Differential Pulse Voltammetric Simultaneous Determination of Paracetamol and Omnipaque on Boron Doped Diamond Electrode: Application to Natural Tomato, Carrot, Cucumber Juices and Wastewater ()
1. Introduction
Numerous pharmaceutical molecules are increasingly consumed throughout the world, particularly in Côte d’Ivoire, and are liable to contaminate environmental waters (surface water, groundwater, etc.). A review of the literature has shown that pharmaceuticals are present in various aqueous matrices such as influents and effluents from wastewater treatment plants, surface water, groundwater, marine water and drinking water [1] [2] .
Among the pharmaceutical products present in water are paracetamol and omnipak, an iodinated contrast agent commonly known as iohexol.
Paracetamol (PCM) or acetaminophen (C8H9NO2) is a widely used analgesic and antipyretic. It is well tolerated and does not have the side effects of aspirin.
It is therefore commonly used for the relief of fever, headaches and minor pain, as well as for the management of more severe pain, where lower doses than other non-steroidal anti-inflammatory drugs are permitted, minimizing overall side effects [3] [4] .
Omnipaque (OMP) or iohexol (C19H26I3N3O9), like most contrast agents, is not biodegradable and highly soluble in water, in neutral and acidic media [5] . Iohexol is an iodine X-ray contrast media member, a class of drugs mainly used to enhance soft tissue imaging in diagnostic tests [6] . Consumption of iodine X-ray contrasts can reach 3.5 million kilograms in one year worldwide [7] .
Humans can be treated with up to 200 g of iohexol per day [8] and consume up to 4 g of paracetamol per day [9] . Humans cannot digest these pharmaceutical products in a short space of time, and most of them are eliminated in the urine and faeces.
Persistent products like OMP and PCM [5] [10] are a danger to the aquatic environment and our environment in general. Duirk and al. showed in a 2011 study that iodine contrasts were frequently detected in raw water at a concentration of 2.7 μg/L [8] and Nikoulaou and al showed so in a 2007 study the presence of trace paracetamol in drinking water [11] .
Because of their preference in clinical practice, paracetamol and iohexol are increasingly present either individually or simultaneously in the aquatic environment in the form of used products, products not consumed or products released by the human body. To protect this environment, paracetamol and iohexol must be detected, quantified and degraded individually or simultaneously before being released into the aqueous environment (wastewater, groundwater, etc.)
It is therefore essential to find techniques or implement simple, rapid, effective and inexpensive methods for the individual and simultaneous detection and quantification of these discharges of pharmaceutical products, which become bioresistant pollutants in our soil and water. These pollutants then contaminate the plants and animals in our environment, leading to a growing risk of human intoxication.
Analytical methods have been developed for the individual determination of PCM and OMP, as well as for the simultaneous determination of these analytes with other pharmaceutical and biologically active compounds. These include chromatography [12] [13] [14] [15] , spectrophotometry [16] [17] [18] and electroanalytical methods [19] - [27] . However, very little work has been done with a method for the simultaneous voltammetric determination of PCM and OMP in pharmaceutical dosage forms or biological fluids.
Voltammetric methods, in particular differential pulse voltammetry (DPV), are considered an important electrochemical technique used in the development of analytical procedures for the individual and/or simultaneous determination of a wide range of compounds of pharmaceutical interest because they provide reliable, rapid, sensitive, precise, accurate and simple results [28] [29] .
Boron-doped diamond electrode (BDDE) has been widely used in electroanalytical applications for pharmaceutical compounds [24] [30] [31] [32] [33] [34] , because of unique properties, such as a very wide working potential window, very low and stable background current, long term stability, low sensitivity to dissolved oxygen, and an extreme electrochemical stability in both alkaline and acidic media [35] . In this work, the development of a reliable, low cost and selective method based on DPV using a cathodically pretreated boron-doped diamond electrode for the individual and simultaneous determination of PCM and OMP in pharmaceutical formulations has been proposed.
2. Experimental Method
2.1. Reagents and Solutions
The sulfuric acid (H2SO4, 95% - 98%) was purchased from SIGMA-ALDRICH, USA, and the perchloric acid (HClO4, 60%) solution was obtained from Panreac, GERMANY. Paracetamol 500 mg and Iohexol (OmnipaqueTM 300 mg I/mL) were supplied respectively by the laboratories of BAILY-CREAT (France) and GE Healthcare (France). A stock solution of paracetamol was prepared by dissolving an accurate mass of the drug in an appropriate volume of HClO4. Stocks of paracetamol and omnipaque solutions were prepared by disolving a precise quantity of paracetamol or omnipaque in a certain volume of perchloric acid (HClO4 0.1 M).
