Dicationic imidazolium ionic liquid modified silica as a novel reversed‐phase/anion‐exchange mixed‐mode stationary phase for high‐performance liquid chromatography

A dicationic imidazolium ionic liquid modified silica stationary phase was prepared and evaluated by reversed‐phase/anion‐exchange mixed‐mode chromatography. Model compounds (polycyclic aromatic hydrocarbons and anilines) were separated well on the column by reversed‐phase chromatography; inorganic anions (bromate, bromide, nitrate, iodide, and thiocyanate), and organic anions (p‐aminobenzoic acid, p‐anilinesulfonic acid, sodium benzoate, pathalic acid, and salicylic acid) were also separated individually by anion‐exchange chromatography. Based on the multiple sites of the stationary phase, the column could separate 14 solutes containing the above series of analytes in one run. The dicationic imidazolium ionic liquid modified silica can interact with hydrophobic analytes by the hydrophobic C₆ chain; it can enhance selectivity to aromatic compounds by imidazolium groups; and it also provided anion‐exchange and electrostatic interactions with ionic solutes. Compared with a monocationic ionic liquid functionalized stationary phase, the new stationary phase represented enhanced selectivity owing to more interaction sites.     

A strong inorganic acid‐initiated methacrylate polymerization strategy for room temperature preparation of monolithic columns for capillary electrochromatography

A facile strong inorganic acid‐initiated methacrylate polymerization strategy was developed for fabricating monolithic columns at room temperature. The prepared monoliths were characterized by FTIR spectrometry, mercury intrusion porosimeter and SEM, while their performance was evaluated by CEC for the separation of various types of compounds including alkyl benzenes, polycyclic aromatic hydrocarbons, nonsteroidal anti‐inflammatory drugs, anilines, and nitrophenol isomers. The column‐to‐column and batch‐to‐batch reproducibility for the prepared monoliths in terms of the RSD of EOF flow velocity, retention factor, and the minimum plate height of naphthalene ranged from 3.4 to 12.4%. The fabricated monoliths gave excellent performance for the separation of the test neutral compounds with the theoretical plates of 170 000–232 000 plates per meter for thiourea, and 77 400–112 300 plates per meter for naphthalene. The proposed strong inorganic acid‐initiated methacrylate polymerization strategy is a promising alternative for fabricating organic polymer‐based monoliths.     

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.     

Development of a dispersive liquid–liquid microextraction method using a lighter‐than‐water ionic liquid for the analysis of polycyclic aromatic hydrocarbons in water

A dispersive liquid–liquid microextraction method using a lighter‐than‐water phosphonium‐based ionic liquid for the extraction of 16 polycyclic aromatic hydrocarbons from water samples has been developed. The extracted compounds were analyzed by liquid chromatography coupled to fluorescence/diode array detectors. The effects of several experimental parameters on the extraction efficiency, such as type and volume of ionic liquid and disperser solvent, type and concentration of salt in the aqueous phase and extraction time, were investigated and optimized. Three phosphonium‐based ionic liquids were assayed, obtaining larger extraction efficiencies when trihexyl‐(tetradecyl)phosphonium bromide was used. The optimized methodology requires a few microliters of a lighter‐than‐water phosphonium‐based ionic liquid, which allows an easy separation of the extraction solvent phase. The obtained limits of detection were between 0.02 and 0.56 μg/L, enrichment factors between 109 and 228, recoveries between 60 and 108%, trueness between 0.4 and 9.9% and reproducibility values between 3 and 12% were obtained. These figures of merit combined with the simplicity, rapidity and low cost of the analytical methodology indicate that this is a viable and convenient alternative to the methods reported in the literature. The developed method was used to analyze polycyclic aromatic hydrocarbons in river water samples.     

Polycyclic aromatic hydrocarbons as test probes to investigate the retention behavior of 1,3‐alternate calix[4]arene silica‐bonded stationary phases

A series of polycyclic aromatic hydrocarbons (PAHs) of different size and shape has been used to characterize the chromatographic behavior of five calix[4]arene stationary phases in 1,3‐alternate conformation synthesized in our laboratory. The selection of linear, four‐ring nonlinear, and five‐ring PAHs gave data on selectivity changes across range of the calix[4]arene columns. Retention of the 12 aromatic solutes has been evaluated at various methanol contents in the mobile phase (70–100% v/v) and column temperatures (20–45°C). The thermodynamic parameters underlying the retention mechanisms revealed that each of the five calix[4]arene columns exhibited variation in selectivity and retention of PAHs caused by enthalpy and entropy effects. The calixarene stationary phases substituted with electron‐withdrawing groups exhibit enhanced selectivity toward PAHs in comparison to the rest of the investigated columns. The observed divergences are due to differences in solute–stationary phase interactions and originate in π–π and π‐electron transfer specific to the analytes and the type of calix[4]arene functionalization at the upper rim, as well as steric and sorption phenomena.     

