Enantiomeric Separation of S-Epichlorohydrin and R-Epichlorohydrin by Capillary Gas Chromatography with FID Detector

The aim of this study was to develop a simple and derivatization free method for the Quantification of S-Epichlorohydrin in R-Epichlorohydrin by using a gas chromatography coupled with flame ionization detector (FID). Enantiopure epichlorohydrin was a valuable epoxide key starting material for preparing optically active Rivaroxaban. The enantiomeric separations of S-Epichlorohydrin and R-Epichlorohydrin were achieved on Gamaa-Dex-225 (30 meters × 0.25 mm I.D, 0.25 μm) column with a total run time of 30 min. Nitrogen was used as a carrier gas with constant pressure 25.0 psi. The critical experimental parameters such as, column selection, flow rate, injection volume and diluent were studied and optimized. Excellent correlation coeffient between peak responses and concentrations was >0.9998. The recoveries of S-Epichlorohydrin spiked in R-Epichlorohydrin were in the range from 98.2% to 102.8%. Limit of quantitation for S-Epichlorohydrin was sufficiently lower than limits specified by ICH. The method has validated as per International Conference on Harmonization (ICH) guidelines. A precise, accurate, linear and robust Gas Chromatography method was developed for the quantification of S-Epichlorohydrin in R-Epichlorohydrin for Rivaroxaban.


Introduction
Separation of the enantiomers of chiral drugs has become an important issue in analyt-How to cite this paper: Kumar, C.V., Vasa, P.K., Kumar, Y.R., Aparna, P. and Pratyusha, P. (2016) Enantiomeric Separation of S-Epichlorohydrin and R-Epichlorohydrin by Capillary Gas Chromatography with FID Detector.American Journal of Analytical Chemistry, 7, 772-784.http://dx.doi.org/10.4236/ajac.2016.711069ical chemistry in recent years, because of differences in the biological activity and pharmacokinetic properties of drug enantiomers [1] [2].Epichlorohydrin is an organochlorine compound and it is a chiral molecule and exists as R-Epichlorohydrin and S-Epichlorohydrin mentioned in Figure 1(a) and Figure 1(b).Epichlorohydrin is a versatile precursor in the synthesis of many organic compounds.Epichlorohydrin is a colorless, volatile and highly reactive liquid.It is soluble in most organic solvents and slightly soluble in petroleum hydrocarbons and in water [3].Epichlorohydrin (1-chloro-2,3-epoxypropane) was used mainly for the manufacture of pharmaceutical products, glycerol, unmodified epoxy resins and to a lesser extent elastomers, water-treatment resins, surfactants, ion exchange resins, plasticizers, dyestuffs, oil emulsifiers, lubricants, and adhesives [4].Epichlorohydrin has been classified as a probable carcinogen (group A) to human by the International Agency for Research on Cancer (IARC) [5].
Gas chromatography is the most commonly used technology for the analysis of Epichlorohydrin.From the literature review there were few analytical methods which have been reported for Epichlorohydrin such as GC methods [8] [9] [10] [11] [12] and GC-MS methods [13] [14].There was no reported method for the determination of S-Epichlorohydrin in R-Epichlorohydrin by gas chromatographic method for Rivaroxaban.The major objective of the present work is to develop a simple and robust GC method for determination of S-Epichlorohydrin in R-Epichlorohydrin for Rivaroxaban.
Hence, a reproducible gas chromatography with FID detector method was developed for the quantitative determination of S-Epichlorohydrin in R-Epichlorohydrin.This method was successfully validated according to the International Conference Harmonization (ICH) guidelines (Validation of Analytical Procedures: Test and Methodology Q2).

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loromethane was purchased from Merck, Germany.

Instruments and Software
A calibrated electronic single pan balance Mettler Toledo.All analysis performed on Agilent 6890 and 7890 modules equipped with FID detectors.Empower-3 software was used for signal monitoring and data processing.Microsoft Excel 2007 was used for analysis of validation results.

Chromatographic Conditions
The method was developed by using Gamaa-Dex-225 (30 meters × 0.25 mm I.D, 0.25 µm) column.The separation was achieved using an isothermal oven program 50˚C for 30 min.The injector temperature was maintained at 250˚C.Nitrogen was used as a carrier gas with constant pressure 25.0 psi.The detector temperature was maintained at 250˚C.The injection volume was 1.0 μL.Split ratio 1:50 and Runtime was 30.0 min.Dichloromethane was used as diluent during the standard and test samples preparation.Chromatograms were summarized in Figures 2(a)-(d).

