Imidazolium‐embedded iodoacetamide‐functionalized silica‐based stationary phase for hydrophilic interaction/reversed‐phase mixed‐mode chromatography

A novel imidazolium‐embedded iodoacetamide‐functionalized silica‐based stationary phase has been prepared by surface radical chain‐transfer polymerization. The stationary phase was characterized by Fourier transform infrared spectrometry, thermogravimetric analysis, and element analysis. Fast and efficient separations of polar analytes, such as nucleosides and nucleic acid bases, water‐soluble vitamins and saponins, were well achieved in hydrophilic interaction chromatography mode. Additionally, a mixed mode of hydrophilic interaction and reversed‐phase could be also obtained in the analysis of polar and nonpolar compounds, including weak acidic phenols, basic anilines and positional isomers, with high resolution and molecular‐planarity selectivity, outperforming the commercially available amino column. Moreover, simultaneous separation of polar and nonpolar compounds was also achieved. In conclusion, the multimodal retention capabilities of the imidazolium‐embedded iodoacetamide‐functionalized silica‐based column could offer a wide range of retention behavior and flexible selectivity toward hydrophilic and hydrophobic compounds.     

Quinine‐modified polymer monolithic column with reversed‐phase /strong anion‐exchange mixed‐mode for pressurized capillary electrochromatography

Via the facile ring‐opening reaction of epoxy groups with quinine, a novel polymer monolith with quaternary ammonium for reversed‐phase/strong anion‐exchange mixed‐mode has been fabricated for pressurized capillary electrochromatography (pCEC). Optimization on the preparation of quinine‐modified monoliths has been investigated, and characteristics including morphology, permeability, mechanical stability, reproducibility, and column performance have been also studied. Active quaternary ammonium groups were conveniently produced to generate cationic action sites and stable anodic electroosmotic flow. Multiple interactions including reversed‐phase, strong anion‐exchange, electrostatic repulsion and π–π stacking interactions were obtained. Satisfactory separation capability of various analytes such as alkylbenzenes, polycyclic aromatic hydrocarbons, benzoic acid and its homologs, and β₂‐receptor excitants has been achieved. Applied to the real sample, the good resolution of three alkaloids in Corydalis yanhusuo were achieved by pCEC with the quinine‐modified monolith. The results light a potential access to facilely fabricating quaternary ammonium‐functionalized polymer monolith with multiple interactions for efficient electrochromatography profiling of various compounds.     

Poly(l‐lactic acid)‐modified silica stationary phase for reversed‐phase and hydrophilic interaction liquid chromatography

Poly(l ‐lactic acid) is a linear aliphatic thermoplastic polyester that can be produced from renewable resources. A poly(l ‐lactic acid)‐modified silica stationary phase was newly prepared by amide bond reaction between amino groups on aminopropyl silica and carboxylic acid groups at the end of the poly(l ‐lactic acid) chain. The poly(l ‐lactic acid)‐silica column was characterized in reversed‐phase liquid chromatography and hydrophilic interaction liquid chromatography with the use of different mobile phase compositions. The poly(l ‐lactic acid)‐silica column was found to work in both modes, and the retention of test compounds depending on acetonitrile content exhibited “U‐shaped” curves, which was an indicator of reversed‐phase liquid chromatography/hydrophilic interaction liquid chromatography mixed‐mode retention behavior. In addition, carbonyl groups included into the poly(l ‐lactic acid) backbone work as an electron‐accepting group toward a polycyclic aromatic hydrocarbon and provide π–π interactions.     

Preparation of a weak anion exchange/hydrophobic interaction dual‐function mixed‐mode chromatography stationary phase for protein separation using click chemistry

In this study, 3‐diethylamino‐1‐propyne was covalently bonded to the azide‐silica by a click reaction to obtain a novel dual‐function mixed‐mode chromatography stationary phase for protein separation with a ligand containing tertiary amine and two ethyl groups capable of electrostatic and hydrophobic interaction functionalities, which can display hydrophobic interaction chromatography character in a high‐salt‐concentration mobile phase and weak anion exchange character in a low‐salt‐concentration mobile phase employed for protein separation. As a result, it can be employed to separate proteins with weak anion exchange and hydrophobic interaction modes, respectively. The resolution and selectivity of the stationary phase were evaluated in both hydrophobic interaction and ion exchange modes with standard proteins, respectively, which can be comparable to that of conventional weak anion exchange and hydrophobic interaction chromatography columns. Therefore, the synthesized weak anion exchange/hydrophobic interaction dual‐function mixed‐mode chromatography column can be used to replace two corresponding conventional weak anion exchange and hydrophobic interaction chromatography columns to separate proteins. Based on this mixed‐mode chromatography stationary phase, a new off‐line two‐dimensional liquid chromatography technology using only a single dual‐function mixed‐mode chromatography column was developed. Nine kinds of tested proteins can be separated completely using the developed method within 2.0 h.     

