Polar-Embedded and Polar-Endcapped Stationary Phases for LC

Design and evaluation of semicarbazide-embeddedd stationary phases for liquid chromatography

Semicarbazide, as a derivative of urea, constitutes a great variety of functional molecules for different needs. Herein, novel stationary phases with an incorporated semicarbazide group were proposed. Using aliphatic (docosanoyl, C22) and aromatic (benzoyl, Bz) hydrazides, the semicarbazide-embedded ligands were synthesized before chemical modification of the silica gel, allowing for an accurate interpretation of the chromatographic properties of the corresponding packings. The new stationary phases were water-wettable, due to the presence of highly polar groups. In particular, Bz-semicarbazide (Bz-SCD) stationary phase was sufficiently hydrophilic to run in hydrophilic interaction (HILIC) mode, whilst the C22 (C22-SCD) equivalent, in spite of its reversed-phase nature, was markedly less hydrophobic than the referenced polar-embedded ones. The versatility of C22-SCD was demonstrated with a large selection of analytes, including geometric isomers and standard mixtures of polycyclic aromatic hydrocarbons, sulfonamides, sulfonylurea, substituted ureas, pyridines and carbamates, fat-soluble colorants, antifungal metabolites, angiotensin II receptor blockers and calcium channel blockers.

Facile preparation of embedded polar group-containing pentafluorophenyl stationary phases for highly selective separations of diverse analytes

Pentafluorophenyl (PFP) stationary phase is one of the most important phases after the C18 phase in terms of its applications. Three embedded polar groups (EPG)-containing stationary phases were newly synthesized to act the EPGs as additional interaction sites. The silica surface was initially modified with (3-aminopropyl)trimethoxysilane (APS). The APS-modified silicas were coupled with 2,3,4,5,6-pentafluorobenzoic acid, 2,3,4,5,6-pentafluorophenylacetic acid, and 2,3,4,5,6-pentafluoro-anilino(oxo)acetic acid to obtain Sil-PFP-BA, Sil-PFP-AA, and Sil-PFP-AN phases, respectively. The new phases were characterized by elemental analysis, ATR-FTIR, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The phases were evaluated with the Tanaka and Neue tests in reversed-phase liquid chromatography (RPLC). In addition, they were characterized as hydrophilic phases by the Tanaka test protocol used in hydrophilic interaction chromatography (HILIC) separation mode. The Sil-PFP-AA phase showed the highest molecular shape selectivity in RPLC, while Sil-PFP-AN achieved the highest separability in HILIC compared to the commercial PFP reference column. The Sil-PFP-AA phase was successfully applied for the analysis of capsaicinoids from real samples of fresh chili peppers (Capsicum spp.) in RPLC and the Sil-PFP-AN phase for vitamin C (ascorbic acid) in HILIC.

Surface Modification of Silica with β-Alanine Derivatives for Unique Applications in Liquid Chromatography

Column purchasing cost is an important issue for an analyst to analyze complex sample matrices. Here, we report the development of an amino acid (β-alanine)-derived stationary phase (Sil-Ala-C12) with strategic and effective interaction sites (amide and urea as embedded polar groups with C12 alkyl chain) able to separate various kinds of analytes. Owing to the balanced hydrophobicity and hydrophilicity of the phase, it showed exceptional separation abilities in both reversed-phase high-performance liquid chromatography (RP-HPLC) as a hydrophobic phase and hydrophilic interaction chromatography (HILIC) as a hydrophilic phase. Remarkably, the baseline separation was achieved for the challenging β- and γ-isomers of tocopherol. Usually, three columns such as pentafluorophenyl or C30, C18, and sulfobetaine HILIC are required for the analysis of vitamin E, capsaicinoids, and vitamin C in chili peppers (Capsicum spp.), respectively. However, only Sil-Ala-C12 was able to separate these analytes. A single column can serve 3-4 purposes, which suggests that Sil-Ala-C12 had the potential to reduce column purchasing costs.

