Table of Contents Author Guidelines Submit a Manuscript
Journal of Analytical Methods in Chemistry
Volume 2017, Article ID 2321572, 8 pages
https://doi.org/10.1155/2017/2321572
Research Article

Spectrophotometric Methods for Simultaneous Determination of Oxytetracycline HCl and Flunixin Meglumine in Their Veterinary Pharmaceutical Formulation

1Analytical Chemistry Department, Faculty of Pharmacy, Cairo University, Kasr El Aini, Cairo 11562, Egypt
2National Organization for Drug Control and Research (NODCAR), 6 Abu Hazem Street, Pyramids Ave., P.O. Box 29, Giza, Egypt

Correspondence should be addressed to Hanan A. Merey; moc.oohay@afotabib

Received 23 April 2017; Accepted 29 May 2017; Published 24 July 2017

Academic Editor: Sibel A. Ozkan

Copyright © 2017 Hanan A. Merey et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Four precise, accurate, selective, and sensitive UV-spectrophotometric methods were developed and validated for the simultaneous determination of a binary mixture of Oxytetracycline HCl (OXY) and Flunixin Meglumine (FLU). The first method, dual wavelength (DW), depends on measuring the difference in absorbance (ΔA 273.4–327 nm) for the determination of OXY where FLU is zero while FLU is determined at ΔA 251.7–275.7 nm. The second method, first-derivative spectrophotometric method (1D), depends on measuring the peak amplitude of the first derivative selectively at 377 and 266.7 nm for the determination of OXY and FLU, respectively. The third method, ratio difference method, depends on the difference in amplitudes of the ratio spectra at ΔP 286.5–324.8 nm and ΔP 249.6–286.3 nm for the determination of OXY and FLU, respectively. The fourth method, first derivative of ratio spectra method (1DD), depends on measuring the amplitude peak to peak of the first derivative of ratio spectra at 296.7 to 369 nm and 259.1 to 304.7 nm for the determination of OXY and FLU, respectively. Different factors affecting the applied spectrophotometric methods were studied. The proposed methods were validated according to ICH guidelines. Satisfactory results were obtained for determination of both drugs in laboratory prepared mixture and pharmaceutical dosage form. The developed methods are compared favourably with the official ones.

1. Introduction

Oxytetracycline HCl (OXY) (Figure 1(a)) is chemically designated as 4S,4aR,5S,5aR,6S,12aS-4-dimethylamino-1,4,4a,5,5a,6,11,12a octahydro-3,5,6,10,12,12a-hexahydroxy-6-methylene-1,11-dioxonaphthacene-2-carboxamide,5β-hydroxytetracycline as hydrochloride salt. It has a broad spectrum antimicrobial activity. It binds reversibly with 30S subunit of ribosome preventing the binding of aminoacyl transfer RNA and inhibiting proteins synthesis and so cell growth [1].

Figure 1: Chemical structures of (a) Oxytetracycline HCl and (b) Flunixin Meglumine.

Flunixin Meglumine (FLU) (Figure 1(b)) is chemically designated as (2-[[2-methyl-3-(trifluoromethyl)-phenyl]amino]-3-pyridinecarboxylic acid compounded with 1-deoxy-1-(methylamino)-D-glucitol (meglumine salt). It is an NSAIDS used in veterinary medicine to relieve pain and inflammation in acute and chronic disorders [1]. It blocks some part of the cyclooxygenase enzyme pathway and thereby suppresses the synthesis of several chemical mediators of inflammation [2].

The mixture of OXY and FLU is formulated as injectable solution FLOXON® which is widely used as a veterinary drug indicated for the treatment of infectious diseases where concurrent analgesic, anti-inflammatory therapy is desired [3].

Oxytetracycline HCl and Flunixin Meglumine are official drugs in British Pharmacopeia [4], United State Pharmacopoeia [5], and European Pharmacopoeia [6].

Literature survey represented that several methods have been reported for determination of OXY alone or in combination with other drugs. These include spectrophotometric method [7], HPLC [811], colorimetric [12, 13], fluorometric [14, 15], and electrochemical methods [16], while FLU was determined by spectrophotometric [17], HPLC [1823], and voltammetric methods [24, 25]. To the best of our knowledge, no method has been reported for the simultaneous determination of OXY and FLU in their pharmaceutical preparation.

