Amino-endcapped Octadecylsilane Bonded Silica Mixed-mode Stationary Phase For The Simultaneous Separation Of Neutral And Ionizable Drugs

Fixed-dose combinations（FDCs）are single preparations containing two or more active pharmaceutical components.Those active pharmaceutical components can simultaneously interact with multiple targets to provide synergistic efficacy.In recent years,FDCs have been increasingly used for the diseases treatment due to their increased efficacy,decreased drug resistance and adherence.However,there are great challenges in the analysis due to the various properties of the active components in FDC.Researchers have worked on developing methods for simultaneous analysis of various active components in FDC for quality control.Reversed-phase liquid chromatography（RPLC）based on silica support is one of the most popular and efficient methods for FDC analysis.However,some drawbacks impede its application,such as weak retention for polar ionic compounds and peak tailing for basic compounds.Suppressing the dissociation of analytes by modulating the p H of mobile phase to extreme p H value can address the problems to some extent.However,simultaneous separation of various components in FDC,which contain neutral,acidic and basic compounds,cannot be always achieved by only adjusting the p H of mobile phase.Moreover,long-term use of the silica-based column beyond p H 2-8 will sacrifice its life-time.Adding ion-pair reagents in the mobile phase（IPC）is one of the other methods to improve the selectively for neutral,acidic and basic compounds.However,the problems,including long equilibration time,difficult in gradient elution,and incompatibility with MS in IPC,limit its widely application.Another effective approach to simultaneously separate neutral and ionizable compounds is to use the stationary phases containing multiple functional groups.In recent years,a variety of reversed-phase stationary phases containing endcapped polar groups have been performed.Although polar-endcapped reversed-phase stationary phase generally exhibit reversed-phase behavior,the presence of the polar groups can increase the selectivity for acidic compounds,improve peak symmetry for basic compounds and enhance compatibility with highly aqueous mobile phases.This provides alternative selectivity for separation of complex samples.Therefore,the preparation of the novel stationary phases for simultaneous separation of compounds with different polarities is still one of the hotspots in chromatography.The retention mechanism of polar-endcapped reversed-phase stationary phases will become more complicated because of the multiple interactions between the ligands on the stationary phase and the analytes.Although there are the increased production and application of polar-endcapped reversed-phase stationary phases,the systematic study of their retention mechanism still needs to be further explored.Moreover,it is necessary to further expand the application of polar-endcapped reversed-phase stationary phase in the separation of the complex samples.In this work,an amino-endcapped octadecylsilane bonded silica stationary phase（ODS-APS）was prepared and used for the simultaneous separation of neutral and ionizable compounds in FDC.The stationary phase was prepared by a three-step vapour deposition method without using organic solvent and environmental-friendly.Physicochemical characterization containing Fourier transform infrared spectroscope,solid-state ¹³C CP/MAS NMR and elemental analysis exhibited the presence of octadecyl chains and amino-endcapped moieties on the ODS-APS phase.The chromatographic performance of the synthesized phase was thoroughly evaluated and compared with the ODS phase.Furthermore,the retention mechanism for acidic compounds was evaluated by studying the effect of p H,ionic and solvent strength of the mobile phase.Finally,acidic,neutral,basic drugs and several FDCs were successfully separated on the ODS-APS phase under neutral conditions.The results demonstrate that the ODS-APS phase provides better peak shape and higher selectivity while separating basic and acidic analytes under neutral conditions,compared to the traditional ODS phase.The ODS-APS phase also exhibits superior simultaneous separation performance during the analysis of neutral and ionizable components in FDC.