Preparation of S-Epichlorohydrin Stock Solution
Transfer accurately 0.1 mL of S-Epichlohydrin standard in 50 mL volumetric flask, containing 10.0 mL diluent and made up to volume with diluent.

Preparation of System Suitability Solution
Transfer accurately 1.0 mL of standard in 50 mL volumetric flask, containing 10.0 mL diluent and to this add accurately 0.5 mL of above S-Epichlorohydrin stock solution and made up to volume with diluent.

Preparation of Standard Solution
Transfer accurately 1.0 mL of standard in 50 mL volumetric flask, containing 10.0 mL diluent and made up to volume with diluent.

Sample Solution Preparation
Transfer accurately 1.0 mL of test sample in 50 mL volumetric flask, containing 10.0 mL diluent and made up to volume with diluent.

Method Validation
The method has been validated by GC as per ICH guidelines [15].The method was validated for the following parameters: Precision, Linearity, Accuracy, Robustness, Solution stability, Limit of Detection, Limit of Quantification and Ruggedness.

Precision
The precision of the method was verified by repeatability and by intermediate precision.Repeatability of the method was checked by (Agilent 7890 module equipped with FID detector) injecting six individual preparations of R-Epichlorohydrin sample spiked with 0.10% of S-Epichlorohydrin (0.10% of S-Epichlorohydrin isomer with respect to 20 μL/mL R-Epichlorohydrin).The RSD of peak area for S-Epichlorohydrin was calculated.The intermediate precision of the method was also evaluated using different analysts, different instruments and different columns and performing the analysis on three different days.

Accuracy
For determination of accuracy of method recovery study was carried out by analyzing the spiked samples.Known amount of S-Epichlorohydrin was spiked in triplicate at three different concentration levels of 0.01, 0.02 and 0.03 μL/mL (50%, 100% and 150% of the analyte concentration i.e. 20 μL/mL) to the drug product.The % recoveries for S-Epichlorohydrin was calculated based on mentioned in Equation ( 1).
%S Epichlorohydrin Found % Recovery of S Epichlorohydrin 100 % S Epichlorohydrin added

Linearity
To establish linearity of the method was prepared by diluting stock solution to the required concentration.The solutions were prepared at seven concentration levels from LOQ to 150% of the specification level (LOQ, 0.025, 0.050, 0.075, 0.10, 0.125and 0.150%) with respect to the normal sample concentration (20 μL/mL).The correlation coefficients, slopes and Y-intercepts of the calibration curve were determined.

Limit of Detection (LOD) and Limit of Quantitation (LOQ)
The LOD and LOQ for S-Epichlorohydrin were determined at a signal-to-noise ratio of 3:1 and 10:1, respectively, by injecting a series of dilute solutions with known concentrations.Precision study was also carried out at the LOQ level by injecting six (n = 6) individual preparations and calculating the RSD of the area of S-Epichlorohydrin.

Robustness
The robustness of an analytical procedure is a measure of its capability to remain unaltered by small, but deliberate variations in method parameters and provides an indication of its reliability during normal usage.
To determine the robustness of the method, the chromatographic conditions were deliberately changed and relative standard deviation of the S-Epichlorohydrin peak was evaluated.As the flow rate was 25 psi to study the effect of flow rate on relative standard deviation of the S-Epichlorohydrin peak, the flow rate was changed to 20 psi and 30 psi.The effects of the column oven temperature were studied at 45˚C and 55˚C instead of 50˚C.

Solution Stability
The stability of S-Epichlorohydrin in R-Epichlorohydrin solution was determined by leaving test solutions of the sample and spiked solution in tightly capped volumetric flasks at room temperature for 24 hrs during which they were analysed at 12 hrs intervals.

Method Development and Optimization
The main goal of method development was to achieve separation of S-Epichlorohydrin in R-Epichlorohydrin without derivatization.An understanding of the nature of the racemic Epichlorohydrin is the foremost prerequisite for successful method development in GC.Following were the stepwise strategies for the method development in our case.