Tetra‐proline‐modified calix[4]arene‐bonded silica stationary phase for simultaneous reversed‐phase/hydrophilic interaction mixed‐mode chromatography

A new water‐soluble tetra‐proline‐modified calix[4]arene‐bonded silica stationary phase was prepared straightforwardly by an indirect method and characterized by elemental analysis, energy dispersive Spectrometry, solid‐state ¹³C NMR spectroscopy, Fourier‐transform infrared spectroscopy, and thermogravimetric analysis. Due to the simultaneous introduction of polar tetra‐proline and nonpolar calix[4]arene, the developed column possessing a double retention mode of reverse‐phase liquid chromatography and hydrophilic interaction liquid chromatography. A series of hydrophobic and hydrophilic test samples, including nucleosides and nucleotides, amines, monosubstituted benzenes, chiral compounds, and phenols, were used to evaluate the developed stationary phase. A rapid separation capability, high separation efficiency, and selectivity were achieved based on the multiple interactions between solutes and tetra‐proline‐modified calix[4]arene‐bonded silica stationary phase. Moreover, the developed stationary phase was further used to detect and separate hexamethylenetetramine in rice flour. All the results indicated the potential merits of the developed stationary phase for simultaneous separation of complex hydrophobic and hydrophilic samples with high selectivity.     

Improvement of the chromatographic separation performance of an imidazolium ionic liquid functionalized silica column by in situ anion‐exchange with dodecyl sulfonate and dodecylbenzene sulfonate anions

The anionic part of ionic liquids can provide additional interactions during chromatographic separations. In this work, the chromatographic separation performance of a silica column functionalized with 1‐propyl‐3‐methylimidazolium chloride ionic liquid was improved by in situ anion‐exchange from chloride anions to dodecyl sulfonate anions and dodecylbenzene sulfonate anions. The separation performances of these ionic liquid functionalized phases were investigated and compared with each other using polycyclic aromatic hydrocarbons, phthalates, parabens, and phenols as model compounds. Results indicated that the new columns presented a better chromatographic separation than the original one. This was ascribed retention mechanism from organic anions. The introduction of dodecyl sulfonate anions increased the hydrophobicity of stationary phase. Furthermore, the phenyl groups of dodecylbenzene sulfonate anions could provide an enhanced selectivity to aromatic compounds such as polycyclic aromatic hydrocarbons by π–π interactions. Analysis repeatability of the new columns was satisfactory (RSD of retention time, 0.10–0.40%; RSD of peak area, 0.66–0.84%).     

Evaluation and application of a mixed‐mode chromatographic stationary phase in two‐dimensional liquid chromatography for the separation of traditional Chinese medicine

In this study, two mixed‐mode chromatography stationary phases (C8SAX and C8SCX) were evaluated and used to establish a two‐dimensional liquid chromatography system for the separation of traditional Chinese medicine. The chromatographic properties of the mixed‐mode columns were systematically evaluated by comparing with other three columns of C8, strong anion exchanger, and strong cation exchanger. The result showed that C8SAX and C8SCX had a mixed‐mode retention mechanism including electrostatic interaction and hydrophobic interaction. Especially, they were suitable for separating acidic and/or basic compounds and their separation selectivities could be easily adjusted by changing pH value. Then, several off‐line 2D‐LC systems based on the C8SAX in the first dimension and C8SAX, C8SCX, or C8 columns in the second dimension were developed to analyze a traditional Chinese medicine—Uncaria rhynchophylla. The two‐dimensional liquid chromatography system of C8SAX (pH 3.0) × C8SAX (pH 6.0) exhibited the most effective peak distribution. Finally, fractions of U. rhynchophylla prepared from the first dimension were successfully separated on the C8SAX column with a gradient pH. Thus, the mixed‐mode stationary phase could provide a platform to separate the traditional Chinese medicine in practical applications.     

