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

Impurity profiling of synthetic cyclic peptides based on orthogonality between hydrophilic-interaction and reversed-phase liquid chromatography

Owing to their high selectivity and low toxicity, synthetic cyclic peptides hold promise as bioactive therapeutics. The corresponding impurity analysis typically relies on reversed-phase high-performance liquid chromatography (RP-HPLC), which, however, often provides inaccurate results because of the abundance of chemically similar impurities. To bridge this gap, hydrophilic-interaction liquid chromatography (HILIC), a technique orthogonal to RP-HPLC, was herein used with an ultraviolet detector to analyze the impurities of four cyclic peptides. Each of these peptides comprises nine amino acids (both natural and nonnatural) and features neutral, acidic, basic, or zwitterionic residues incorporated at positions 3 and 6. Three polymer-based HILIC columns (acidic, basic and zwitterionic) were newly employed, and seven mobile phases were used to investigate the effects of additives and pH. Derringer's desirability functions based on five criteria (purity, impurity factor, peak symmetry factor, theoretical plate number, and retention time) were employed to identify optimal screening conditions. Ammonium acetate was identified as an effective additive, and polymeric selectors with vinylpyridine and phosphoryl choline residues were shown to be effective irrespective of the peptide features. Optimized HILIC conditions were established for each peptide, and the corresponding impurity elution profiles were compared with those obtained by RP-HPLC using a column filled with octadecylsilyl-functionalized silica. RP-HPLC and HILIC were revealed to be mutually orthogonal, i.e., most impurities with small RP-HPLC retention times had large HILIC retention times, and vice versa. The results help make the purity evaluation of synthetic peptides more accurate, thus paving the way for their efficient purification.

Quantitative Assessment of Retention Mechanisms for Ionized Compounds in Hydrophilic Interaction Chromatography (HILIC)

The retention mechanisms for polar compounds in HILIC are only qualitatively understood to include hydrophilic partitioning, surface adsorption, and electrostatic interactions if both the analytes and stationary phases are charged. However, the main retention mechanism may be different for different compounds under different chromatographic conditions, and it is difficult to identify the main retention mechanism based on the existing knowledge and methods. We previously developed a methodology to quantitatively determine the retention contributions of hydrophilic partitioning and surface adsorption for nonionized compounds in HILIC. In this study, the methodology has been expanded to include the retention contribution of electrostatic interactions for the ionized compounds on charged stationary phases. When electrostatic interactions are sufficiently shielded at high salt concentrations, the partitioning coefficient of ionized compounds is determined using the same method for nonionized compounds. Then, the retention contributed by partitioning and adsorption is calculated. The retention contribution by electrostatic interactions (both attractive and repulsive) is determined by subtracting the retention contributed by partitioning and adsorption from the observed retention at each salt concentration. This is the first study that evaluated the retention contributions of hydrophilic partitioning, surface adsorption, and electrostatic interactions for ionized compounds. Quantitative information on retention mechanisms will be helpful to better understand selectivity in HILIC and facilitate the development of retention models.

Optimization of an analytical method based on the use of zwitterionic- phosphorylcholine -HILIC column for the determination of multiple polar emerging contaminants in reclaimed water

The aim of this study was to optimize a Liquid Chromatography Mass Spectrometry (LC-MS) method using a zwitterionic phosphorylcholine HILIC column for the determination of several Persistent and Mobile Organic Contaminants (PMOC) in wastewater samples. An experimental design approach was implemented to both better understand the retention mechanisms of several polar compounds and to find the optimal operating conditions for their detection and quantification. Eleven PMOCs, with logDpH=7 ranging from -5.27 to 0.24, were considered, including pesticides, artificial sweeteners, pharmaceuticals, and central nervous system stimulants. Key chromatographic variables-such as the initial percentage of the organic mobile phase, temperature, flow rate, gradient time, acid percentage, and the type and concentration of two different salts- were studied to assess their influence on peak areas, retention times and separation efficiency. The results indicated buffer type, flow rate, and initial percentage of organic mobile phase as the most influential factors affecting retention, though the effects were closely related to the chemical and physicochemical properties of the analytes. The optimized instrumental method demonstrated acceptable figures of merit, with recoveries ranging from 49 % to 100 % for all analytes (except taurine, which may require a different experimental preprocessing step). The method also showed satisfactory precision (repeatability of the entire experimental procedure), in terms of Relative Standard Deviation (RSD %), which was <10 % for all analytes. The developed method was successfully applied to the analysis of reclaimed water samples collected in six wastewater treatment plants in two regions of northern Italy. All target ECs were detected and quantified, except for clenbuterol, terbutaline, acesulfame K and 2,4-dichlorophenoxyacetic acid, which were below the detection limit.