Comparing the separation performance of poly(ethyleneimine) embedded butyric and octanoic acid based chromatographic stationary phases

Poly(ethyleneimine) (PEI) modified silica spheres were used to graft butyric acid and octanoic acid onto their surfaces, forming two stationary phases named Sil-PEI-BAD and Sil-PEI-CAD, respectively. Characterized methods including fourier transform infrared spectroscopy (FT-IR), elemental analysis (EA) and thermogravimetric analysis (TGA) were utilized to determine the successful synthesis of these two stationary phase materials. The chromatographic performance of these two stationary phases was analyzed with hydrophobic and hydrophilic compounds as analytes. Compared with Sil-PEI-CAD column, Sil-PEI-BAD column was more effective in separating hydrophilic compounds including nucleosides, alkaloids and vitamins. Hydrophobic substances including polycyclic aromatic hydrocarbons (PAHs) and alkylbenzenes obtained excellent separation results on Sil-PEI-CAD column than Sil-PEI-BAD column. Additionally, according to the separation of phenols, Sil-PEI-CAD column can be used in HILIC/RPLC mixed-mode. The results showed that the properties and retention mechanisms of the prepared stationary phases depended on the length of the alkyl chains bonded on the silica surface.

p-Terphenyl-based rigid stationary phases with embedded polar groups for liquid chromatography

Terphenyls are important building blocks for a wide range of functional molecules. Among the three isomers, p-terphenyl (C₁₈H₁₄) is particularly useful for the construction of optical devices on account of its unique structure. Herein, two rigid stationary phases bearing p-terphenyl as an external moiety and variable embedded carbamate groups (p-TerC with one embedded carbamate group and p-TerC² with two embedded carbamate group) were presented. The proposed stationary phases were characterized by various means and evaluated in reversed-phase (RP) mode, using different classes of analytes, including polycyclic aromatic hydrocarbons (PAHs), alkylbenzenes, 4-alkylbiphenyls, substituted ureas, sulfonylureas, substituted sulfanilamides and aromatic acids. The comparison with conventional C18, several other polar-embedded aromatic and C18 equivalents indicated p-terphenyl-based stationary phases were featured by multiple retention mechanisms, involving π-π interaction, charge-transfer interaction, hydrogen-bonding and hydrophobic interaction in RP mode. A unusually high specificity to the analytes with linear structures was observed, as exemplified by an irreversible adsorption of tetracene and a readily separation of tetraphene and chrysene. The aliphatic linker used in the proposed stationary phases was an influential factor for retentivity, selectivity and column efficiency. Interestingly, p-TerC² was operable in normal-phase mode for the separation of certain PAHs through polar-related interactions. The linear, rigid polyphenyl structure of p-terphenyl endowed the new stationary phase with distinctive chromatographic properties, in contrast to those of the preceding counterparts bonded with alkyl and/or polynuclear aromatic moieties.

Effect of phenyl numbers in polyphenyl ligand on retention properties of aromatic stationary phases

Aromatic phase, as one type of reversed-phase stationary phases, shows complementary selectivity to the n-alkyl counterparts especially for certain challenging separation tasks. However, effect of phenyl numbers in aromatic ligands on retention behaviors has rarely been addressed compared with the alkyl stationary phases. To illustrate the issue, a series of polyphenyl stationary phases were facially prepared via the coupling chemistry of isocyanate with amine, including aniline (π¹), 4-aminobiphenyl (π²), 4-amino-p-terphenyl (π³) and [1,1':4',1'':4'',1'''-quaterphenyl]-4-amine (π⁴), respectively. The chromatographic behaviors of the new stationary phases as well as the traditional C18 were systematically compared in terms of retention mode, hydrophobic and aromatic selectivity, shape selectivity and π-π interaction by various analytes, including alkylbenzenes, polycyclic aromatic hydrocarbons congeners and substituted benzenes with electron-withdrawing groups. Due to the homologous structure of four polyphenyl ligands, the hydrophobic selectivity, aromatic selectivity and shape selectivity of stationary phases increase with phenyl numbers in the bonded polyphenyl ligands, whereas the increment becomes insignificant from U-π³ to U-π⁴. This phenomenon is explained by the insertion degree of analytes in the polyphenyl ligand brushes. Compared with the homemade C18, the polyphenyl phases indicate insignificant changes of shape selectivity with temperature. Notably, the new polyphenyl phases demonstrate the great selective separation towards the electron-deficient compounds through the π-π interaction. These findings make up for the understanding of the retention behavior of aromatic stationary phases.

