Kazuhiro Kimata

Molecularly imprinted uniform-size polymer-based stationary phase for high-performance liquid chromatography structural contribution of cross-linked polymer network on specific molecular recognition

Abstract Non-covalently molecularly imprinted, uniform-size, polymer-based, stationary phases were prepared by a two-step swelling technique using isomers of diaminonaphthalene or a chiral amide derived from ( S )- α -methylbenzylamine as the template molecule. Methacrylic acid worked as an effective host molecule for diaminonaphthalene templates, however, an imprinted, cross-linked, polymer-based, stationary phase without such a relatively strong host functionality unexpectedly showed moderate molecular recognition, which suggested that the cross-linked polymer network could memorize the shape of a template. In chiral separation of amide derivatives, a cross-linked polymer network also memorized the shape of a chiral template, resulting in chiral resolution. In addition, a chiral cross-linking agent having similar functionality to the chiral amide template could enhance molecular recognition drastically. Further studies suggested that this enhancement in chiral recognition was due to a favorable structural interaction within the specific recognition sites.

Selectivity of carbon packing materials in comparison with octadecylsilyl- and pyrenylethylsilylsilica gels in reversed- phase liquid chromatography☆

Abstract The retention selectivity of graphitic carbon packing materials was compared with those of octadecylsilyl- and pyrenylethylsilylsilica packing materials in reversed-phase liquid chromatography. A carbon stationary phase showed the highest hydropobicity and selective retention of planar, unsaturated compounds. The presence of hydrophilic groups did not cause as great a retention decrease on carbon as on silica-based materials. A much greater influence of structural planarity of solutes was observed with carbon than with silica-based phases, presumably because the solute planarity emphasizes the interaction with the graphite surface based on charge transfer and dispersion forces. Pyrenylethyl-bonded silica gel was found to be intermediate between alkylsilica and graphitized carbon packing materials with respect to the selectivity based on the electronic steric interaction with the fused-ring aromatic systems on the stationary phase.

Performance of polymer-coated silica C18 packing materials prepared from high-purity silica gel : The suppression of undesirable secondary retention processes

Abstract Coating with silicone polymer of silica surfaces was shown to be very effective in suppressing the undesirable peak tailing in reversed-phase liquid chromatography caused by hydrogen bonding, ion-exchange and chelate formation processes. Silica particles containing various amounts of metal impurities were derivatized to octadecylsilylated (ODS) silica phases with or without subsequent trimethylsilylation, and to a polymer-coated phase with subsequent introduction of octadecyl groups. The performance of endcapped ODS phases prepared from high-purity silica gel was satisfactory for hydrogen-bond acceptors and protonated amines, but not acceptable for chelating compounds. The extent of tailing seen with chelating compounds depends on the solute structure and the metal content of silica particles. Coating with silicone polymer was more effective than trimethylsilylation of ODS phases for the suppression of tailing for chelating compounds, and afforded alkyl-bonded silica packing materials free from undesirable secondary effects by using metal-free silica particles.

Stationary phase effects in reversed-phase liquid chromatography

Abstract Selectivity of reversed-phase packing materials was discussed based on the solute-stationary phase interaction. Solute retention on a silica C18 phase is primarily determined by the hydrophobicity of a solute, whereas the long alkyl groups result in preferential retention of rigid, planar solutes over non-planar, bulky ones. An organic solvent imbibed in the stationary phase also contributes to the retention. Thus an electron donor, such as tetrahydrofuran, tends to give longer retention for acidic compounds or electron acceptors. The presence of micropores in polymer gel packing materials results in the preferential retention of solutes with rigid, compact structures, and the presence of dipolar groups (ester and ether linkages) contributes to the preferential retention of dipolar and/or aromatic compounds based on dipole-dipole or dipole-π interactions. Attractive interaction between the graphite carbon surface and solutes, presumably based on the dispersion force, results in selective retention of planar compounds compared with non-planar ones. Attractive interactions provided by electron-donor-acceptor bonded phases are also shown to be very effective for structural recognition.

Molecularly imprinted chiral stationary phase prepared with racemic template.

The first example of molecularly imprinted chiral stationary phase prepared using a racemic template is shown. N-(3,5-Dinitrobenzoyl)-α-methylbenzylamine (DNB) was chirally discriminated on the molecularly imprinted stationary phase prepared using racemic DNB as the template. A chiral monomer, (S)-(-)-N-methacryloyl-1-naphthylethylamine, was utilized as the functional monomer toward the racemic template, and its chiral recognition ability was, interestingly, found to be enhanced through racemic molecular imprinting. A thermodynamic discussion briefly suggests that the observed chiral recognition ability of the racemic imprinting was proper value.

