited (by reduction) during the injection step (dispensing rate: 28 mL s 1; injection time: 3.6 s). During this step, the solution in the BIA cell is unstirred; (3) the potential was scanned from 1.5 to þ1.8 V; Zn is oxidized at 1.0 V and NPZ at þ1.4 V. As the standard or sample aliquot was slowly injected, the solution remains close to the electrode surface for a period after the end of the injection step [45,58] such that the NPZ determination is also possible (the solution in the BIA cell is unstirred); (4) the solution inside the cell is again stirred, and the injected aliquot is diluted by 1500 times (total volume of the cell E150 mL). A new analysis procedure can be performed sequentially. In the next step, the experimental parameters that affect the performance of the BIA–SWASV system were optimized using
Table 3 Comparison of results (mean7 RSD; n¼ 3) obtained for the simultaneous determination of Zn and NPZ in pharmaceutical samples using the proposed BIA–SWASV system versus FAAS (Zn) and HPLC (NPZ). Samples
Label value (mg mL 1) Zn
1 2
0.122 0.122
Found value (mg mL 1) NPZ
0.150 0.150
BIA
FAAS
HPLC
Zn
NPZ
Zn
NPZ
0.1247 0.003 0.1237 0.003
0.1537 0.004 0.1327 0.001
0.1207 0.001 0.1267 0.002
0.146 7 0.003 0.132 7 0.003
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acetate buffer solution (pH 4.7) as the supporting electrolyte. In the optimization of the SW parameters (step potential, amplitude and frequency), special attention was given to NPZ determination, because NPZ is at a lower concentration than Zn (e.g. 0.15:0.30 of NPZ:Zn, mg mL 1); the Zn concentration is at least two times higher than that of NPZ in commercial eye drops samples. In addition, the sensitivity of Zn detection (due to the prior preconcentration step) is better than that of NPZ (direct detection). The optimized values are given in Table 1. Under the optimized conditions, the interference of one analyte in the determination of the other was performed by changing one analyte concentration and keeping the concentration of the other constant. Fig. 4A shows the separate analysis of solutions containing increasing concentrations of Zn (10 to 60 mmol L 1) with the presence of NPZ at the fixed concentration of 25 mmol L 1. In this condition, the peak area of NPZ remained relatively constant (RSD ¼0.6%) and a good linearity (r¼ 0.997) was observed between Zn peak area and respective concentration. Fig. 4B shows the independent determination of NPZ in the concentration range of 5– 25 mmol L 1 with the presence of Zn at the fixed concentration of 60 mmol L 1. The peak area of Zn also remained relatively constant (RSD ¼2.1%) and good linearity (r ¼0.998) was observed between the NPZ peak area and the respective concentration. It can be concluded that variation of in the concentration of one studied compound did not present a significant influence on the peak current and peak potential of the other one. Next, the repeatability of the BDD response coupled to the BIA– SWASV system was checked by repetitive injections (n ¼20) of standard solutions containing 30 þ10 mmol L 1 and 60þ 20 mmol L 1 of Zn and NPZ, respectively (not shown). The RSD values were 0.5% (30 mmol L 1) and 1.0% (60 mmol L 1) for Zn, and 0.9% (10 mmol L 1) and 1.0% (20 mmol L 1) for NPZ. These results show that no memory effect occurs during the repeatability test, even after alternating injections of solutions with different concentrations. The inter-day repeatability was evaluated by measuring of peak current of solutions containing both analytes on three different days. The obtained RSD values were 4.1% and 3.1% for NPZ and Zn, respectively. The performance of the proposed BIA–SWASV method was also tested by determining both NPZ and Zn in two samples of eye drops from different suppliers by using the external calibration method. Fig. 5 presents the BIA–SWASV responses obtained for injections of 100 mL of standard solutions containing simultaneously increasing concentrations of Zn and NPZ and the respective calibration curves (inset). Linear behavior with good correlation coefficients ( 40.99) was observed from 10 to 50 mmol L 1 of Zn (slope value of 0.145 mA V L mmol 1) and from 3.5 to 17.5 mmol L 1 of NPZ (slope value of 0.067 mA V L mmol 1). The detection limits under the optimized conditions were 0.126 mmol L 1 and 0.040 mmol L 1 for Zn and NPZ, respectively. Table 2 shows the analytical characteristics of the proposed method. The performance of the proposed BIA–SWASV method was also evaluated through simultaneous determination of Zn and NPZ in two pharmaceutical samples. Table 3 lists the concentrations of Zn and NPZ found by the BIA– SWASV method. For comparison, the samples were also analyzed by FAAS (Zn) and HPLC (NPZ). No significant differences were observed between the values found for the contents of NPZ and Zn in the commercial pharmaceutical samples using the proposed method and FAAS (Zn) and HPLC (NPZ). At the 95% confidence level, the calculated t-test values were smaller than the critical value (2.78; n ¼ 3). In addition, recovery tests were performed through the analysis of commercial sample solutions without and with addition of known amounts NPZ and Zn. Recovery values (n ¼3) of 967 4% and 967 3% were obtained for NPZ and Zn, respectively, which also attests the
accuracy of the proposed method and showed the absence of interference from sample matrix.