Spectrophotometry has been widely accepted and extensively used in pharmaceutical analysis as an alternative to high performance liquid chromatography due to its simplicity, low coast, and short turnaround time.

The aim of this work was to develop different simple, sensitive, accurate, fast, and low-cost spectrophotometric methods, namely, dual wavelength, first derivative, derivative ratio, and ratio difference spectrophotometric methods capable of simultaneous determination of the aforementioned drugs in their dosage form. Different factors affecting each of the applied spectrophotometric methods were considered and experimental conditions were optimized.

2. Experimental

2.1. Apparatus

Shimadzu UV-2450 PC Series Spectrophotometer (Tokyo, Japan) with two matched 1 cm quartz cells using the following spectral parameters, a single fast scan mode and a slit width (2 nm), connected to a computer loaded with Shimadzu UV-PC software and used for all the absorbance measurements and data manipulation.

2.2. Pure Samples

OXY standard was kindly supplied by Farmachem SA, Mendrisio, Switzerland, and its purity was found to be 99.16 ± 0.896% according to the reported method [26], and FLU was supplied by Norbrook Lab. Ltd., Northern Ireland, UK. Its purity was found to be 99.84 ± 0.891% according to the official method [5].

2.3. Pharmaceutical Formulations

FLOXON injectable solution, labelled to contain 108 mg/mL of OXY and 33 mg/mL of FLU, batch number 160142, is manufactured by Pharma Swede Company [27]. It was purchased from local Egyptian market.

2.4. Reagents

All chemicals and solvents used were of analytical grade, hydrochloric acid, ADWIC (Cairo, Egypt); water used was distilled.

2.5. Standard Solutions

OXY and FLU standard stock solutions are 100 µg/mL in 0.1 N HCl. They are stored in refrigerator whenever they are not used (they are stable for 14 days).

2.6. Laboratory Prepared Mixtures Containing Different Ratios of OXY and FLU

Into a series of 10 mL volumetric flasks, different aliquots OXY and FLU were transferred from their corresponding standard stock solutions (100 µg/mL) of each, and then the volume was completed with 0.1 N HCl.

3. Procedures

3.1. Linearity

Aliquots equivalent to 50–600 µg of OXY and FLU were accurately transferred from their respective standard stock solutions (100 µg/mL) into sets of 10 mL volumetric flasks. The volume was completed to the mark with 0.1 N HCl. The zero-order absorption spectra of the prepared standard solutions were scanned from 200 to 400 nm under the previously mentioned spectral parameters and stored.(i) Dual Wavelength Method. Linear relationships were directly constructed from zero-order spectra relating the difference in absorbance at and to the corresponding concentration of OXY and FLU, respectively.(ii) First-Derivative Spectrophotometric Method. The first derivative of the stored spectra of OXY and FLU was manipulated at Δλ = 8 nm and scaling factor = 10. Calibration graphs were constructed relating the amplitude of the obtained first-derivative spectra at 377 nm and 266.7 nm to the corresponding concentrations of OXY and FLU (zero contribution of FLU and zero crossing of OXY), respectively.(iii) Ratio Difference Spectrophotometric Method. Ratio spectra of OXY and FLU were obtained by dividing the stored zero-order absorption spectra of OXY and FLU by the normalized divisor of FLU and OXY for the determination of OXY and FLU, respectively. Calibration graphs were constructed relating the difference in amplitudes of the ratio spectra at and to the determination of OXY and FLU, respectively.(iv) First Derivative of Ratio Spectra1DD Spectrophotometric Method. The first derivatives of the previously ratio spectra were obtained using Δλ = 8 nm and scaling factor = 10. Calibration graphs were constructed relating peak to peak amplitudes of first derivative of the ratio spectra at and to the corresponding concentrations of OXY and FLU, respectively.The regression equations were then computed relating the response in each method to corresponding concentrations.

3.2. Analysis of Laboratory Prepared Mixtures

The absorption spectra of laboratory prepared mixtures were scanned and stored. Then procedures were performed as described under linearity and the concentrations of OXY and FLU in the prepared mixtures were obtained from the corresponding regression equation of each method.