Preparation and evaluation of a novel bonded imidazolium ionic liquid as stationary phase for gas chromatography

1‐Butyl‐3‐[(3‐trimethoxysilyl)propyl]imidazolium chloride ionic liquid was synthesized and chemically modified onto the inner wall of a fused capillary column as a stationary phase for gas chromatography. The 1‐butyl‐3‐[(3‐trimethoxysilyl)propyl]imidazolium chloride ionic liquid bonded capillary column was evaluated in detail. The results revealed that the ionic liquid bonded capillary column exhibited high column efficiency of 1.08 × 10⁴ plates/m, and good chromatographic separation selectivity (α ) for polar and non‐polar substances, and a good thermal stability between room temperature and 400°C. Moreover, the determination of thermodynamic parameters and the linear solvation energy relationship were further carried out. The results indicated that the chromatographic retention of each probe molecule on the ionic liquid bonded stationary phase was an enthalpy‐driven process, and the system constants of the linear solvation energy relationship signified that the dispersion interaction, the hydrogen bonding acidity and hydrogen bonding basicity were dominant interactions between probes and stationary phase among five interactions during the chromatographic separation. However, the contribution of each specific interaction for the stationary phase is ranked as the dispersion interaction > the hydrogen bonding basicity > the hydrogen bonding acidity.     

Development of a decaaza‐cyclophane stationary phase for high‐performance liquid chromatography

A new stationary phase for high‐performance liquid chromatography was prepared by covalently bonding a heteroatom‐bridged cyclophane onto silica gel using 3‐aminopropyltriethoxysilane as the coupling reagent. The structure of the new material was characterized by infrared spectroscopy, elemental analysis, and thermogravimetric analysis. The linear solvation energy relationship method was successfully employed to evaluate the new phase with a set of 25 solutes, and compared with octadecylsilyl and p‐tert‐butyl‐calix[4]arene bonded stationary phases. The retention characteristics of the new phase are similar to the octadecylsilyl and conventional calixarene phases, and it also has distinctive features. In addition, the chromatographic behavior of the phase was illustrated by eluting alkylbenzenes and inorganic anions in the reversed‐phase mode and anion‐exchange mode, respectively. Thus, multi‐interaction mechanisms and mixed‐mode separation of the new phase can very likely guarantee its promising application in the analysis of complex samples. The column has been successfully employed for the analysis of triazines in milk, and it is demonstrated to be a competitive alternative analytical method for the determination of triazine herbicide residues.     

Gold nanoparticle decorated graphene oxide/silica composite stationary phase for high‐performance liquid chromatography

In the initial phase of this study, graphene oxide (GO)/silica was fabricated by assembling GO onto the silica particles, and then gold nanoparticles (GNPs) were used to modify the GO/silica to prepare a novel stationary phase for high‐performance liquid chromatography. The new stationary phase could be used in both reversed‐phase chromatography and hydrophilic interaction liquid chromatography modes. Good separations of alkylbenzenes, isomerides, amino acids, nucleosides, and nucleobases were achieved in both modes. Compared with the GO/silica phase and GNPs/silica phase, it is found that except for hydrophilicity, large π‐electron systems, hydrophobicity, and coordination functions, this new stationary phase also exhibited special separation performance due to the combination of 2D GO with zero‐dimensional GNPs.     

Preparation of a monolithic column with a mixed‐mode stationary phase of reversed‐phase/hydrophilic interaction for capillary liquid chromatography

A monolithic capillary column with a mixed‐mode stationary phase of reversed‐phase/hydrophilic interaction chromatography was prepared for capillary liquid chromatography. The monolith was created by an in‐situ copolymerization of a homemade monomer N,N‐dimethyl‐N‐acryloxyundecyl‐N‐(3‐sulfopropyl) ammonium betaine and a crosslinker pentaerythritol triacrylate in a binary porogen agent consisting of methanol and isopropanol. The functional monomer was designed to have a highly polar zwitterionic sulfobetaine terminal group and a hydrophobic long alkyl chain moiety. The composition of the polymerization solution was systematically optimized to permit the best column performance. The columns were evaluated by using acidic, basic, polar neutral analytes, as well as a set of alkylbenzenes and Triton X100. Very good separations were obtained on the column with the mixed‐mode stationary phase. It was demonstrated that the mixed‐mode stationary phase displayed typic dual retention mechanisms of reversed‐phase/hydrophilic interaction liquid chromatography depending on the content of acetonitrile in the mobile phase. The method for column preparation is reproducible.     

