Novel PEI/Zein core-shell composite as mixed-mode stationary phase for high performance liquid chromatography.

Based on the adhesion of polyethyleneimine (PEI), a novel PEI/zein co-modified core-shell stationary phase (PEI/Zein@SiO2) was prepared by doping zein to form a composite modification layer. The stationary phase achieved effective separation of nucleosides, bases and antibiotics in hydrophilic interaction mode on account of the hydrophilic groups of composite coating. With the hydrophobicity of zein, the flavones could be separated in reversed-phase mode. In short, the separation and analysis of hydrophilic/hydrophobic compounds were accomplished excellently by the PEI/Zein@SiO2 column with mixed double mode. The prepared chromatographic stationary phase not only avoided the dissolution of zein, but also covered the strong adsorption of some analytes caused by silica hydroxyl groups on the surface of silica spheres. The morphological structure and specific surface area of the material were reflected by various characterization techniques. Hydrophilic/hydrophobic compounds were used as tested analytes to research separation performance and retention mechanisms of PEI/Zein@SiO2 column. The stability and reproducibility of the PEI/Zein@SiO2 stationary phase were satisfied. Therefore, the modification of zein could improve the separation selectivity of stationary phase effectively for complex samples, which had the potential to be one of the significant potential application materials in stationary phase packing.

Adjustable chromatographic performance of silica-based mixed-mode stationary phase through the control of co-grafting amounts of imidazole and C18 chain.

In this paper, three imidazole- and C18- bifunctional silica stationary phases (Sil-Im-C18) were prepared by adjusting introduction interval of octadecyltrichlorosilane (ODS) and 3-imidazol-1-ylpropyl(trimethoxy)silane (TMPImS), which can be used for reversed-phase liquid chromatography (RPLC) and ion exchange chromatography (IEC) with adjustable performance. The successful preparation of Sil-Im-C18 were confirmed by the characterizations of elemental analysis, infrared spectroscopy (FTIR) and contact angle (CA). Chromatographic performance of Sil-Im-C18 were evaluated by the separation of Tanaka test mixture, alkylbenzenes, linear PAHs and a set of analytes with different properties (uracil, phenol, 1,2-dinitrobenzene and naphthalene), and compared with commonly used C18 column. It was found that the chromatographic performance of Sil-Im-C18 changed significantly with the difference in bonding amount of imidazole and C18. Sil-Im-C18 demonstrated the excellent separation performance towards polycyclic aromatic hydrocarbons (PAHs), phenylesters, phenylamines, phenols and inorganic anions, and notably, nucleobases and nucleosides can be separated using pure water as mobile phases. The van Deemter plot showed that the column efficiency of Sil-Im-C18-3 was 64,933 plate·m-1 for naphthalene, indicated that Sil-Im-C18 was reasonably chromatographic columns. The RSD values of retention time were 0.22 %-0.61 % for 10 needles alkylbenzenes injected continuously at 50 °C to investigate thermal stability and repeatability, all the fluctuations of k of naphthalene were less than 2.3 % for Sil-Im-C18-1 during flushing 24 h with the mobile phase at different pH values (pH = 3 and 8), the retention time of alkylbenzenes were almost same for Sil-Im-C18-1 at different time, the RSD values of retention time of alkylbenzenes were 0.45 %-2.28 % for two batches Sil-Im-C18-1, revealing the excellent repeatability, thermal stability, durability and reproducibility of Sil-Im-C18, and implying a commercial prospect.

Immobilization of two dendritic organic phases onto silica and their molecular shape recognition for polycyclic aromatic hydrocarbons, tocopherols and carotenoid isomers.