3.3. Application to Pharmaceutical Formulation

One mL FLOXON solution (equivalent to 108 mg and 33 mg of OXY and FLU, resp.) was accurately transferred into 100 mL measuring flask; the volume was completed to the mark with 0.1 N HCl.

Suitable dilution was done using 0.1 N HCl to prepare a solution of final concentration equal to 27 µg/mL and 8.25 µg/mL of OXY and FLU, respectively. The procedures under linearity were followed to determine the concentrations of both drugs from the corresponding regression equation of each method.

4. Results and Discussion

OXY and FLU are coformulated in FLOXON injection that is widely used as a veterinary dosage form indicated for the treatment of infectious diseases where concurrent analgesic, anti-inflammatory therapy is desired [3]. No publications were reported for the simultaneous determination of OXY and FLU in their pharmaceutical formulation; this acquire our attention to develop simple, accurate, and precise spectrophotometric methods for the simultaneous determination of both drugs without any preliminary separation to be used in quality control laboratories. Reviewing the structures of OXY and FLU illustrated that both drugs have different functional group affected by pH (pKa = 2.84 and 1.88 for OXY and FLU, res.), thus shifting the spectra of both drugs. Therefore different solvents were tried with different pH (water-NaOH-HCl-methanol); the best spectra of both drugs are obtained when using 0.1 N HCl as a solvent regarding the selectivity and reproducibility. The zero-order absorption spectra of OXY and FLU show severe overlapping between OXY and FLU (Figure 2) which prevents their direct determination. Therefore, we started with the simplest spectrophotometric method which is dual wavelength method. It is used to determine binary mixtures with overlapped spectra, as the difference in absorbance between two wavelengths is proportional to the absorbance of one component and zero contribution of the other component in the mixture [2830]. For the determination of OXY, the difference in absorbance at 273.4 and 327 nm was selected where the difference in absorbance of FLU, at the same wavelengths, equals zero. Depending on the same principle, FLU could be determined by measuring the difference in absorbance at 251.7 and 275.7 nm where the absorbance difference of OXY at these two wavelengths equals zero (Figure 2). Linear relationships were obtained between the absorbance difference and the corresponding drug concentrations in the range of 5–60 µg/mL for OXY and FLU.

Figure 2: Zero-order spectra of 25 µg/mL OXY (—) and 25 µg/mL FLU (- - - - -) using 0.1 N HCl as a blank.

Derivative spectrophotometry is a valuable technique for resolving overlapped spectra of the binary mixture and for eliminating the effect of baseline shifts [3032]. So, the first-derivative spectrophotometric method was tried to solve the overlapped spectra of OXY and FLU. The main parameters that affect the shape of the derivative spectra such as wavelength, scanning speed, and the wavelength increment over which the derivative is calculated (Δλ) were studied. It was found that fast scanning speed, Δλ = 8, and scaling factor 10 gave the best compromise in terms of signals to noise ratio, peak resolution, and sensitivity throughout the determination. By viewing the first-derivative spectra of OXY and FLU we found that OXY can be selectively determined at 377 nm where FLU has zero contribution, while FLU can be selectively determined at 266.7 nm as OXY shows zero crossing (Figure 3). Linear relationships were obtained between the peak amplitude of the first-derivative spectra at the selected wavelength and the corresponding drug concentrations in the range of 5–60 µg/mL for OXY and FLU.

Figure 3: First-derivative absorption spectra of 60 µg/mL OXY (—) and 60 µg/mL FLU (- - - - - -) using 0.1 N HCl as a blank.

