Additional investigations into the retention mechanism of hydrophilic interaction liquid chromatography by linear solvation energy relationships

Dicationic imidazole ionic liquid stationary phase for preservative detection and its application under mixed mode of HILIC/RPLC/IEC

BACKGROUND

Food safety has always been a great concern, and the detection of additives is vital to ensuring food safety. Therefore, there is a necessity to develop a method that can quickly and efficiently separate and detect additives in food. High performance liquid chromatography is widely used in the analysis and testing of food additives. Ionic liquids have attracted wide attention in the preparation of high performance liquid chromatography stationary phases owing to their high stability, low vapor pressure and adjustable structure.

RESULTS

We developed a novel dicationic imidazole ionic liquid stationary phase for the simultaneous determination of organic preservatives (sodium benzoate, potassium sorbate) and inorganic preservatives (nitrate and nitrite) in foodstuffs under mixed-mode chromatography. The method had the advantages of easy operation, high reproducibility, good linearity and precision. In the detection of these four preservatives, the limit of detection ≤0.4740 mg⋅L-1 and the limit of quantification ≤1.5800 mg⋅L-1. The intra-day and inter-day precision were less than 4.02%, and the recovery rate was 95.90∼100.19 %. At the same time, we also characterized the stationary phase, explored the mechanism and evaluated the chromatographic performance. The stationary phase was able to operate under the mixed mode of reversed phase/hydrophilic interaction/ion exchange chromatography, and it was capable of separating hydrophilic substances, hydrophobic substances, acids, and inorganic anionic substances with good separation efficiency and had high column efficiency.

SIGNIFICANCE

In summary, the stationary phase has a promising application in the routine analysis of organic and inorganic preservatives in food. In addition, the stationary phase has good separation ability for hydrophilic, hydrophobic, ionic substances and complex samples, making it a prospective material for chromatographic separation.

Separation of carbohydrates using dynamically adsorbed borate stationary phase for hydrophilic interaction liquid chromatography

In this work, a chromatographic method for the separation of carbohydrates was proposed. Tris-(hydroxymethyl)-amine (TRIS) functionalized silica-based hydrophilic interaction liquid chromatography (HILIC) stationary was synthesized. The dynamically absorbed borate layer is generated by using borate buffer as a polar modifier due to the complexation of borate with TRIS ligand in the stationary phase. The chromatographic systems were analyzed by the linear solvation energy relationship model. The calculated system constants revealed the enhancement of anionic exchange by the addition of borate in the mobile phase system. In addition, ligand exchange is critical for the retention and elution order of sugars and sugar alcohols. Carbohydrates displayed prolonged retention with different selectivity profiles relating to their complexation coefficients with borate. Experiment results showed that the effect of borate in this chromatographic system was stable within the range of pH 3-7 and borate concentration of 5-15 mM. This work provides a complementary solution for the separation of carbohydrates. It can also be extended to the separation of glycosides.

Preparation and evaluation of a piperidinium-sulfonate based zwitterionic monolith for HILIC separation

In this study, a novel piperidinium-sulfonate based zwitterionic hydrophilic monolith was prepared through thermally initiated co-polymerization of a piperidinium-sulfonate monomer 3-(4-((methacryloyloxy)methyl)-1-methylpiperidin-1-ium-1-yl)propane-1-sulfonate (MAMMPS), and a hydrophilic crosslinker N,N'-methylenebisacrylamide (MBA) using n-propanol and H2O as porogenic system. Satisfactory mechanical and chemical stabilities, good repeatability and high column efficiency (120,000 N/m) were obtained on the optimal monolith. The resulting poly(MAMMPS-co-MBA) monolith showed a typical HILIC retention behavior over an ACN content range between 5 and 95 %. Furthermore, this column exhibited good separation performance for various polar compounds. Compared to quaternary ammonium-sulfonate based zwitterionic hydrophilic monolith, i.e. poly(N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl)ammonium betaine-co-MBA), the poly(MAMMPS-co-MBA) monolith displayed stronger retention and better selectivity for the tested phenolic and amine compounds at different pH conditions. Finally, this column was applied for the separation of six sulfonamide antibiotics, and the analytical characteristics of the method were evaluated in terms of precision, repeatability, limits of detection (LOD) and quantitation (LOQ). Overall, this study not only developed a novel HILIC monolithic column, but also proved the potential of piperidinium-sulfonate based zwitterionic chemistry as stationary phase, which further increased the structure diversity of zwitterionic HILIC stationary phases.