Synthesis and applications of ionic liquids for chromatographic analysis

Ionic liquids (ILs) have emerged as more desirable liquids than conventional solvents for chemistry, material science, engineering and environmental science. The scientific literature reveals an exponential increase in the number of research projects aimed at exploring the chromatographic features of ionic liquids. The review provides sound scientific data to examine the structural characteristics of ionic liquids that make them ideal for use in chromatography. This contribution is distinctive since it integrates the synthesis, benefits, drawbacks, and possible uses of ionic liquids in several chromatographic separation processes. Keeping the cation the same, the introduction of different anions is also possible, and this strategy leads to the synthesis of a series of different ionic liquids with varying properties. A detailed probe is given on the influence of ionic liquid structure and properties on their chromatographic behavior, both as stationary phase and mobile phase and/or mobile phase additives. Ionic liquid based immobilized stationary phases and their analyte retention mechanisms (hydrogen bonding, electrostatic forces of attraction, π-π stacking, ion exchange, and hydrophilic interactions, etc.) are critically discussed. Finally, a thorough analysis of the literature suggests that IL-based stationary phases may undergo multi-mode and more flexible retention mechanisms. Their dual polarity can facilitate interaction with both polar and non-polar compounds. Similarly, using IL as a mobile phase can offer more pragmatic and sustainable options for enantiomer separation.

Applications of hydrophilic interaction and mixed-mode liquid chromatography in pharmaceutical analysis

Hydrophilic Interaction Liquid Chromatography (HILIC) and Mixed-Mode Chromatography (MMC) excel in separating polar, hydrophilic, and charged analytes due to unique hydrophilic or mixed-mode retention mechanisms. They represent a complementary approach to the widely used Reversed Phase Liquid Chromatography (RPLC). Often, where RPLC struggles, HILIC and MMC thrive. The applications of HILIC and MMC in pharmaceutical analysis are expanding rapidly across a variety of drug modalities. This article reviews advances in the applications of HILIC and MMC in seven major areas of pharmaceutical analysis: synthetic small molecules, counterions and salts, lipids and surfactants, carbohydrates, amino acids and peptides, proteins, and nucleic acids in the past two decades. We aim to provide comprehensive information and strategic guidance to facilitate future research, development and applications in these areas.

Separation of nucleobases, nucleosides, nucleotides and oligonucleotides by hydrophilic interaction liquid chromatography (HILIC): A state-of-the-art review

The polar nature of nucleobases, nucleosides and nucleotides makes hydrophilic interaction chromatography (HILIC) a good choice of technology for separation. Both naturally occurring and modified nucleosides and nucleotides have been successfully separated in HILIC. A wide range of stationary phases with different retention and selectivity are suitable for the separation of nucleobases, nucleosides and nucleotides; and a sufficient knowledge base is also available to guide method development. Although oligonucleotides are significantly different from nucleotides in terms of polarity and charges, HILIC has been shown to be a viable alternative to ion-pairing reversed-phase liquid chromatography (IP-RPLC). Only a few polar stationary phases have been shown to provide satisfactory performance; however, the requirements for the mobile phase composition including organic solvent, mobile phase pH and salt concentration are sufficiently understood. This review provides a comprehensive evaluation of the chromatographic conditions with a historical perspective on adopting and developing HILIC for the separation of nucleobases, nucleosides, nucleotides and oligonucleotides. The areas for more research and potential directions for future development activities are identified and discussed.