A simple and selective technique with minimal data manipulation for the resolution of overlapped spectra by rapid calculation of the difference of the amplitude of the ratio spectra at appropriate wavelengths [29, 30, 33] was suggested as a third method for resolving the aforementioned drugs. The important factor in obtaining the ratio spectra is the choice of a good divisor. Different divisors including 60 µg/mL and normalized divisor spectra of OXY and FLU were tried to obtain the ratio spectra of OXY and FLU. The best results regarding selectivity and baseline noise were obtained upon using normalized divisor spectra of OXY and FLU for determination of FLU and OXY, respectively. The used normalized spectra of OXY and FLU as divisors facilitate the optimization of the working conditions and diminish quantitation errors obtained by dividing the spectra for several standards of variable concentration into the corresponding concentration [34]. Different wavelength pairs were investigated to meet the method requirements. The difference of the amplitude of the ratio spectra at 286.5 nm–324.8 nm and 249.6 nm–286.3 nm was selected for the determination of OXY and FLU, respectively (Figures 4 and 5).

Figure 4: The ratio spectra of 10 µg/mL OXY (—) and laboratory prepared mixture containing 10 µg/mL FLU and 10 µg/mL OXY (⋯) using normalized spectra of FLU as a divisor.
Figure 5: The ratio spectra of 10 µg/mL FLU (- - - -) and laboratory prepared mixture containing 10 µg/mL FLU and 10 µg/mL OXY (⋯) using normalized spectra of OXY as a divisor.

The first derivative of ratio spectra spectrophotometric method (1DD) is a popular method for resolving a mixture of two interfering components [35]. Different parameters that affect the shape of the derivative of the ratio spectra such as wavelength, scanning speed, and the wavelength increment over which the derivative is obtained (Δλ) were studied. It was found that fast scanning speed, Δλ = 8, and scaling factor 10 give the best compromise in terms of signals to noise ratio, peak resolution, and sensitivity throughout the determinations. The obtained first derivative of the ratio spectra of OXY and FLU is shown in Figures 6 and 7, respectively. OXY can be successfully determined by measuring peak to peak maximum at 296.7 nm and 369 nm, while FLU can be successfully determined by measuring peak to peak maximum at 259.1 nm and 304.7 nm.

Figure 6: First derivative of ratio spectra of OXY (5–60 µg/mL) using normalized divisor of FLU and 0.1 N HCl as a blank.
Figure 7: First derivative of ratio spectra of FLU (5–60 µg/mL) using a normalized divisor of OXY and 0.1 N HCl as a blank.

Linear relationships were obtained between the concentrations of OXY and FLU in the range of 5–60 µg/mL and responses in each proposed spectrophotometric method. The regression equations and correlation coefficient were computed and shown in Table 1.

Table 1: Assay validation sheet of the proposed methods for the determination of Oxytetracycline HCl and Flunixin Meglumine.

The stability of OXY and FLU in 0.1 N HCl has been studied by keeping samples of the drugs in tightly capped volumetric flasks, covered with aluminum foil and stored in the refrigerator. The samples were checked for assay in fourteen successive days of storage and compared with the freshly prepared sample. We found that the RSD values of the assay are below 2.0%, which indicates that both OXY and FLU are stable in their 0.1 N HCl solutions for 2 weeks.

4.1. Method Validation

Validation was done according to ICH guidelines [36]. Linearity, accuracy, range, and precision (repeatability and intermediate precision) were determined. Satisfactory results were obtained and illustrated in Table 1. Selectivity was also determined by applying the proposed methods for the determination of cited drugs in laboratory prepared mixtures containing different ratios of OXY and FLU (one of them represents the pharmaceutical formulation ratio). Good results are obtained indicating good selectivity of the proposed methods (Table 2).

Table 2: Result obtained for the determination of Oxytetracycline HCl and Flunixin Meglumine in laboratory prepared mixtures by the proposed spectrophotometric methods.

The suggested methods are fruitfully applied for the determination of OXY and FLU in their pharmaceutical formulation. Satisfactory results indicate that there is no interference from dosage form excipients. Standard addition technique was applied to evaluate the accuracy of the developed methods for the analysis of drugs in the dosage form. The obtained results are listed in Tables 3 and 4.

Table 3: Determination of Oxytetracycline HCl and Flunixin Meglumine in FLOXON by the proposed dual wavelength and first-derivative spectrophotometric methods and application of standard addition technique.
Table 4: Determination of Oxytetracycline HCl and Flunixin Meglumine in FLOXON by the proposed ratio difference and first derivative of ratio spectra spectrophotometric methods and application of standard addition technique.