Magnetic solid‐phase extraction of angiotensin II receptor antagonists in human urine and plasma with a reversed‐phase/cation‐exchange mixed‐mode sorbent

Biocompatible magnetic nanoparticles that featured divinylbenzene and sulfonate functionalities were used for the magnetic solid‐phase extraction of five angiotensin II receptor antagonists from human urine and plasma samples based on a reversed‐phase and cation‐exchange mixed‐mode mechanism. Under the optimized extraction conditions, coupled to high‐performance liquid chromatography with fluorescence detection, this proposed method was found to be accurate and precise with relative standard deviations of less than 11.7%, and a good recovery of 80.1–119.5% for both samples. The linear ranges were 0.2–2000 and 0.2–2500 ng/mL along with correlation coefficients above 0.9923 and 0.9928 for urine and plasma samples, respectively. Limits of detection were 0.01–5.74 and 0.01–1.31 ng/mL, respectively. The proposed magnetic solid‐phase extraction based on the magnetic nanoparticles functionalized with divinylbenzene and sulfonate was a reliable and convenient sample pretreatment method and had the potential for isolating and enriching the angiotensin II receptor antagonists in biological samples.     

Incorporation of ionic liquid into porous polymer monoliths to enhance the separation of small molecules in reversed‐phase high‐performance liquid chromatography

An ionic liquid was incorporated into the porous polymer monoliths to afford stationary phases with enhanced chromatographic performance for small molecules in reversed‐phase high‐performance liquid chromatography. The effect of the ionic liquid in the polymerization mixture on the performance of the monoliths was studied in detail. While monoliths without ionic liquid exhibited poor resolution and low efficiency, the addition of ionic liquid to the polymerization mixture provides highly increased resolution and high efficiency. The chromatographic performances of the monoliths were demonstrated by the separations of various small molecules including aromatic hydrocarbons, isomers, and homologues using a binary polar mobile phase. The present column efficiency reached 27 000 plates/m, which showed that the ionic liquid monoliths are alternative stationary phases in the separation of small molecules by high‐performance liquid chromatography     

Open tubular columns with mixed‐mode reversed‐phase and weak anion‐exchange stationary phase for capillary electrochromatography

Columns for open tubular capillary electrochromatography, coated with a mixed‐mode (RP/ion‐exchange) stationary phase, were prepared by using the sol–gel method. The synthetic procedure was optimized by changing the ratios of tetraethoxysilane, octyltriethoxysilane, and 3‐aminopropyltriethoxysilane in the initial sol. SEM studies reveal that a coating with about 400 nm thickness can be obtained. The inner surface properties of these capillaries were probed by measuring the EOF as a function of pH. The surface of this stationary phase contains octyl, amine, and residual silanol moieties; the amine and silanol groups determine the net charge on the inner surface of the capillary and can produce a switchable EOF (anodal/cathodal). The performances of the columns were evaluated by open tubular capillary electrochromatography using a wide range of compounds (polycyclic aromatic hydrocarbons, aromatic acids, and aromatic amines).     

A reversed phase/hydrophilic interaction/ion exchange mixed-mode stationary phase for liquid chromatography

A new stationary phase demonstrated effective separation towards polar analytes or their counterions within a single run.     

Anion separations for liquid chromatography using propylpyridinium silica as the stationary phase

This work describes the characterization and potential applications of a silica-based anion-exchange phase prepared by a two-step modification process that incorporates a propylpyridinium group. The effects of pH and eluent concentration on anion separation were examined using 150 mm × 3.9 mm HPLC columns packed with the new phase. The mobile phase pH values ranged from 3.8 to 6.6 using phthalic acid/Tris solutions. The best separation was achieved using 2.5 mmol L⁻¹ phthalate/2.4 mmol L⁻¹ Tris solution at pH 4.2 as mobile phase with non-suppressed conductivity detection. The new stationary phase was used for the separation of some inorganic and organic anions showing good resolution. The stability of the silica-based anion exchange phase was also evaluated.Analytical curves, for concentrations ranging from 0.25 to 10 mg L⁻¹ for the inorganic anions chloride, nitrite, bromide and nitrate, showed good linear correlations (r > 0.998). The method was tested with certified rainwater samples. The measured and certified values were in good agreement, indicating that the new phase holds significant promise for the analysis of these anions in environmental samples.     

