Polar-copolymerized approach based on horizontal polymerization on silica surface for preparation of polar-modified stationary phases

Two-dimensional liquid chromatography with reversed phase in both dimensions: A review

Two-dimensional liquid chromatography (2D-LC), and in particular comprehensive two-dimensional liquid chromatography (LC×LC), offers increased peak capacity, resolution and selectivity compared to one-dimensional liquid chromatography. It is commonly accepted that the technique produces the best results when the separation mechanisms in the two dimensions are completely orthogonal; however, the use of similar separation mechanisms in both dimensions has been gaining popularity as it helps avoid difficulties related to mobile phase incompatibility and poor column efficiency. The remarkable advantages of using reversed phase in both dimensions (RPLC×RPLC) over other separation mechanisms made it a promising technique in the separation of complex samples. This review discusses some physical and practical considerations in method development for 2D-LC involving the use of RP in both dimensions. In addition, an extensive overview is presented of different applications that relied on RPLC×RPLC and 2D-LC with reversed phase column combinations to separate components of complex samples in different fields including food analysis, natural product analysis, environmental analysis, proteomics, lipidomics and metabolomics.

A nitrogenous heterocyclic ring-bonded stationary phase for separating alkaloids in supercritical fluid chromatography

A novel silica stationary phase was designed and prepared through thiol-epoxy click chemistry for supercritical fluid chromatography (SFC). The developed stationary phase was characterized by elemental analysis, Fourier transform infrared spectrometry and solid-state 13C/CP MAS NMR spectroscopy. In order to evaluate the chromatographic performance and retention mechanisms of the prepared column, a variety of alkaloids were used, including indoles, isoquinolines, pyrrolidines, piperidines, quinolizidines and organic amines. The stationary phase showed more symmetrical peak shapes and better performance for these compounds compared to the conventional SFC stationary phases. The investigations on the effects of pressure and temperature on retention provided information that the selectivity of the compounds can be improved by changing the density of the supercritical fluids. Moreover, it shows improved separation efficiency of three natural products with alkaloids as the main components at high sample loading. In conclusion, the developed stationary phase could offer flexible selectivity toward alkaloids and complex samples.

Phosphodiester Stationary Phases as Universal Chromatographic Materials for Separation in RP LC, HILIC, and Pure Aqueous Mobile Phase

Modern analytical chemistry techniques meet the need for greater attention to ecological and economic aspects. It is becoming necessary to seek solutions to reduce harmful waste production, especially in large quantities. High-performance liquid chromatography is a technique widely used in many industries, including mainly pharmaceuticals, and requires an approach to reduce the significant amount of organic solvent waste. One of the green chemistry solutions is using environmentally benign substitutes, such as pure water, supercritical dioxide, and ethanol. Our work focuses on the preparation and application of new stationary phases with embedded hydrophilic groups for separations using pure water in liquid chromatography. Polar-embedded stationary phases are obtained by attaching a phosphodiester group and 4 different hydrophobic molecules. The studies consisted of hydrophobicity measurements, concentration dependence of retention of the organic additive to the mobile phase, and chromatographic separations of polar and non-polar substance mixtures in RP-LC and HILIC systems. Three mixtures were studied: purine alkaloids, benzene, and polycyclic aromatic hydrocarbons and nucleosides. The stationary phases interact differently with the analytes depending on the attached hydrophobic group. It is possible to use pure water to separate each mixture under study. It is also significant that it has been possible to separate a mixture of completely non-polar compounds using pure water for the first time. The research being carried out is crucial in synthesizing new polar-embedded stationary phases, providing work versatility and high environmental performance.