Statistically comparison was done between the proposed methods and the reported direct spectrophotometric method for OXY [26] and official direct spectrophotometric method for FLU [5]. The obtained results show no significant difference between them (Table 5).

Table 5: Statistical comparison of the results obtained by the proposed spectrophotometric methods and the reported method for the determination of Oxytetracycline HCl [26] or official method for the determination of Flunixin Meglumine [5] in their pure form, respectively.

5. Conclusion

The proposed methods give the first contribution for the simultaneous determination of Oxytetracycline HCl and Flunixin Meglumine in the veterinary pharmaceutical dosage form. The developed spectrophotometric methods are validated and successfully applied for the simultaneous determination of OXY and FLU as a binary mixture in their pure form and in their available dosage form without any preliminary separation steps, which do not require complex algorithm, software programs (like Matlab), or sophisticated calculation. The proposed methods are simple, accurate, sensitive, selective, and reproducible and therefore can be applied for the routine work in QC laboratories of any poor country lacking liquid chromatographic instrument.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

References

  1. A. Brayfield, Martindale: The complete drug reference, 38th, Parmaceutical Press, UK, 2014.
  2. P. Lees and A. J. Higgins, “Clinical pharmacology and therapeutic uses of non-steroidal anti-inflammatory drugs in the horse,” Equine Veterinary Journal, vol. 17, no. 2, pp. 83–96, 1985. View at Publisher · View at Google Scholar
  3. J. H. Foreman, B. E. Bergstrom, K. S. Golden et al., “Dose titration of the clinical efficacy of intravenously administered flunixin meglumine in a reversible model of equine foot lameness,” Equine Veterinary Journal, vol. 44, no. 43, pp. 17–20, 2012. View at Publisher · View at Google Scholar · View at Scopus
  4. The British Pharmacopeia (Veterinary), COMMISSION, BRITISH PHARMACOPOEIA, London, UK, 2016. http://libguides.rgu.ac.uk/c.php?g=260908&p=1999961.
  5. “The United States Pharmacopoeia and National Formulary,” Tech. Rep. (USP 34-NF 29), United States Pharmacopeia, Rockville, MD, USA, 2011.
  6. European pharmacopoeia, Council Of Europe: European Directorate for the Quality of Medicines and Healthcare, Strasbourg, Strasbourg, 7th edition, 2010, http://www.worldcat.org/title/european-pharmacopoeia/oclc/662704167.
  7. M. Inés Toral, T. Sabay, S. L. Orellana, and P. Richter, “Determination of oxytetracycline from salmon muscle and skin by derivative spectrophotometry,” Journal of AOAC International, vol. 98, no. 3, pp. 559–565, 2015. View at Publisher · View at Google Scholar · View at Scopus
  8. G. Anna, J. Artur, B. Tomasz, and P. Andrzej, “Oral fluid as a biological material for antemortem detection of Oxytetracycline in Pigs by Liquid Chromatography–Tandem Mass Spectrometry,” Journal of Agricultural and Food Chemistry, vol. 65, pp. 494–500, 2017. View at Publisher · View at Google Scholar
  9. I. Ghorbel-abid, H. Belhassen, R. Lahsini, D. C. Ben Hassen, and M. Trabelsi-Ayadi, “Development of a high- performance liquid chromatography with diode- array detection method using monolithic column for simultaneous determination of five tetracyclines residues in fish muscles,” Journal of Food & Nutritional Disorders, vol. 5, no. 5, pp. 10–4172, 2016. View at Publisher · View at Google Scholar
  10. F. Mgonja, R. Mosha, F. Mabiki, and K. Choongo, “A simple and sensitive method for the detection of Oxytetracycine levels in ready to eat beef by liquid chromatography-mass spectrometry,” African Journal of Pharmacy and Pharmacology, vol. 10, no. 28, pp. 571–578, 2016. View at Publisher · View at Google Scholar