BACKGROUND: Molecular shape selectivity, based on the size and shape parameters of the molecule, such as length and planarity, is a separation process that can be used for compounds with restricted shapes, such as isomers. The separation of geometric isomers is challenging because these compounds have similar physicochemical properties but differ slightly in molecular shape. The ability to separate and quantify these isomers is important in high performance liquid chromatography (HPLC), which is one of the most widely used techniques in separation science today, because the shape of the molecule has a strong influence on biological processes. RESULTS: We prepared symmetrical discoidal dendrimeric organomolecule gelators (GSDM) and o-phenylenediamine-derived low-molecular-weight dendrimeric organomolecule gelators (G1) and bonded them to silica surfaces. The dendritic organic compound-grafted silica (SiO2@GSDM and SiO2@G1) was used as HPLC stationary phases for the separation of shape-restricted isomers of polycyclic aromatic hydrocarbons (PAHs), carotenoids and tocopherols. The two phases exhibit a very high molecular shape selectivity compared to the commercially available alkyl phases. There are differences in molecular shape selectivity between the two stationary phases. Changes in the chemical structure of dendritic organic compounds can alter the orientation of the molecules, as well as changes in the molecular recognition ability. It was found that SiO2@GSDM has high molecular linear selectivity for PAHs at different temperatures, even at 50 °C. The planar selectivity of SiO2@GSDM was better for triphenylene and o-terphenyl benzenes compared to SiO2@G1. SIGNIFICANCE: This separation behavior may be attributed to the combined effect of weak interaction centers, which allowed the effective separation of bioactive and shape-restricted isomers through multiple interactions. Furthermore, SiO2@GSDM showed better separation of tocopherols and carotenoids, suggesting that the backbone and ordered structure of organic molecular gelators is an effective way to improve the shape selectivity of the molecules, whereas the molecular orientation of the functional groups influences the separation mechanism of the shape-restricted isomers.

Factors affecting mixed-mode retention properties of cation-exchange stationary phases.

Cation-exchange stationary phases were characterized in different chromatographic modes (HILIC, RPLC, IC) and applied to the separation of non-charged hydrophobic and hydrophilic analytes. The set of columns under investigation included both commercially available cation-exchangers and self-prepared PS/DVB-based columns, the latter consisting of adjustable amounts of carboxylic and sulfonic acid functional groups. The influence of cation-exchange site and polymer substrate on the multimodal properties of cation-exchangers was identified using selectivity parameters, polymer imaging and excess adsorption isotherms. Introducing weakly acidic cation-exchange functional groups to the unmodified PS/DVB-substrate effectively reduced hydrophobic interactions, whilst a low degree of sulfonation (0.09 to 0.27% w/w sulphur) mainly influenced electrostatic interactions. Silica substrate was found to be another important factor for inducing hydrophilic interactions. The presented results demonstrate that cation-exchange resins are suitable for mixed-mode applications and offer versatile selectivity.

Dendritic organic molecular gel coating with molecular shape selectivity and its application in selective separation by liquid chromatography.

Dendritic organic molecular gels are a promising class of three-dimensional network compounds. Here, we have synthesized a new type of dendritic organic molecular gel stationary phase (SiO2-G3) by using benzyl alcohol as raw material and dimethyl 5-hydroxyisophthalate as growth unit to synthesize a third-generation organic molecular gel G3, which grafted onto the silica surface by cyanogen chloride (CC). The developed stationary phase not only exhibits high molecular shape selectivity but also has a RPLC/HILIC/IEC mixed-mode characteristic for HPLC due to the ordered structure, the multiple strong π-π stacking interactions and the introduction of a hydrophilic triazine fraction during the grafting process. Compared with a commercial C18 column, the developed column exhibited flexible selectivity, enhanced separation performance and excellent separation of monosubstituted benzene, polycyclic aromatic hydrocarbons (PAHs), positional isomers, nucleosides and nucleobases, benzoic acid and aniline compounds. In addition, the new column provided baseline separation of polycyclic aromatic hydrocarbon contaminants in Yellow River water, verifying its potential for application in the analysis of real samples.

Mesoporous nanomaterial-assisted hydrogel double network composite for mixed-mode liquid chromatography.