Polydopamine‐functionalized monolithic silica column for mixed‐mode chromatography†

Dopamine is easy to self‐polymerize under alkaline conditions and the resultant polydopamine is easy to adhere to the surface of many organic and inorganic materials. Based on the characteristics of dopamine, in this paper, a new polydopamine functionalized monolithic silica column was successfully prepared for performing mixed‐mode chromatography. The performance of the column was evaluated by the separation of different types of samples including alkylbenzenes, polycyclic aromatic hydrocarbons, aromatic acids, phenols, and bases. The mechanism for the separation of these compounds was studied and appeared to involve the mixed interactions containing π?π, hydrophobic, electrostatic, and hydrophilic interactions.     

Investigating retention characteristics of a mixed‐mode stationary phase and the enhancement of monolith selectivity for high‐performance liquid chromatography

The synthesis and chromatographic behavior of an analytical size mixed‐mode bonded silica monolith was investigated. The monolith was functionalized by an in situ modification process of a bare silica rod with chloro(3‐cyanopropyl)dimethyl silane and chlorodimethyl propyl phenyl silane solutions. These ligands were selected in order to combine both resonance and nonresonance π‐type bonding within a single separation environment. Selectivity studies were undertaken using n‐alkyl benzenes and polycyclic aromatic hydrocarbons in aqueous methanol and acetonitrile mobile phases to assess the methylene and aromatic selectivities of the column. The results fit with the linear solvent strength theory suggesting excellent selectivity of the column was achieved. Comparison studies were performed on monolithic columns that were functionalized separately with cyano and phenyl ligands, suggesting highly conjugated molecules were able to successfully exploit both of the π‐type selectivities afforded by the two different ligands on the mixed‐mode column.     

Preparation,characterization and application of a new 25,27‐bis‐[2‐(5‐methylthiadiazole)thioethoxyl]‐26,28‐dihydroxy‐para‐tert‐butyl calix[4]arene stationary phase for HPLC

A new para‐tert‐butylcalix[4]arene column containing thiadiazole functional groups was prepared and used for the separation of polycyclic aromatic hydrocarbons, phenolic compounds, aromatic amines, benzoic acid and its derivatives by high‐performance liquid chromatography (HPLC). The effect of organic modifier content in the mobile phase on retention and selectivity of these compounds were investigated. The results indicate that the stationary phase behaves like reversed‐phase packing. However, hydrogen bonding, π–π and inclusion interactions seem to be involved in the separation process. The column has been successfully employed for the analysis of clenbuterol in pork and pig casing; the limit of detection and the limit of quantitation for this method by HPLC‐UV detection was 0.03 and 0.097 μg/mL, respectively; the method is demonstrated to be suitable and a competitive alternative analytical method for the determination of clenbuterol.     

Polytetrafluoroethylene‐jacketed stirrer modified with graphene oxide and polydopamine for the efficient extraction of polycyclic aromatic hydrocarbons