Adjusting the chromatographic properties of poly(ionic liquid)‐modified stationary phases by substitution on the imidazolium cation

Poly(ionic liquid)‐modified stationary phases can have multiple interactions with solutes. However, in most stationary phases, separation selectivity is adjusted by changing the poly(ionic liquid) anions. In this work, two poly(ionic liquid)‐modified silica stationary phases were prepared by introducing the cyano or tetrazolyl group on the pendant imidazolium cation on the polymer chains. Various analytes were selected to investigate their mechanism of retention in the stationary phases using different mobile phases. Two poly(ionic liquid)‐modified stationary phases can provide various interactions toward solutes. Compared to the cyano‐functionalized poly(ionic liquid) stationary phase, the tetrazolyl‐functionalized poly(ionic liquid) stationary phase provides additional cation‐exchange and π‐π interactions, resulting in different separation selectivity toward analytes. Finally, applicability of the developed stationary phases was demonstrated by the efficient separation of nonsteroidal anti‐inflammatory drugs.     

Characterization of hexacationic imidazolium ionic liquids as effective and highly stable gas chromatography stationary phases

Polycationic ionic liquids (ILs) are an attractive class of ILs with great potential applicability as gas chromatography stationary phases. A family of hexacationic imidazolium ILs derived from the cycloalkanol family was chemically first prepared in a straightforward manner and then applied for analytical separation purposes. Four tuneable engineering vectors, namely cation ring size structure, anion nature, spatial disposition of cycloalkanol substituents and O‐substitution, were considered as experimental parameters for the design of the desired ionic liquids. A total number of five new phases based on a common benzene core respectively exhibited column efficiencies around to 2500 plates/m, broad operating temperature ranges and also, even more importantly, good thermal stabilities (bleeding temperature between 260 and 365°C), finding variations in the selectivity and analytes elution orders depending on the IL structures. Their solvation characteristics were evaluated using the Abraham solvation parameter model, establishing clear correlations between their cation structure and retention capability with respect to certain analytes. The study of relationships between the ILs structure and solvation parameters gives us an idea of the IL stationary phase to be used for specific separations.     

Comprehensive analysis of pharmaceutical products using simultaneous mixed‐mode (ion‐exchange/reversed‐phase) and hydrophilic interaction liquid chromatography

Liquid chromatographic assays were developed using a mixed‐mode column coupled in sequence with a hydrophilic interaction liquid chromatography column to allow the simultaneous comprehensive analysis of inorganic/organic anions and cations, active pharmaceutical ingredients, and excipients (carbohydrates). The approach utilized dual sample injection and valve‐mediated column switching and was based upon a single high‐performance liquid chromatography gradient pump. The separation consisted of three distinct sequential separation mechanisms, namely, (i) ion‐exchange, (ii) mixed‐mode interactions under an applied dual gradient (reversed‐phase/ion‐exchange), and (iii) hydrophilic interaction chromatography. Upon first injection, the Scherzo SS C₁₈ column (Imtakt) provided resolution of inorganic anions and cations under isocratic conditions, followed by a dual organic/salt gradient to elute active pharmaceutical ingredients and their respective organic counterions and potential degradants. At the top of the mixed‐mode gradient (high acetonitrile content), the mobile phase flow was switched to a preconditioned hydrophilic interaction liquid chromatography column, and the standard/sample was reinjected for the separation of hydrophilic carbohydrates, some of which are commonly known excipients in drug formulations. The approach afforded reproducible separation and resolution of up to 23 chemically diverse solutes in a single run. The method was applied to investigate the composition of commercial cough syrups (Robitussin®), allowing resolution and determination of inorganic ions, active pharmaceutical ingredients, excipients, and numerous well‐resolved unknown peaks.     

Separation behavior of basic compounds on unbonded silicon oxynitride and silica high‐performance liquid chromatography stationary phases with reversed‐phase eluents

Unbonded silicon oxynitride and silica high‐performance liquid chromatography stationary phases have been evaluated and compared for the separation of basic compounds of differing molecular weight, pK_a, and log D using aqueous/organic mobile phases. The influences of percentage of organic modifier, buffer pH, and concentration in the mobile phase on base retention were investigated on unbonded silicon oxynitride and silica phases. The results confirmed that unbonded silicon oxynitride and silica phases demonstrated excellent separation performance for model basic compounds and both the unbonded phases examined possessed a hydrophobic/adsorption and ion‐exchange character. The silicon oxynitride stationary phase exhibited high hydrophilicity compared with silica with a reversed‐phase mobile phase. An ion‐exclusion‐type mechanism becomes predominant for the separation of three aimed bases on the silicon oxynitride column at pH 2.8. Different from silicon oxynitride stationary phase, no obvious change for the retention time of three model bases on silica stationary phase at pH 2.8 can be observed.