By introducing functional groups such as quaternary amine groups, sulfonic acid groups, triazine groups, and other mespore nanomaterials into the hydrogel, better separation effect of some organic framework materials has been obtained. Due to a reasonable design and preparation strategy, the hydrogel composite-modified silica can be used in the selective separation of various analytes such as pesticides, alkylbenzenes, polycyclic aromatic hydrocarbons, nucleosides/bases, benzoic acids, antibiotics, and carbohydrates. Through the exploration of chromatographic retention behavior, it is proved that the column can be used in mixed-mode liquid chromatography. The intra-day relative standard deviation for retention time of this new stationary phase is 0.12-0.16% (n = 10), and the inter-day relative standard deviation is less than 0.39% (n = 5). This new stationary phase can also be used for separation in complex samples. The limit of detection (LOD) for chlorotoluron in farm irrigation water is 0.21 µg/L and the linear range is 2-250 µg/L. After optimizing the chromatographic conditions, the highest efficiency of the hydrogel column in RPLC and HILIC modes has reached 32,400 plates/m (chlorobenzuron) and 41,300 plates/m (galactose). This new type of hydrogel composite is a porous network material with flexible functional design and simple preparation method and its application has been expanded in liquid chromatography separation successfully. The hydrogel composed of triallyl cyanate cross-linking agent and 3-(2-(methacryloyloxy) ethyl) dimethylamine) propane-1-sulfonate (SBMA) monomer which were co-modified on the surface of mesoporous silica with MOF-919 for separation in mixed-mode liquid chromatography.

Fabrication of two-dimensional metal-organic framework nanosheets/PDA composites as mixed-mode stationary phase for chromatographic separation.

The synthesis of two-dimensional metal-organic frameworks (2D MOFs)/polymer core-shell composites is reported which were composed of polydopamine modified 2D Zr-1,3,5-(4-carboxylphenyl)-benzene (2D Zr-BTB) nanosheets and silica microspheres via a double-solvent approach. In this way, the composites were obtained under the condition of two solvents with different polarities to avoid agglomeration and uneven modification of most MOFs particles on the surface of the silica, existing inevitably in the one-pot method. Compared with the reported MOFs@silica composites adopting one-pot solvent method, the prepared composites exhibited significantly enhanced separation performance for sulfonamides, antibiotics, nucleosides, and polycyclic aromatic hydrocarbons compounds. Furthermore, the retention mechanisms were demonstrated by studying the relationships of chromatographic retention factors of tested analytes versus a variety of parameters under RPLC and HILIC modes, respectively. The superior chromatographic repeatability and stability were validated through the relative standard deviations of the retention time and/or column efficiency, which were found to be less than 0.8% and 0.9%, respectively. The material showed efficient separation ability for several types of compounds and provided another selectivity for preparing composites based on 2D MOFs nanosheets and other functional molecules.

An alternative strategy to construct uniform MOFs-Grafted silica core-shell composites as mixed-mode stationary phase for chromatography separation.

The preparation of the metal-organic frameworks (MOFs)@silica core-shell microspheres as the stationary phases mainly relied on the method of electrostatic interaction between the metal ions of MOFs and the silanol groups. Herein, the ligands of MOFs were preferentially modified to the surface of silica as connection points and seed crystals to connect or form the MOFs. In this way, the evenness of the MOFs particles on the silica surface was effectively improved, and the prepared composites possessed excellent reproducibility and stability, including acid-base stability. The relative standard deviation of the retention time for repeatability ranged from 0.1% to 0.26% and for stability retention time from 0.3% to 0.6%. Compared with commercial columns, the prepared stationary phase showed enhanced separation selectivity for separation of both hydrophilic and hydrophobic compounds containing alkaloids, nucleosides, antibiotics and alkylbenzenes, etc. The obtained column was used as a matrix for fast separation and analysis of antibiotics in actual samples. In short, the composites showed superior reproducibility, stability and satisfactory separation performance towards a variety of compounds in the studied conditions. It also provided another way to improve the evenness of MOFs particles on the surface of silica and enhance the stability of them under polar conditions.

A new ionic liquid bridged periodic mesoporous organosilicas stationary phase for per aqueous liquid chromatography and its application in the detection of biogenic amines.

