Applications of two-dimensional ion chromatography for analytes determination in environmental matrix: A review

Ion chromatography (IC) has grown in usage rapidly since its first introduction in 1975. However, IC is still sometimes unable to separate target analytes from coexisting components well with identical elution time, due to the limited resolution and column capacity, especially in the presence of high-level salt matrix. These limitations hence drive IC to develop two-dimensional IC (2D-IC). In this review, we capture the 2D-IC applications in environmental samples via the perspective of coupling different IC columns, which aim to summarize where these 2D-IC methods fit in. In sequence, we firstly review the principles of 2D-IC and emphasize one-pump column-switching IC (OPCS IC) because it is a simplified 2D-IC that only uses one set of IC system. We then compare typical 2D-IC and OPCS IC performances in terms of application scope, method detection limit, drawbacks, and expectations. Finally, we propose some challenges of current methods and opportunities for future research. For instance, it is challenging to couple anion exchange column and capillary column in OPCS IC due to the incompatibility between flow path dimensions and suppressor; coupling ion exclusion column and mixed-bed column may be promising to simultaneously determine anions and cations in weak acids or salts. The details of this study may help practitioners to better understand and implement 2D-IC methods and meanwhile motivate researchers to fill in the knowledge gap in the future.

Preparation of an anion stationary phase modified by quaternary ammoniated allyl glycidyl ether

制备了一种季铵化烯丙基缩水甘油醚(allyl glycidyl ether, AGE)改性聚合物基质的阴离子固定相应用于离子色谱系统。它是利用AGE与水解的聚甲基丙烯酸缩水甘油酯-二乙烯基苯(poly(glycidylmethacrylate-divinylbenzene, GMA-DVB)微球表面残留双键通过表面自由基共聚,再通过N,N-二甲基乙醇胺(一种叔胺)进行开环反应制备得到的。通过考察有机叔胺类型、微球水解、单体和引发剂用量、反应温度和时间对7种阴离子分离性能的影响,优化了制备条件。采用扫描电镜、元素分析对所得阴离子固定相进行了表征。结果表明,采用预先水解的GMA-DVB微球(水解过程中微球表面丰富的环氧基团转化为羟基)相对于直接采用GMA-DVB微球有助于降低固定相的交换容量和微球自身的非离子吸附作用;通过淋洗液浓度和目标离子保留因子的拟合结果证实了该固定相保留机理为典型的离子交换作用。使用碳酸根淋洗液,在优化的色谱条件下,该固定相可在13 min内实现常见7种无机阴离子的基线分离,并表现出较高的柱效(Cl^-理论塔板数为49000块/m)。该色谱柱实用性通过分析自来水实际样品进行了验证。

Contribution of the "Click Chemistry" Toolbox for the Design, Synthesis, and Resulting Applications of Innovative and Efficient Separative Supports: Time for Assessment

The last two decades have seen the rapid expansion of click chemistry methodology in various domains closely related to organic chemistry. It has notably been widely developed in the area of surface chemistry, mainly because of the high-yielding character of reactions of the "click" type. Especially, this powerful chemical reaction toolbox has been adapted to the preparation of stationary phases from the corresponding chromatographic supports. A plethora of selectors can thus be immobilized on either organic, inorganic, or hybrid stationary phases that can be used in different chromatographic modes. This review first highlights the few different chemical ligation strategies of the "click" type that are up to now mainly devoted to the development of functionalized supports for separation sciences. Then, it gives in a second part an up-to-date survey of the different studies dedicated to the preparation of click chemistry-based chromatographic supports while highlighting the powerful and versatile character of the "click" ligation strategy for the design, synthesis, and developments of more and more complex systems that can find promising applications in the area of analytical sciences, in domains as varied as enantioselective separation, glycomics, proteomics, genomics, metabolomics, etc.