Steel stirrers jacketed with polytetrafluoroethylene can be regarded as an ideal substrate for stirrer bar sorptive extraction. However, it is still a great challenge to immobilize graphene onto a polytetrafluoroethylene stirrer due to the high chemical resistance of the surface of a polytetrafluoroethylene stirrer. We describe here a method to modify the surface of polytetrafluoroethylene stirrers with graphene. In this work, graphene was used as the sorbent due to its excellent adsorption capability for aromatic compounds, such as polycyclic aromatic compounds. Graphene was successfully immobilized onto polytetrafluoroethylene‐stirrer by a bio‐inspired polydopamine functionalization method. The graphene‐modified polytetrafluoroethylene‐stirrer shows good stability and tolerance to stirring, ultrasonication, strong acidic and basic solutions, and to organic solvents. The multilayer coating was characterized by scanning electronic microscopy and Fourier transform infrared spectroscopy. After the optimization of some experimental conditions, the graphene‐modified polytetrafluoroethylene stirrer was used for the stirrer bar sorptive extraction of polycyclic aromatic hydrocarbons, in which the binding between the polycyclic aromatic hydrocarbons and the graphene layer was mainly based on π–π stacking and hydrophobic interactions. The graphene‐modified polytetrafluoroethylene‐stirrer‐based stirrer bar sorptive extraction and high‐performance liquid chromatography method was developed for the determination of polycyclic aromatic hydrocarbons with great extraction efficiency, with enrichment factors from 18 to 62. The method has low limits of detection of 1–5 pg/mL, wide linear range (5–100 and 10–200 pg/mL), good linearity (R ≥ 0.9957) and good reproducibility (RSD ≤ 6.45%). The proposed method has been applied to determine polycyclic aromatic hydrocarbons in real dust samples. Good recoveries were obtained, ranging from 88.53 to 109.43%.     

Porous silica particles grafted with an amphiphilic side‐chain polymer as a stationary phase in reversed‐phase high‐performance liquid chromatography

The amphiphilic polymer‐grafted silica was newly prepared as a stationary phase in high‐performance liquid chromatography. Poly(4‐vinylpyridine) with a trimethoxysilyl group at one end was grafted onto porous silica particles and the pyridyl side chains were quaternized with 1‐bromooctadecane. The obtained poly(octadecylpyridinium)‐grafted silica was characterized by elemental analysis, diffuse reflectance infrared Fourier transform spectroscopy and Brunauer–Emmett–Teller analysis. The degree of quaternization of the pyridyl groups on the obtained stationary phase was estimated to be 70%. The selective retention behaviors of polycyclic aromatic hydrocarbons including some positional isomers were investigated using poly(octadecylpyridinium)‐grafted silica as an amphiphilic polymer stationary phase in high‐performance liquid chromatography and results were compared with commercially available polymeric octadecylated silica and phenyl‐bonded silica columns. The results indicate that the selectivity toward polycyclic aromatic hydrocarbons exhibited by the amphiphilic polymer stationary phase is higher than the corresponding selectivity exhibited by a conventional phenyl‐bonded silica column. However, compared with the polymeric octadecylated silica phase, the new stationary phase presents similar retention behavior for polycyclic aromatic hydrocarbons but different retention behavior particularly for positional isomers of disubstituted benzenes as the aggregation structure of amphiphilic polymers on the surface of silica substrate has been altered during mobile phase variation.     

氨基甲酸酯功能化离子液体高效液相色谱固定相的制备与初步评价

合成一种了氨基酸衍生物：4,4′-二苯亚甲基桥联-二[2-(1-咪唑基)-3-苯基丙醇氨基甲酸酯](ImPh-Carb),并将其键合到硅胶上制备了一种新的氨基甲酸酯功能化的离子液体HPLC固定相（ImPh-Carb-Silica）。 利用1H NMR、13C NMR、MS和FTIR对ImPh-Carb进行了表征;通过FTIR和元素分析对ImPh-Carb-Silica固定相进行了表征,根据N含量计算得到ImPh-Carb-Silica的键合量为0.19 mmol/g。 以5种芳烃、5种酚类化合物和4种有机磷农药为分析物,分别在正相和反相色谱模式下对固定相的色谱分离性能进行了评价,同时考察了流动相的变化与溶质保留因子lg k之间的关系。 结果表明,该固定相与溶质分子间存在多重作用力,如疏水、氢键、π-π和偶极-诱导偶极作用等,使其能同时在正相和反相色谱模式下使用;在正相色谱条件下固定相对酚类化合物和有机磷农药表现出较好的分离选择性。     

Study of a monolithic silica capillary column coated with poly(octadecyl methacrylate) for the reversed-phase liquid chromatographic separation of some polar and non-polar compounds

The performance of a monolithic silica capillary column coated with poly(octadecyl methacrylate) (ODM column) for the reversed-phase liquid chromatographic separation of some polar and non-polar compounds was studied, and the results were compared to those obtained by using a monolithic silica capillary column modified with octadecylsilyl-(N,N-diethylamino)silane (ODS column). Benzene and naphthalene derivatives, polycyclic aromatic hydrocarbons (PAHs), steroids, alkyl phthalates, and tocopherol homologues were used as test samples. In general, compounds with aromatic character, rigid and planar structures, and lower length-to-breadth ratios (more compacted structures) seem to have more preference for the polymer coated stationary phase (ODM). Compounds with acidic character have also a higher retention on ODM columns because of the presence of ester groups in the stationary phase. The polymer coated column allowed the separation of some PAHs, alkyl phthalates, steroids, and of beta- and gamma-tocopherol isomers which cannot be separated under the same conditions on ODS columns, while keeping similar column efficiency. These results allowed to suggest ODM columns as a good alternative to conventional ODS columns for reversed-phase liquid chromatography.     