4. Conclusion This work has demonstrated, for the first time, the application of a BIA system for the simultaneous determination of Zn (an inorganic compound) and NPZ (an organic compound). The proposed method is precise, accurate (confirmed by comparison tests and recovery studies), sensitive, fast (70 injections h 1) and environmentally friendly (minimum consumption of samples and solvents). Such notable analytical characteristics allow the replacement of two rather expensive instrumental techniques, such as HPLC and FAAS, for routine analysis of pharmaceutical dosage forms. Moreover, the method proposed here may be appropriate for use in laboratories with minimal infrastructure requirements or on-site analysis.
Acknowledgments The authors are grateful to FAPEMIG (APQ-02118-15), and CAPES (PRO FORENSES–Process number: 23038.007073/2014-12) for the financial support. EMR (Process number: 307333/2014-0) and RAAM (Process number: 308174/2013-5) thank CNPQ for the fellowship.
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A batch injection analysis system with square-wave voltammetric detection for fast and simultaneous determination of naphazoline and zinc Thiago da Costa Oliveira, Jhonys Machado Freitas, Rodrigo Alejandro Abarza Munoz, Eduardo Mathias Richter n Instituto de Química, Universidade Federal de Uberlândia, Av. João Naves de Ávila, 2121 Uberlândia, MG, Brazil
art ic l e i nf o
a b s t r a c t
Article history: Received 4 January 2016 Received in revised form 11 February 2016 Accepted 12 February 2016 Available online 15 February 2016
In this work, a batch-injection analysis system with square-wave voltammetric (BIA–SWV) detection was applied for the first time to the simultaneous determination of inorganic (zinc) and organic (naphazoline) species. Both compounds were detected in a single run (70 injections h 1) with a small injection volume (∼100 mL). The calibration curves exhibited linear response range between 3.0 and 21.0 μmol L 1 (r ¼0.999) for naphazoline and between 10.0 and 60.0 μmol L 1 (r ¼0.992) for zinc. The detection limits were 0.13 and 0.04 μmol L 1 for zinc and naphazoline, respectively. Good reproducibility was achieved for multiple measurements of a solution containing both species (RSDo 1.0%; n ¼ 20). The results obtained with the BIA–SWV method for the simultaneous determination of naphazoline and zinc were compared to those obtained by HPLC (naphazoline) and by FAAS (zinc); no statistically significant differences were observed (95% confidence level). & 2016 Elsevier B.V. All rights reserved.