The working solutions for the voltammetric investigations were prepared by dilution of stocks solutions. All solutions were protected from light and used within 24 h to avoid decomposition. 0.1 M of HClO4 is used to prepare the supporting electrolytes. Distilled water was used to prepare all the solutions.
2.2. Equipment and Electrodes
The voltammetric measurements were performed using an AUTOLAB PGSTAT 20 (Ecochemie) connected to a potentiostat equipped with a USB electrochemical interface. This system is connected to a three-electrode single compartment glass cell and a computer for data storage and processing. A GPES 4 software was employed to get the voltammograms. The glass electrochemical cell consisted of saturated calomel electrode (SCE) and platinum wire as reference and counter electrode respectively. BDD electrode was used as working electrode. The surface of the BDD in contact with the electrolyte is 1 cm2. All the pH values were measured with pH meter. All the potentials reported in this paper were given against (SCE) electrode at an ambient temperature of 25˚C.
2.3. Analytical Procedures
Analytical curves in the supporting electrolyte (HClO4) were constructed and used to produce a calibration curve. Simultaneous DPV measurements were carried out, for iohexol concentrations varying alone up to 262.6 µM with that of fixed PCM, PCM concentrations varying alone up to 210 µM with that of fixed OMP, PCM and OMP concentrations varying.
Prior to the experiments, the BDD electrode was electrochemically pretreated in a 0.5 mol/L H2SO4 solution. For this pre-treatment, an anodic pre-treatment (+2 V, 15 s) is followed by a cathodic pre-treatment (−2 V, 90 s). In this way, the BDD surface was cleaned of all impurities, then rendered predominantly hydrogenated [36] .
By the metered addition method, suitable quantities of PCM and/or OMP were taken from the stock solutions and introduced into the measuring cell containing 100 ml of the carrier.
After stirring, with a magnetized bar, to homogenize the mixture obtained, DPV curves were recorded at each test.
The calibration curves for each of the species were then represented for each series of experiments.
The detection limits (LOD) and quantification limits (LOQ) of PCM and OMP were then calculated from the following equations: LOD = 3 * SD/b and LOQ = 10 * SD/b with SD the standard deviation and b the slope of the calibration curve.
3. Results and Discussion
3.1. Electrochemical Behavior of PCM and OMP
In order to determine the best conditions for studying the electrochemical behavior of PCM and OMP on the boron-doped diamond electrode (BDDE), our previous work [37] enabled us to choose the perchloric acid solution with a concentration of 0.1 M at pH = 0.217 as the support medium in which the best voltammetric responses are obtained.
The electrochemical behavior of PCM and OMP on BDDE has been studied by CV (Figure 1) respectively illustrates the cyclic voltammetric profile of the electrochemical oxidation of the PCM with a concentration of 2 g/L (curve (b)) and of the OMP (curve (c)) with a concentration of 4 g/L in a 0.1 M HClO4 solution.
As can be seen, PCM and OMP have only a well-defined oxidation peak at 0.8 V/SCE (curve (b)), 0.6 V/SCE (curve (c)), respectively, without the true presence of a cathode peak. It is clear that the electrochemical reactions of these compounds at the BDDE level are irreversible. These curves were recorded under the pretreatment conditions indicated above, in order to have better resolutions of the oxidation peaks of the compounds studied.
On the same Figure 1, curve (d) shows the simultaneous behavior of the PCM and of the OMP on the BDDE in the same support electrolyte. Two oxidation peaks are observed at very distinct potentials, that is to say 0.8 V/SCE and 0.6 V/SCE respectively as previously.
The good resolution and the good separation of these oxidation peaks clearly show that studies of detection and quantification of PCM and OMP can be carried out simultaneously in the reaction medium on the surface of our BDDE without interference.
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Figure 1. Cv voltammograms of BDD electrode in HClO4 0.1 M electrolytes: (a) blank , (b) only 2 g/L of PCM , (c) only 4 g/L of OMP , (d) 2 g/L of PCM + 4 g/L of OMP : Working electrode: DDB (1 cm2), Counter electrode: Pt wound, Reference electrode: SCE. Scanning rate: 50 mV/s.
3.2. Individual and Simultaneous Determination of PCM and OMP
Before recording the voltammograms with differential pulses for the individual or simultaneous determination of the PCM and of the OMP using a BDDE pretreated anodially then cathodically, the influence of the key parameters in DPV on the peak potential and the peak current were studied in the following ranges: pretreatment potential (−2.5 ≤ Ep ≤ 3 V/SCE); pretreatment time (15 ≤ tEp ≤ 300 s); modulation amplitude (0.01 ≤ AM ≤ 0.5 V/SCE); modulation time (0.005 ≤ tM ≤ 0.25 s); step potential (0.001 ≤ Ep ≤ 0.02 V/SCE). The optimized parameters obtained and used in this work are therefore: Ep = −2 V/SCE, tEp = 90 s, AM = 0.1 V/SCE, tM = 0.05 s and Ep = 0.007 V/SCE [37] .