Biogenic amines and bacterial spoilage in Plant-Based grill sausage alternatives

Vegan and vegetarian diets are increasing in popularity. Consequently, the supply and demand of plant-based meat alternatives has increased steadily over the past few years. However, scientific research on spoilage processes for such products is still inadequate as compared to research on traditional meat products. In traditional meat products, biogenic amines are prominent spoilage markers and potential sources of food toxicity, especially for people sensitive to biogenic amines. Plant-based meat alternatives are manufactured to mimic the taste, look, texture, and nutritional value of meat, and they have a protein-rich basis. It is therefore hypothesized that biogenic amines could be markers for spoilage in such products as well. Further analysis of their presence and concentrations and comparison to conventional meat products is necessary. If biogenic amines are lower in plant-based meat alternatives, these products are possibly better suited for consumption by people with biogenic amine sensitivities. A simple and rapid extraction method, followed by HILIC-MS/MS separation and detection was therefore developed as a first step and validated for nine biogenic amines in plant-based meat alternatives. This method showed a strong linear correlation between amine concentration and detector response, high accuracy, and precision (< 12 %), as well as high sensitivity, as proven by the lowest limits of quantification (i.e., the lowest concentration within the calibration model) of 1 mg/kg for all analytes, which also compares well with other methods. Subsequently, as part of a pilot spoilage study, the method was applied to one vegetarian and nine vegan grill sausage alternatives during a period of 32 days of open-package storage at refrigerator temperature. Correlations with the results of microbiological testing of the same samples, as well as with the storage time were investigated. However, the results of the correlation analysis showed that biogenic amines are not suitable as spoilage indicators for plant-based meat alternatives, as almost no increase in biogenic amines was identified during the spoilage study for all the samples investigated. Differences in the microbiota of conventional meat products versus plant-based meat alternatives, as well as possibly lower concentrations of free amino acids, are proposed as reasons for biogenic amines not being similarly prevalent, and consequently, not being suitable spoilage markers in plant-based meat alternatives. However, as spoilage of the analyzed products was evident, both via sensory assessment and appearance of mold growth, further targeted and non-targeted research on potential spoilage markers for plant-based meat alternatives is required in the future.

Preparation and evaluation of a pyridine sulfonate betaine-based zwitterionic stationary phase for hydrophilic interaction chromatography

A zwitterionic stationary phase comprising pyridinium cations and sulfonate anions was successfully developed through thiol-ene click chemistry. Using seven polar small molecules as probes, the zwitterionic stationary phase showed high separation selectivity and excellent column efficiency (35,200-54,800 plates/m) compared with two commercial columns. The influence of water proportion, salt concentration, and pH in the mobile phase, and column temperature, on the retention of six polar compounds was examined. The retention mechanism was explored by three hydrophilic retention models, Tanaka test and linear solvation energy relationship analysis. For the analysis of sample dairy products (milk powder, milk, and yogurt), the stationary phase was operated in hydrophilic interaction chromatography mode without the addition of buffer salts, facilitating rapid and efficient detection and quantification of melamine. The LOD and LOQ are 0.04 mg⋅g-1 and 0.13 mg⋅g-1, respectively, and the recovery rate is 90.3 - 102.8 %. The zwitterionic stationary phase has the advantages of simple preparation, good method reproducibility, good selectivity and high precision.

Separation of polyphenols by HILIC methods with diode array detection, charged aerosol detection and mass spectrometry: Application to grapevine extracts rich in stilbenoids

The characterization of plant extracts is usually accomplished by reverse-phase liquid chromatography, but the development of new complementary approaches, such as HILIC, offers an orthogonal method. In this study, five HILIC stationary phases were evaluated to assess their ability to retain polyphenols. They were selected to cover the main different HILIC mechanisms: bare silica; silica with ethylene bridge; neutral amide; amino; zwitterionic. A total of 31 polyphenol standards were used for the screening, including 9 stilbenes, 8 flavonoids, 6 anthocyanins, and 8 phenolic acids. Three different detections were tested: diode array detector, charged aerosol detector and mass spectrometry. Results indicated that silica supports were not suitable for retaining polyphenols, with no or low retention observed except for anthocyanins. The effectiveness of stationary phases in retention of phenolics following the order related to increased retention: zwitterionic, amide, and amino. The choice of mobile phase also influenced retention. Mobile phases containing TFA as pH modifier limited retention, while formic acid was found to be more effective for polyphenol retention. Ammonium buffers also improved retention but often compromised peak shape. pH changes mainly impacted ionizable compounds, such as phenolic acids, by increasing their retention when they were ionized. DAD was wellsuited for detecting polyphenols that possess aromatic rings, though peak wavelengths depend on the structures of the polyphenols. CAD, while less sensitive than DAD and MS, provided an almost similar response for structurally related compounds, even with gradient elution. MS was the preferred detector for quantification when resolution between compounds was challenging, as it is often the case with natural extracts. The study successfully demonstrated that best HILIC conditions were obtained using an amino stationary phase composed of a polyethylenimine and formic acid-based mobile phase. These conditions were successfully applied to the analysis of stilbenoid-rich extracts from different parts of the vine. The elution order of stilbenoids followed the degree of polymerization. With CAD, the chromatographic profile was more representative of sample composition. It was demonstrated for the first time the interest of a combination of HILIC and CAD for analyzing stilbenes, offering a complementary approach to the classic RP analysis.