Keywords: Simultaneous analysis Inorganic and organic species Fast analysis Boron doped diamond Portable system
1. Introduction Naphazoline (NPZ) or [2-(1-naphtylmethyl)-2-imidazoline] is a sympathomimetic drug used in over-the-counter eye and nasal preparations [1]. The compound has vasoconstrictive and decongestive properties [2,3] and acts on β-adrenergic receptors in the arterioles of the conjunctiva to induce vasoconstriction, resulting in decreased conjunctival congestion. NPZ is often combined with zinc salts in pharmaceutical formulations. According to the literature, the astringent properties of zinc increase the rate at which the therapeutic effect of naphazoline is achieved without increasing its concentration (synergistic effect) [4]. The quantification of NPZ in different samples has been addressed in some reports. Various analytical techniques including potentiometry [5], voltammetry [6], photometry [7], thin layer chromatography [8], gas chromatography [9], high performance liquid chromatography [10,11], capillary electrophoresis [12–14], atomic absorption/emission spectrometry [15], fluorimetry [16] and phosphorimetry [17,18] have been reported for the determination of NPZ. In addition, a significant number of analytical techniques are suitable for the determination of Zn(II), such as spectrophotometry [19–22], atomic absorption spectroscopy [23– n
Corresponding author. E-mail address: [email protected] (E.M. Richter).
http://dx.doi.org/10.1016/j.talanta.2016.02.031 0039-9140/& 2016 Elsevier B.V. All rights reserved.
26], atomic emission spectroscopy (ICP-AES) [27–31], titrimetry [32,33], capillary electrophoresis [34], polarography [35], and neutron activation analysis [36]. Ideally, the quality control of pharmaceutical formulations containing more than one active ingredient should be carried out using only one analytical technique and in a single analytical run. However, to the best of our knowledge, there has been no previous work that reports the possibility of the simultaneous determination of NPZ and Zn. In recent years, boron-doped diamond (BDD) electrodes have attracted much interest due to their attractive properties, including low background currents, a wide working potential window, favorable electron-transfer kinetics, and surface inertness, which result in high resistance to deactivation. The electrode itself is resistant to fouling and can be reused many times [37,38]. As previously demonstrated by other research groups, Zn can be determined by stripping voltammetry using BDD as the working electrode [37,39,40]. Batch injection analysis (BIA) with electrochemical detection represents an alternative way of performing rapid assays even in on-site applications (portable characteristics) [41]. This system involves the injection of a sample plug through a micropipette tip directly onto the working electrode surface (wall–jet configuration) which is immersed in a large-volume blank solution [42,43]. The association of BIA with electrochemical techniques provides additional precision and rapidness for the development of
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electroanalytical methods. Moreover, BIA eliminates the need for a reproducible stirring condition in stripping analysis since the deposition step occurs simultaneously with sample injection by the micropipette, and then facilitates the development of portable analytical methods. Methods showing the association of BIA with stripping analysis employing conventional or electronic micropipettes have been described in the literature [44–49]. In this work, we demonstrate, for the first time, that a BIA system with square-wave voltammetric (BIA–SWV) detection can be used for the simultaneous determination of NPZ and Zn in pharmaceutical formulations. This unique method, with a single run, allowed for this analysis, which to date has only been possible using two different methods.
2. Material and methods 2.1. Reagents and samples Highly-pure deionized water (R4 18 MΩ cm) obtained from a Millipore Direct-Q3 water purification system (Bedford, MA, USA) was used to prepare all aqueous solutions. Acetic, phosphoric, hydrochloric and sulfuric acids, sodium hydroxide and metallic zinc were purchased from Synth (Diadema – Brazil), and naphazoline hydrochloride (NPZ) from Sigma-Aldrich (St. Louis, United States). All reagents were used without further purification. The Britton–Robinson 0.12 mol L 1 buffer solutions (pH 2.0–10.0) were prepared using 0.04 mol L 1 acetic, boric, and phosphoric acid solutions; the adjustment of pH was carried out using a 2.0 mol L 1 sodium hydroxide solution. The stock solution of Zn was prepared by dissolving 100 mg of metallic zinc in a mixture of 1.0 mL of water and 2.0 mL of HCl in a 100 mL calibrated flask. The solution was diluted to volume with water and the concentration of the standard solution was 1000 mg L 1. A stock solution of NPZ was freshly prepared just before the experiments by dissolution in water. Standard solutions were prepared by the dilution of stock solutions in the supporting electrolyte (0.05 mol L 1 sodium acetate buffer; pH 4.7). Two different pharmaceutical formulations (eye drops) containing NPZ and Zn (0.15 and 0.30 mg mL 1, respectively) were purchased from a local drugstore. An adequate amount of the solution was dissolved (under sonication for 3 min) in buffer solution (supporting electrolyte) for subsequent injection in the BIA system.