3.3. Individual Determination of PCM and OMP
Figure 2(a) shows the voltammetric response of the oxidation peaks for each of the concentrations of PCM ranging from 0 μΜ to 13.87 μΜ. Voltamograms recorded under the optimum conditions obtained show that the oxidation peaks of the PCM are all observed at the same potential (E = 0.7 V/SCE), whatever its concentrations. Figure 2(b) shows the intensity of the oxidation peak current of the PCM increases linearly with paracetamol concentration. The limit of detection and quantification are 0.167 μΜ and 0.559 μΜ, respectively, using the equations LOD = 3 SD/b and LOQ = 10 SD/b.
Three series of independent measurements, each with five (05) different concentrations of paracetamol, were carried out and used to determine the recovery rate, which is close to 98%. A calibration curve of R2 = 0.9999 was obtained [37] .
All these results show the reliability, reproducibility, sensitivity and selectivity of the method.
In Figure 3(a), the voltamograms recorded under the same optimum conditions show that the oxidation peaks of the OMP are all observed at the same potential (E = 0.5 V/SCE) for the concentrations varying from 0 to 52.43 μΜ. An increase in the intensity of the oxidation peak current of the OMP with its concentration is observed in Figure 3(b), giving a linear calibration curve of R2 = 0.9996. Applying the same experimental conditions, the calculated limit of detection and quantification are 0.77 µM and 2.57 µM respectively with recovery rates ranging from 98.7% to 100.7%.
3.4. Simultaneous Determination of PCM and OMP
For the simultaneous determination of OMP and PCM, DPV were performed for their mixture keeping the concentration of one species constant while changing that of the other. Thus, the separate determination of the OMP in the concentration range 71.63 - 262.6 µM was carried out in solutions containing PCM at the fixed concentration of 63.91 µM, using BDDE (Figure 4(a)). Then, the concentration of the OMP was fixed at 115.4 µM and that of the PCM varied in the range from 62.71 to 210 µM (Figure 4(b)). Examination of Figure 4(a) makes it possible to conclude that the oxidation peak current of the OMP
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(b)
Figure 2. (a) Differential pulse voltammograms at different concentrations of paracetamol in HClO4 solution on BDD electrode by DPV; the concentration varies from 0 µM to 19.8 µM, (b) Calibration curve of method.
increases regularly as its concentration increases in the presence of a fixed concentration of PCM. Similarly, the oxidation peak current for the PCM increases regularly as its concentration increases in the presence of a fixed concentration of OMP, as shown in Figure 4(b). In addition, the slope of the linear regression line for the calibration graph for each species in the mixture was almost the same as for the individual calibration graph, concluding that these two species do not interfere in the determination of the other.
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Figure 3. (a) Differential pulse voltammograms at different concentrations of omnipaque in HClO4 solution on BDD electrode by DPV; the concentration varies from 0 µM to 52.43 µM, (b) Calibration curve of method.
The performance of the boron-doped diamond electrode was evaluated by varying the concentrations of OMP and PCM. Peak currents increased linearly with OMP concentrations in the range of 259.8 to 467.2 µM and PCM concentrations in the range of 58.73 to 116.3 µM with good correlation coefficients (Figure 6(c)).
The calibration curves for OMP and PCM (Figure 5(b) and Figure 5(c)) each show a good linear response.
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(b)
Figure 4. DPV curves with different concentrations of OMP (71.63 - 262.6 µM) in the presence of a fixed concentration of 63.91 µM PCM (a); of PCM (62.71 - 210 µM) in the presence of a fixed concentration of 115.4 µM de OMP (b). Epr = −2 V/SCE, tEp = 90 s, MA = 0.1 V/SCE, tM = 0.05 s and ∆Ep = 0.007 V/SCE.
The equations of these curves are respectively J(A/cm2) = −0.0001 + 0.9109 [C/(mol∙L−1)] and J(A/cm2) = −0.0001 + 4.547 [C/(mol∙L−1)], with regression coefficients R2 = 0.9998 and R2 = 0.9993. The calculated detection limits are 7.23 µM and 3.6 µM, respectively.
It should be noted that when comparing these results with those obtained for the separate determination of OMP and PCM, the detection limits obtained are
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(c)
Figure 5. DPV curves with different concentrations of OMP (259.8 - 467.2 µM) and different concentrations of PCM (58.73 - 116.3 µM) (a); Calibration curves for OMP (b) and PCM (c). Epr = −2 V/SCE, tEp = 90 s, MA = 0.1 V/SCE, tM = 0.05 s and ∆Ep = 0.007 V/SCE.
of the same order of magnitude. Determination of OMP and PCM in the same matrix is as effective as separate determination.
