1,720,986 research outputs found
Innovative materials and methods for HPLC and UHPLC: design of new bidentate urea-type stationary phases. Stereodynamic studies of artemisinin derivatives.
Full text linkThis Ph.D. thesis focuses on three main topics:
1) Design and evaluation of new stable bidentate urea-type stationary phases for HPLC and UHPLC. New totally synthetic bidentate urea-type phases were envisioned for HPLC and UHPLC, characterized by a high stability, a wide application range and an excellent suitability for LC-MS. Four conceptually new stationary phases have been developed for the separation of a wide variety of polar compounds by hydrophilic interaction chromatography (HILIC) through a facile one-pot two-step procedure with the aim of obtaining high hydrolytic stability in a variety of elution conditions. A full chemical characterization of the new materials has been obtained through solid-state NMR (both 29Si and 13C CPMAS) spectroscopy. A major application field of the bidentate urea-type stationary phase with free amino groups USP-HILIC-NH2-sil was sugars analysis, usually hampered by anomer peak splitting and instability of the stationary phases under conditions normally employed to suppress it. Complex mixtures of mono-, di- and oligosaccharides were successfully resolved under mild chromatographic conditions, which also allowed an easy interface with mass spectrometry. The potential of such materials was shown in the separation of other highly polar compounds, including polyols, hydroxybenzoic acids, nucleobases, and vitamins. A parallel project was developed using a very similar synthetic scheme where enantiopure diamines were treated with (3-isocyanatopropyl) triethoxysilane to yield reactive ureido selectors that were eventually attached to unmodified silica particles through a stable, bidentate tether. The two steps were part of an easy and time-saving one-pot procedure, as was the case for the HILIC phases. These new monomeric brush-type chiral stationary phases, hereafter called “Crab-like” CSPs, are characterized by an excellent stability and enantioselectivity, a high permeability and efficiency, as well as an extended compatibility with all solvents and no memory effect, which allowed us to operate under normal phase, reversed phase and polar organic mode conditions. Comparative studies were performed with commmercial polymeric brush-type CSPs bearing the same chiral selector (1,2-diaminocyclohexane and 1,2-diphenyl-1,2-ethylenediamine) in order to evaluate the resolution and performance skills of the new “Crab-like” CSPs in different elution modes.
2) Extending the use of “Inverted Chirality Columns Approach (ICCA)” for enantiomeric excess determination in absence of reference samples: application to a water-soluble camptothecin derivative. The aim of the present study was to extend the use of the “Inverted Chirality Columns Approach (ICCA)” previously developed for the identification and quantitation of the trace enantiomer in highly enriched samples of the camptothecin (CPT) family of drugs to a novel water-soluble CPT derivative, namely namitecan (ST1968), currently undergoing phase I clinical trials as anticancer agent. Namitecan, identified from a series of hydrophilic 7-oxyiminomethyl derivatives, contains a free terminal amino group, which traditionally hampers the analysis under normal-phase HPLC conditions. Nevertheless, commercially available Pirkle-type chiral stationary phases (CSPs) available in both the enantiomeric forms (i.e., having the same bound selector with opposite configuration) mainly operate under normal-phase HPLC conditions. For this reason, namitecan was pre-columnN-derivatized with isocyanates A–D and their sulfur analogues E–H to reduce its polarity by converting the amino group into a fragment compatible with the chiral recognition mechanism (i.e., ureido and thioureido groups). Once the optimal columns system and derivatizing agents were selected, an original enantioselective HPLC–MS/MS technique was developed on the Whelk-O1 CSPs.