Fig. 1. Cyclic voltammograms of 1 mmol L 1 of NPZ obtained in: (A) different supporting electrolytes: (1) 0.1 mol L 1 H2SO4; (2) 0.05 mol L 1 acetate buffer solution (pH 4.7); (3) phosphate buffer solution (pH 2.0) and (4) phosphate buffer solution (pH 7.0); (B) 0.12 mol L 1 BR buffer solutions of different pH values (2.0– 10). Inset in (B) is the plot of Ep vs. pH. Working electrode: BDD; scan rate: 50 mV s 1; step potential: 5 mV.
2.2. Instrumentation and apparatus Electrochemical measurements using cyclic voltammetry and square-wave voltammetry were performed using a m-Autolab type III potentiostat (Metrohm Autolab B.V., the Netherlands) interfaced to a microcomputer and controlled by GPES 4.9.007 software. A mini Ag/AgCl (saturated KCl) [50] and platinum wire were employed as the reference and auxiliary electrodes, respectively. Boron-doped diamond (BDD) from Neocast SA, La Chaux-de-Fonts, Switzerland was used as the working electrode. The working electrode consisted of a thin film of BDD ( 1.2 μm thickness; boron doping level of 8000 ppm) deposited on a polycrystalline silicon wafer (7 7 mm) with a thickness of 1.0 mm. Prior to use for the first use (new electrode), the BDD electrode was anodically pre-treated by applying þ0.01 A for 1000 s in 0.12 mol L 1 Britton-Robinson buffer solution and then cathodically pretreated by applying 0.01 A for 1000 s in a 0.1 mol L 1 H2SO4 solution. These pretreatments are similar to that used in previously published studies [51,52]. After the first pretreatment, the BDD electrode was pretreated only cathodically once at the beginning of the workday. All results presented in the proposed work were performed with
Fig. 2. Proposed electrochemical oxidation mechanism of NPZ (pH¼ 4.7).
the same BDD electrode and without removal of the dissolved oxygen. The HPLC measurements (NPZ) were performed using a Shimadzu LC-10 VP equipped with an UV–vis detector (SPD-10AV), a LC column (Phenomenex 110 A° Gemini-C18, 250 mm 4.6 mm, 5 mm), a manual injector (20 mL) and a pump (LC-10AD-VP). The mobile phase consisted of a mixture (32:68, v/v) of methanol and 0.01 mol L 1 H3PO4 (pH adjusted to 2.8 by adding triethylamine). The detector was fixed at 280 nm. The flow rate was 1.0 mL min 1. A Varian SpectrAA 220 atomic absorption spectrometer was used for the zinc measurements. The following analytical parameters were used: slit width: 0.7 nm; wavelength: 213.9 nm; lamp
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Fig. 3. Scheme of the BIA–SWASV procedure used for the simultaneous determination of Zn and NPZ: (a) reference electrode, (b) auxiliary electrode, (c) pipette tip, (d) micro DC motor (for solution stirring). The stages of the measurement are: (A) conditioning step (the solution in the BIA cell stirred using the micro DC motor); (B) deposition step (the solution in the BIA cell not stirred and injection of 100 mL of the standard or sample solution is performed using an electronic pipette); (C) SWASV scanning (the solution in the BIA cell not stirred); and (D) the BIA system is ready for another injection (the solution inside the cell is stirred, and the injected aliquot is diluted by 1500 times).