The high recovery rates of OMP and PCM (Table 1) obtained in this study demonstrate the efficacy of the method and its applicability to the simultaneous detection of these two pharmaceutical compounds in perchloric acid medium.
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Table 1. Recovery rates for simultaneous detection of OMP and PCM in perchloric acid.
3.5. Interference Studies
The selectivity of the proposed method was evaluated by the addition of any interfering agents (frequently present in the pharmaceutical formulations analyzed), such as chlorine, magnesium, sulfate, potassium and sodium ions with variable contractions in solution containing a fixed concentration of PCM and OMP. The selectivity of this method was also tested in the presence of ceftriaxone, an organic pharmaceutical compound.
The corresponding oxidation peak currents were compared with those obtained in the absence of each interferer. Analysis of the responses obtained led to the conclusion that these elements do not significantly interfere (<5%) in the simultaneous determination the PCM and the OMP under the proposed working conditions.
3.6. Comparison Method
Voltammetric studies for the individual determination of PCM [38] [39] [40] or OMP [41] by DPV have given low limits of detection and quantification, and other studies have introduced techniques for the simultaneous determination of PCM and one or two other pharmaceutical organic substances with also low limits of detection and quantification. However, the simultaneous determination of PCM and OMP in HClO4 0.1 M medium has been the subject of very little or no work, while consulting the literature. In this work, the technique used to determine the PCM and the OMP simultaneously is selective, sensitive and precise in view of the results obtained.
3.7. Application to the Simultaneous Detection of OMP and PCM in Complex Media
To assess the applicability of the proposed method, OMP and PCM were detected simultaneously in complex media such as wastewater, tomato, cucumber and carrot juice. These complex media were obtained by diluting 1 mL of wastewater or juice in 0.1 M perchloric acid to obtain 100 mL of solution for each series of voltammetric measurements.
The analysis technique developed with these optimized DPV parameters was applied to the simultaneous detection of OMP and PCM in these complex media.
Well-defined quantities of OMP and PCM were introduced by simultaneous dosed additions to the same carrier media. The different voltammogram responses (Figure 6) obtained were used to calculate the overlap rates of the added quantities with the equations of the calibration curves obtained with each of the pharmaceutical substances (Table 2).
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Table 2. Recovery rates for simultaneous detection of OMP and PCM in perchloric acid, tomato, carrot, cucumber juice and wastewater.
The voltammograms presented on Figure 6 show an increase in the current density of the OMP and PCM oxidation peaks in the same order of magnitude with their concentrations. Moreover, in each electrolyte, the potential of the oxidation peaks related to OMP is 0.5 V/SCE and that related to PCM is 0.7 V/SCE as in the case of their individual study in 0.1 M HClO4 medium.
The reproducibility and repeatability of the measurements were also demonstrated during these series of experiments.
Good linear dependencies between the recorded current densities and the concentrations of OMP and PCM in tomato, carrot, cucumber juice, and wastewater were obtained.
The calculated recovery rates are shown in Table 2. They vary from 71.3% ± 0.1% to 101.2% ± 0.4% from one complex medium to another. These fairly high values show that OMP and PCM can be clearly detected simultaneously in complex media of tomato juice, carrot juice, cucumber juice and wastewater without interfering with each other, hence the reliability of the method employed.
4. Conclusions
To our knowledge, this is one of the first reports on the simultaneous determination of OMP and PCM by a cyclic or differential voltammetric technique using BDDE. This work demonstrates that BDDE, which has excellent electrochemical properties, is suitable for the simultaneous voltammetric determination of OMP and PCM.
The results obtained allow us to conclude that DPV can be used with many advantages for the quantitative determination of these drugs, alone or in combination, as they are commonly found in pharmaceutical formulations.
This DPV technique, operated with optimized parameters, enabled the simultaneous detection of OMP and PCM in 0.1 M HClO4 solution with very low limits and fairly high recoveries. The same analytical procedure was used to simultaneously detect OMP and PCM in fresh natural juices from tomatoes, carrots and cucumbers, and in wastewater from Treichville University Hospital. The voltammograms recorded show the same characteristics as those obtained in 0.1 M HClO4. The high recovery rates determined, ranging from 71.3% ± 0.1% to 101.4% ± 0.1% in the various complex media, show that the method developed is effective. Thus, the use of BDDE in the DPV technique for the simultaneous detection of OMP and PCM appears to have significant potential due to its application in various natural matrices.