Chromatographic evaluation and application of nitrogenous heterocyclic ring-bonded stationary phase in hydrophilic interaction liquid chromatography

Hydrophilic interaction liquid chromatography (HILIC) based on polar stationary phases has vital research significance in the separation of polar compounds. Numerous HILIC stationary phases with different structures have been developed, which do not have universal properties and broad selectivity, making it a challenge to select the suitable column based on the properties of the samples. Consequently, it is particularly important to develop a bonded phase capable of separating a wide variety of samples, while having enhanced retention, improved selectivity, symmetric peak shape and good stability. Herein, a novel nitrogen-containing heterocyclic bonded phase with multiple functionalities, such as thioether, amino and hydroxyl groups (named AMTA) was employed as HILIC stationary phase. Detailed chromatographic evaluations were carried out, and the results showed that it was superior to other hydrophilic chromatographic columns in terms of selectivity, peak shapes and practical sample separation. Lastly, it has been verified that AMTA exhibited high orthogonality with the XBridge C18 column of reversed-phase liquid chromatography (RPLC) mode. In summary, we anticipate our assay to be instructive to other researchers in developing the HILIC stationary phase.

Understanding retention and intra-particle diffusivity of alkylsulfobetaine-bonded Ethylene Bridged Particles with different mesopore sizes for hydrophilic interaction liquid chromatography applications

The role of the average pore diameter (APD) of 1.7μm AtlantisTM Premier BEHTM Particles derivatized with a zwitterionic group (propylsulfobetaine) on the efficiency of their 2.1 × 50 mm hydrophilic interaction liquid chromatography (HILIC) packed columns is investigated experimentally. Van Deemter plots for toluene (neutral, hydrophobic), cytosine (neutral, polar), tosylate (negatively charged), bretylium and atenolol (positively charged) were measured on three HILIC columns packed with BEH Z-HILIC Particles having APDs of 95 Å, 130 Å, and 300 Å. The intraparticle diffusivities of the analytes across these three BEH Z-HILIC Particles were measured by the peak parking method. The experimental data reveal that the slope of the C-branch of the van Deemter plots can be reduced by factors of about 15 for toluene, 2.5 for cytosine, 6 for atenolol, 5 for tosylate, and 14 for bretylium with increasing the APD from 95 Å to 300 Å. This observation is explained by: (1) the reduced amount of the highly viscous water diffuse layer and subsequent increase of the amount of acetonitrile-rich eluent in the mesopores, (2) the localized electrostatic adsorption of the retained analytes onto the zwitterion-bonded BEH Particles, and (3) depletion/excess of the analytes into the water diffuse layer. A general model of intraparticle diffusivity was then proposed to account for the impact of the APD of Z-HILIC Particles on the solid-to-liquid mass transfer resistance of small molecules. The model highlights the relevance of the thickness of the water diffuse layer, the access of the bulk eluent into the mesopore, the localized electrostatic adsorption, and the partitioning constant of the retained analyte between the bulk eluent and the water diffuse layer.

