“Click oligo(ethylene glycol)”: An excellent orthogonal stationary phase to C18 for two-dimensional reversed-phase/reversed-phase liquid chromatography

Contribution of the "Click Chemistry" Toolbox for the Design, Synthesis, and Resulting Applications of Innovative and Efficient Separative Supports: Time for Assessment

The last two decades have seen the rapid expansion of click chemistry methodology in various domains closely related to organic chemistry. It has notably been widely developed in the area of surface chemistry, mainly because of the high-yielding character of reactions of the "click" type. Especially, this powerful chemical reaction toolbox has been adapted to the preparation of stationary phases from the corresponding chromatographic supports. A plethora of selectors can thus be immobilized on either organic, inorganic, or hybrid stationary phases that can be used in different chromatographic modes. This review first highlights the few different chemical ligation strategies of the "click" type that are up to now mainly devoted to the development of functionalized supports for separation sciences. Then, it gives in a second part an up-to-date survey of the different studies dedicated to the preparation of click chemistry-based chromatographic supports while highlighting the powerful and versatile character of the "click" ligation strategy for the design, synthesis, and developments of more and more complex systems that can find promising applications in the area of analytical sciences, in domains as varied as enantioselective separation, glycomics, proteomics, genomics, metabolomics, etc.

Enrichment and separation of high-polar compounds from Saussurea obvallata using solid-phase extraction combining with offline two-dimensional liquid chromatography

The high-polar compounds from natural products are often used as medicines due to their good bioactivities. However, owing to the complexity and diversity of their structure, the separation of high-polar compounds is still a challenging work. For this, an efficient method for enrichment and separation of the high-polar compounds from Saussurea obvallata was developed. First, the target compounds were enriched from the total extract using a solid-phase extraction method. An offline two-dimensional liquid chromatography method was used for the separation of pure compounds from the enriched sample. After optimization of chromatographic conditions, high separation selectivity of target compounds was obtained on a polar-modified C18 column and a HILIC XAmide column. Hence, a two-dimensional reversed-phase × hydrophilic interaction liquid chromatography system was constructed and enlarged from the analytical level to the preparative level. In the first dimension, four fractions were obtained on the XCharge C18 column with a recovery rate of 71.2%. In the second-dimension preparation on the XAmide column, eight high-polar compounds with more than 96% purity were isolated. All compounds were isolated from Saussurea obvallata for the first time. The results demonstrated that this developed strategy is effective for preparative-scale isolation of high-polar compounds from natural products.

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.

Synthesis, characterization, and application of a novel multifunctional stationary phase for hydrophilic interaction/reversed phase mixed-mode chromatography

A novel multifunctional stationary phase based on silica gel was synthesised starting from L- isoleucine and 4-phenylbutylamine and evaluated as a hydrophilic interaction/reversed-phase mixed-mode stationary phase for high-performance liquid chromatography (HPLC). The prepared stationary phase was characterized by elemental analysis, infrared spectroscopy (IR), scanning electron microscopy (SEM), Brunauer, Emmett and Teller (BET) and solid-state ¹³C nuclear magnetic resonance (NMR). The mechanisms involved in the chromatographic separation are multi-interaction, including hydrophobic, π-π, hydrogen-bonding, dipole-dipole and ion-dipole interactions. Based on these interactions, successful separation could be achieved among several aromatic compounds having different polarities under both hydrophilic interaction liquid chromatography (HILIC) and reversed phase (RP) condition. Nucleotides/nucleosides were separated in the HILIC mode. The effects of different separation conditions, such as pH value, mobile-phase content, column temperature, buffer concentration and flow rate, on the separation of nucleotides/nucleosides in HILIC mode were investigated. The seven nucleotides/nucleosides were separated within 22min, while six of them were separated within 10min by isocratic elution. To determine the influence of the new multifunctional stationary phase under the RP condition, a number of moderately and weakly polar and nonpolar compounds, such as 10 substituted anilines and eight substituted phenols were separated successfully under the RP condition within 14 and 15min, respectively. Additionally, nine mixtures of polar/nonpolar test compounds were simultaneously separated within 19min, while seven of them were separated within 12min, under HILIC/RP mixed-mode conditions. Chromatographic parameters, such as the retention factor and peak asymmetry factor, were calculated for all of the analytes, while the theoretical plate number was calculated for analytes separated by isocratic elution. Compared to traditional C18 and commercial HILIC columns, the new stationary phase exhibited both HILIC and RPLC performance, and the scope of analyte separation was thus enlarged.

