A green TLC densitometric method for the simultaneous detection and quantification of naphazoline HCl, pheniramine maleate along with three official impurities

Impurity profiling of a pharmaceutical compound is now taking great attention during quality assessment of pharmaceuticals, as presence of small amount of impurities may affect safety and efficacy. In this work, a novel TLC chromatographic method coupled with densitometric detection was established for the simultaneous quantification of naphazoline HCl, pheniramine maleate and three of their official impurities, namely; naphazoline impurity B, pheniramine impurities; A & B. Chromatographic separation was carried out on TLC aluminum silica plates F254, as a stationary phase, using methanol: ethyl acetate: 33.0% ammonia (2.0: 8.0: 1.0, by volume), as a mobile phase. Plates were examined at 260.0 nm and International Council for Harmonisation (ICH) guidelines were followed for method’s validation. Important factors, such as; composition of mobile phase and detection wavelengths were optimized. Linearity was achieved over the ranges of 2.0–50.0 µg band−1 for naphazoline, 10.0–110.0 µg band−1 for pheniramine, 0.1–10.0 µg band−1 for naphazoline impurity B and 2.0–50.0 µg band−1 for both pheniramine impurities. The proposed method was assessed in terms of accuracy, precision and robustness where satisfactory results (recovery % ≈ 100% and RSD < 2) were obtained. The method was also applied for the simultaneous determination of naphazoline HCl and pheniramine maleate, in Naphcon-A® eye drops, with respective recoveries of 101.36% and 100.94%. Method greenness was evaluated and compared to the reported HPLC one via environmental, health and safety tool. The developed method has much potential over the reported one of being simple, selective, economic and time saving for the analysis of the five cited compounds.

NPZ and PHN are usually co-formulated together in optic dosage forms used for eye inflammation treatment. The literature survey revealed some methods for their simultaneous quantification, such as HPLC [17][18][19]27] and one capillary electrophoresis [36]. One of the reported HPLC methods described their determination in presence of three selected impurities [19]. As a result, the aim of this work was to develop a first validated, as per ICH guidelines [50], TLC densitometric method for detection and quantification of NPZ, PHN, NPZ impurity B, PHN impurities; A and B, (Fig. 1). The proposed method was successfully applied for their simultaneous determination in a quinary mixture as well as in pharmaceutical eye drops. Furthermore, the organic solvents used in this work were assessed and compared to that used in the reported HPLC one [19] by the aid of environmental, health and safety (EHS) tool [51].

Materials and chemicals
Pure standard NPZ and PHN were kindly provided by Eva pharma pharmaceutical company, Cairo, Egypt. Their purities were assessed to be 100.12 ± 0.102% and 99.58 ± 0.124%, respectively [5]. The impurities (NPZ impurity B, PHN impurities; A & B) were purchased from the German company Alfa Aesar Company. Their certified potency values were found to be 99.00%, 100.30%, and 99.70%, respectively.

Pharmaceutical dosage form
Naphcon-A ® eye drops (Batch no.H13949-0615); manufactured by Alcon laboratories INC (Novartis Company), labeled to contain 0.25 & 3.0 mg mL −1 of NPZ and PHN, respectively, and has been purchased from Egyptian market.

Solutions
Standard solutions In 10-mL volumetric flasks, standard solutions of 20.0 mg mL −1 , for NPZ, PHN and two PHN impurities, were separately prepared in methanol. For NPZ impurity B, 1.0 mg mL −1 standard solution was prepared.
Laboratory prepared mixtures Different aliquots, from the five standard solutions, were transferred into separate 10-mL volumetric flasks to prepare laboratory prepared mixtures of various ratios. The volume of each flask was then completed to the mark using methanol. plates (10 × 20 cm). A mobile phase of methanol: ethyl acetate: 33.0% ammonia (2.0: 8.0: 1.0, by volume) was used for elution over 8.5 cm distance. The elution time was around 6.0 min. After that, the plates were removed, air dried and scanned at 260.0 nm. Calibration curves, for the five cited drugs, representing the polynomial relationship between peak area and corresponding concentration were constructed and regression parameters were computed.

Assay of laboratory prepared mixtures
Different mixtures of NPZ, PHN and their official impurities were prepared as mentioned under solution section and mixed with different ratios. The prepared mixtures were then analyzed by the proposed method as mentioned above.

Forced degradation study
Stability of the two cited drugs were studied under different conditions, namely; acidic, alkaline, oxidative, photolytic and thermal ones. For acidic and alkaline hydrolyses, a mass of 10 mg of each drug was separately dissolved in least amounts of methanol then refluxing in 1 M HCl or methanolic solutions of 1 M NaOH at 100 °C for 2 h. For oxidative studies, 0.5 mL of 5% H 2 O 2 aqueous solution was separately added to 10 mL of 1.0 mg mL −1 solutions. The two drugs solutions were then kept at room temperature for 24 h. For photolytic study, thin layers of each powdered drug was uniformly spread in two Petri dishes, and exposed to UV light at 254 nm for 10 h at a distance of 15 cm. Thermal degradation was assessed through sealing each powdered drug in glass ampoules and heating in a thermostatic oven at 100 °C for 7 h. Finally, samples were periodically withdrawn for observing the forced degradation process.