A Polymer-Based Polar Stationary Phase Grafted With Modified Lysine for Hydrophilic Interaction Chromatography

The high hydrophobicity and chemical inertness of poly(styrene-divinylbenzene) (PS-DVB) microspheres make their surface hydrophilic modification difficult. Here we describe a facile way to convert PS-DVB microspheres to hydrophilic, then can be used as polar stationary phase for hydrophilic interaction chromatography. This approach utilizes the grafting of an acrylamide-terminated lysine zwitterionic monomer onto PS-DVB microspheres via free radical polymerization. The obtained stationary phase shows good hydrophilicity and a typical retention mechanism of hydrophilic interaction chromatography toward several model polar analytes. It also exhibits obvious zwitterionic properties and is capable of separating cationic and anionic analytes simultaneously. The column shows negligible bleeding level, much superior to silica-based ones.

Solving the retention time repeatability problem of hydrophilic interaction liquid chromatography

Hydrophilic interaction (liquid) chromatography (HILIC) has become the first choice LC mode for the separation of hydrophilic analytes. Numerous studies reported the poor retention time repeatability of HILIC. The problem was often ascribed to slow equilibration and insufficient re-equilibration time to establish the sensitive semi-immobilized water layer at the interface of the polar stationary phase and the bulk mobile phase. In this study, we compare retention time repeatability in HILIC for borosilicate glass and PFA (co-polymer of tetrafluoroethylene and perfluoroalkoxyethylene) solvent bottles. During this study, we observed peak patterns shifting towards higher retention times (for metabolites and peptides) and lower retention times (oligonucleotide sample) with ongoing analysis time when standard borosilicate glass bottles were used as solvent reservoirs. It was hypothesized that release of ions (sodium, potassium, borate, etc.) from the borosilicate glass bottles leads to alterations (thickness and electrostatic screening effects) in the semi-immobilized water layer which is adsorbed to the polar stationary phase surface under acetonitrile-rich eluents in HILIC with concomitant shifts in retention. When PFA solvent bottles were employed instead of borosilicate glass, retention time repeatability was greatly improved and changed from average 8.4 % RSD for the tested metabolites with borosilicate glass bottles to 0.14 % RSD for the PFA solvent bottles (30 injections over 12 h). Similar improvements were observed for peptides and oligonucleotides. This simple solution to the retention time repeatability problem in HILIC might contribute to a better acceptance of HILIC, especially in fields like targeted and untargeted metabolomics, peptide and oligonucleotide analysis.

Insights into the selectivity of polar stationary phases based on quantitative retention mechanism assessment in hydrophilic interaction chromatography

Hydrophilic interaction chromatography (HILIC) offers different selectivity than reversed-phase liquid chromatography (RPLC). However, our knowledge of the driving force for selectivity is limited and there is a need for a better understanding of the selectivity in HILIC. Quantitative assessment of retention mechanisms makes it possible to investigate selectivity based on understanding the underlying retention mechanisms. In this study, selected model compounds from the Ikegami selectivity tests were evaluated on different polar stationary phases. The study results revealed significant insights into the selectivity in HILIC. First, hydroxy and methylene selectivity is driven by hydrophilic partitioning; but surface adsorption for 2-deoxyuridine or 5-methyluridine reduces the selectivity factor. Furthermore, the retention of 2-deoxyuridine or 5-methyluridine by surface adsorption in combination with the phase ratio explain the difference in hydroxy or methylene selectivity observed among different stationary phases. Investigations on xanthine positional isomers (1-methylxanthine/3-methylxanthine, theophylline/theobromine) indicate that isomeric selectivity is controlled by surface adsorption; however, hydrophilic partitioning may contribute to resolution by enhancing overall retention. In addition, two pairs of nucleoside isomers (adenosine/vidarabine, 2'-deoxy and 3'-deoxyguanosine) provide an example that isomeric selectivity can also be controlled by hydrophilic partitioning if their partitioning coefficients are significantly different in HILIC. Although more data is needed, the current study provides a mechanistic based understanding of the selectivity in HILIC and potentially a new way to design selectivity tests.