Rapid polymerization of polyhedral oligomeric siloxane-based zwitterionic sulfoalkylbetaine monolithic column in ionic liquid for hydrophilic interaction capillary electrochromatography

A novel polyhedral oligomeric siloxane (POSS)-based zwitterionic monolithic capillary column was prepared via one-pot polymerization in ionic liquid porogen, using N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl)ammonium betaine (DMMSA) and methacrylic ethyl trimethylammonium chloride (META) as binary functional monomers, and methacryl substituted POSS as cross-linker. The pore structure, permeability and homogeneity were well tuned by optimizing the polymerization conditions. The resultant monolith was characterized by scanning electron microscopy, nitrogen adsorption/desorption isotherm and Fourier transform infrared spectroscopy. The incorporation of zwitterionic ligand (DMMSA), quaternary amine group (META) and rigid POSS skeleton endows the hybrid organic-silica monolith with high hydrophilicity, electrostatic interaction and good mechanical stability, as well as a tunable electroosmotic flow over wide pH range. A close investigation of capillary electrochromatographic separations of different types of polar compounds such as bases, nucleosides and benzoic acids on such stationary phase exhibited a retention independent column efficiency up to 118,000 plates/m (thiourea), as well as a mixed-mode hydrophilic interaction chromatography (HILIC) retention mechanism including weak electrostatic interaction, hydrophobic interaction and anion exchange.

Layer-by-layer coating and chemical cross-linking of multilayer polysaccharides on silica for mixed-mode HPLC application

A facile, controllable and environmentally friendly method for fabricating a novel polysaccharide-silica composite stationary phase (SiO₂@(HA-CS)₁₂) was developed in this report. Two natural polysaccharides (hyaluronan acid and chitosan) were controllably coated on the silica surface using a layer-by-layer assembly technique, and then the polysaccharide shell was chemically cross-linked to improve the stability. The column efficiency of the SiO₂@(HA-CS)₁₂ column reached 74 000 plates per m in HILIC mode and 20 100 plates per m in IEC mode, which indicates great potential for separating polar and charged samples.

A hydrolytically stable amide polar stationary phase for hydrophilic interaction chromatography

A novel amide functionalized polar stationary phase has been described by grafting acrylamide polymer coating onto hydrolysed poly (glycidyl methacrylate-divinylbenzene) (GMA-DVB) microsphere. The grafting of acrylamide coating is performed via free radical polymerization of acrylamide with pendant double bonds of hydrolysed GMA-DVB microsphere. The obtained stationary phase (G-pAM) possesses favorable hydrophilicity, as proved by strong retention and good separation ability towards several types of model polar analytes (e.g. 71913 plates/m plate count of sucrose), and excellent hydrolytically stability, as indicated by extremely low bleed level (much superior to commercial ones, ~23.7-fold-~77.4-fold lower). Negligible baseline drift under gradient elution (water fraction even up to 50%) was observed. It also exhibits good selectivity in the separation of isomers and homologue sugars.

Preparation of two amide-bonded stationary phases and comparative evaluation under mixed-mode chromatography

In this work, the conventional reactions were used to functionalize the silica surface with amide and hydrocarbon chain groups affording two different mixed-mode stationary phases (Sil-amide-C11 and Sil-C12-amide). The prepared stationary phases were analyzed by elemental analysis and thermogravimetric analysis. The retention of benzene, phenol, pyridine, and aniline was investigated and compared with synthesized and commercial columns, and this led to prove the existence of different interactions on the synthesized stationary phases. The mixed-mode stationary phases showed multiple interactions, and different chromatography modes were found under distinct chromatographic conditions. According to the type of amide group (either free or within the hydrocarbon chain), different interactions can be made on the columns. The alkylbenzenes and polycyclic aromatic hydrocarbons, as nonpolar hydrocarbons, were chromatographed under reversed-phase liquid chromatography modes, in which amide groups on the silica could efficiently separate polar analytes under hydrophilic interaction liquid chromatography mode in both prepared stationary phases. The performance of the columns was compared by the separation of the carboxylic acid group and biological samples. The bonding method and the type of amide group showed different interactions leading to different separation and performance.