Table 1 Optimized parameters for simultaneous determination of NPZ and Zn by BIA– SWASV. Electrochemical parameters Conditioning step Conditioning potential (V) Conditioning time (s)
Studied range
Optimized value
0.0–1.0
1.0
5–60
30
Deposition step
Deposition potential (V) Deposition time (s) Dispensing rate (mL s 1)
1.4 to −1.6 1.5 5–30 5 28–100 28
SWASV scanning
Initial potential (V) End potential (V)
1.4 to −1.6 1.5 þ1.6 to þ1.8 þ1.9 1–10 6 10–70 60 10–140 100
Step potential (mV) Amplitude (mV) Frequency (Hz)
current 10 mA; hollow cathode lamp (HCL): L1788-30NE; flame temperature: about 1700 K; flame type: C2H2/air – 1.25/ 5.25 L min 1 (adapted from [53]).
3. Results and discussion 3.1. Electrochemical behavior of NPZ In the initial studies, the electrochemical behavior of NPZ was investigated by cyclic voltammetry in different electrolyte solutions: 0.1 mol L 1 H2SO4 (pH 1.0), 0.05 mol L 1 acetate buffer (pH 4.7), and 0.1 mol L 1 phosphate buffers (pH 2.1 and 7.2). Fig. 1 shows the effects of the pH value and electrolyte composition on the electrochemical oxidation of NPZ. Voltammograms obtained at the BDD electrode presented an irreversible electrochemical behavior for NPZ in different electrolyte solutions (Fig. 1A), with an oxidation peak around þ 1.5 V (vs Ag/AgCl). The best results (peak shape and current intensity) were obtained using acetate buffer and H2SO4 as the supporting
Fig. 4. Square-wave voltammograms obtained for different concentrations of (A) Zn (10.0–60.0 mmol L 1) in the presence of 25.0 mmol L 1 NPZ and of (B) NPZ (5.0–25.0 mmol L 1) in the presence of 60.0 mmol L 1 Zn in acetate buffer solution (pH 4.7). Insets: (A) Calibration curve for Zn (r¼ 0.997); (B) Calibration curve for NPZ (r ¼ 0.998). Step potential: 6 mV; amplitude: 60 mV; frequency: 100 Hz; deposition potential: 1.5 V; deposition time: 5 s; dispensing rate: 28 mL s 1.
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less positive potential values with an increase in pH (Fig. 1B). The plot of peak potential vs. pH values presented a slope of 40 mV per pH unit (inset of Fig. 1B), which indicates that the same number of protons and electrons were involved in the electro-oxidation process for NPZ. The mechanism of the electrode reaction for NPZ was assigned to oxidation of imidazole group, consuming one electron and yielding a cation radical which, after losing a further proton, can undergo dimerization or polymerization [55,56]. Fig. 2 shows the proposed mechanism for the oxidation of NPZ. 3.2. Simultaneous determination of Zn and NPZ using BIA–SWASV
Fig. 5. Square-wave voltammetric curves obtained after the injection (n ¼3) of five standard solutions containing simultaneously increasing concentrations of Zn (10.0–50.0 mmol L 1) and NPZ (3.5–17.5 mmol L 1) in 0.05 mol L 1 acetate buffer solution (pH 4.7). Inset: (A) Zn and (B) NPZ analytical curves. Table 2 Analytical characteristics of the proposed BIA–SWASV method for simultaneous determination of NPZ and Zn (confidence interval ¼ 95%). Characteristic
Zn
NPZ
r Sensibility Intercept LOD (mmol L 1) LOQ (mmol L 1) AF (injections h 1) Intra-day RSD (n¼ 20) Inter-day RSD (n¼ 3)
0.992 0.1457 0.009 0.2047 0.030 0.126 0.420 70 r 1.0% 3.1%
0.999 0.0677 0.001 0.1397 0.012 0.040 0.132 70 r 1.0% 4.1%
r: correlation coefficient; LOD: limit of detection; AN: analytical frequency; RSD: relative standard deviation.