Retention behavior of nucleosides and nucleobases on a 3 μm undecylenic acid-functionalized silica column in per aqueous liquid chromatography and hydrophilic interaction liquid chromatography separation modes

A 3 μm undecylenic acid-functionalized stationary phase (UAS) was prepared for the separation of nucleosides and nucleobases using per aqueous liquid chromatography (PALC) and hydrophilic interaction liquid chromatography (HILIC). The retention behaviors of nucleosides and nucleobases in PALC and HILIC modes were explored by adjusting parameters such as water content, buffer concentration, pH of the mobile phase and column temperature. The experimental data and separation chromatogram demonstrated that PALC could provide retention comparable to that of HILIC for nucleosides and nucleobases. Comparative studies using diluted adenosine solutions evaluated theoretical plates and peak shape for the same retention factors (between 0.25 and 5.0) in PALC and HILIC. There was no buffer component in the mobile phases used to operate the comparisons. HILIC mode is more efficient for adenosine than PALC mode at low retention factors. It's the exact opposite phenomenon for high retention factors. It is proposed that the mass transfer of adenosine between the UAS, the water-rich layer and the ACN-rich mobile phase in HILIC is relatively slow. Given the significant use of toxic ACN in HILIC, PALC emerges as a safer and more effective alternative for separating nucleosides and nucleobases.

A lysine and amide functionalized polymer-based polar stationary phase for hydrophilic interaction chromatography

A novel polymer-based polar stationary phase for hydrophilic interaction chromatography (HILIC) is described. It was obtained by grafting lysine and acrylamide onto poly (glycidyl methacrylate-divinylbenzene) (GMA-DVB) microspheres via ring-opening reaction of epoxy groups and free radical polymerization with pendant double bonds of the microspheres. Multiple types of polar groups including zwitterionic (carboxylate and amine), amide and diol onto the microspheres make them highly hydrophilic. It showed typical HILIC character and good separation performance towards model polar analytes. Negligible bleed level under gradient elution mode (up to 50% fraction of water) was observed. It also exhibited specific separation selectivity to ionic analytes and simultaneous separation of anions and cations could be achieved in ideal electrostatic selectivity elution order, e.g. I-< NO3- Collapse

Key Words

• Bleed
• Glycidyl methacrylate
• Hydrophilic interaction chromatography
• Polymer
• Zwitterionic stationary phase

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MESH Headings

• Amides
• Lysine
• Acrylamide
• Amines
• Chromatography

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Grants Collapse

Affiliation(s)

Ziteng Yu Engineering Research Center of Pharmaceutical Process Chemistry, Shanghai Frontiers Science Center of Optogenetic Techniques for Cell Metabolism, School of Pharmacy, East China University of Science and Technology, Shanghai 200237, China
Zongying Li Engineering Research Center of Pharmaceutical Process Chemistry, Shanghai Frontiers Science Center of Optogenetic Techniques for Cell Metabolism, School of Pharmacy, East China University of Science and Technology, Shanghai 200237, China.
Feifang Zhang Engineering Research Center of Pharmaceutical Process Chemistry, Shanghai Frontiers Science Center of Optogenetic Techniques for Cell Metabolism, School of Pharmacy, East China University of Science and Technology, Shanghai 200237, China
Bingcheng Yang Engineering Research Center of Pharmaceutical Process Chemistry, Shanghai Frontiers Science Center of Optogenetic Techniques for Cell Metabolism, School of Pharmacy, East China University of Science and Technology, Shanghai 200237, China.

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Hydrophilic Partitioning or Surface Adsorption? A Quantitative Assessment of Retention Mechanisms for Hydrophilic Interaction Chromatography (HILIC)

Retention mechanisms in HILIC have been investigated and reported in literature. However, the current understanding of retention mechanisms is qualitative and lacks quantitative details. Previously, mechanism elucidation was based on indirect evidence, and unambiguous assignment of retention mechanisms has not been reported based on direct data. This study aims to quantitatively determine the contributions of two major retention mechanisms in HILIC, hydrophilic partitioning and surface adsorption to the overall retention of neutral compounds. Using the methodologies we developed previously, the phase ratio for adsorbed water layer and distribution coefficients were measured and used to calculate the retention factors contributed by hydrophilic partitioning. The methodology allows the determination of the contribution of surface adsorption simultaneously. The evaluation of five test compounds demonstrates that the retention may be controlled by hydrophilic partitioning, surface adsorption or both depending on compound characteristics. Quantitative assessment of retention mechanisms also makes it possible to better understand the effect of acetonitrile on retention in HILIC.