Pharmaceutical application
The content of 10 Naphcon-A ® eye drops were emptied. 20.0 mL aliquot was transferred into a 25-mL measuring flask. 3.0 mL methanol was added and the flask was then sonicated for 20.0 min. Volume was completed to the mark using methanol to obtain final concentration of 200.0 µg mL −1 NPZ and 2400.0 µg mL −1 PHN. 10.0 µLs from this solution were applied onto TLC plates. Finally, solutions were analyzed as mentioned before under construction of the calibration curves.

Results and discussion
The importance of impurity detection and determination evokes the requirement for developing simple, economical, rapid and accurate analytical techniques which can be utilized easily in quality control laboratories wherein cost and time are essential. Owing to simplicity, cost effectiveness, time saving, no need for tedious sample preparation and high sensitivity, TLC densitometry could be considered as one of the best options for that purpose [2,52,53] Here, we present a novel TLC densitometric method for the simultaneous determination of NPZ, PHN and three related official impurities (NPZ impurity B, PHN impurities; A & B) in their quinary mixture. Moreover, EHS tool is applied for greenness evaluation of this method in comparison to our previously reported HPLC one [19].

Development and optimization of TLC densitometric method
Various mobile phases were tried to get optimum separation and resolution between the five cited components. Firstly, mixtures with different ratios of methanol and ethyl acetate have been tried, but the separation between the five cited components was not achieved. Thus, different solvents were added individually to the previous mixture for improving the separation between the studied components such as chloroform, acetone and ammonia. Table 1 summarizes the obtained resolution values during this optimization phase. It was noticed that addition of ammonia to the conventional mixture enhanced separation and resolution between the studied drugs. Finally, a mixture of methanol-ethyl acetate-ammonia (2.0: 8.0: 1.0, by volume) was chosen for optimum suitability parameters. Moreover, different wavelengths were tried for evaluating the densitometric measurement as 260.0 nm and 280.0 nm. 260.0 nm was the wavelength of choice as it gave the highest sensitivity with minimum noise for measuring the five cited components. Retardation factor (R f ) values were sequentially at 0.18 ± 0.02, 0.35 ± 0.02, 0.49 ± 0.02, 0.63 ± 0.02 and 0.83 ± 0.02 for NPZ impurity B, NPZ, PHN, PHN impurity A and PHN impurity B (Fig. 2). Scanning profiles were obtained at 260.0 nm, and five calibration curves were then plotted.

System suitability parameters
To evaluate the performance of the proposed TLC method, system suitability parameters were calculated manually [54]. The results of retardation, resolution, capacity and tailing factors for the five components were obtained in Table 2.

Method validation
Method's validation was conducted in agreement to ICH guidelines [50].

Linearity and range
Polynomial relationships were established between the integrated peak area and the corresponding concentration in the ranges of 2.

Accuracy
Accuracy was assessed by applying the previously mentioned procedures on pure samples with various concentrations within the defined ranges. Satisfactory results regarding recovery % were computed in Table 3.     lyzed intra-daily three times. Results were obtained preliminary to RSD calculation, Table 3.

Precision
Intermediate precision Inter-daily analysis was also conducted for the formerly selected concentrations. Results are represented in Table 3.

Robustness
It was evaluated by studying the effect of deliberately changing the mobile phase composition; methanol  Table 3.

Specificity
It was assessed by analysis of different laboratory prepared mixtures containing various ratios of the five studied components. Table 3 shows good recovery percentages and RSDs for analyzing those mixtures. Furthermore, forced degradation study was conducted whereas the two drugs were subjected to different stress condi-  (Fig. 3).

Analysis of pharmaceutical eye drops
The two active pharmaceutical ingredients (NPZ and PHN) were simultaneously quantified in their combined dosage form. Excipients did not have an impact on the obtained TLC chromatograms. In addition, method's validity was proved using standard addition technique, Table 4.

Statistical analysis
Statistical comparison between results of the suggested TLC method and that of official HPLC ones [5] were performed. The calculated values of student's t-test and F-test indicated that there is no significant difference observed between those methods, Table 5.

Greenness evaluation and methods comparison
In order to assess and compare this work with our previously reported HPLC one [19], EHS tool was applied. In this tool, nine categories representing safety, health and environmental hazards are utilized for organic solvents assessment whereas the lower the calculated score, the greener the solvent will be [51]. The calculated scores for methanol, ethyl acetate (used in this work) and acetonitrile (used in reported HPLC) revealed the dominance of the proposed method over our previously reported one in terms of environmental sustainability, Table 6. Finally, a comparative overview on those two methods along with a statistical F-test for their variances are shown in Table 7.

Conclusion
A novel simple TLC densitometric method was established for the simultaneous detection and quantification of NPZ, PHN as well as three of their official impurities (NPZ impurity B, PHN impurities; A & B). The proposed method was validated in agreement to ICH guidelines. NPZ and PHN were successfully determined in their combined eye drops. EHS tool was utilized for greenness assessment of the organic solvents used in this work as well as the previously reported HPLC one. The proposed TLC densitometric method provides simplicity, low cost, fast analysis and environmental sustainability compared to the reported one. In addition, the capacity of the method to detect low concentrations of NPZ and PHN official impurities highlights it as a promising one for impurity profiling of those drugs.