Preparation and evaluation of a polymer-based sulfobetaine zwitterionic stationary phase

We describe a polymer-based sulfobetaine zwitterionic stationary phase for hydrophilic interaction liquid chromatography (HILIC). It was prepared by grafting acrylamide-type sulfobetaine monomer instead of common methacrylate-type sulfobetaine onto hydrolysed poly (glycidyl methacrylate-divinylbenzene) (GMA-DVB) microspheres via pendant double bonds of DVB. The phase has been characterized by elemental analysis, scanning electron micrograph and N₂ adsorption-desorption experiment. It shows wider pH tolerance range (from 2 to 12) and excellent separation ability towards common strong polar analytes such as nucleosides and nucleic bases, water-soluble vitamins, amino acids, inorganic anions and cations. Notably, it exhibits negligible baseline noise level (~0.15 pA) under typical HILIC mobile phase. Excellent selectivity in separation of α- and β-anomers of reducing sugars and lactose/lactulose has also been observed.

Preparation and evaluation a mixed-mode stationary phase with C18 and 2-methylindole for HPLC

A modified C₁₈ column (Silpr-2MI-C18) was prepared using 2-methylindole and C₁₈ reagent. The extent of C₁₈ hydrocarbon chain, conjugative rings and anion exchange site provided multiple retention mechanisms, including reversed-phase liquid chromatography (RPLC), π-π interaction, hydrophilic interaction liquid chromatography (HILIC) and anion exchange chromatography (AEC). The separation of protected amino acids was investigated on the commercial C₁₈ and Silpr-2MI-C18 columns, while the chromatographic conditions, including methanol content and pH of the mobile phase, were studied. The separation arrangement of the hydrophilic amino acids was different on the Silpr-2MI-C18 column compared to the commercial C₁₈ column under RPLC mode. Furthermore, these amino acids were separated on the Silpr-2MI-C18 column under HILIC mode. The modified C₁₈ column was employed to separate amino acids, alkylbenzenes and polycyclic aromatic hydrocarbons under RPLC mode and inorganic anion under AEC mode. The results confirm that this new stationary phase of RPLC/HILIC/AEC has multiple interactions with different analytes. Effective retention of biological samples was found on the Silpr-2MI-C18 column by comparing the results obtained from the commercial C₁₈ column.

Silica modification with 9-methylacridine and 9-undecylacridine as mixed-mode stationary phases in HPLC

In this research, 9-methylacridine and 9-undecylacridine were synthesized through Bernthsen's reaction and well characterized using gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR). Two mixed-mode stationary phases were developed by functionalizing silica with 9-methylacridine and 9-undecylacridine. Then, two modified silicas were characterized by elemental analysis, thermogravimetric analysis (TGA), and fourier transform-infrared spectroscopy (FT-IR). Due to the extent of conjugative rings, the hydrophobic hydrocarbon chain, and anion exchange sites of 9-methylacridinium and 9-undecylacridinium group on the silica gel of columns, mixed-mode stationary phases were designed with multiple interactions including π-π stacking interaction, reverse phase, hydrophilic interaction, and anion exchange. According to the type of acridine, different interactions may be formed in the target column. Polycyclic aromatic hydrocarbons (PAHs), alkylbenzenes, pyridines and parabens were chromatographed on π-π stacking modes and RPLC, where anion exchange sites can be applied for the separation of inorganic anions on AEC mode. Considering the structure of the stationary phases, these columns were used to separate organic compounds with higher polarity on the HILIC retention. The performance of the columns was investigated by the chromatographic parameters in terms of column efficiency (N/m), asymmetry factor (A_f), retention factor (k), and resolution (Rs). The mixed-mode stationary phases can be successfully employed to conduct chromatographic separation on a wide range of samples with a single column.

A click tyrosine zwitterionic stationary phases for hydrophilic interaction liquid chromatography

New zwitterionic (ZIC) stationary phases (SPs) are synthesized with the click and conventional bonding of tyrosine to silica gel. Infrared spectra and elemental analysis demonstrate the successful click and conventional bonding of this ZIC group on silica particles by the surface coverage including 2.36 and 0.75 µm m^-2, respectively. Given the above-mentioned explanation, the present study evaluated the retention mechanism and chromatographic manners of polar compounds on these new materials under hydrophilic interaction liquid chromatography (HILIC) conditions. Based on the results, the Click-Tyrosine Stationary Phase provided good HILIC characteristics when it was applied to separate phenolic compounds, amino acids, alkaloids, and nucleobases compared to bare silica gel SP and even conventional tyrosine SPs. Further, this new Click-Tyrosine-SP represented appropriate HILIC features and column efficiency (the theoretical plate number was up to 50,000 plates m^-1 for thebaine). Furthermore, the study investigated the effect of solute polarity (the number of the hydroxyl group of phenolic compounds) and hydrophobicity (the number of the side chain of aliphatic amino acids) on retention behaviors. Finally, some important factors were studied as the potential variables for guiding the retention behavior of the polar compound in HILIC condition including solvent composition, salt concentration, and the buffer pH of the mobile phase.