Progress and prediction of multicomponent quantification in complex systems with practical LC-UV methods

Complex systems exist widely, including medicines from natural products, functional foods, and biological samples. The biological activity of complex systems is often the result of the synergistic effect of multiple components. In the quality evaluation of complex samples, multicomponent quantitative analysis (MCQA) is usually needed. To overcome the difficulty in obtaining standard products, scholars have proposed achieving MCQA through the "single standard to determine multiple components (SSDMC)" approach. This method has been used in the determination of multicomponent content in natural source drugs and the analysis of impurities in chemical drugs and has been included in the Chinese Pharmacopoeia. Depending on a convenient (ultra) high-performance liquid chromatography method, how can the repeatability and robustness of the MCQA method be improved? How can the chromatography conditions be optimized to improve the number of quantitative components? How can computer software technology be introduced to improve the efficiency of multicomponent analysis (MCA)? These are the key problems that remain to be solved in practical MCQA. First, this review article summarizes the calculation methods of relative correction factors in the SSDMC approach in the past five years, as well as the method robustness and accuracy evaluation. Second, it also summarizes methods to improve peak capacity and quantitative accuracy in MCA, including column selection and two-dimensional chromatographic analysis technology. Finally, computer software technologies for predicting chromatographic conditions and analytical parameters are introduced, which provides an idea for intelligent method development in MCA. This paper aims to provide methodological ideas for the improvement of complex system analysis, especially MCQA.

Updated clinical and glycomic features of mannosyl-oligosaccharide glucosidase deficiency: Two case reports

BACKGROUND

Mannosyl-oligosaccharide glucosidase (MOGS) deficiency is an extremely rare type of congenital disorder of glycosylation (CDG), with only 12 reported cases. Its clinical, genetic, and glycomic features are still expanding. Our aim is to update the novel clinical and glycosylation features of 2 previously reported patients with MOGS-CDG.

CASE SUMMARY

We collected comprehensive clinical information, and conducted the immunoglobulin G1 glycosylation assay using nano-electrospray ionization source quadruple time-of-flight mass spectrometry. Novel dysmorphic features included an enlarged tongue, forwardly rotated earlobes, a birth mark, overlapped toes, and abnormal fat distribution. Novel imaging findings included pericardial effusion, a deep interarytenoid groove, mild congenital subglottic stenosis, and laryngomalacia. Novel laboratory findings included peripheral leukocytosis with neutrophil predominance, elevated C-reactive protein and creatine kinase, dyslipidemia, coagulopathy, complement 3 and complement 4 deficiencies, decreased proportions of T lymphocytes and natural killer cells, and increased serum interleukin 6. Glycosylation studies showed a significant increase of hypermannosylated glycopeptides (Glc3Man7GlcNAc2/N2H10 and Man5GlcNAc2/N2H5) and hypersialylated glycopeptides. A compensatory glycosylation pathway leading to an increase in Man5GlcNAc2/N2H5 was indicated with the glycosylation profile.

CONCLUSION

We confirmed abnormal glycomics in 1 patient, expanding the clinical and glycomic spectrum of MOGS-CDG. We also postulated a compensatory glycosylation pathway, leading to a possible serum biomarker for future diagnosis.

C18-Functionalized Amine-Bridged Hybrid Monoliths for Mass Spectrometry-Friendly Peptide Separation and Highly Sensitive Proteomic Analysis

For proteomic analysis based on mass spectrometry (MS), high-performance peptide separation under MS-friendly conditions is of importance. To this end, a novel kind of amine-bridged hybrid monolith was developed by the sol-gel reaction of bis[3-(trimethoxysilyl)propyl]amine and allyltrimethoxysilane, followed by "thiol-ene" click functionalization of C18 groups. With the secondary amino groups bridged in the framework, the nonspecific adsorption from silanol groups could be decreased, so that peptide peak tailing under MS-friendly conditions was reduced, and half peak width was narrowed. Furthermore, such materials were facilely in situ prepared in the very narrow bore capillary with low backpressure for proteomic analysis of limited amounts of samples. Finally, 16,692 unique peptides corresponding to 3698 protein groups could be averagely identified from 10 ng Hela cell digests in a single 65 min run, and 5257 peptides corresponding to 1062 protein groups could be averagely identified from 200 pg digests in a single 60 min run. Such high sensitivity could be attributed to the decreased nonspecific adsorption, the narrowed peak width, and the miniaturization of the column. It is shown that such monoliths are promising for highly sensitive proteomic analysis, including single-cell proteomics.