In order to solve the problems of using a large proportion of acetonitrile on the hydrophilic interaction liquid chromatography (HILIC) columns that was not environmentally friendly, and the poor acid and base resistance of traditional bonded silica columns, we reported a novel stationary phase of Au nanoparticles (Au NPs) covalently bonded to ionic liquid (ILs) bridged periodic mesoporous organosilicas (PMO) hydrophilic microspheres (PMO-ILs-Au NPs) for per aqueous liquid chromatography (PALC). The PMO hydrophilic microspheres were prepared by condensation of 1,3-bis(trimethoxysilylpropyl)imidazoliumchloride and 1, 2-Bis (triethoxysilyl) ethane and then modified with Au NPs the surface. The obtained materials were characterized by elemental analysis, FT-IR spectra, scanning electron microscope and transmission electron microscopy. The retention behavior was evaluated by investigating the effect of various chromatographic factors on the retention of different types of solutes. The retention mechanism of the stationary phases in PALC was a mixed type of anion-exchange and hydrophobic interaction. Compared with C18-SiO2 column, the acid and base resistance of the stationary phase were greatly improved. Compared with the HILIC column and C18 column, some hydrophilic compounds such as six organic acids and eight biogenic amines were baseline separated with the enhanced resolution of the PMO-ILs-Au NPs column under the PALC mode. The efficiency of the new column was significantly higher than that of the HILIC column. Furthermore, the analysis of PALC-triple quadrupole mass spectrometry was developed for simultaneous detection of eight biogenic amines. This method could improve detection efficiency, save reagent and reduce environmental pollution. PALC as a green chromatography analytical method was suitable for the replacement of HILIC.

Striped covalent organic frameworks modified stationary phase for mixed mode chromatography.

Covalent organic frameworks (COFs) have showed expected potential in chromatographic separation due to unique structure and excellent performance. Nowadays, COF materials applied as chromatographic stationary phases is still in its infancy. Here, we modified COF materials on silica using benzene-1,4,5-tetracarboxylic dianhydride (PMDA) and 1,3,5-tris-(4-aminophenyl)triazine (TAPT) monomers by one-pot synthetic method for performing mixed-mode function, named as SiO2@COF. Five characterization methods including thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier transform infrared spectrometry (FT-IR), elemental analysis (EA) and powder X-ray diffraction (XRD) verified the morphology, structure characteristics and physicochemical properties of the materials. SiO2@COF for performing the separation of polar and nonpolar analytes on high performance liquid chromatography (HPLC) achieved the desired results. Retention mechanisms of the constructed SiO2@COF were researched via observing the effects of mobile phase with retention times. Results exhibited that the prepared stationary phase can provide various interaction modes, including hydrophobic, hydrophilic, hydrogen bonding and π-π interactions. In conclusion, the prepared SiO2@COF stationary phase can execute mixed-mode separation abilities and show potential for complex samples analysis.

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

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

Prospects of using hyperbranched stationary phase based on poly(styrene-divinylbenzene) in mixed-mode chromatography.

Evaluation of the chromatographic properties of covalently bonded hyperbranched stationary phase based on poly(styrene-divinylbenzene) (PS-DVB) and containing zwitterionic fragments in the structure of functional layer was conducted in suppressed ion chromatography (IC), reversed phase high performance liquid chromatography (RP HPLC), and hydrophilic interaction liquid chromatography (HILIC) modes. Besides the possibility of resolving 20 inorganic anions and organic acids using KOH eluent in suppressed IC, prepared resin provided the separation of alkylbenzenes in RP HPLC, water-soluble vitamins, amino acids, and sugars in HILIC mode. Trends in the retention of hydrophobic and polar analytes on the prepared stationary phase indicated the dominating effect of analyte nature on the retention mechanism and proved satisfactory hydrophilization of PS-DVB surface with hyperbranched functional layer for retaining polar compounds. The obtained results revealed good prospects of using hydrophobic PS-DVB substrate for preparing stationary phases for mixed-mode chromatography.

Design and evaluation of novel MOF-polymer core-shell composite as mixed-mode stationary phase for high performance liquid chromatography.

A general method was developed for preparing a metal-organic framework-polymer composite coated silica core-shell stationary phase. Silica microspheres were comodified with metal-organic framework and polyvinylpyrrolidone rather than the in situ method of silica modification by original metal-organic framework particles. Metal-organic framework particles and polyvinylpyrrolidone on silica surface were beneficial to suppress silanol activity and enhance composite material tolerance, as well as increasing the water compatibility of the original metal-organic framework-based stationary phases. The stationary phase exhibited superior hydrophilic and hydrophobic performance in terms of separation for various analytes including seven alkaloids, six sulfonamides, five antibiotics, and five polycyclic aromatic hydrocarbons. Moreover, the composite material also showed excellent stability with the relative standard deviation of the retention time of 0.4 to 0.7%. The separation performance with real samples proved that the column has good practical application potential. In summary, the poposed method provides a general way for preparing metal-organic framework-polymer composite material and changed the current status of original metal-organic framework particles modified silica as a single mode chromatographic stationary phase.