A story of peptides, lipophilicity and chromatography - back and forth in time

Peptides, as part of the beyond the rule of 5 (bRo5) chemical space, represent a unique class of pharmaceutical compounds. Because of their exceptional position in the chemical space between traditional small molecules (molecular weight (MW) < 500 Da) and large therapeutic proteins (MW > 5000 Da), peptides became promising candidates for targeting challenging binding sites, including even targets traditionally considered as undruggable - e.g. intracellular protein-protein interactions. However, basic knowledge about physicochemical properties that are important for a drug to be membrane permeable is missing but would enhance the drug discovery process of bRo5 molecules. Consequently, there is a demand for quick and simple lipophilicity determination methods for peptides. In comparison to the traditional lipophilicity determination methods via shake flask and in silico prediction, chromatography-based methods could have multiple benefits such as the requirement of low analyte amount, insensitivity to impurities and high throughput. Herein we elucidate the role of peptide lipophilicity and different lipophilicity values. Further, we summarize peptide analysis via common chromatographic techniques, in specific reversed phase liquid chromatography, hydrophilic interaction liquid chromatography and supercritical fluid chromatography and their role in drug discovery and development process.

Solute–Solvent Interactions in Hydrophilic Interaction Liquid Chromatography: Characterization of the Retention in a Silica Column by the Abraham Linear Free Energy Relationship Model

The Abraham linear free energy relationship model has been used to characterize a hydrophilic interaction liquid chromatography (HILIC) silica column with acetonitrile/water and methanol/water mobile phases. Analysis by the model for acetonitrile/water mobile phases points to solute volume and hydrogen bond basicity as the main properties affecting retention, whereas solute hydrogen bond acidity, dipolarity and polarizability practically do not affect it. Formation of a cavity is easier in acetonitrile-rich mobile phases than in the aqueous stationary phase, and hence increase of solute volume decreases retention. Conversely, hydrogen bond acidity is stronger in the aqueous stationary phase than in the acetonitrile-rich mobile phase and thus an increase of solute hydrogen bond basicity increases retention. Results are similar for methanol/water mobile phases with the difference that solute hydrogen bond acidity is significant too. Increase in hydrogen bond acidity of the solute decreases retention showing that methanol mobile phases must be better hydrogen bond acceptors than acetonitrile ones, and even than water-rich stationary phases. The results are like the ones obtained in zwitterionic HILIC columns bonded to silica or polymer supports for acetonitrile/water mobile phases, but different for solute hydrogen bond acidity for a polymer bonded zwitterionic column with methanol/water mobile phases, indicating that bonding support plays an important role in HILIC retention. Comparison to RPLC characterized systems confirms the complementarity of HILIC systems to RPLC ones because the main properties affecting retention are the same but with reversed coefficients. The least retained solutes in RPLC are the most retained in HILIC.

Contribution of ionic interactions to stationary phase selectivity in hydrophilic interaction chromatography

We compared the separation selectivities of 19 different hydrophilic interaction chromatography columns. The stationary phases included underivatized silica and hybrid particles, cyano‐bonded silica, materials with neutral ligands such as amide, diol, pentahydroxy, and urea, zwitterionic sorbents, and mixed‐mode materials with amine functionalities. A set of 77 small molecules was used to evaluate the columns. We visualized the retention behavior of the different columns using retention time correlation plots. The analytes were classified as cations, anions, or neutral based on their estimated charge under the separation conditions. This involved adjusting the dissociation constants of the analytes for the acetonitrile content of the mobile phase and experimentally determining the pH of the mobile phase containing 70% acetonitrile. The retention correlation plots show that the selectivity differences strongly depended on ionic interactions. Comparisons of the neutral stationary phases (e.g., diol vs. amide) showed more similar selectivity than did comparisons of neutral columns versus columns with cation or anion exchange activity (bare silica or amine columns, respectively). The zwitterionic columns did not behave as perfectly neutral. The correlation plots indicated that they exhibited either cation or anion exchange activity, although to a lesser degree than the silica and amine‐containing stationary phases.

A Survey of Polar Stationary Phases for Hydrophilic Interaction Chromatography and Recent Progress in Understanding Retention and Selectivity

Various polar stationary phases have become available for hydrophilic interaction chromatography (HILIC) and help drive continuous applications in biomedical, environmental and pharmaceutical areas in the past decade. Although the stationary phases for HILIC have been reviewed previously, it is an appropriate time to take another look at the progresses during the past five years. The current review provides an overview of the polar stationary phases commercially available for HILIC applications in an effort to assist scientists in selecting suitable columns. New types of stationary phase that were published in literature in the past five years are summarized and discussed. The trend in stationary phase research and development is also highlighted. Of particular interest is the experimental evidence for direct interactions of polar analytes with the ligands of the stationary phases under HILIC conditions. In addition, two different approaches have been developed to delineate the relative significance of the partitioning and adsorption mechanisms in HILIC, representing an important advancement in our understanding of the retention mechanisms in HILIC.