Evaluation and application of a positively-charged phenylaminopropyl bonded stationary phase for separation of basic compounds

Silica-based positively-charged stationary phase bonding phenylaminopropyl (named PHN) was found to produce symmetrical peak shape and higher sample loading for basic compounds. In this work, firstly, surface charge property of the PHN was evaluated by ζ-potential and retention of NO₃^-. A considerable amount of pH-dependent positive charges was confirmed more than that on CSH Phenyl-Hexyl, a commercial positively-charged phenyl stationary phase. Then chromatographic evaluation of standard alkaloids revealed that PHN could offer better peak shape and higher column efficiency at lower pH, and it functioned well under a wide range of buffer ionic strength. The PHN also showed different selectivity for basic compounds compared to the CSH Phenyl-Hexyl. Furthermore, it provided superior peak shape for high sample mass, demonstrating potential applications of this stationary phase in a preparative scale. These results can be explained by the strong charge intensity of the PHN stationary phase. Finally, the PHN was applied to separate a fraction from rhizomes of Corydalis decumbens, and purify dehydrocorybulbine from Corydalis yanhusuo W.T. Wang. Our study indicated the advantages and potential applications of the phenylaminopropyl bonded PHN stationary phase for basic compound separation.

Analysis of alkaloids in Gelsemium elegans Benth. Using an online heart-cutting + comprehensive RPLC×RPLC system tandem mass spectrometry

Characterization of alkaloids and new compound discovery become increasing challenging for Gelsemium elegans Benth. (G. elegans), due to the lack of an effective separation method. In this study, we developed a new online heart-cutting + comprehensive (HC) RPLC × RPLC system with pH difference, which was coupled to a mass detector to realize the separation and characterization of alkaloids from G. elegans. 18 Gelsemium standards were used to construct the RPLC × RPLC system with pH difference (pH 3 and 11), and good orthogonality (correlation coefficient 0.3) was obtained. A heart-cutting valve was introduced into the traditional online comprehensive RPLC × RPLC system to remove principal components and improve detection of minor components. The online HC RPLC × RPLC system achieved good resolving power (effective peak capacity 687) in condition of optimized practical factors, like the first- and second-dimension flow rates, modulation period and elution gradient et al. Finally, a total of 256 alkaloids were grouped and tentatively identified, among which 156 were unreported, including a new alkaloid type in G. elegans and many dimeric indole alkaloids, which was an important supplement to the study on chemical constituents of G. elegans.

A strategy for efficient enrichment of steroidal alkaloids from Fritillaria based on fluorinated reversed-phase stationary phase

Plant-derived alkaloids are bioactive natural ingredients, but their contents are relatively low in plants. Therefore, the efficient enrichment of alkaloids is a prerequisite for purification and further pharmacological research. In this study, an efficient and simple strategy for enrichment of steroidal alkaloids in Fritillaria was developed for the first time based on the fluorinated reverse-phase stationary phase (FC8HL). Superior selectivity between alkaloids and non-alkaloids was achieved in a non-aqueous system, and a simple solvent system containing low-content additives was applied to elute alkaloids. Key parameters that affected the elution were investigated, including different types of buffer salts and optimized concentrations. The optimized elution system was then applied to selectively enrich alkaloids from five species of Fritillaria. Its practicability was further demonstrated by enrichment of alkaloids from Fritillaria cirrhosa D.Don at a preparative level. This developed method has great potential for other types of hydrophobic alkaloids.

Methoxy terminated poly dimethylsiloxane bonded stationary phase for reversed-phase liquid chromatography

In this work, a methoxy terminated poly dimethylsiloxane modified silica gel material was proposed as a novel stationary phase for reversed-phase liquid chromatography. With 5 μm silica gel as matrix, methoxy terminated poly dimethylsiloxane polymer was grafted by one step chemical bonding reaction. The obtained stationary phase was characterized by scanning electron microscopy, fourier transform infrared spectroscopy and element analysis. To our knowledge, this is the novel methoxy terminated poly dimethylsiloxane bonded stationary phase with good separation efficiency (42107-46988 plates/m for benzene homologues) and high stability (RSD is 0.08-5.09%). Comparing to other traditional columns of the same type, the proposed stationary phase has a wider polarity separation scale and shorter analysis time. In addition to the rapid separation of hydrophobic compound, such column also exhibited great potential in the separation of hydrophilic analytes.