3) Kinetic and thermodynamic studies of the epimerization processes in Artemisinin-derived antimalarial drugs by Dynamic High Performance Liquid Chromatography (D-HPLC). Stereodynamic investigations have been performed on two active artemisinin derivatives having labile stereogenic centres, namely dihydroartemisinin (DHA), and an in vivo degradation product of DHA containing an endoperoxide, an aldehyde and a ketonic moiety, namely MKA (monoketoaldehyde). We were able to obtain MKA through pyrolysis or basic degradation of DHA and have fully characterized its structure through a variety o analytical techinques. The stereodynamic studies performed have focused on a stereolabile hemiacetalic function present in both molecules. Linear Solvation Energy Relationships (LSER) analysis was used to study the thermodynamics of the epimerization process in DHA and MKA. Computational studies have provided further data on the differential stability and acidity of the two epimeric forms of MKA, while a kinetic investigation is introduced
Design and Evaluation of a New Tweezers-Like Stable Stationary Phase for Hydrophilic Interaction Chromatography-Mass Spectrometry (HILIC-MS)
No full textUn approccio razionale verso la sintesi di nuove fasi stazionarie chirali "Crab-like": sintesi e valutazione
No full textA rational Apporache to the design of new "Crab-like" chiral stationary phases for HPLC and UHPLC: Synthesis and Evaluation
No full textIntroducing enantioselective UHPLC in the chiral science separation scenario: ultra-fast separations on a new sub-2 μm Whelk-O1 stationary phase
No full textA new chiral stationary phase for UHPLC applications was prepared by covalent attachment of the Whelk-O1 selector to spherical high-surface-area 1.7-μm porous silica particles. Columns of varying dimensions (50-150 mm length and 3.0 or 4.6 mm internal diameter) were packed and characterized in terms of permeability, efficiency, retention and enantioselectivity.1 Van Deemter plots, performed on a 100 x 4.6 mm I.D. column, generated Hmin values of 3.53 μm for 1,3-dinitrobenzene, at an interstitial mobile phase linear velocity (μinter) of 5.07 mm/s, and Hmin of 4.26 and 4.17 μm for the two enantiomers of acenaphthenol, at μinter of 4.85 mm/s and 4.24 mm/s, respectively. Resolution of 21 enantiomeric pairs including alcohols, epoxides, sulfoxides, phosphine oxides, benzodiazepines and 2-aryloxyproprionic esters used as herbicides, were obtained with significant advantages in terms of efficiency (with N/m values as high as 270 000) and analysis time (in the seconds-time-scale with trans-stilbene oxide being resolved in only 10 s).2 The excellent efficiency and selectivity, conjugated with the very high-throughput and the ultra-fast analysis time, prove the potentials of the eUHPLC Whelk-O1 columns in the development of enantioselective UHPLC methods and represent a powerful tool in organic chemistry
Separation of complex sugar mixtures on a hydrolytically stable bidentate urea-type stationary phase for hydrophilic interaction near ultra high performance liquid chromatography
No full textA stationary phase bearing both bridged bis-ureido and free amino groups (USP-HILICNH2–2.5SP) for high-speed hydrophilic interaction liquid chromatography separations was prepared using a one-pot two-step procedure starting from 2.5 m totally porous silica particles. Highly polar compounds, such as polyols, hydroxybenzoic acids, and sugars, were successfully analyzed in shorter times and with higher peak efficiency, when compared to
results obtained with a bidentate urea-type column packed with 5 m particles. Increased sugarophilicity and better peak shape were attested for the USP-HILIC-NH2–2.5SP column (100 × 3.2 mm id) when compared with two commercially available UHPLC columns, namely an acquity BEH amide packed with totally porous 1.7 m microparticles and a HILIC Kinetex column packed with core–shell 2.6 m particles. Finally, the new column was employed in the separation of complex mixture of sugars (mono-, di-, and oligosaccharides) and in the analysis of beer samples. The resulting chromatograms showed good selectivity and overall resolution, while the catalyzing effect of the free amino moieties resulted in excellent peak shapes and in the absence of split peaks due to sugar anomerization
phenomen
Enantioselective ultra-high and high performance liquid chromatography: A comparative study of columns based on the Whelk-O1 selector
