Two-dimensional liquid chromatography separation of peptides using reversed-phase/weak cation-exchange mixed-mode column in first dimension

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.

Preparation and chromatographic evaluation of a mixed polymer brush-silica stationary phase with temperature-sensitive property

In this study, a developed chromatographic stationary phase combines the high selectivity of mixed-mode retention with a temperature-responsive property to boost separation efficiency. Copolymer brushes were grafted onto silica gels through surface initiated-atom transfer radical polymerization by polymerizing two types of monomer, temperature-responsive vinylcaprolactam (VCl) and quinine (Qun) containing benzopyridine, a tertiary ammonium positive center, and hydroxyl groups. The obtained silica@poly(Qun-co-VCl) stationary phases were packed as a chromatographic column, and the retention behavior of hydrophobic polycyclic aromatics, highly polar nucleosides, charged organic acids and β-agonists was studied for this column under different separation modes. The ability to separate different types of analyte shows that the silica@poly(Qun-co-VCl) column provides multiple hydrophobic, hydrophilic and electrostatic interactions toward analytes, achieving the separation of various compounds in one column. In addition, temperature-dependent resolution of polycyclic aromatics, nucleosides, organic acids and β-agonists was investigated using modulation of the column temperature, and the column exhibited adjustable separation selectivity by simply changing the column temperature. These results demonstrate that the grafting of copolymer brushes on a silica surface, consisting of temperature-responsive poly-VCl and multifunctional groups of poly-Qun, is useful as a mixed-mode chromatographic stationary phase for thermally-modulated multiple interactions. Additionally, this column was also used for the quantitative detection of uridine and inosine from cordyceps.

pH-dependent selective separation of acidic and basic proteins using quaternary ammoniation functionalized cysteine-zwitterionic stationary phase with RPLC/IEC mixed-mode chromatography

In this paper, a cysteine-functionalized zwitterionic stationary phase (Cys-silica) was prepared based on the "thiol-ene" click chemistry between cysteine and vinyl-functionalized silica, and was further modified with bromoethane, 1-bromooctane and 1-bromooctadecane, respectively, to obtain a series of quaternary ammoniation-functionalized stationary phases (Cys-silica-C_n, n = 2, 8 and 18). These zwitterionic stationary phases were regarded as reversed-phase/ion-exchange (RP/IEC) mixed-mode chromatography (MMC) stationary phases for protein separation. The retention behaviors of proteins on these zwitterionic stationary phases were carefully investigated. The results indicated that the retentions of acidic and basic proteins on these zwitterinonic stationary phases were significantly influenced by the acetonitrile and salt concentrations, pH of mobile phase as well as the hydrophobicity of the ligand. The separation selectivity of proteins on these zwitterionic stationary phases strongly depended on the pH value of mobile phase. The baseline separation of 6 kinds of basic proteins can be achieved at pH 8.0 using Cys-silica-C2 or Cys-silica-C8 column, and 5 kinds of acidic proteins can also be separated completely at pH 4.0 with Cys-silica-C2 column. Moreover, owing to the quaternary ammoniation-functionalization on Cys-silica by using appropriately hydrophobic bromoalkanes, the selectivity and separation efficiency of proteins can be enhanced greatly. As a result, the acidic and basic proteins can be separated completely step by step from the complex sample by adjusting pH of mobile phase using a single Cys-silica-C2 column, which illustrates that the cysteine-functionalized zwitterionic stationary phase has a great potential for protein separation.

Preparation and evaluation a mixed-mode stationary phase with C18 and 2-methylindole for HPLC

A modified C₁₈ column (Silpr-2MI-C18) was prepared using 2-methylindole and C₁₈ reagent. The extent of C₁₈ hydrocarbon chain, conjugative rings and anion exchange site provided multiple retention mechanisms, including reversed-phase liquid chromatography (RPLC), π-π interaction, hydrophilic interaction liquid chromatography (HILIC) and anion exchange chromatography (AEC). The separation of protected amino acids was investigated on the commercial C₁₈ and Silpr-2MI-C18 columns, while the chromatographic conditions, including methanol content and pH of the mobile phase, were studied. The separation arrangement of the hydrophilic amino acids was different on the Silpr-2MI-C18 column compared to the commercial C₁₈ column under RPLC mode. Furthermore, these amino acids were separated on the Silpr-2MI-C18 column under HILIC mode. The modified C₁₈ column was employed to separate amino acids, alkylbenzenes and polycyclic aromatic hydrocarbons under RPLC mode and inorganic anion under AEC mode. The results confirm that this new stationary phase of RPLC/HILIC/AEC has multiple interactions with different analytes. Effective retention of biological samples was found on the Silpr-2MI-C18 column by comparing the results obtained from the commercial C₁₈ column.