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.

A new strategy for the preparation of mixed-mode chromatographic stationary phases based on modified dialdehyde cellulose

A new strategy for the preparation of mixed-mode chromatographic stationary phases based on modified dialdehyde cellulose was proposed. Two novel mixed-mode chromatographic stationary phases, dicarboxyl cellulose-modified silica (DCC/SiO₂) and (S)-α-phenylethylamine-bonded DCC/SiO₂ ((S)-α-PEA/DCC/SiO₂), were prepared by utilizing the easy functionalization characteristics of dialdehyde cellulose. The chromatographic evaluation showed that DCC/SiO₂ column could be used in hydrophilic interaction liquid chromatography (HILIC) and ion exchange chromatography (IEC) modes, (S)-α-PEA/DCC/SiO₂ column could be used in HILIC, IEC and chiral separation modes. The DCC/SiO₂ column and (S)-α-PEA/DCC/SiO₂ column exhibited excellent chromatographic performance by separating strongly polar compounds, phenylamines and chiral compounds in the above separation modes. The preparation method of modified dialdehyde cellulose-based mixed-mode chromatographic stationary phases was simple, and also provided a new idea for the development of the subsequent novel mixed-mode chromatographic stationary phases.

Poly(vinyl alcohol)–cationic cellulose copolymer encapsulated SiO2 stationary phase for hydrophilic interaction liquid chromatography

A poly(vinyl alcohol)–cationic cellulose copolymer encapsulated SiO₂ HILIC stationary phase is described, which exhibits excellent separation for various analytes.

An arginine-functionalized stationary phase for hydrophilic interaction liquid chromatography

An arginine-functionalized stationary phase for hydrophilic interaction liquid chromatography (HILIC) has been prepared for the first time by clicking arginine onto silica gel. It offers an efficient separation of organic acids, nucleotides and sugars. More interestingly, it exhibited excellent selectivity and enrichment toward acidic glycopeptides, even at a ratio of 1 : 150 to non-glycopeptides.

A polyvinyl alcohol-coated silica gel stationary phase for hydrophilic interaction chromatography

A facile and green way is proposed to prepare a polyvinyl alcohol coated silica gel stationary phase for HILIC. It demonstrated high separation efficiency and high chemical stability.

Facile preparation of polyvinyl alcohol coated SiO2 stationary phases for high performance liquid chromatography

A facile physical method to prepare polyvinyl alcohol coating-based silica stationary phase for HPLC was proposed.

A novel amide stationary phase for hydrophilic interaction liquid chromatography and ion chromatography

A novel amide stationary phase (ASP) for hydrophilic interaction liquid chromatography (HILIC) has been prepared via the Click chemistry method. It was based on the strategy that the amino group of Asparagine was easily transferred to the corresponding azido group and then clicked onto terminal alkyne-silica gel in the presence of Cu(I)-based catalyst. For the tested polar compounds including nucleosides and nucleic acid bases, ASP-based column has demonstrated good performance in terms of separation efficiency and column stability, and the retention mechanism was found to match well the typical HILIC retention. In addition, the ASP described here showed much better selectivity in separation of inorganic anions under ion chromatography mode relative to other kinds of commercial ASP.

A facile versatile polymeric monolith for multiple separations

A hydrophilic versatile polymeric monolith with multiple retention mechanisms including hydrogen-bonding, π-π and electrostatic interactions, was developed by a simple one-step in situ polymerization. High mechanical stability and excellent separation capabilities of various nonpolar and polar analytes were successfully achieved and employed for multiple separations in mixed-mode chromatography.