  11. Z. I. Kimera, R. H. Mdegela, C. J. N. Mhaiki et al., “Determinaton of Oxytetracycline residues in cattle meat marketed in the Kilosa district,” Journal of Veterinary Research, vol. 82, pp. 1–5, 2015. View at Google Scholar
  12. Y. Fang, Z. Huimin, W. Xiaodan, and Q. Xie, “Determination of oxytetracycline by a graphene - gold nanoparticle based colorimetric aptamer sensor,” Analytical Letters Journal, vol. 50, pp. 544–553, 2017. View at Google Scholar
  13. R. Su, J. Xu, Y. Luo et al., “Highly selective and sensitive visual detection of oxytetracycline based on aptamer binding-mediated the anti-aggregation of positively charged gold nanoparticles,” Materials Letters, vol. 180, pp. 31–34, 2016. View at Publisher · View at Google Scholar · View at Scopus
  14. Z. Xu, X. Yi, Q. Wu, Y. Zhu, M. Ou, and X. Xu, “First report on a BODIPY-based fluorescent probe for sensitive detection of oxytetracycline: application for the rapid determination of oxytetracycline in milk,” RSC Advances, vol. 6, no. 92, pp. 89288–89297, 2016. View at Publisher · View at Google Scholar
  15. C. Liu, C. Lu, Z. Tang, X. Chen, G. Wang, and F. Sun, “Aptamer-functionalized magnetic nanoparticles for simultaneous fluorometric determination of oxytetracycline and kanamycin,” Microchimica Acta, vol. 182, Article ID 00263672, pp. 2567–2575, 2015. View at Google Scholar
  16. J. Ghodsi, A. A. Rafati, and Y. Shoja, “First report on electrocatalytic oxidation of oxytetracycline by horse radish peroxidase: Application in developing a biosensor to oxytetracycline determination,” Sensors and Actuators, B: Chemical, vol. 224, pp. 692–699, 2015. View at Publisher · View at Google Scholar · View at Scopus
  17. M. M. Fouad, S. A. Abd El-Razeq, F. F. Belal, and F. A. Fouad, “Spectrophotometric methods for the determination of flunixin meglumine and menbutone in bulk and dosage forms,” International Journal of Pharmaceuticals Analysis, vol. 4, pp. 30–35, 2013. View at Google Scholar
  18. F. F. Belal, S. A. Abd El-Razeq, M. M. Fouad, and F. A. Fouad, “Micellar high performance liquid chromatographic determination of flunixin meglumine in bulk, pharmaceutical dosage forms, bovine liver and kidney,” Analytical Chemistry Research, vol. 3, pp. 63–69, 2015. View at Publisher · View at Google Scholar · View at Scopus
  19. V. Meucci, M. Minunni, M. Vanni, M. Sgorbini, M. Corazza, and L. Intorre, “Selective and simultaneous determination of NSAIDs in equine plasma by HPLC with molecularly imprinted solid-phase extraction,” Bioanalysis, vol. 6, no. 16, pp. 2147–2158, 2014. View at Publisher · View at Google Scholar · View at Scopus
  20. A.-L. Zhua, T. Pengb, L. Liuc et al., “Ultra-performance liquid chromatography–tandem mass spectrometry determination and depletion profile of flunixin residues in tissues after single oral administration in rabbits,” Journal of Chromatography B, vol. 934, pp. 8–15, 2013. View at Google Scholar
  21. K. C. Chang, J. S. Lin, and C. Cheng, “Online eluent-switching technique coupled anion-exchange liquid chromatography-ion trap tandem mass spectrometry for analysis of non-steroidal anti-inflammatory drugs in pig serum,” Journal of Chromatography A, vol. 1422, pp. 222–229, 2015. View at Publisher · View at Google Scholar · View at Scopus
  22. Z.-Y. Liu, K. Yang, F.-H. Chen et al., “Development of a Rapid Method for the Confirmatory Analysis of Flunixin Residue in Animal Tissues Using Liquid Chromatography–Tandem Mass Spectrometry,” Food Analytical Methods, vol. 8, no. 2, pp. 352–362, 2014. View at Publisher · View at Google Scholar · View at Scopus