Graph Convolutional Networks for Improved Prediction and Interpretability of Chromatographic Retention Data

Machine learning is a popular technique to predict the retention times of molecules based on descriptors. Descriptors and associated labels (e.g., retention times) of a set of molecules can be used to train a machine learning algorithm. However, descriptors are fixed molecular features which are not necessarily optimized for the given machine learning problem (e.g., to predict retention times). Recent advances in molecular machine learning make use of so-called graph convolutional networks (GCNs) to learn molecular representations from atoms and their bonds to adjacent atoms to optimize the molecular representation for the given problem. In this study, two GCNs were implemented to predict the retention times of molecules for three different chromatographic data sets and compared to seven benchmarks (including two state-of-the art machine learning models). Additionally, saliency maps were computed from trained GCNs to better interpret the importance of certain molecular sub-structures in the data sets. Based on the overall observations of this study, the GCNs performed better than all benchmarks, either significantly outperforming them (5-25% lower mean absolute error) or performing similar to them (<5% difference). Saliency maps revealed a significant difference in molecular sub-structures that are important for predictions of different chromatographic data sets (reversed-phase liquid chromatography vs hydrophilic interaction liquid chromatography).

A novel approach for the preparation of core-shell MOF/polymer composites as mixed-mode stationary phase

The nickel organic framework capped with polyvinylpyrrolidone was prepared and synergistically immobilized onto porous silica surface as the mixed-mode stationary phase for high-performance liquid chromatography. Here, polyvinylpyrrolidone firstly was chosen as functional molecules to change morphology and size of the metal organic framework. The silica microspheres were then modified by them via a simple bonding method rather than in-situ growth method with the aid of electrostatic interaction commonly used before. The stationary phase showed flexible selectivity for separation of both hydrophilic and hydrophobic compounds, especially for hydrophilic compounds such as carbohydrates, alkaloids and sulfonamides etc. The chromatographic behaviors were evaluated by investigating various factors, and a typical mixed-mode retention feature of the column was observed. The composites could be prepared repetitively, and relative standard deviations of retention time of objective compounds among different batches were less than 1.75%. It also showed excellent chromatographic reproducibility, stability and potentiality for application in real samples. In short, the composites can be used for a feasible option for analysis of multiple compounds as the mixed-mode stationary phase and it provides a general approach for preparing MOFs-based composites by changing morphology and size of MOFs.

Linear free energy relationship models for the retention of partially ionized acid-base compounds in reversed-phase liquid chromatography

The LFER model of Abraham is applied to the retention of the neutral and ionic forms of 94 solutes in a C18 column and 40% v/v acetonitrile/water mobile phase. The results show that polarizability and cavity formation interactions increase retention, whereas dipole and hydrogen bonding interactions favours partition to the mobile phase and thus, they decrease retention. The coefficients of the ionic descriptors measure the effect of the electrostatic interactions and their contribution to partition of the cation or anion between the two mobile and stationary chromatographic phases. A new LFER model for application to the retention of partially dissociated acids and bases is derived averaging the descriptors of the neutral and ionic forms according to their degrees of ionization in the mobile phase. This new LFER model is satisfactorily compared to other literature modified Abraham models for a set of 498 retention data of partially dissociated acids and bases. All tested models require the calculation of the ionization degrees of the compounds at the measuring pH. Calculation of the ionization degrees in the chromatographic mobile phase (i.e. from pH and pK_a in the eluent) give good correlations for all tested models. However, estimation of these ionization degrees from pH - pK_a data in pure water gives biased estimations of the retention of the partially ionized solutes.

Surface-bonded amide-functionalized imidazolium ionic liquid as stationary phase for hydrophilic interaction liquid chromatography

The amide group modified silica materials are popular stationary phases for hydrophilic interaction liquid chromatography (HILIC). Meanwhile, surface-confined imidazolium ionic liquids (ILs) have been proved to be useful HILIC stationary phases and possess many unique properties. In this study, the synthesis of an amide-functionalized imidazolium IL was conducted which was then bonded onto silica surface to obtain a novel imidazolium-embedded amide stationary phase for HILIC. The combination of the amide group and imidazolium IL moiety might bring some advantages in selectivity or retention and therefore extended its applications. After characterizing the prepared IL and the resulting modified silica materials, the chromatographic performance and separation selectivity of the packed column were evaluated and compared with a commercial amide column. Then, the retention behavior was investigated through observing the retention factors at different chromatographic conditions using a wide range of compounds. Exceptionally, the prepared amide IL column exhibited superior separation performance towards complex samples such as flavonoids mixture, soybean flavonoids and human urine. All the results indicated that the novel amide IL column possessed an anion-exchange/HILIC mixed-mode retention mechanism and could be useful in the sample analysis as a promising candidate for HILIC stationary phase.