Structural selectivity provided by starburst dendrimers as pseudostationary phase in electrokinetic chromatography

Abstract Starburst dendrimers (SBDs) were used as a pseudostationary phase in electrokinetic chromatography (EKC) of hydrophobic compounds. The selectivity of SBD-mediated EKC (SBD-EKC) was different from those in micellar EKC (MEKC) systems, in spite of the apparent structural resemblance between micelles and SBDs. The SBDs provided similar selectivity as polymer gel packing materials in reversed-phase liquid chromatography (RPLC), showing little selectivity for alkyl groups and clear preference for aromatic compounds, especially for rigid, planar polynuclear aromatic hydrocarbons. The alkylation of SBDs resulted in the increased retention and hydrophobic selectivity while maintaining the preference toward rigid, planar compounds. These SBDs can be used in a full range of methanol-water mixtures, showing the retention decrease with the increase in methanol content as in RPLC. The results suggest that SBDs can make a support for various pseudostationary phases for EKC.

Polymer-supported pseudo-stationary phase for electrokinetic chromatography Electrokinetic chromatography in a full range of methanol-water mixtures with alkylated starburst dendrimers

Abstract Alkylated polymidoamine starburst dendrimers (PAMAM-SBDs) are used as high-performance carriers in electrokinetic chromatography (EKC) in a full range of water-methanol (up to 90%) mixtures. The sodium dodecyl sulfate-micellar EKC (SDC-MEKC) system showed a greater migration time window at 20% methanol, but its applicability is limited above 40% methanol. The SBDs modified with dodecyl groups provided high efficiency and much wider migration time windows than SDS micelle carriers at high methanol content, especially for hydrophobic compounds, based on their more hydrophobic properties and reduced electroosmotic mobility. Different selectivities were observed between octyl and dodecyl derivatives of SBDs, the latter showing some similarity with the SDS micelle carrier. The results, showing a nearly linear relation between log k ′ and methanol content, indicate the possibility for the optimization of EKC separation with polymeric carriers by simply changing the organic solvent content to manipulate the solute retention as in reversed-phase liquid chromatography.

Method for the preparation of internal-surface reversed-phase packing materials starting from alkylsilylated silica gels

Abstract A simple method for the preparation of internal-surface reversed-phase (ISRP) packing materials was developed. Partial decomposition of alkylsilylated silica gel with an aqueous acid followed by the introduction of diol functionalities produced stationary phases possessing the properties of ISRP packing materials. The method is applicable to various types of reversed-phase packing materials irrespective of the structure of the bonded alkyl moieties. The alkyl/diol-type packing materials possess greater hydrophobic properties than other ISRP packing materials currently available for high-performance liquid chromatography. Reversed-phase separation of low-molecular-weight compounds on the alkyl/diol phase by direct injection of serum samples is demonstrated. The present packing materials prepared from relatively large particles with or without the introduction of the diol phase can also be used for open-column chromatography in the reversed-phase mode with a wide range of aqueous-organic mobile phases, as the external surfaces are wettable with water.

Performance of wide-pore silica- and polymer-based packing materials in polypeptide separation: effect of pore size and alkyl chain length

The effects of pore size and alkyl chain length of silica- and polymer-based packing materials in the elution of polypeptides with an acetonitrile gradient in the presence of trifluoroacetic acid were studied. Considerable differences were found in the performance of alkylsilylated phases prepared from various wide-pore silica particles assumed to have 30-50-nm pores. The pore size of such silica gels was found to be the critical factor in determining the efficiency for high-molecular-weight polypeptides. Silica C18 phases having small pore volumes below 20 nm pore diameter showed comparable performances to C4 and C8 phases for polypeptides with molecular weights of up to 80,000, and were more stable. Polymer-based packing materials with adequate pore size provided excellent column efficiencies and recoveries for polypeptides with higher chemical stabilities than silica-based materials.

Selectivity of stationary phases in reversed-phase liquid chromatography based on the dispersion interactions.

Selectivity of 15 stationary phases was examined, either commercially available or synthesized in-house. The highest selectivity factors were observed for solute molecules having different polarizability on the 3-(pentabromobenzyloxy)propyl phase (PBB), followed by the 2-(1-pyrenyl)ethyl phase (PYE). Selectivity of fluoroalkane 4,4-di(trifluoromethyl)-5,5,6,6,7,7,7-heptafluoroheptyl (F13C9) phase is lowest among all phases for all compounds except for fluorinated ones. Aliphatic octyl (C8) and octadecyl (C18) phases demonstrated considerable selectivity, especially for alkyl compounds. While PBB showed much greater preference for compounds with high polarizability containing heavy atoms than C18 phase, F13C9 phase showed the exactly opposite tendency. These three stationary phases can offer widely different selectivity that can be utilized when one stationary phase fails to provide separation for certain mixtures. The retention and selectivity of solutes in reversed-phase liquid chromatography is related to the mobile phase and the stationary phase effects. The mobile phase effect, related to the hydrophobic cavity formation around non-polar solutes, is assumed to have a dominant effect on retention upon aliphatic stationary phases such as C8, C18. In a common mobile phase significant stationary phase effect can be attributed to dispersion interaction. Highly dispersive stationary phases such as PBB and PYE retain solutes to a significant extent by (attractive) dispersion interaction with the stationary phase ligands, especially for highly dispersive solutes containing aromatic functionality and/or heavy atoms. The contribution of dispersion interaction is shown to be much less on C18 or C8 phases and was even disadvantageous on F13C9 phase. Structural properties of stationary phases are analyzed and confirmed by means of quantitative structure-chromatographic retention (QSRR) study.