Preparation and evaluation of a poly(N-isopropylacrylamide) derived graphene quantum dots based hydrophilic interaction and reversed-phase mixed-mode stationary phase for complex sample analysis.

The poly(N-isopropylacrylamide) (NIPAAm) was first polymerized onto the surface of graphene quantum dots (GQDs) functionalized silica as packing materials via reversible addition-fragmentation chain transfer (RAFT) polymerization reaction, which can expand the interaction modes between stationary phase and analytes. A series of characteristic methods were selected to estimate the chemical bonding results of silica, involving Fourier transform infrared spectroscopy (FT-IR), elemental analysis (EA), thermogravimetric analysis (TGA) and scanning electron microscope (SEM). The prepared column can exhibit reversed-phase and hydrophilic interaction modes, which were demonstrated by the retention of eight kinds of target analytes with different Log P values. The column was then applied to separate banlangen granules and further verified by HPLC tandem time-of-flight mass spectrometry (HPLC/QTOF-MS). In conclusion, Sil-GQDs-PNIPAAm stationary phase improved the analysis range and performance of traditional phases, exhibiting flexible selectivity and application prospect for both hydrophobic and hydrophilic analytes.

Ionic liquid functionalized ß-cyclodextrin and C18 mixed-mode stationary phase with achiral and chiral separation functions.

A novel reversed-phase/hydrophilic interaction/ion-exchange (RPLC/HILIC/IEC) and chiral recognition mixed-mode stationary phase material was synthesized using 3-n-octadecyl-1-vinylimidazolium bromide and 6-(1-allylimidazolium)-cyclodextrin tosylate as functional monomers. After identifications of intermediates and stationary phase materials using nuclear magnetic resonance analysis, Fourier transform infrared spectrometer, element analysis and thermogravimetric analysis, the synthesized Sil-VMBD material was packed into an empty column under high pressure. The mixed-mode retention performance was researched by hydrophobic, hydrophilic and ionic compounds. And the retention mechanism of the multi-mode stationary phase was studied via changing the mobile phase composition and pH value. Moreover, the enantiomers of 1-phenyl-1-propanol, warfarin and styrene oxide were separated rapidly under reverse-phase mode. Fast enantioseparation of ibuprofen and ketoprofen were obtained within 4 min under polar organic mode. The experimental results show that the as-prepared mixed-mode column possesses reversed-phase, hydrophilic and ion-exchange interactions with various analytes, and these interactions also enable the stationary phase with multi-mode and chiral separation abilities.

Systematic evaluation and optimization of high-performance liquid chromatography separation of polyoxins.

The chromatographic behavior of a kind of nucleoside peptides, polyoxins, was investigated in this study. Molecular simulation technique was used to elucidate the temperature-dependent peak sharpening of polyoxins. There was a relatively small energy barrier between the global minimum conformer and the local minimum conformer of polyoxin A and the high temperature helped to quickly cross the energy barrier and accelerate the conformational transformation for getting the global minimum, so that stationary phase could not identify these two conformations and presented a sharp peak. Two kinds of mixed-mode columns, strong cation exchange or strong anion exchange ligands bonded with C18 (C18SCX and C18SAX) were used to improve separation selectivity of four polyoxins (A, K, F, H). The electrostatic attraction was necessary to increase the retention to ensure that the alkyl chain can give better play to its hydrophobic effect. Therefore, four polyoxins were well separated on C18SCX at pH 2 and they were also well separated on C18SAX at pH 7. In the small-scale purification of polyoxins, the sample loading of the C18SCX was five times than that of the C18SAX and the purity of the collected four polyoxins was all over 90%.

[Preparation of mixed-mode stationary phase for high performance liquid chromatography with ground hybrid silica monolithic material].

With polyethylene glycol as a porogen, vinyltrimethoxysilane (VTMS) and tetramethoxysilane (TMOS) as silica precursors, a hybrid silica monolithic material was obtained under the catalysis of acetic acid and thermally decomposed urea. The silica monolithic material was ground by a ballmill, treated with tris(hydroxymethyl)aminomethane (Tris), then washed and dried to obtain silica particles with particle size~3 µm. The effects of different reaction conditions on the particle size, surface morphology and dispersibility of silica particles were investigated. When the volume ratio of TMOS to VTMS was 3:1, it was observed that silica particles with a pore diameter of 7.5 nm and a specific surface area of 245 m2/g were obtained. The resultant silica particles were modified by binding with chlorodimethyloctadecylsilane (C18) and by the thiol-ene click reaction to obtain a mixed-mode type stationary phase. The test results showed that the silica packing materials prepared in this work has certain applicability.