electrolytes. The effect of the potential scan rate on the oxidation peak current of NPZ was also evaluated by cyclic voltammetry using acetate buffer (pH 4.7) as the supporting electrolyte. The results show that the logarithm of the peak current was proportional to the logarithm of the scan rate (ν) over the 10 to 220 mV s 1 range, which indicated that electrode process was diffusion-controlled. Moreover, the value of αn could be calculated from the slope of Ep vs. log ν. In this system, the slope was 0.106, and the αn value was found to be 0.559. Generally, α is assumed to be 0.5 for organic molecules with irreversible electrode processes [54]. Thus, the number of electron transferred (n) in the electrooxidation of NPZ was calculated to be 1. The effect of pH on the voltammetric oxidation of NPZ was also investigated in 0.12 mol L 1 Britton-Robinson buffer solutions (in pH range from 2.0 to 10). The results show that the pH of the solution affects the peak potential (Ep) of NPZ, with the Ep shifting linearly toward
Recently, Honório et al. [40] developed a method for the determination of Zn by differential pulse anodic stripping voltammetry (DPASV) using BDD as the working electrode. In this work, the best performance for Zn determination was achieved when acetate buffer (pH 4.7) was used as the supporting electrolyte. As the oxidation of NPZ also presented good results in acetate buffer solution (Fig. 1A), this electrolyte was used in subsequent studies. Initially, attempts were made to perform the simultaneous determination of NPZ and Zn by employing a conventional batch type electrochemical cell (E10 mL) with square wave voltammetric detection and using the standard addition method. Zn was determined by stripping voltammetry (after a pre concentration step) and NPZ was determined by direct electrooxidation. Under these conditions, satisfactory results were obtained for NPZ determination; however, the results obtained for Zn were unstable and presented low accuracy. It is likely that the preconditioning step (usually required in stripping analysis to achieve reproducible responses) was not effective when NPZ was also present in the solution (standard addition method). To overcome this limitation, studies were performed using a BIA–SWASV system. Fig. 3 shows an illustration of the BIA–SWASV system and the scheme proposed for the simultaneous determination of NPZ and Zn. More details on the construction of the BIA cell can be found in a previous study [57]. In the procedure shown in Fig. 3, an aliquot of 100 mL of the standard or sample solution was used for the analysis. Basically, for each determination, the following steps are required: (1) first, a conditioning potential was applied for 30 s. During this step, the solution in the BIA cell is stirred; (2) Zn is deposited (by reduction) during the injection step (dispensing rate: 28 mL s 1; injection time: 3.6 s). During this step, the solution in the BIA cell is unstirred; (3) the potential was scanned from 1.5 to þ1.8 V; Zn is oxidized at 1.0 V and NPZ at þ1.4 V. As the standard or sample aliquot was slowly injected, the solution remains close to the electrode surface for a period after the end of the injection step [45,58] such that the NPZ determination is also possible (the solution in the BIA cell is unstirred); (4) the solution inside the cell is again stirred, and the injected aliquot is diluted by 1500 times (total volume of the cell E150 mL). A new analysis procedure can be performed sequentially. In the next step, the experimental parameters that affect the performance of the BIA–SWASV system were optimized using
Table 3 Comparison of results (mean7 RSD; n¼ 3) obtained for the simultaneous determination of Zn and NPZ in pharmaceutical samples using the proposed BIA–SWASV system versus FAAS (Zn) and HPLC (NPZ). Samples
Label value (mg mL 1) Zn
1 2
0.122 0.122
Found value (mg mL 1) NPZ
0.150 0.150
BIA
FAAS
HPLC
Zn
NPZ
Zn
NPZ
0.1247 0.003 0.1237 0.003
0.1537 0.004 0.1327 0.001
0.1207 0.001 0.1267 0.002
0.146 7 0.003 0.132 7 0.003