Isolation of three polyoxins by reversed-phase liquid chromatography with pure aqueous mobile phase

Developing methods for the isolation of highly polar compounds from complex samples is of great significance. In this study, three polyoxins were successfully isolated from a complex sample (PN1-1^# ) by preparative high-performance liquid chromatography. Separation was carried out on five polar reversed-phase stationary phases, using pure aqueous as mobile phase, where the C18HC column can provide the best performance for PN1-1^# . Next, the effects of the mobile phase composition were studied. It was found that adding NaClO₄ can enhance the retention and resolution, and adding NaH₂ PO₄ was beneficial to maintain good peak shapes when the sample loading increased. Therefore, the optimized mobile phase consisting of 20 mmol NaH₂ PO₄ and 20 mmol NaClO₄ (adding H₃ PO₄ to adjust pH 2) was used to separate PN1-1^# . This method of using 100% aqueous phase can effectively improve both the retention and the solubility of polar samples. Eventually, through further purification, three compounds, namely, polyoxins B, D, and G, were obtained. This paper provided an effective and eco-friendly strategy for the preparative-scale separation of polar samples.

Purification of tertiary and quaternary alkaloids from Rhizoma Corydalis using reversed-phase/weak cation-exchange mixed-mode class separation combined with preparative C18 and silica based strong cation-exchange chromatography

A new optimization strategy for purification of alkaloids from Rhizoma Corydalis using preparative liquid chromatography was developed, featuring a selective separation of different types of alkaloids into different parts by a reversed-phase/weak cation-exchange mixed-mode column (named C18WCX) at first. The total alkaloids of Rhizoma Corydalis were divided into four fractions with fraction III and IV corresponding to the tertiary type medium bases and the quaternary type strong bases, respectively. For fraction III, a conventional C18 column was used to isolate tertiary alkaloids using acetonitrile and 0.1% phosphoric acid (adjusted with triethylamine to pH 6.0) as mobile phases. High selectivity and symmetrical peak shapes of tertiary alkaloids were obtained, resulting in six main tertiary alkaloids isolated in a single run. As strong bases, quaternary alkaloids often suffer from serious peak tailing problem on conventional C18 columns. Therefore, a silica-based strong cation-exchange (SCX) column was used for purification of fraction IV. On the SCX column, good peak shapes in high sample loading were achieved. Five main quaternary alkaloids were isolated and identified from the fraction in one-step. The procedures presented effective for the preparative isolation and purification of alkaloids from Rhizoma Corydalis.

A novel two-dimensional liquid chromatography - Mass spectrometry method for direct drug impurity identification from HPLC eluent containing ion-pairing reagent in mobile phases

In this study, a novel two dimensional liquid chromatography - mass spectrometry (2D-LC-MS) method with use of a weak anion exchange column between the 1st DLC RP column and the 2nd DLC RP column (RP1-WAX-RP2) was developed and applied to identify drug impurities from MS incompatible mobile phases containing sodium 1-octanesulfonate and non-volatile buffer. The 1st DLC conditions follow exactly the original standard HPLC method recorded in Chinese Pharmacopeia (ChP), European Pharmacopeia (EP) or US Pharmacopeia (USP). An impurity fraction was collected with a built-in sample loop (100 μL) and transferred to the WAX column where 1-octanesulfonate and phosphate were trapped and removed. While, the impurity and other cations were eluted to the 2nd D column (RP2) for separation and identification by connected IT-TOF MS. Methods were programmed and applied to identify impurities in two generic drugs, sulpiride (hydrophilic drug with logP 0.57) and dobutamine (hydrophobic drug with logP 3.6). The results indicate that the methods based on RP1-WAX-RP2 column configuration offer a feasible solution for direct impurity identification in generic drug product or API without needs of off-line desalting from the MS incompatible mobile phases containing ion-pairing reagent and non-volatile buffer.