Investigation of π–π and ion–dipole interactions on 1-allyl-3-butylimidazolium ionic liquid-modified silica stationary phase in reversed-phase liquid chromatography

1-Allyl-3-butylimidazolium bromide ionic liquid [AyBIm]Br was prepared and used for the modification of mercaptopropyl-functionalized silica through surface radical chain-transfer addition. The obtained ionic liquid-modified silica (SiImBr) was characterized by elemental analysis, infrared spectroscopy, NMR spectroscopy, and thermogravimetric analysis. The selective retention behaviours of polycyclic aromatic hydrocarbons (PAHs) including some positional isomers were investigated using SiImBr as a stationary phase in reversed-phase liquid chromatography. The results showed that SiImBr presented multiple interactions including hydrophobic, π–π, and ion–dipole interactions during the separation of PAHs and dipolar compounds. However, it is proposed that π–π and ion–dipole interactions play important roles in the separation of PAHs and dipolar compounds. These results indicate that the ionic liquid-modified silica stationary phase is promising for future applications. A commercially available monomeric octadecylated silica (ODS) column and a custom-made poly(styrene)-grafted silica (Sil-St_n) column were used as references.     

N-methylimidazolium-functionalized monolithic silica column for mixed-mode chromatography

The development of mixed-mode stationary phase to achieve multiple separation capabilities in one column is very important for high performance liquid chromatography. In this paper, a new specific stationary phase based on grafting N-methylimidazolium to a monolithic silica column was successfully prepared for performing capillary liquid chromatography. The characteristics of the column were evaluated by the separation of different types of compounds including inorganic anions, aromatic acids, nucleotides, polycyclic aromatic hydrocarbons, alkylbenzenes, and phenols. The mechanisms for the separation of these compounds were investigated and appeared to involve the mixed interactions including anion-exchange, hydrophilic, π-π, dipole-dipole, and hydrophobic interactions.     

Trennung polyzyklisher aromatischer kohlenwasserstoffe durch hochdruckflössigkeitschromatographie: Vergleich einer bestimmung von benzo[a]pyren nach trennung an saäulen mit quervernetztem celluseacetat und einem reversed-phase system

Separation of polycyclic aromatic hydrocarbons by high-pressure liquid chromatography. Comparison of the determination of benzo(a)pyrene with separation on columns of cross-linked cellulose acetate and a reversed-phase system.

The behaviour of a new high-pressure liquid chromatographic support, i.e. a cross-linked cellulose acetate, as selective separation material for polycyclic aromatic hydrocarbons is discussed. The determination of benzo[a]pyrene is described by an example of separating a so-called benzpyrene fraction. The separation of the benzpyrene fraction was possible by combining column systems with aluminium oxide, cross-linked cellulose acetate or a reversed-phase system. By means of a fluore- scence detector 0.1-0.8 ng benzo[a]pyrene could be detected in 5μl injection volume.     

Separations by Shape: Molecular Shape Recognition in Liquid Chromatography

Molecular shape can provide a basis for chromatographic separations that is distinct from other interaction mechanisms often considered in liquid chromatography. Molecular shape recognition, or shape selectivity, is most evident for the separation of isomeric compounds that have constrained molecular structures, such as polycyclic aromatic hydrocarbons. A central feature of shape-selective columns is conformational order within the stationary phase; this aspect of stationary phase morphology is revealed through spectroscopic studies and molecular dynamics simulations, and is correlated with chromatographic performance.