A carbonylative coupling approach to alkyl stationary phases with variable embedded carbamate groups for high-performance liquid chromatography

A highly efficient carbonylative coupling method for the preparation of alkyl stationary phases with variable numbers of carbamate groups was established. The effectiveness of such method was verified through elemental analysis, X-ray photoelectron spectroscopy and solid-state ¹³C nuclear magnetic resonance spectroscopy of three as-synthesized stationary phases bearing different alkyl chains and different numbers of carbamate groups (octadecyl/one carbamate group, C18C; docosyl/three carbamate groups, C22C³; triacontyl/two carbamate groups, C30C²). The comparative evaluation of these stationary phases using a great variety of analytes, including three sets of isomers of alkylbenzenes, two sets of standard mixtures of polycyclic aromatic hydrocarbons (SRM 1647e and 869b), nine polychlorinated biphenyls, fiveteen N-substituted ureas, ten sulfonylureas, five xanthines and some other phytonutrients, revealed their remarkable applicability in reversed-phase liquid chromatography. Notably, the intercalated carbamate groups rendered the resultant stationary phases compatible with 100% aqueous mobile phase. The suppression of silanol activity was positively related to the number of polar groups embedded in the bonded selector, and the smallest peak tailing factor (1.14) for amitriptyline was obtained by C22C³. The molecular shape-related selectivity was found to be more closely related to the length of the selector's aliphatic chain, as supported by the lowest α_TBN/BaP value (0.31) by C30C². These carbamate-embedded alkyl stationary phases constituted another class of polar-embedded stationary phases possessing a single type of functional ligand.

A systematic approach to development of analytical scale and microflow-based liquid chromatography coupled to mass spectrometry metabolomics methods to support drug discovery and development

As the reliance on metabolic biomarkers within drug discovery and development increases, there is also an increased demand for global metabolomics methods to provide broad metabolome coverage and sensitivity towards differences in metabolite expression and reproducibility. A systematic approach is necessary for the development, and evaluation, of metabolomics methods using either conventional techniques or when establishing new methods that allow for additional gains in sensitivity and a reduction in requirements for amounts of a biological sample, such as those seen with methods based on microseparations. We developed a novel standard mixture and used a systematic approach for the development and optimization of optimal, ion-pair free, liquid chromatography-mass spectrometry (LC-MS) global profiling methods. These methods were scaled-down to microflow-based LC separations and compared with analytical flow ion-pairing reagent containing methods. Average peak volume improvements of 7- and 22-fold were observed in the positive and negative ionization mode microflow methods as compared to the ion-pairing reagent analytical flow methods, respectively. The linear range of the newly developed microflow methods showed up to a 10-fold increase in the lower limit of detection in the negative ionization mode. The developed microflow LC-MS methods were further evaluated using wild-type mouse plasma where up to a 9-fold increase in peak volume was observed.

Surface properties of stationary phases with embedded polar group based on secondary interaction, zeta potential measurement and linear solvatation energy relationship studies

The chromatographic properties of six non-commercially available stationary phases with ester or phosphodiester functional groups embedded into alkyl chain were studied. Zeta potential values of stationary phases suspended in water, organic solvent and their mixtures were measured. Moreover, the selectivity coefficients were calculated on the basis of the retention factor for the test solutes. Separations were performed under hydrophilic interaction liquid chromatographic conditions. Hydrophobic and polar properties of the investigated columns were compared. Based on the chromatographic properties, polar embedded packing materials were classified. Also two phases with different spacer but the same embedded polar group and alkyl chain (Amino-P-C18 and Diol-P-C18) were used for comparison with homemade materials. Amino-P-C18 stationary phase exhibits positive values of zeta potential which is in accordance with the observed anion exchange properties. The highest negative values of zeta potential were observed for Diol-P-C18, together with cation exchange properties. The highest methylene selectivity, polar selectivity and configurational selectivity were observed for the stationary phase with an ester bond and a phenyl group.