A facile approach to undecylenic acid-functionalized stationary phases for per aqueous liquid chromatography

Green chromatography techniques using low-toxic mobile phase are getting increasingly attention in recent years. The core is developing stationary phases that possess adequate retention and separation under the mobile phase of high content water. Using thiol-ene click chemistry, an undecylenic acid-bonded silica stationary phase (UAS) was prepared in a facile manner. Elemental analysis (EA), solid-state ¹³C NMR spectroscopy and Fourier transform infrared spectrometry (FT-IR) confirmed the successful preparation of UAS. The synthesized UAS was employed for per aqueous liquid chromatography (PALC), which uses little organic solvent during separation. Due to the hydrophilic carboxy, thioether group and hydrophobic alkyl chains of the UAS, various categories of compounds (including nucleobases, nucleosides, organic acids and basic compounds) with different properties can achieve enhanced separation under the mobile phase of high content water compared with commercial C18 and silica stationary phases. Overall, our present UAS stationary phase shows excellent separation ability toward highly polar compounds and meets the requirements of green chromatography.

Studies on the retention mechanism of solutes in hydrophilic interaction chromatography using stoichiometric displacement theory II. HILIC/RPLC dual-retention mechanism of solutes in hydrophilic interaction chromatography over the entire range of water concentration in mobile phase

This paper is a continuation of research into the retention behavior and mechanism of solutes in hydrophilic interaction chromatography (HILIC) using stoichiometric displacement theory (SDT). A HILIC/reversed-phase liquid chromatography (RPLC) dual-retention mechanism was studied in detail using a β-CD HILIC column. The retention behaviors of three groups of solutes with varying polarities were studied over the entire range of water concentrations in the mobile phase on the β-CD column, resulting in the formation of "U-shape" curves when lgk' was plotted against lg[H₂O]. Additionally, the effect of hydrophobic distribution coefficient lgP_O/W on the retention behaviors of solutes in HILIC and RPLC modes was also examined. A four-parameter equation derived from the SDT-R was found to accurately describe the "U-shaped" curves of solutes with RPLC/HILIC dual-retention mechanisms on β-CD column. The theoretical lgk' values of solutes calculated using the equation were found to be in agreement with their experimental values, with correlation coefficients greater than 0.99. This indicates that the four-parameter equation derived from SDT-R can effectively describe the retention behaviors of solutes over the entire range of water concentrations in the mobile phase in HILIC. As such, SDT can be used as a theoretical guide for the development of HILIC, including the exploration of new dual-function stationary phases to enhance separation efficiency.

Retention and mass transfer properties of the series of unbonded, amide-bonded, and alkylsulfobetaine-bonded ethylene bridged hybrid hydrophilic interaction liquid chromatography columns

This work investigates the link between the retentivity and the stationary phase to mobile phase mass transfer resistance of hydrophilic interaction liquid chromatography (HILIC) columns packed with the same base ethylene-bridged hybrid particles (BEH). The retention volumes, the plate heights, and the volume of the adsorbed water layer were measured for the ACQUITY^TM UPLC^TM BEH^TM 130 Å HILIC Column (unbonded BEH), ACQUITY UPLC BEH 130 Å Amide Column (amide group attached), and Atlantis^TM Premier BEH 95 Å Z-HILIC (zwitterionic group attached) Column. The method of Guo (toluene retention volumes in pure acetonitrile and in the HILIC eluent) was validated from the UNIFAC group-contribution method and applied to measure accurately the water layer volumes in these columns. A strong correlation was found between the retention volumes of most neutral polar analytes and the volume of the water layer adsorbed in the HILIC column. The fraction of the pore volume occupied by the water layer increases significantly from the BEH HILIC Column to the BEH Amide Column, and to the BEH Z-HILIC Column. This is explained by the water solvation of the attached ligands in the pore volume of the BEH Particles and to the smaller average mesopore size of the BEH Z-HILIC Particles. A second and strong correlation is also observed between the water content in the HILIC particle and the stationary phase to mobile phase mass transfer resistance of the HILIC columns at high mobile phase linear velocities. The measured intra-particle diffusivity normalized to the bulk diffusion coefficient decreased from 0.33 (BEH HILIC Column) to 0.10 (BEH Amide Column) and to only 0.03 (BEH Z-HILIC Column) for comparable retention of cytosine. These results are fully consistent with the higher viscosity of the internal eluent (higher water content) and higher internal obstruction for diffusion (smaller mesopores and internal porosity) in the BEH Z-HILIC Particles. Still, in gradient elution mode, the peak capacity was found to be 18% higher for the BEH Z-HILIC Column than that on the BEH Amide Column because the retention factors at elution were smaller when maintaining the same analysis time and starting eluent composition.