     

Properties of two amide‐based hydrophilic interaction liquid chromatography columns

Hydrophilic interaction liquid chromatography is a separation technique suitable for the separation of moderately and highly polar compounds. Various stationary phases (SPs) for hydrophilic interaction liquid chromatography are commercially available. While the SPs based on the same type of ligand are available from different providers, they can display a distinct retention characteristics and separation selectivity. The current work is focused on characterization and comparison of the separation systems of two amide‐based HPLC columns from two producers, i.e. XBridge Amide column and TSK gel Amide‐80 column. Several characterization procedures (tests) were used to investigate the differences between these columns. The chromatographic behavior of selected analytes indicates that multimodal interactions are responsible for retention and separation on these columns. Multiple testing approaches were used in order to reveal subtle differences between the SPs. Both amide‐based columns showed certain differences in retention, selectivity, and plate counts. Based on the tests used in this study, we conclude that the investigated columns provide a different degree of H‐bonding interactions.     

Retention behaviour of strong acid anions in ion-exclusion chromatography on sulfonate and carboxylate stationary phases

Some factors influencing the retention of strong-acid anions on ion-exclusion columns were investigated using columns with sulfonate and carboxylate functional groups. The nature of the functional group on the resin, the eluent pH and the eluent ionic strength all significantly affected the retention and separation of these analytes. Retention was observed for all strong-acid anions over the eluent pH range 2.2-5.7 and increased with both decreasing eluent pH and increasing eluent ionic strength. Some separation of strong-acid anions was possible when using a resin with carboxylate functional groups. It has also been demonstrated that strong-acid anions are poor markers of column void volume for ion-exclusion chromatography. A more accurate value was obtained using the neutral polymeric material dextran blue. When using eluents of low ionic strength, poor or fronted peak shapes were observed. A mechanism for these observations is proposed that relates the shape to ionic strength changes across the peak. A system peak was encountered under most experimental conditions. The properties of this peak are discussed and a cause for the system peak postulated.     

Silica-Based Phenyl and Octyl Bifunctional Imidazolium as a New Mixed-Mode Stationary Phase for Reversed-Phase and Anion-Exchange Chromatography

In this study, octylbenzimidazolium-modified silica (BeImC8-Sil) was prepared by covalent attachment of 1-octylbenzimidazole to γ-chloropropyl silica. The synthesized materials were characterized by the elemental analysis, IR spectrum, and thermogravimetric analysis. Due to the introduction of phenyl and octyl groups on the quaternary imidazolium, the developed BeImC8-Sil column can function via both reversed-phase and anion-exchange retention mechanisms. The chromatographic properties of the synthesized material were investigated by the separations of polycyclic aromatic hydrocarbons, mono-substituted derivatives of benzene, anilines, and phenols, revealing the existence of multiple interactions, including hydrogen bonding, π–π stacking, electrostatic forces, and hydrophobic interactions in reversed-phase mode; inorganic and organic anions were also separated mainly through anion-exchange interaction. The proposed BeImC8-Sil is a promising mixed-mode stationary phase for the separation of complex samples in high-performance liquid chromatography.

     

Terminal‐vinyl liquid crystal crown ether‐modified,vinyl‐functionalized hybrid silica monolith for capillary electrochromatography

A novel terminal‐vinyl liquid crystal crown ether (2‐[4‐(3‐undeceny‐1‐yloxy)‐phenyl]‐2‐[4′‐(4′‐carboxybenzo‐15‐crown‐5)‐phenyl] propane) (LCCE) was synthesized and used to modify hybrid silica‐based monolithic column possessing vinyl ligands for CEC. The monolithic silica matrix containing vinyl functionalities was prepared by in situ co‐condensation of tetramethoxysilane and vinyl‐trimethoxysilane via sol–gel process and chemically modified with LCCE by free radical polymerization procedure using α,α'‐azobisisobutyronitrile as an initiator. Morphology of the monolithic column was examined by SEM and mercury porosimetry and the successful incorporation of terminal‐vinyl LCCE to the vinyl‐hybrid monolith was characterized by infrared spectra. Polycyclic aromatic hydrocarbons, benzenediols, carbamate pesticides and steroids, were successfully separated on the column. The separations were dominated hydrogen bonding supplied by crown ether and hydrophobic interaction offered by the liquid crystal. The effect of ACN concentration on separation performance was studied and the result indicated that RP retention mechanism played an important role. Reproducibilities of migration times for the six selected polycyclic aromatic hydrocarbons were reasonable, with relative standard deviation less than 3.50% for five consecutive within‐column runs and were 8.38–9.11% for column‐to‐column measurements of three columns.