Protonation of polyaniline-coated silica stationary phase affects the retention behavior of neutral hydrophobic solutes in reversed-phase capillary liquid chromatography

Because of its high conductivity when acid doped, polyaniline is known as a synthetic metal and is used in a wide range of applications, such as supercapacitors, biosensors, electrochromic devices, or solar and fuel cells. Emeraldine is the partly oxidized, stable form of polyaniline, consisting of alternating diaminobenzenoid and iminoquinoid segments. When acidified, the nitrogen atoms of emeraldine become protonated. Due to electrostatic repulsion between positive charges, the polarity and morphology of emeraldine chains presumably change; however, the protonation effects on emeraldine have not yet been clarified. Thus, we investigated these changes by reversed-phase capillary liquid chromatography using a linear solvation energy relationship approach to assess differences in dominant retention interactions under a significantly varied mobile phase pH. We observed that hydrophobicity dominates the intermolecular interactions under both acidic and alkaline eluent conditions, albeit to different extents. Therefore, by tuning the mobile phase pH, we can even modulate the retention of neutral hydrophobic solutes, such as aromatic hydrocarbons, because the pH-dependent charge and structure of polymer chains of the emeraldine-coated silica stationary phase show a mixed-mode separation mechanism.

Analysis of oligonucleotides by ion-pairing hydrophilic interaction liquid chromatography/electrospray ionization mass spectrometry

RATIONALE

Hydrophilic interaction liquid chromatography/electrospray ionization mass spectrometry (HILIC-LC/ESI-MS) has been proved to be useful for the quality control of oligonucleotides. However, the lack of separation for some oligonucleotides using HILIC-LC/MS has proved problematic. This study aimed to improve the resolving ability of HILIC-LC/MS.

METHODS

The study was performed on a Waters UPLC® system coupled to a Waters LCT premier XE ESI-TOF mass spectrometer using a Zorbax® RRHD HILIC column (2.1 mm × 100 mm, 1.8 μm). Buffer systems contained triethylammonium acetate (TEAA) and acetonitrile. The effects of the concentration of TEAA and the type of organic modifiers on the separation of oligonucleotides were investigated.

RESULTS

The results showed that the optimum concentration of TEAA is 10 mM and acetonitrile is a better organic solvent than methanol. The addition of TEAA in the HILIC mobile phase improved the separation of N from N + A significantly compared to the HILIC method buffered with ammonium acetate. The IP-HILIC chromatography has demonstrated that the separation of oligonucleotides is sequence dependent. In addition, the IP-HILIC method produces a much simpler mass spectrum of an oligonucleotide with very efficient desalting.

CONCLUSIONS

The HILIC-LC/MS method with the addition of TEAA at a MS-compatible concentration has improved the separation of oligonucleotides. The IP-HILIC-LC/MS method also produces very simple mass spectra with high desalting efficiency.

Chemometric-assisted method development in hydrophilic interaction liquid chromatography: A review

With an enormous growth in the application of hydrophilic interaction liquid chromatography (HILIC), there has also been significant progress in HILIC method development. HILIC is a chromatographic method that utilises hydro-organic mobile phases with a high organic content, and a hydrophilic stationary phase. It has been applied predominantly in the determination of small polar compounds. Theoretical studies in computer-aided modelling tools, most importantly the predictive, quantitative structure retention relationship (QSRR) modelling methods, have attracted the attention of researchers and these approaches greatly assist the method development process. This review focuses on the application of computer-aided modelling tools in understanding the retention mechanism, the classification of HILIC stationary phases, prediction of retention times in HILIC systems, optimisation of chromatographic conditions, and description of the interaction effects of the chromatographic factors in HILIC separations. Additionally, what has been achieved in the potential application of QSRR methodology in combination with experimental design philosophy in the optimisation of chromatographic separation conditions in the HILIC method development process is communicated. Developing robust predictive QSRR models will undoubtedly facilitate more application of this chromatographic mode in a broader variety of research areas, significantly minimising cost and time of the experimental work.