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acetate buffer solution (pH 4.7) as the supporting electrolyte. In the optimization of the SW parameters (step potential, amplitude and frequency), special attention was given to NPZ determination, because NPZ is at a lower concentration than Zn (e.g. 0.15:0.30 of NPZ:Zn, mg mL 1); the Zn concentration is at least two times higher than that of NPZ in commercial eye drops samples. In addition, the sensitivity of Zn detection (due to the prior preconcentration step) is better than that of NPZ (direct detection). The optimized values are given in Table 1. Under the optimized conditions, the interference of one analyte in the determination of the other was performed by changing one analyte concentration and keeping the concentration of the other constant. Fig. 4A shows the separate analysis of solutions containing increasing concentrations of Zn (10 to 60 mmol L 1) with the presence of NPZ at the fixed concentration of 25 mmol L 1. In this condition, the peak area of NPZ remained relatively constant (RSD ¼0.6%) and a good linearity (r¼ 0.997) was observed between Zn peak area and respective concentration. Fig. 4B shows the independent determination of NPZ in the concentration range of 5– 25 mmol L 1 with the presence of Zn at the fixed concentration of 60 mmol L 1. The peak area of Zn also remained relatively constant (RSD ¼2.1%) and good linearity (r ¼0.998) was observed between the NPZ peak area and the respective concentration. It can be concluded that variation of in the concentration of one studied compound did not present a significant influence on the peak current and peak potential of the other one. Next, the repeatability of the BDD response coupled to the BIA– SWASV system was checked by repetitive injections (n ¼20) of standard solutions containing 30 þ10 mmol L 1 and 60þ 20 mmol L 1 of Zn and NPZ, respectively (not shown). The RSD values were 0.5% (30 mmol L 1) and 1.0% (60 mmol L 1) for Zn, and 0.9% (10 mmol L 1) and 1.0% (20 mmol L 1) for NPZ. These results show that no memory effect occurs during the repeatability test, even after alternating injections of solutions with different concentrations. The inter-day repeatability was evaluated by measuring of peak current of solutions containing both analytes on three different days. The obtained RSD values were 4.1% and 3.1% for NPZ and Zn, respectively. The performance of the proposed BIA–SWASV method was also tested by determining both NPZ and Zn in two samples of eye drops from different suppliers by using the external calibration method. Fig. 5 presents the BIA–SWASV responses obtained for injections of 100 mL of standard solutions containing simultaneously increasing concentrations of Zn and NPZ and the respective calibration curves (inset). Linear behavior with good correlation coefficients ( 40.99) was observed from 10 to 50 mmol L 1 of Zn (slope value of 0.145 mA V L mmol 1) and from 3.5 to 17.5 mmol L 1 of NPZ (slope value of 0.067 mA V L mmol 1). The detection limits under the optimized conditions were 0.126 mmol L 1 and 0.040 mmol L 1 for Zn and NPZ, respectively. Table 2 shows the analytical characteristics of the proposed method. The performance of the proposed BIA–SWASV method was also evaluated through simultaneous determination of Zn and NPZ in two pharmaceutical samples. Table 3 lists the concentrations of Zn and NPZ found by the BIA– SWASV method. For comparison, the samples were also analyzed by FAAS (Zn) and HPLC (NPZ). No significant differences were observed between the values found for the contents of NPZ and Zn in the commercial pharmaceutical samples using the proposed method and FAAS (Zn) and HPLC (NPZ). At the 95% confidence level, the calculated t-test values were smaller than the critical value (2.78; n ¼ 3). In addition, recovery tests were performed through the analysis of commercial sample solutions without and with addition of known amounts NPZ and Zn. Recovery values (n ¼3) of 967 4% and 967 3% were obtained for NPZ and Zn, respectively, which also attests the
accuracy of the proposed method and showed the absence of interference from sample matrix.
4. Conclusion This work has demonstrated, for the first time, the application of a BIA system for the simultaneous determination of Zn (an inorganic compound) and NPZ (an organic compound). The proposed method is precise, accurate (confirmed by comparison tests and recovery studies), sensitive, fast (70 injections h 1) and environmentally friendly (minimum consumption of samples and solvents). Such notable analytical characteristics allow the replacement of two rather expensive instrumental techniques, such as HPLC and FAAS, for routine analysis of pharmaceutical dosage forms. Moreover, the method proposed here may be appropriate for use in laboratories with minimal infrastructure requirements or on-site analysis.
Acknowledgments The authors are grateful to FAPEMIG (APQ-02118-15), and CAPES (PRO FORENSES–Process number: 23038.007073/2014-12) for the financial support. EMR (Process number: 307333/2014-0) and RAAM (Process number: 308174/2013-5) thank CNPQ for the fellowship.
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