Direct quantification of inorganic iodine in seawater by mixed-mode liquid chromatography-electrospray ionization-mass spectrometry.

Atmospheric iodine plays a relevant role in climate change. Bearing in mind that most of this iodine comes from the oceans, analytical methods capable of determining iodine in a challenging matrix as seawater are necessary. In this work, the first method capable of direct determination of total inorganic iodine in seawater at subnanomolar level based on mixed-mode liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) without any sample treatment is presented. Analytical characteristics of the developed method were studied in terms of linear range, limits of detection and quantification, precision, trueness, matrix effect, and robustness. The detection limit for iodide was as low as 0.16 nM, injecting 5 µL of seawater without any sample treatment and the working linear range of four orders of magnitude was wide enough to cover the broad concentration range observed in seawater samples. Average values for repeatability and intermediate precision were 4.1% and 8.1%, respectively. The suitability of the method was demonstrated through its application to the analysis of several types of samples, including seawater samples taken at different locations along the Spanish Mediterranean coast and some domestic iodized salts. According to the results obtained, the method developed is rapid, easy to apply and to be automated, avoids sample treatment and requires only few microliters of sample. Furthermore, it has a low detection limit and allows the quantification of inorganic iodine over a wide concentration range.

Investigation and prediction of retention characteristics of imidazoline and serotonin receptor ligands and their related compounds on mixed-mode stationary phase.

Investigation of the retention behavior of a wide range of analytes, 43 nitrogen containing heterocyclic and guanidine derivatives such, as imidazoline and serotonin receptor ligands or their related compounds, was performed on mixed-mode stationary phase in the combined reversed-phase (RP) and hydrophilic interaction liquid chromatography (HILIC) modes. Suitability of the linear retention modelling in the HILIC and RP modes was tested including separate contributions from adsorption and partition. For the HILIC retention, the partition model was found to provide better description compared with the adsorption model. In a wider range of the aqueous eluent volume fractions, φ(aq), retention was described as a function of volume fractions and total polarity of mobile phase using the mixed-mode retention modelling. The obtained results revealed that the shift of the chromatographic mode can be calculated from the change of total polarity of mobile phase in a multi-modal relation, logarithm of retention factor vs. total polarity, with the minimum value representing the turning point between the HILIC and the RP mode. Molecular properties of the investigated compounds that influence the retention behavior and the turning point were selected using Multiple Linear Regression (MLR) and Support Vector Machine (SVM). Slightly better statistical results were found for the logkwRP(aq)/MLR, logkwHILIC(org)/MLR, logkbHILIC(aq)/MLR, and φmin (aq)/SVM (RBF) QSRR models than for the logkwRP(aq)/SVM, logkwHILIC(org)/SVM, logkbHILIC(aq)/SVM, and φmin(aq)/MLR modelling. With this insight, it is possible to precisely define and predict the retention characteristics based on physico-chemical properties of imidazoline and piperazine related compounds.

Strong electrostatic repulsive interaction used for fast and effective alkaloid enrichment from plants.

Since the content of alkaloids is usually low in plants and they are easily co-eluted with other constituents, enrichment of alkaloids is essential in the discovery of bioactive lead compounds from natural products. In this paper, an easy SPE enrichment method was developed in a buffer-free solvent system based on electrostatic repulsion mechanism. The feasibility of the new method was verified by successful enrichment of alkaloids from Scopolia tangutica (S. tangutica) with an optimized eluting condition. Then this developed method was applied to other representative plants in different families, including Przewalskia tangutica and Peganum harmala L, Lycoris radiata and Menispermum dauricum DC, which enlarged the scope of applicability. Additionally, the new SPE procedure avoided possible structural change destruction caused by pH change. Simple solvent system, including formic acid (FA) and methanol, would benefit subsequent mass analysis, quantity determination and bioactivity screening, and so on.