Column Selection
The primary goal of column selection was to separate S-Epichlorohydrin and R-Epichlorohydrin from each other, which were used during the synthesis of Rivaroxaban.As part of method development screened various columns, namely Chiral GTA (30 meters × 0.25 mm I.D, 0.12 µm) and Chiralsil (30 meters × 0.25 mm I.D, 0.25 µm) were employed but no adequate separation was found with above columns.After careful screening of columns, it was observed that Gamaa-Dex-225 (30 meters × 0.25 mm I.D, 0.25 µm) column provides better resolution between S-Epichlorohydrin and R-Epichlorohydrin and it showed good system suitability parameters.

Flow Rate
As the flow rate increase, the viscosity of carrier gas decreased and velocity increased.Check the flow rate from 6 psi to 40 psi.At 6 psi, the retention time was very high and runtime is long, poor separation was observed at 40 psi.25 psi was selected as finalized flow rate.

Selection of Diluent
Diluent selection study was conducted for the analysis of R-Epichlorohydrin and S-Epichlorohydrin.Four diluents had been tried N-methyl-2-pyrrolidone, Dimethyl formamide, Dimethyl imidazolidine and Dichloromethane.Dichloromethane was finalized as diluent because of no interference at the S-Epichlorohydrin and R-Epichlorohydrin peaks.

Injection Volume
The effect of injection volume on the quantification of the S-Epichlorohydrin and R-Epichlorohydrin were investigated by injecting volume between 0.5 μL to 2 μL of the standard solution.The results show that the peak widths of S-Epichlorohydrin and R-Epichlorohydrin were independent of injection volume within the tested range.
System suitability results are shown in Table 1 and Figures 3(a)-(c).

Precision
The % RSD for the content of S-Epichlorohydrin in the method precision was found to be less than 1.1.The % RSD for the content of S-Epichlorohydrin in the intermediate precision (Ruggedness) was found to be less than 2.2 (Table 2).The results confirmed the high precision of the developed GC method.

Limit of Detection and Quantification
The obtained limit of detection and limit of quantification, precision and accuracy at limit of quantification values are given in Table 2 and Figure 2(e), Figure 2(f).

Accuracy
Individual and average recoveries of three preparations and at three concentrations for S-Epichlorohydrin were within 100% ± 5% results shown in

Linearity
The calibration curve was drawn by plotting the peak area against the concentration.The correlation coefficient (r 2 ) obtained was ˃0.9998.The % Y-intercept with respect to response at 100% level was ˃± 5%.The results for the Correlation coefficient (r 2 ) and % Y-intercept with respect to response at 100% level were shown in Table 4 & Table 5.
The results demonstrate that an excellent correlation existed between the peak area and concentration of S-Epichlorohydrin.

Robustness
In all the deliberately varied chromatographic conditions, no effect on the Relative standard deviation of the S-Epichlorohydrin peak (Table 2) the method was more robust within the normal operating range, i.e., column oven temperature, 50˚C ± 5˚C and flow rate, 25 ± 5 psi, demonstrating the robustness of the method results shown in Table 2.

Solution Stability
No significant change in the amounts of S-Epichlorohydrin was observed during solution stability experiments.The results from solution stability experiments confirmed that sample and spiked solutions were stable for up to 24 hrs results shown in Table 6.

Discussion
Based on the results, the successful separation of S-Epichlorohydrin and R-Epichlorohydrin from each other.All the validated parameters were found to be within limits.System suitability for 6 injections % RSD was found to be NMT 0.54%.Precision at LOQ, 100% and 150% were found to be NMT 3.29%, Accuracy at LOQ, 50% 100% and 150% were found to be 98.2% to 102.8%.Linearity was performed from LOQ to 150% and graph obtained was linear showing correlation coefficient >0.9998.

Conclusion
A simple gas chromatographic method was developed and validated for the quantitative S-Epichlorohydrin in R-Epichlorohydrin for Rivaroxaban.S-Epichlorohydrin and R-Epichlorohydrin were well separated from each other, indicating that the developed GC method was specific.The method validation data showed satisfactory results for all tested method parameters.This simple GC method is precise, accurate, linear and rugged.Hence, it is proved that developed method can be used for routine testing in quality control laboratories for estimation S-Epichlorohydrin in R-Epichlorohydrin for Rivaroxaban.The method is user-friendly and robust to operate.

Figure 2 .
Figure 2. (a) Typical chromatograms of blank; (b) Typical chromatograms of system suitability; (c) Typical chromatograms of R-Epichlorohydrin sample; (d) Typical chromatograms of R-Epichlorohydrin sample spiked at specification level; (e) Typical chromatograms of LOQ; (f) R-Epichlorohydrin sample spiked at LOQ level.