Further Evaluation of the Base Stability of Hydrophilic Interaction Chromatography Columns Packed with Silica or Ethylene-Bridged Hybrid Particles

One of the fundamental attributes of a liquid chromatography column is its stability when exposed to acidic and basic mobile phases. However, there have been relatively few reports to date on the stability of hydrophilic interaction chromatography (HILIC) columns. Here, we report the results of stability evaluations carried out for HILIC columns packed with ethylene-bridged hybrid or silica particles using accelerated conditions, employing a 100% aqueous pH 11.3 ammonium bicarbonate mobile phase at 70 °C. Under these conditions, the primary mode of column failure was a loss of efficiency due to the formation of voids resulting from the hydrolysis of the particles. We investigated the dependence of stability on the surface area of both unbonded and sulfobetaine-bonded ethylene-bridged hybrid stationary phases. The results show a clear trend of stability increasing as the surface area decreases. Several commercially available HILIC columns that are recommended for use with high-pH mobile phases were also evaluated. The results show times to 50% loss of the initial efficiency ranging from 0.3 to 9.9 h. Columns containing unbonded, sulfobetaine-bonded or diol-bonded ethylene-bridged hybrid stationary phases had longer lifetimes than amino-bonded silica or sulfobetaine-bonded, hybrid-coated, superficially porous silica columns.

Multi-2D LC × LC as a Novel and Powerful Implement for the Maximum Separation of Complex Samples

Achieving complete information about the chemical composition of complex samples requires the use of multianalytical platforms able to maximize the acquisition of high-quality data for unequivocal identification. However, this process requires long analysis times and several instruments. Food analysis is one of the analytical fields where the analysis of very complex samples has a huge impact. One of these complex samples is vermouth, a fortified wine based on the maceration of a large number of herbs, fruits, barks, seeds, and leaves. The application of conventional or even advanced analytical techniques like comprehensive two-dimensional (2D) liquid chromatography (LC × LC) does not provide enough separation power to resolve the complete profile of this sample. In this work, a novel 2DLC strategy called multi-²D LC × LC is developed. This new setup consists of the use of two different columns with different separation properties in the second dimension (²D) that can be selected during the LC × LC analysis accordingly to the chemical nature of the compounds eluted from the first dimension (¹D). The vermouth sample was analyzed using a ¹D-PFP and a combination of HILIC (from 0 to 30 min) and C18 (from 30 to the end) columns in the ²D. This setup increased both the peak capacity and the orthogonality of the analysis in comparison to the use of only one of the columns in the ²D. Multi-²D LC × LC is presented as an integrated 2DLC tool that maximizes the separation capacity for very complex samples.

A Compendium of the Principal Stationary Phases Used in Hydrophilic Interaction Chromatography: Where Have We Arrived?

Hydrophilic interaction liquid chromatography (HILIC) today is a well-known and largely applied technique to analyse polar compounds such as pharmaceuticals, metabolites, proteins, peptides, amino acids, oligonucleotides, and carbohydrates. Due to the large number of stationary phases employed for HILIC applications, this review aims to help the reader in choosing a proper stationary phase, which often represents the critical point for the success of a separation. A great offer is present for achiral applications in contrast to the chiral phases developed for HILIC enantioseparations. In the last case, up-to-date solutions are presented.

Pursuing the Polar Routes with Penguins and Polar Bears: Balancing the Choice of Chromatographic Mode

Reversed-phase liquid chromatography (RPLC) may have historical popularity, but it is not the only LC technique available to chromatographers. To help balance the choice of chromatographic technique, a discussion of the merits of being flexible when making the choice of LC technique for a given application is presented. Because having a flexible perspective is particularly important for biotherapeutics, it is important that choosing which technique to use is determined based on good science and begins with a physicochemical property assessment of the analytes, followed by subsequent alignment with the most appropriate retentive mode. An Earth-based figurative analogy is used to help highlight several popular LC techniques.