High-performance liquid chromatography coupled with tandem mass spectrometry technology in the analysis of Chinese Medicine Formulas: A bibliometric analysis (1997-2015)

There is a recognized challenge in analyzing traditional Chinese medicine formulas because of their complex chemical compositions. The application of modern analytical techniques such as high-performance liquid chromatography coupled with a tandem mass spectrometry has improved the characterization of various compounds from traditional Chinese medicine formulas significantly. This study aims to conduct a bibliometric analysis to recognize the overall trend of high-performance liquid chromatography coupled with tandem mass spectrometry approaches in the analysis of traditional Chinese medicine formulas, its significance and possible underlying interactions between individual herbs in these formulas. Electronic databases were searched systematically, and the identified studies were collected and analyzed using Microsoft Access 2010, Graph Pad 5.0 software and Ucinet software package. 338 publications between 1997 and 2015 were identified, and analyzed in terms of annual growth and accumulated publications, top journals, forms of traditional Chinese medicine preparations and highly studied formulas and single herbs, as well as social network analysis of single herbs. There is a significant increase trend in using high-performance liquid chromatography coupled with tandem mass spectrometry related techniques in analysis of commonly used forms of traditional Chinese medicine formulas in the last 3 years. Stringent quality control is of great significance for the modernization and globalization of traditional Chinese medicine, and this bibliometric analysis provided the first and comprehensive summary within this field.

Evaluation of separation properties of a modified strong cation exchange material named MEX and its application in 2D-MEX × C18 system to separate peptides from scorpion venom

Peptides from scorpion venom represent one of the most promising sources for drug discovery in some specific disease.

Effective 2D-RPLC/RPLC enrichment and separation of micro-components from Hedyotis diffusa Willd. and characterization by using ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry

An effective method aiming at enrichment and analysis of micro-components in traditional Chinese medicine (TCM) was developed. One fraction (fraction E) from the extract of Hedyotis diffusa Willd. was selected as test sample, which was isolated by using the XAD-4 macroporous resin. To study the micro-components, a two-dimensional reverse-phase liquid chromatography (2D-RPLC/RPLC) method was developed, comprising Click OEG and C18 stationary phases as the first and second dimensions, respectively. Of the eight sub-fractions isolated from the first dimension, three sub-fractions (fractions II-IV) containing micro-components were further separated with the second dimension. The 2D-RPLC/RPLC system was proved to possess high orthogonality. Furthermore, the micro-components were characterized by using ultra-performance liquid chromatography-diode array detector/quadrupole time-of-flight mass spectrometry (UPLC-DAD/Q-TOF MS) with electrospray ionization (ESI) source. With the optimized separation and characterization method, a large number (>400) of micro-components were enriched and detected from the extracts of H. diffusa Willd., the majority of which has not been isolated from the herb before. Among these isolated micro-components, 38 compounds involving 24 phenylpropanoids, 7 flavonoids and 7 iridoid glucosides (IGs), were identified or tentatively identified from the H. diffusa extracts on the basis of spectral data of the authentic standards and the fragmentation characteristics information available in literatures. The proposed method made it possible to effectively screen and analyze the micro-components in TCMs or other complex natural medicines.

Combination of off-line two-dimensional hydrophilic interaction liquid chromatography for polar fraction and two-dimensional hydrophilic interaction liquid chromatography×reversed-phase liquid chromatography for medium-polar fraction in a traditional Chinese medicine

Two-dimensional liquid chromatography (2-D LC) has been widely used for the analysis of complex samples owing to its great improvement in separation selectivity and peak capacity. However, one 2-D LC system may not be enough to meet the separation requirements due to the complexity of certain samples and respective limitations of two separation modes. In this work, water extract of Scutellaria barbata D. Don, a traditional Chinese medicine, was fractionated into polar fraction and medium-polar fraction by means of solid phase extraction (SPE). The fraction preparation made it easy to select the corresponding combination of 2-D LC method from hydrophilic interaction chromatography (HILIC) and reversed-phase liquid chromatography (RP-LC). An off-line 2-D HILIC×HILIC to analyze the polar fraction and an off-line 2-D HILIC×RP-LC to analyze the medium-polar fraction were developed, respectively. In total, 749 peaks were detected: 206 peaks from the polar fraction by the 2-D HILIC×HILIC and 543 from the medium-polar fraction by the 2-D HILIC×RP-LC. The practical peak capacities obtained in both systems were 2698 and 2879, and the orthogonality reached 63.18% and 90.62%, respectively. The results demonstrated that the two systems were both highly orthogonal, and the peak capacities greatly increased.

Separation and identification of flavonoids from complex samples using off-line two-dimensional liquid chromatography tandem mass spectrometry

In this study, a LC-based identification strategy was proposed and off-line two-dimensional liquid chromatography was developed for the separation and identification of flavonoids from complex samples. The highly orthogonal separation system was composed of a CD column, an OEG column and an XTerra C(18) column. The CD column was employed for the first dimensional separation and the OEG column and XTerra C(18) column were used for the second dimensional separation in parallel. A mixture of six traditional Chinese medicines was used as a complex sample in testing this method. Unknown peaks were identified by matching their retention times and accurate mass molecular weights with those of references. The identification efficacy of parallel unidimensional liquid chromatography and off-line two-dimensional liquid chromatography was compared, and the latter was demonstrated to be more efficient for the identification of target compounds-especially co-eluted compounds or minor compounds from complex samples.