Preparation and evaluation of maltose modified polymer-silica composite based on cross-linked poly glycidyl methacrylate as high performance liquid chromatography stationary phase

A new maltose modified polymer-silica composite was fabricated and applied as high performance liquid chromatography (HPLC) stationary phase. The cross-linked poly glycidyl methacrylate (pGMA) layer was chemically bonded to the outer surface as well as pore inner surface of silica beads via in-situ polymerization, and then maltose was modified onto the polymer layer via a [3 + 2] "click" reaction. The porous spherical silica (4 μm diameter) with 300 Å pore size was selected as the matrix so that the 3.25 nm-thick polymer layer fabricated on the pore inner surface would not affect its permeability. The typical 'U-shape' retention curves indicated a mixed-mode retention mechanism of the as-synthesized stationary phase. Both polar and non-polar analytes could be well separated on the stationary phase with column efficiency reaching 123809 plates/m for guanosine in hydrophilic interaction liquid chromatography (HILIC) mode and 46808 plates/m for fluorene in reversed-phase liquid chromatography (RPLC) mode, respectively. Nucleotides and their bases were baseline separated with good peak shape without any buffer salt in mobile phase, suggesting the effective shielding of the silanol groups. The packing material also showed excellent chromatographic repeatability with intraday RSDs of the retention time of five nucleosides less than 0.048% (n = 3) and interday RSDs less than 0.33% (n = 7) and great pH stability (from 1.5 to 10.2). Finally, the stationary phase was applied to the separation of ginseng extract.

Synthesis and chromatographic evaluation of phenyl/tetrazole bonded stationary phase based on thiol-epoxy ring opening reaction

A silica-based reversed-phase stationary phase bonding with phenyl and tetrazole groups was synthesized by thiol-epoxy ring opening reaction. The bonded groups could not only provide hydrophobic interaction, but also π-π, hydrogen bonding, electrostatic interactions, and so on. The results of characterization with elemental analysis and solid-state ¹³ C cross-polarization magic-angle-spinning NMR spectroscopy indicated the successful preparation of phenyl/tetrazole sulfoether bonded stationary phase. Chromatographic evaluation revealed that phenyl/tetrazole sulfoether bonded stationary phase behaved well under the reversed-phase mode. The column parameters (H, S*, A, B, and C) showed different selectivity compared with some typical commercial columns, and it was validated by the separation of estrogen, ginsenoside, alkaloid samples. Based on the different selectivity between phenyl/tetrazole sulfoether bonded stationary phase and C18 columns, phenyl/tetrazole sulfoether bonded stationary phase also showed potential to construct a 2D reversed-phase liquid chromatography system with C18. And it was verified by the separation of corydalis tuber and curcuma zedoary extracts.

Mixed-mode reversed phase/positively charged repulsion chromatography for intact protein separation

A mixed-mode reversed phase/positively charged repulsion stationary phase C8PN composed of octyl and amino group has been developed for separation of intact protein. Before the separation of proteins, a set of probe compounds were employed to evaluate the chromatographic properties of C8PN, demonstrating typical reversed phase/positively charged repulsion interaction on this stationary phase as estimated. Then the new C8PN stationary phase was used to separate a standard protein mixture on the reversed phase mode. Compared with a commercial C4 stationary phase, it showed different selectivity for some proteins. In order to better understand the properties of C8PN, the effect of acetonitrile content was investigated based on retention equation. Higher values of the equation parameters on C8PN demonstrated that the protein retentions were more sensitive to the change of acetonitrile content. Besides, the influences of buffer salt additives on the protein retentions were also studied. The retention factors of the proteins got larger with the increase of buffer salt concentration, which confirmed the positively charged repulsion interaction on the column. Finally, the C8PN was further applied to separate oxidized- and reduced- forms of Recombinant Human Growth Hormone. Our study indicated the advantages and application potential of mixed-mode reversed phase/positively charged repulsion stationary phase for intact protein separation.

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.