No full textThis paper reports on the thermodynamic and kinetic evaluation of a new ultra-high performance liquid chromatography broad-spectrum Pirkle-type chiral stationary phase (CSP) for enantioselective applications (eUHPLC). The well-known Whelk-O1 selector was covalently immobilized onto 1.7-mu m high-surface-area, porous spherical silica particles to produce a totally synthetic, covalently bonded CSP that was packed into 150 mm, 100 mm, 75 mm and 50 mm columns, either 4.6 or 3.0 mm ID. A 100 mm x 4.6 mm ID column was fully characterized from a kinetic and thermodynamic point of view, using as reference a conventional HPLC Whelk-O1 column, 250 mm x 4.6 mm ID, based on 5-mu m porous silica particles. On the eUHPLC column, van Deemter plots generated H-min values of 3.53 mu m for 1.3-dinitrobenzene, at an interstitial mobile phase linear velocity (mu(inter)) of 5.07 mm/s. and H-min of 4.26 and 4.17 mu m for the two enantiomers of acenaphthenol, at mu(inter) of 4.85 mm/s and 4.24 mm/s, respectively. Resolution of 21 enantiomeric pairs including alcohols, epoxides, sulfoxides, phosphine oxides, benzodiazepines and 2-aryloxyproprionic esters used as herbicides, were obtained with significant advantages in terms of efficiency and analysis time. Speed gain factors were calculated for the different column geometries (150 mm, 100 mm, 75 mm and 50 mm, either 4.6 or 3.0 mm ID), with respect to the standard HPLC column (250 mm x 4.6 mm ID), and were as high as 13, in the case of the 50-mm-long column, affording sub-minute separations of enantiomers with excellent resolution factors. In particular, transstilbene oxide was resolved in only 10 s, while a 50 mm x 3.0 mm ID column was used as a compromise between reduced mobile phase consumption (less than 1 mL per analysis) and smaller extra-column band-broadening effect. Given the relatively low viscosity in NP mode, and the excellent permeability of these eUHPLC columns, with backpressure values under 600 bar for a wide range of flow rates, the use of standard HPLC hardware is possible. In this case, however, a significant loss in resolution is observed, compared to the UHPLC instrumentation, if no modifications are introduced in the HPLC apparatus to reduce extra-column variance. The excellent efficiency and selectivity, conjugated with the very high-throughput and the ultra-fast analysis time, prove the potentials of the eUHPLC Whelk-O1 columns in the development of enantioselective UHPLC methods. (C) 2012 Elsevier B.V. All rights reserved
A rational approach to the design of new "Crab-like" chiral stationary phases: synthesis and evaluation
No full textDesign and evaluation of hydrolytically stable bidentate urea-type stationary phases for hydrophilic interaction chromatography
No full textWe have developed conceptually new stationary phases containing two bidentate urea-type functions suitable for the separation of a wide variety of polar compounds by hydrophilic interaction chromatography (HILIC) through a facile one-pot two-step procedure with the aim of obtaining high hydrolytic stability in a variety of elution conditions. The preparation of the new phases involves a first reaction of 12-ethylendiamine with (3-isocyanatopropyl)triethoxysilane to give an intermediate bis-urea with two pendant triethoxysilane functions, followed by anchoring on the silica surface. Two stationary phases were prepared, namely an urea-type stationary phase (USP-HILIC) and an urea-type phase bearing free amino groups (USP-HILIC-NH2), whereas silanization with 1,2-bis(trichlorosilyl)ethane yielded USP-HILIC-sil and USP-HILIC-NH2-sil phases, respectively. The silanization step aimed at forming a hydrophilic stable coating through cross-linking between adjacent silanols which prevents silica dissolution at alkaline pH. A full chemical characterization of the new materials has been obtained through solid-state NMR (both Si-29 and C-13 CPMAS) spectroscopy. A major application field of the bidentate urea-type stationary phase with free amino groups USP-HILIC-NH2-sil was sugars analysis, usually hampered by alpha/beta anomer peak splitting and instability of the stationary phases under conditions normally employed to suppress it. Complex mixtures of mono-, di- and oligosaccharides were successfully resolved under mild chromatographic conditions, which also allowed an easy interface with mass spectrometry. The potential of such materials was shown in the separation of other highly polar compounds, including polyols, hydroxybenzoic acids, nucleobases, and vitamins. (c) 2011 Elsevier B.V. All rights reserved
The evolution of chiral stationary phases from HPLC to UHPLC.
No full textThere is a need for chiral stationary phases (CSPs) designed for high performance liquid chromatography (HPLC) to switch to enantioselective applications using ultrahigh-performance liquid chromatography (UHPLC). Although important goals have been achieved to rapidly separate achiral compounds, enantioselective LC remains solidly attached to 3-μm and 5-μm totally porous particles and pressure values in the HPLC domain. This article describes strategies aimed at immobilizing or coating well established chiral selectors onto sub-2-μm silica particles, and aims to illustrate the potential of enantioselective UHPLC (eUHPLC) in terms of high speed, throughput and resolution
- …