Liquid chromatography of short chain carboxylic acids using a glutamic acid surfactant coated C18 stationary phase

A C18 column was modified with the anionic amino acid surfactant lauroyl-l-glutamate (LLG) to facilitate the separation of ten short-chain aliphatic carboxylic acids (oxalic, tartaric, malic, malonic, lactic, acetic, maleic, citric, fumaric, and succinic). The developed method was proven to be fast, versatile, and environmentally friendly. After the coating of the column using 1% LLG solution and optimizing chromatographic conditions such as pH and temperature, near baseline resolution of the ten carboxylic acids within 4 min with excellent peak shape at pH = 1.8 using 100% H₂O acidified with sulfuric acid was possible. Although the design of this stationary phase, with the hydrophilic group at the end of the alkyl chain, seems to be in contrast to such columns designed for a totally aqueous mobile phase that have a polar (often amide) group embedded near the silica surface, no evidence of phase collapse was noted. Linear relationships of ln retention factor (k) versus 1/Temperature (T) (van't Hoff plots) were generated for all the acids indicating a single retention mechanism was likely. As the pH of the mobile phase decreased, the analyte retention factors increased due to the increase of the fraction of the analyte with neutral charge (alpha zero). The surfactant amide linkage, being electron donating, increased the pKa of the more acidic carboxyl group of glutamic acid so both carboxyl groups were protonated (neutral) at pH 1.8. The exact nature of the retention mechanism is uncertain but there certainly seems to be a pronounced hydrophobic component due to the large difference in retention of fumaric acid and methyl fumarate at pH 1.8. In addition, eleven beverage samples were analyzed for their aliphatic carboxylic acid contents. The results showed that malic, fumaric, and citric acids were the most common carboxylic acids in natural beverages with concentrations as high as 6432 ppm of malic acid in organic apple juice, 64 ppm of fumaric acid in organic concord juice, and 6543 ppm citric acid in strawberry lemonade juice.

Ultra-high performance separation of basic compounds on reversed-phase columns packed with fully/superficially porous silica and hybrid particles by using ultraviolet transparent hydrophobic cationic additives

The use of the tetrabutylammonium additive was investigated in the ultra-high performance reversed-phase liquid chromatographic elution of basic molecules of pharmaceutical interest. When added to the mobile phase at low pH, the hydrophobic tetrabutylammonium cation interacts with the octadecyl chains and with the residual silanols, thus imparting a positive charge to the stationary phase, modulating retention and improving peak shape of protonated basic solutes. Two sources of additive were tested: a mixture of tetrabutylammonium hydroxide/trifluoroacetic acid and tetrabutylammonium hydrogen sulfate. Retention and peak shape of 11 basic pharmaceutical compounds were evaluated on commercially available ultra-fast columns packed with octadecyl stationary phases (Ascentis Express C18 2.0 µm, Acquity BEH C18 1.7 µm, Titan C18 1.9 µm). All columns benefit from the use of additive, especially tetrabutylammonium hydrogen sulfate, providing very symmetric peaks with reasonable retention times. Focusing on the probe compounds amitriptyline and sertraline, efficiency and asymmetry values were investigated at increasing retention factor. The trend is very different to that obtained in reversed-phase conditions and the effect lies in the complex molecular interaction mechanisms based on hydrophobic and ion exchange interactions as well as electrostatic repulsion.