Evaluation and application of a mixed-mode chromatographic stationary phase in two-dimensional liquid chromatography for the separation of traditional Chinese medicine

In this study, two mixed-mode chromatography stationary phases (C8SAX and C8SCX) were evaluated and used to establish a two-dimensional liquid chromatography system for the separation of traditional Chinese medicine. The chromatographic properties of the mixed-mode columns were systematically evaluated by comparing with other three columns of C8, strong anion exchanger, and strong cation exchanger. The result showed that C8SAX and C8SCX had a mixed-mode retention mechanism including electrostatic interaction and hydrophobic interaction. Especially, they were suitable for separating acidic and/or basic compounds and their separation selectivities could be easily adjusted by changing pH value. Then, several off-line 2D-LC systems based on the C8SAX in the first dimension and C8SAX, C8SCX, or C8 columns in the second dimension were developed to analyze a traditional Chinese medicine-Uncaria rhynchophylla. The two-dimensional liquid chromatography system of C8SAX (pH 3.0) × C8SAX (pH 6.0) exhibited the most effective peak distribution. Finally, fractions of U. rhynchophylla prepared from the first dimension were successfully separated on the C8SAX column with a gradient pH. Thus, the mixed-mode stationary phase could provide a platform to separate the traditional Chinese medicine in practical applications.

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.

A non-derivative method for the quantitative analysis of isosteroidal alkaloids from Fritillaria by high performance liquid chromatography combined with charged aerosol detection

A non-derivative method was developed for the qualitative and quantitative analysis of isosteroidal alkaloids from Fritillaria thunbergii. During method development the performance of two universal detectors, the charged aerosol detector (CAD) and evaporative light scattering detector (ELSD), were evaluated. The CAD was found to be 30 to 55 times more sensitive than ELSD enabling the measurement of low levels of reference compound impurities that could not be detected by ELSD. The peak area percent of the reference compound, peimisine, obtained by CAD was 50.10%, but 91.66% by ELSD showing that CAD is suitable to estimate the presence of impurities. The CAD showed good reproducibility with overall intra- and inter-day peak area RSD values of less than 1.8% and 2.7%, respectively and had a linear dynamic range of up to 4 orders of magnitude (0.06-44mg/L) for peimine and peiminine. The optimized method was used for the quantitative analysis of peimine and peiminine from F. thunbergii.

Efficient purification of active bufadienolides by a class separation method based on hydrophilic solid-phase extraction and reversed-phase high performance liquid chromatography

Traditional Chinese medicines (TCMs) have played a significant role in the process of discovering natural bioactive compounds, especially in anticancer therapeutics. However, the components of TCMs are complex mixtures with wide variation in polarity and content, which leads to inefficiency in the process of active compound discovery from TCMs. In this paper, the popular strategy of utilizing "pre-fractionated natural product libraries" has been improved by a new class separation approach to accelerate the process. As an example, the skin of Bufo bufo gargarizans Cantor, a well-known TCM, mainly contains two distinct bufadienolide classes: amino acid-conjugated bufadienolides (AACBs) and free form bufadienolides (AAUBs). We utilized hydrophilic interaction liquid chromatography solid-phase extraction (HILIC-SPE) to resolve the two types of bufadienolides, which co-eluted on C18 columns. By this strategy, twelve bufadienolides of the two types were purified via prep-HPLC from one active fraction, and eight of them were identified by (1)H NMR and (13)C NMR. These results indicated that the class separation method not only overcame the limited orthogonality in a 2D-RPLC×RPLC system but also accelerated the process of active compound discovery.

Preparation of a novel carboxyl stationary phase by "thiol-ene" click chemistry for hydrophilic interaction chromatography

A novel carboxyl-bonded silica stationary phase was prepared by "thiol-ene" click chemistry. The resultant Thiol-Click-COOH phase was evaluated under hydrophilic interaction liquid chromatography (HILIC) mobile phase conditions. A comparison of the chromatographic performance of Thiol-Click-COOH and pure silica columns was performed according to the retention behaviors of analytes and the charged state of the stationary phases. The results indicated that the newly developed Thiol-Click-COOH column has a higher surface charge and stronger hydrophilicity than the pure silica column. Furthermore, the chromatographic behaviors of five nucleosides on the Thiol-Click-COOH phase were investigated in detail. Finally, a good separation of 13 nucleosides and bases, and four water-soluble vitamins was achieved.