Imidazolium ionic liquid-enhanced poly(quinine)-modified silica as a new multi-mode chromatographic stationary phase for separation of achiral and chiral compounds

Highlights

• Imidazolium-enhanced poly(quinine)-modified silica stationary phase was synthesized.

• Advantages of imidazolium and quinine co-functional on silica were presented.

• Multi-mode chromatographic performances of the Sil-PQn-MIm column were proved.

• The stationary phase possessed good enough reproducibility and high efficiency.

Abstract

In this work, a novel imidazolium ionic liquid-functionalized poly(quinine)-modified silica stationary phase (Sil-PQn-MIm) was successfully synthesized via surface radical chain transfer and nucleophilic substitution reaction. The modified silica was confirmed by series of characterizations including Fourier transform infrared spectroscopy (FT-IR), elemental analysis (EA), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The multi-mode chromatographic performances of the Sil-PQn-MIm column were investigated by anion-exchange mode for separation of aromatic acid samples, hydrophilic interaction mode for separation of nucleosides/nucleobases and sulfanilamides, and reversed-phase mode for separation of alkylbenzenes, benzene and polycyclic aromatic hydrocarbons (PAHs), and the Tanaka test mixtures, respectively. As expected, compared to the Sil-PQn column only with quinine as functional group, the Sil-PQn-MIm column further modified by imidazolium ionic liquid possessed higher separation performance, especially for the separation of nucleosides/nucleobases. The effects including buffer concentration, pH, organic solvent content and column temperature on chromatographic performance were studied, which proved that multiple interactions including electrostatic, hydrophobic and hydrophilic interactions can be simultaneously existed between the stationary phase and the analytes. In addition, reproducibility and efficiency of the Sil-PQn-MIm column were also investigated, the results illustrated that the stationary phase have good enough reproducibility (RSDs 0.15%–0.72%, n = 7) and high efficiency (plates per meter, ~90000 plates/m). In conclusion, the prepared stationary phase with multiple-mode retention capabilities could realize separation for various types of samples by optimizing chromatographic conditions, even for some chiral compounds.

Introduction

High performance liquid chromatography (HPLC) with high speed, high resolution and high sensitivity plays a dominantly pivotal role for qualitative or quantitative analysis of multifarious samples in separation science [[1], [2], [3], [4], [5], [6], [7]]. The chromatographic stationary phase (SP), whose chromatographic separation performance is mainly determined by functional groups on the surface of SP [[8], [9], [10], [11]], is conventionally regarded as the core of chromatographic separation. Therefore, the exploitation of advanced SP with reasonable functional modification is evidently quite important to promote the development of chromatographic separation science.

Nowadays, single mode separation in HPLC consisting of ion-exchange chromatography (IEC) [12,13], hydrophilic interaction chromatography (HILIC) [[14], [15], [16], [17], [18]], reversed-phase liquid chromatography (RPLC) [19,20], normal phase liquid chromatography (NPLC) [21] and size-exclusion chromatography (SEC) [22], has been widely used. The single mode HPLC will inevitably exhibit its inherent defects and insufficiency at the separation of complex samples. For example, for RPLC, it can only provide hydrophobic interaction between the stationary phase and the analytes, which limits its application range for polar or hydrophilic compounds [23] and cannot be used in high concentration aqueous phase due to bending of alkyl chains [24]. And the HILIC with polar functional group is exactly the opposite of the RPLC, which is only applicable to the separation of polar hydrophilic compounds [25]. The IEC relies solely on the ion pair interaction between the stationary phase and the analytes, which is only limited to analytes that possess oppositely charged to the stationary phase [26]. Therefore, it is urgent to develop a chromatographic stationary phase with multiple different types of interaction between the stationary phase and the analytes.

Multi-mode HPLC (MHPLC) with different types of functional groups, which is completely regarded as an effective substitute of single mode HPLC, recently obtained dominant impetus in a variety of analytical areas, including pharmaceutical analysis, food analysis as well as environmental analysis [[27], [28], [29], [30], [31], [32], [33], [34]]. Therefore, it is important to develop new multi-mode stationary phases with outstanding chromatographic performance for the separation of complex samples.

Quinine, with multiple functional groups consisting of benzoylpyridinium, a tertiary ammonium positively charged center, multiple chiral sites and a hydroxyl group and a terminal olefin functional group that are easy to be modified, which has been widely used as a modified molecule to bond to the surface of silica gel [[35], [36], [37]] or as a monomer to be polymerized into a monolithic material [38,39] for chromatographic separation.

On the other hand, ionic liquids (ILs), which are composed of organic cations and organic or inorganic anions, have gained wide applications in many disciplines including synthesis, catalysis and chromatographic separation [[40], [41], [42]]. The imidazolium cation-based ILs, which were regarded as a significant member of ILs family, possess outstanding characteristics including easy functionalization, controllable hydrophilicity/hydrophobicity, making it broadly used as a functional group for chromatographic stationary phases [43,44]. In our previous work, many SP based on imidazolium cation-based ILs functionalized with different alkyl chains have been reported [[45], [46], [47], [48], [49]].

In this work, combining the excellent properties of quinine and imidazolium ILs in chromatographic separation, a novel multi-mode stationary phase based on imidazolium-functionalized poly(quinine)-modified silica was prepared. For synthesis, as a monomer, quinine was polymerized on the silica surface by surface radical chain transfer reaction, firstly. And then the poly-quinine SP was further modified with N-methylimidazole using 2-chloroethyl isocyanate as a bridge molecule by a nucleophilic substitution reaction to obtain a new stationary phase (Sil-PQn-MIm). And the schematic diagram for the preparation of Sil-PQn and Sil-PQn-MIm was shown in Fig. 1. Chromatographic separation performance of this column was evaluated by a wide variety of compounds. Aromatic acid compounds were separated in IEC. Sulfanilamides and nucleosides/nucleobases were separated in HILIC. Alkylbenzenes, benzene and PAHs, Tanaka test mixtures were separated in RPLC. And three pairs of enantiomers were separated by chiral separation. In addition, Sil-PQn column only with quinine as functional group were used as a reference of Sil-PQn-MIm. We found the chromatographic performances of Sil-PQn-MIm were enhanced by the further grafting of imidazolium ILs.