Analytical strategies for the determination of amino acids: Past, present and future trends

This review describes the analytical methods that have been developed over the years to tackle the high polarity and non-chromophoric nature of amino acids (AAs). First, the historical methods are briefly presented, with a strong focus on the use of derivatization reagents to make AAs detectable with spectroscopic techniques (ultraviolet and fluorescence) and/or sufficiently retained in reversed phase liquid chromatography. Then, an overview of the current analytical strategies for achiral separation of AAs is provided, in which mass spectrometry (MS) becomes the most widely used detection mode in combination with innovative liquid chromatography or capillary electrophoresis conditions to detect AAs at very low concentration in complex matrixes. Finally, some future trends of AA analysis are provided in the last section of the review, including the use of supercritical fluid chromatography (SFC), multidimensional liquid chromatography and electrophoretic separations, hyphenation of ion exchange chromatography to mass spectrometry, and use of ion mobility spectrometry mass spectrometry (IM-MS). Various application examples will also be presented throughout the review to highlight the benefits and limitations of these different analytical approaches for AAs determination.

Column Characterization and Selection Systems in Reversed-Phase High-Performance Liquid Chromatography

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the most popular chromatographic mode, accounting for more than 90% of all separations. HPLC itself owes its immense popularity to it being relatively simple and inexpensive, with the equipment being reliable and easy to operate. Due to extensive automation, it can be run virtually unattended with multiple samples at various separation conditions, even by relatively low-skilled personnel. Currently, there are >600 RP-HPLC columns available to end users for purchase, some of which exhibit very large differences in selectivity and production quality. Often, two similar RP-HPLC columns are not equally suitable for the requisite separation, and to date, there is no universal RP-HPLC column covering a variety of analytes. This forces analytical laboratories to keep a multitude of diverse columns. Therefore, column selection is a crucial segment of RP-HPLC method development, especially since sample complexity is constantly increasing. Rationally choosing an appropriate column is complicated. In addition to the differences in the primary intermolecular interactions with analytes of the dispersive (London) type, individual columns can also exhibit a unique character owing to specific polar, hydrogen bond, and electron pair donor-acceptor interactions. They can also vary depending on the type of packing, amount and type of residual silanols, "end-capping", bonding density of ligands, and pore size, among others. Consequently, the chromatographic performance of RP-HPLC systems is often considerably altered depending on the selected column. Although a wide spectrum of knowledge is available on this important subject, there is still a lack of a comprehensive review for an objective comparison and/or selection of chromatographic columns. We aim for this review to be a comprehensive, authoritative, critical, and easily readable monograph of the most relevant publications regarding column selection and characterization in RP-HPLC covering the past four decades. Future perspectives, which involve the integration of state-of-the-art molecular simulations (molecular dynamics or Monte Carlo) with minimal experiments, aimed at nearly "experiment-free" column selection methodology, are proposed.

Stationary phase based on cellulose dodecanoate physically immobilized on silica particles for high-performance liquid chromatography

The chemical agent free preparation of a stationary phase using a natural macromolecule was the focus of this paper. Thermal immobilization of cellulose dodecanoate on silica particles was used for the preparation of a stationary phase without the use of chemical reagents. Cellulose modification was performed to produce a hydrophobic macromolecule with solubility in common organic solvents. The new stationary phase was characterized morphologically and physico-chemically, presenting as spherical particles immobilized with a thin cellulose dodecanoate layer. The degree of substitution of cellulose dodecanoate was 1.7, which resulted in a separation mechanism in reversed phase mode, but with lower hydrophobicity and higher steric selectivity, which are properties from cellulose. These characteristics resulted in a stationary phase with intrinsic selectivity that was able to separate mixtures of polar drugs, homologs of an anionic surfactant and omeprazole isomers, which are not well resolved in typical C₁₈ phases. Considering that cellulose is a natural polymer and the preparation method of stationary phase involves only physical processes of silica modification, the final material presents as a stationary phase with specific retention properties coming from both dodecanoate and cellulose.

The versatility of N,O-dialkylphosphoramidate stationary phase-separations in HILIC, highly aqueous RP LC conditions and purely aqueous mobile phase

The chromatographic properties of N,O-dialkylphosphoramidate stationary phase with an octadecyl chain (Amino-P-C18) were studied.