Doping control analysis of small peptides: A decade of progress

Small peptides are handled in the field of sports drug testing analysis as a separate group doping substances. It is a diverse group, which includes but is not limited to growth hormone releasing-factors and gonadotropin-releasing hormone analogues. Significant progress has been achieved during the past decade in the doping control analysis of these peptides. In this article, achievements in the application of liquid chromatography-mass spectrometry-based methodologies are reviewed. To meet the augmenting demands for analyzing an increasing number of samples for the presence of an increasing number of prohibited small peptides, testing methods have been drastically simplified, whilst their performance level remained constant. High-resolution mass spectrometers have been installed in routine laboratories and became the preferred detection technique. The discovery and implementation of metabolites/catabolites in testing methods led to extended detection windows of some peptides, thus, contributed to more efficient testing in the anti-doping community.

Facile Synthesis of Mixed-Mode Weak Anion-Exchange Microspheres via One-Step Pickering Emulsion Polymerization for Efficient Simultaneous Extraction of Strongly and Weakly Acidic Drugs from Reservoir Water

Poly(2-(diethylamino)ethyl methacrylate-co-divinylbenzene) (poly(DEAEMA-co-DVB)) microspheres with mixed-mode weak anion-exchange (WAX) character were successfully fabricated for the first time via facile one-step Pickering emulsion polymerization. The obtained poly(DEAEMA-co-DVB) particles had good spherical geometry, uniform particle size in the range of 30-40 µm, a large specific surface area of 575 m²/g, and a pore size range of 5-30 nm, according to the SEM and nitrogen adsorption-desorption results. Using these mixed-mode WAX microspheres as packing material, a reliable and robust analytical method based on solid phase extraction and high performance liquid chromatography with ultraviolet detection (SPE-HPLC-UV) was developed for simultaneous determination of six strongly and weakly acidic nonsteroidal anti-inflammatory drugs (NSAIDs, niflumic acid, diflunisal, naproxen, ketoprofen, mefenamic acid, and diclofenac) in reservoir water. Under optimized conditions, it was applicable to preconcentrate up to 500 mL of reservoir water samples on the WAX cartridges with satisfying recoveries (88-96%) for all the NSAIDs tested. The limits of detection were in the range of 0.002-0.025 μg L^-1, respectively. Our results showed that the developed mixed-mode WAX poly(DEAEMA-co-DVB) phase containing a tertiary amine with a pKa value of approximately 10.7 could be used for simultaneous clean-up and preconcentration of strongly and weakly acidic organic pollutants in real environmental water, which could not be achieved by single use of quaternary ammonium strong anion-exchange phase or weaker primary and secondary amine anion-exchange.

Development of a multi-functional concurrent assay using weak cation-exchange solid-phase extraction (WCX-SPE) and reconstitution with a diluted sample aliquot for anti-doping analysis

RATIONALE

In addition to the development of adequate screening methods for multiple compounds, the World Anti-Doping Agency (WADA) requires anti-doping laboratories to analyze prohibited substances and their metabolites from various classes. This task presents a difficult challenge for all agencies and interests involved in the field of doping control.

METHODS

A screening method is reported in which hybrid sample preparation was performed using a combination of weak cation-exchange solid-phase extraction (WCX-SPE) and the 'Dilute and Shoot' strategy in order to take advantage of both the methodologies. Target substances were extracted using a WCX cartridge and reconstituted with a diluted sample aliquot that included 20% of an untreated urine sample. The target substances were further analyzed by high-performance liquid chromatography/triple quadrupole mass spectrometry (LC/MS).

RESULTS

The SPE procedure was optimized using a cartridge-washing step, elution conditions, and elution volume. The cartridge-washing step, which was performed using 10% methanol, improved the overall recovery of target substances. Since the recovery was observed to vary according to the pH of the eluting solution, we applied an elution step using both an acid and a basic organic solvent to achieve complementary recovery. Reconstitution of the diluted aliquot sample was performed to recover the polar substances.

CONCLUSIONS

The method was validated and applied to real samples in accordance with the external quality assessment scheme of WADA and to the previously reported samples that had provided positive test results. This novel method using hybrid sample preparation and LC/MS could be useful to screen multiple classes of the 264 targeted substances in anti-doping analysis.

Simultaneous quantitative analysis of uremic toxins by LC-MS/MS with a reversed-phase/cation-exchange/anion-exchange tri-modal mixed-mode column

Column choice is crucial to the development of liquid chromatography/tandem mass spectrometry (LC-MS/MS) methods because analyte selectivity is dependent on the nature of the stationary phase. Recently, mixed-mode chromatography, which employs a combination of two or more stationary phases and solvent systems, has emerged as an alternative to multiple, complementary, single-column systems. This report describes the development and validation of a novel analytical method based on LC-MS/MS employing a reversed-phase/cation-exchange/anion-exchange tri-modal column (Scherzo SS-C18; Imtakt) for the simultaneous quantification of various uremic toxins (UTx), including creatinine, 1-methyladenosine, trimethylamine-N-oxide, indoxyl sulfate, p-cresyl sulfate, phenyl sulfate and 4-ethylphenyl sulfate. Stable isotope-labeled compounds were prepared as internal standards (ISs) for each analyte. Mobile phase optimization and appropriate gradient conditions resulted in satisfactory retention and peak resolution that could not have been attained with a single stationary phase LC system. The essential validation parameters, including intra- and inter-assay precision and accuracy, were adequate. The validated method was applied to measure serum levels of the aforementioned compounds in 19 patients with chronic kidney disease. This is the first report detailing the simultaneous quantification of these analytes using stable isotopes as ISs. Our results suggest that Scherzo SS-C18 columns will be considered breakthrough tools in the development of analytical methods for compounds that are difficult to quantify simultaneously in traditional LC systems.