Application of click chemistry on preparation of separation materials for liquid chromatography

With the increasing requirement for analysis and separation of samples related to genomics, proteomics, metabolomics, pharmacology and agrochemistry, diverse stationary phases for liquid chromatography have been prepared by Cu(i)-catalyzed 1, 3-dipolar azide-alkyne cycloaddition reaction (CuAAC). It has been proved that CuAAC is a powerful tool for preparing covalently bonded stationary phases. In this tutorial review, we highlighted the preparation of separation materials by immobilization of functional groups on silica beads, polymer beads and agarose via CuAAC and their applications in liquid chromatography and related purposes, such as separation of polar compounds, enrichment of valuable bio-samples, orthogonal two-dimensional HPLC and chiral separation. Meanwhile, agarose-based separation materials for affinity chromatography are reviewed.

Multi-dimensional liquid chromatography in proteomics--a review

Proteomics is the large-scale study of proteins, particularly their expression, structures and functions. This still-emerging combination of technologies aims to describe and characterize all expressed proteins in a biological system. Because of upper limits on mass detection of mass spectrometers, proteins are usually digested into peptides and the peptides are then separated, identified and quantified from this complex enzymatic digest. The problem in digesting proteins first and then analyzing the peptide cleavage fragments by mass spectrometry is that huge numbers of peptides are generated that overwhelm direct mass spectral analyses. The objective in the liquid chromatography approach to proteomics is to fractionate peptide mixtures to enable and maximize identification and quantification of the component peptides by mass spectrometry. This review will focus on existing multidimensional liquid chromatographic (MDLC) platforms developed for proteomics and their application in combination with other techniques such as stable isotope labeling. We also provide some perspectives on likely future developments.

Recent analytical approaches in quality control of traditional Chinese medicines--a review

Traditional Chinese medicines (TCMs) are gaining more and more attention all over the world, due to their specific theory and long historical clinical practice. But the uncontrollable quality is a bottleneck for its modernization and globalization. This paper reviewed the recent analytical methods in the quality control of TCMs, including screening strategies of bioactive markers from TCMs through biochromatographic methods, the traditional chromatographic methods, DNA methods, as well as the spectroscopic methods, including FT-IR, NIR and NMR. The comprehensive methods, such as fingerprint and multi-component quantification are emphasized; hyphenated techniques, like HPLC-MS, GC-MS, CE-MS, LC-NMR, chemometric methods, and combination of chemical and biological methods, such as biofingerprint, metabolic fingerprint are now more and more widely used in TCMs. In a few word, the analysis and quality control of TCMs are moving towards an integrative and comprehensive direction, in order to better address the inherent holistic nature of TCMs.

A novel click chitooligosaccharide for hydrophilic interaction liquid chromatography

A novel chitooligosaccharide stationary phase for hydrophilic interaction liquid chromatography (HILIC) was developed via click chemistry and showed great HILIC characteristics on separation of polar compounds and enrichment of glycopeptides.

Optimal gradient operation in comprehensive liquid chromatography x liquid chromatography systems with limited orthogonality

A novel strategy is described for designing optimal second dimension (2D) gradient conditions for a comprehensive two-dimensional liquid chromatography system where the two dimensions are not fully orthogonal. Using the approach developed here, the initial and final organic modifier content values resulting in the highest coverage of separation space can be derived for each 2D gradient run. Theory indicates that these values can be determined by adapting 2D gradient operation to the degree of orthogonality. The new method is tested on a comprehensive two-dimensional liquid chromatography system that uses reversed phase (RP) columns showing different selectivities in the two dimensions. A comparison between analyses carried out using normal and optimized 2D gradients showed that the latter allow a more efficient use of analysis time. This can result either in an improved peak capacity or in decreasing total analysis time, depending on the final goal of the experiment. In the latter scenario, the number of separated peaks is comparable to that obtained using gradients spanning a wide range of organic modifier but, now, in half the time. As test samples complex mixtures of peptides were analyzed.

Enantioseparation of a novel "click" chemistry derived native beta-cyclodextrin chiral stationary phase for high-performance liquid chromatography

A novel native beta-cyclodextrin chiral stationary phase was prepared via "click" chemistry with cuprous iodide-triphenylphosphine complex as the catalyst and applied for enantioseparation of Dns-amino acids, substituted phenyl and phenoxy group modified propionic acids, flavonoids, and some pharmaceutical compounds such as nimodipine, propranolol, brompheniramine and bendroflumethiazide in reversed-phase high-performance liquid chromatography. The studied analytes could be resolved under different separation conditions. The resolution of Dns-DL-Leu could reach 5.08 using a mobile phase consisting of 1% (w/w) triethylammonium acetate buffer (pH 4.11) and methanol (50:50 v/v). The effects of buffer pH and the content of organic modifier on enantioseparation of Dns-amino acids by this novel chiral phase were being investigated. The separation results demonstrate that click chemistry, a versatile reaction, affords a facile approach towards the preparation of stable chiral stationary phases.