Evaluation of coverage, retention patterns, and selectivity of seven liquid chromatographic methods for metabolomics

Liquid chromatography-mass spectrometry-based metabolomics studies require highly selective and efficient chromatographic techniques. Typically employed reversed-phase (RP) methods fail to target polar metabolites, but the introduction of hydrophilic interaction liquid chromatography (HILIC) is slow due to perceived issues of reproducibility and ruggedness and a limited understanding of the complex retention mechanisms. In this study, we present a comparison of the chromatographic performance of a traditional RP-C18 column with zwitterionic, amide-, alkyl diol-, and aminoalkyl-based HILIC and mixed-mode columns. Our metabolite library represents one of the largest analyte sets available and consists of 764 authentic metabolite standards, including amino acids, nucleotides, sugars, and other metabolites, representing all major biological pathways and commonly observed exogenous metabolites (drugs). The coverage, retention patterns, and selectivity of the individual methods are highly diverse even between conceptually related HILIC methods. Furthermore, we show that HILIC sorbents having highly orthogonal selectivity and specificity enhance the coverage of major metabolite groups in (semi-) targeted applications compared to RP. Finally, we discuss issues encountered in the analysis of biological samples based on the results obtained with human plasma extracts. Our results demonstrate that fast and highly reproducible separations on zwitterionic columns are feasible, but knowledge of analyte properties is essential to avoid chromatographic bias and exclusion of key analytes in metabolomics studies. Graphical Abstract The chromatographic parameters of 764 authentic metabolite standards provide the basis for a comparison of coverage, selectivity and orthogonality of 7 reversed-phase (RP), mixed-mode (MM) and hydrophilic interaction liquid chromatography (HILIC) methods.

LSER model for organic compounds adsorption by single-walled carbon nanotubes: Comparison with multi-walled carbon nanotubes and activated carbon

LSER models for organic compounds adsorption by single and multi-walled carbon nanotubes and activated carbon were successfully developed. The cavity formation and dispersion interactions (vV), hydrogen bond acidity interactions (bB) and π-/n-electron interactions (eE) are the most influential adsorption mechanisms. SWCNTs is more polarizable, less polar, more hydrophobic, and has weaker hydrogen bond accepting and donating abilities than MWCNTs and AC. Compared with SWCNTs and MWCNTs, AC has much less hydrophobic and less hydrophilic adsorption sites. The regression coefficients (e, s, a, b, v) vary in different ways with increasing chemical saturation. Nonspecific interactions (represented by eE and vV) have great positive contribution to organic compounds adsorption, and follow the order of SWCNTs > MWCNTs > AC, while hydrogen bond interactions (represented by aA and bB) demonstrate negative contribution. These models will be valuable for understanding adsorption mechanisms, comparing adsorbent characteristics, and selecting the proper adsorbents for certain organic compounds.

Recent progress in the fundamental understanding of hydrophilic interaction chromatography (HILIC)

With the exponential growth in the application of the HILIC technique, there has been a significant progress in understanding the fundamental aspects of hydrophilic interaction chromatography.

A novel surface-confined glucaminium-based ionic liquid stationary phase for hydrophilic interaction/anion-exchange mixed-mode chromatography

Glucaminium-based ionic liquids are a new class of recently developed ionic liquids and prepared by functionalizing the amine group of N-methyl-d-glucamine, which renders them good hydrophilicity due to the presence of the glucose structure and charged quaternary ammonium group. In the present study, a glucaminium-based ionic liquid N,N-diallyl-N-methyl-d-glucaminium bromide was synthesized and subsequently bonded to the surface of 3-mercaptopropyl modified silica materials through "thiol-ene" click chemistry. The obtained stationary phase was characterized by elemental analysis and infrared spectroscopy, and then packed as a HPLC column. A mixture of five nucleosides was used to characterize the separation performance of the obtained column under HILIC mode and the column efficiency was determined with cytidine as the test solute, reaching 80,000plates/m. Then, the retention behavior was evaluated by investigating the effect of various chromatographic factors on retention of different types of solutes, and the results revealed that the developed surface-confined glucaminium-based ionic liquid stationary phase exhibited a hydrophilic interaction/anion-exchange mixed-mode retention mechanism. Finally, two mixtures of nucleotides and flavonoids were separated on the glucaminium-based ionic liquid column, respectively under hydrophilic interaction and hydrophilic interaction/anion-exchange mixed-mode chromatography. In conclusion, the multimodal retention capabilities of the glucaminium-based ionic liquid column could offer a wider range of retention behavior and flexible selectivity toward polar and hydrophilic compounds.