Development of an on-line mixed-mode gel liquid chromatography×reversed phase liquid chromatography method for separation of water extract from Flos Carthami

A novel on-line comprehensive two-dimensional liquid chromatography (2D-LC) method by coupling mixed-mode gel liquid chromatography (MMG-LC) with reversed phase liquid chromatography (RPLC) was developed. A mixture of 17 reference compounds was used to study the separation mechanism. A crude water extract of Flos Carthami was applied to evaluate the performance of the novel 2D-LC system. In the first dimension, the extract was eluted with a gradient of water/methanol over a cross-linked dextran gel Sephadex LH-20 column. Meanwhile, the advantages of size exclusion, reversed phase partition and adsorption separation mechanism were exploited before further on-line reversed phase purification on the second dimension. This novel on-line mixed-mode Sephadex LH-20×RPLC method provided higher peak resolution, sample processing ability (2.5mg) and better orthogonality (72.9%) versus RPLC×RPLC and hydrophilic interaction liquid chromatography (HILIC)×RPLC. To the best of our knowledge, this is the first report of a mixed-mode Sephadex LH-20×RPLC separation method with successful applications in on-line mode, which might be beneficial for harvesting targets from complicated medicinal plants.

C18-COOH Silica: Preparation, Characterisation and Its Application in Purification of Quaternary Ammonium Alkaloids from Coptis chinensis

INTRODUCTION

Traditional methods for isolating and purifying quaternary ammonium alkaloids from Coptis chinensis do not discriminate the target alkaloids from co-extractives. Mixed-mode analytical chromatography has the potential to improve the separation of analytes in more complex extracts and, when used in a solid-phase extraction mode, improve the purity of isolated compounds.

OBJECTIVE

To examine the high-performance liquid chromatographic separation capabilities of a mixed-mode, silica-based adsorbent and its application to the purification of quaternary ammonium alkaloids from Coptis chinensis based on solid-phase extraction.

METHODOLOGY

The C18-COOH silica was prepared via "thiol-ene" click chemistry. Its chromatographic performance was firstly investigated. It was employed as the solid-phase extraction sorbent to purify quaternary ammonium alkaloids.

RESULTS

Hydrophobic, attractive/repulsive electrostatic and ion-exchange interactions were demonstrated to be the possible retention mechanisms of a C18-COOH silica stationary phase, which could separate analytes of various properties. In addition, to purify quaternary ammonium alkaloids from Coptis chinensis, the solid-phase extraction approach based on the C18-COOH silica provided a slightly higher purification efficiency (6.7%) than an alkaloid-salt precipitation protocol (5.3%). The method had satisfactory reproducibility, re-using the same solid-phase extraction column three times, with relative standard deviations ranging from 1.99% to 8.02% for the six target alkaloids.

CONCLUSION

A multi-functionalised silica was synthesised via "click chemistry". As the high-performance liquid chromatographic stationary phase, the C18-COOH silica could be operated in trimodal reversed-phase/weak cation exchange/hydrophilic interaction mode. The C18-COOH silica also exhibited potential as solid-phase extraction sorbent in the purification of quaternary ammonium alkaloids from complex matrices. Copyright © 2017 John Wiley & Sons, Ltd.

Controllable preparation of a hydrophilic/ion-exchange mixed-mode stationary phase by surface-initiated atom transfer radical polymerization using a mixture of two functional monomers

Mixed-mode chromatographic stationary phases require functionalization with at least two functional groups to yield multiple interactions with analytes. Departing from reported methods, a mixture of two different monomers, glycidyl methacrylate and 2-dimethylaminoethylmethacrylate, was grafted onto the surface of silica by a one-step surface-initiated atom transfer radical polymerization to prepare a novel hydrophilic interaction/anion-exchange mixed-mode chromatographic stationary phase. The grafted amounts of functional groups were controlled via varying the ratio of monomers in the polymerization system. The influences of water content, salt concentration and pH in the mobile phase were investigated to illustrate the mixed interaction between the stationary phase and analytes. The retention of various solutes on three columns, especially acidic and basic solutes, showed an obvious dependence on the ratio of the two monomers in the polymerization system. The results indicated that the strategy proposed in this work was beneficial to develop various types of mixed-mode chromatographic stationary phases with adjustable selectivity to meet the needs of complex samples. Finally, the column was successfully employed in the isolation of melamine in liquid milk.

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.

Preparation and evaluation of surface-grafted block copolymers and random copolymers via surface-initiated atom transfer radical polymerization for hydrophilic/ion-exchange stationary phases

Mixed-mode chromatographic (MMC) stationary phases must be functionalized with at least two functional groups to yield multiple interactions.

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 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.

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.