Preparative separation of the polar part from the rhizomes of Anemarrhena asphodeloides using a hydrophilic C18 stationary phase

The goal of this study was to develop a method that utilized a hydrophilic C18 stationary phase in the preparative high performance liquid chromatography to isolate the polar part from the rhizomes of Anemarrhena asphodeloides. The results showed that an initial mobile phase of pure water for the separation could greatly increase the retention and solubility of the polar compounds at the preparative scale. Introducing polar groups on the surface of the hydrophilic C18 column together with the use of optimized mobile phase compositions improved the column separation selectivity for polar compounds. Eleven previously undescribed compounds in Anemarrhena asphodeloides were obtained, indicating that the method developed in this study would facilitate the purification and separation of the polar part of traditional Chinese medicines.

Recent development in liquid chromatography stationary phases for separation of Traditional Chinese Medicine components

Traditional Chinese Medicine (TCM) is an ancient medical practice which has been used to prevent and cure diseases for thousands of years. TCMs are frequently multi-component systems with mainly unidentified constituents. The study of the chemical compositions of TCMs remains a hotspot of research. Different strategies have been developed to manage the significant complexity of TCMs, in an attempt to determine their constituents. Reversed-phase liquid chromatography (RPLC) is still the method of choice for the separation of TCMs, but has many problems related to limited selectivity. Recently, enormous efforts have been concentrated on the development of efficient liquid chromatography (LC) methods for TCMs, based on selective stationary phases. This can improve the resolution and peak capacity considerably. In addition, high-efficiency stationary phases have been applied in the analysis of TCMs since the invention of ultra high-performance liquid chromatography (UHPLC). This review describes the advances in LC methods in TCM research from 2010 to date, and focuses on novel stationary phases. Their potential in the separation of TCMs using relevant applications is also demonstrated.

A simple method for the synthesis of a polar-embedded and polar-endcapped reversed-phase chromatographic packing with low activity of residue silanols

Octadecyl bonded silica (ODS) is the most popular packing for reversed-phase chromatography. However, it generally demonstrates bad resolution for polar analytes because of the residue silanols and its poor stability in aqueous mobile phase. To address the problem, a new reversed-phase packing containing both polar-embedded and polar-endcapped moieties was proposed. It was prepared by a very simple method, in which the epoxide addition reaction of 3-glycidoxypropyltrimethoxysilane with 1-octadecanethiol proceeded simultaneously with the reaction of silane coupling onto silica particles. By controlling the molecular ratio of 3-glycidoxypropyltrimethoxysilane to 1-octadecanethiol higher than 1.0 (1.56 for the present study), both polar-embedded and polar-endcapped moieties were achieved onto the packing. The performance of the packing was evaluated in detail. The results demonstrated that neutral, acidic and basic analytes were well separated on the packing. The column efficiency for phenanthrene was 34,200 theoretical plates per meter. In addition, four nucleotides can be separated in 100% phosphate buffered saline solution with good reproducibility, which indicates the packing has good stability in aqueous mobile phase. Amitriptyline, a typical basic analytes, was eluted out with relatively symmetric peak shape (asymmetry factor of 1.36), which implies that the packing has not suffered from the negative effect of residue silanols significantly. Good stability in buffer solution of pH ranging from 2.0 to 10.0 was also documented for the packing.

Recent progress in polar metabolite quantification in plants using liquid chromatography–mass spectrometry

Metabolite analysis or metabolomics is an important component of systems biology in the post-genomic era. Although separate liquid chromatography (LC) methods for quantification of the major classes of polar metabolites of plants have been available for decades, a single method that enables simultaneous determination of hundreds of polar metabolites is possible only with gas chromatography–mass spectrometry (GC–MS) techniques. The rapid expansion of new LC stationary phases in the market and the ready access of mass spectrometry in many laboratories provides an excellent opportunity for developing LC–MS based methods for multi-target quantification of polar metabolites. Although various LC–MS methods have been developed over the last 10 years with the aim to quantify one or more classes of polar compounds in different matrices, currently there is no consensus LC–MS method that is widely used in plant metabolomics studies. The most promising methods applicable to plant metabolite analysis will be reviewed in this paper and the major problems encountered highlighted. The aim of this review is to provide plant scientists, with limited to moderate experience in analytical chemistry, with up-to-date and simplified information regarding the current status of polar metabolite analysis using LC–MS techniques.