  23. B. Lugoboni, A. Barbarossa, T. Gazzotti, E. Zironi, F. Farabegoli, and G. Pagliuca, “A quick LC-MS-MS method for the determination of flunixin in bovine muscle,” Journal of Analytical Toxicology, vol. 38, no. 2, Article ID bkt120, pp. 80–85, 2014. View at Publisher · View at Google Scholar · View at Scopus
  24. A.-E. Radi, N. Abd El-Ghany, and T. Wahdan, “Voltammetric Determination of Flunixin on Molecularly Imprinted Polypyrrole Modified Glassy Carbon Electrode,” Journal of Analytical Methods in Chemistry, vol. 2016, Article ID 5296582, 2016. View at Publisher · View at Google Scholar · View at Scopus
  25. V. Meucci, M. Vanni, M. Sgorbini, R. Odore, M. Minunni, and L. Intorre, “Determination of phenylbutazone and flunixin meglumine in equine plasma by electrochemical-based sensing coupled to selective extraction with molecularly imprinted polymers,” Sensors and Actuators, B: Chemical, vol. 179, pp. 226–231, 2013. View at Publisher · View at Google Scholar · View at Scopus
  26. A. C. Moffat, B. Osselton, and J. Watts, Clark’s Analysis of Drugs and Poisons, Parmaceutical Press, London, UK, 4th edition, 2011.
  27. Pharma Swede, Floxon, (n.d.). http://pharmaswede.com/Class/Large/01ANTIBIOTICS/FLOXON-inj.htm.
  28. S. S. Abbas, H. E. Zaazaa, H. A. M. Essam, and M. G. El-Bardicy, “Stability-indicating determination of rebamipide in the presence of its acid degradation products,” Journal of AOAC International, vol. 97, no. 1, pp. 78–85, 2014. View at Publisher · View at Google Scholar · View at Scopus
  29. L. M. Abdel-Halimb, M. K. Abd-El Rahmana, N. K. Ramadana, H. F. A. EL Sanabaryb, and M. Y. Salem, “Comparative study between recent methods manipilating ratio spectra and classical methods on two-wavelength selection for the determination of binary mixture of anazoline hydrochloride and tetryzoline hydrocholride,” Journal of Spectrochimica Acta, vol. 159, pp. 98–105, 2016. View at Publisher · View at Google Scholar
  30. A. S. Saad, A. K. Attia, M. S. Alaraki, and E. S. Elzanfaly, “Comparative study on the selectivity of various spectrophotometric techniques for the determination of binary mixture of fenbendazole and rafoxanide,” Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy, vol. 150, pp. 682–690, 2015. View at Publisher · View at Google Scholar · View at Scopus
  31. J. Karpińska, A. Wiszowata, and M. Skoczylas, “Simultaneous determination of levomepromazine hydrochloride and its sulfoxide by UV-derivative spectrophotometry and bivariate calibration method,” Analytical Letters, vol. 39, no. 6, pp. 1129–1141, 2006. View at Publisher · View at Google Scholar · View at Scopus
  32. V. S. Rajmane, S. V Gandhi, U. P. Patil, and M. R. Sengar, “Simultaneous determination of drotaverine hydrochloride and aceclofenac in tablet dosage form by spectrophotometry,” Eurasian Journal of Analytical Chemistry, vol. 4, pp. 184–190, 2009. View at Google Scholar
  33. E. S. Elzanfaly, A. S. Saad, and A.-E. B. Abd-Elaleem, “Simultaneous determination of retinoic acid and hydroquinone in skin ointment using spectrophotometric technique (ratio difference method),” Saudi Pharmaceutical Journal, vol. 20, no. 3, pp. 249–253, 2012. View at Publisher · View at Google Scholar · View at Scopus
  34. J. M. Garcia, O. Hernandez, A. I. Jimenez, F. Jimenez, and J. J. Arias, “A contribution to the derivative ratio spectrum method,” Analytica Chimica Acta, vol. 317, pp. 83–93, 1995. View at Google Scholar
  35. H. A. Merey, “Simple spectrophotometric methods for the simultaneous determination of antipyrine and benzocaine,” Bulletin of Faculty of Pharmacy, Cairo University, vol. 54, pp. 181–189, 2016. View at Google Scholar
  36. “International conference on harmonisation, Guidance for industry: Q2B validation of analytical procedures: methodology,” in International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use, vol. 13, p. 62, 1996.