The modiffication of alumina,zirconia, thoria and titania for potential use as HPLC stationary phases through a silanization/hydrosilation reaction scheme

Chromatographia - A process for bonding a monolayer of silane to the surface of alumina, zirconia, thoria and titania by reaction with the hydrolysis product of triethoxysilane is described. The...     

Synthesis and characterization of chemically bonded stationary phases on hydride surfaces by hydrosilation of alkynes and dienes

A hydrosilation reaction was used to bind four compounds with one or more alkyne groups or two alkene functionalities. The diffuse reflectance infrared Fourier transform, the 29Si cross-polarization magic-angle spinning (CP-MAS) NMR, and the 13C CP-MAS NMR spectra were used to characterize the various bonded materials. The bonded phase density was determined from carbon elemental analysis. The two ten-carbon hydrophobic stationary phases were characterized chromatographically and static stability tests were run in acidic and basic solutions.     

Syntheses of stationary phases for reversed-phase LC using silanization and polymer coating

Summary Various methods and reagents for silanization of silicas originating from different manufacturers were investigated in comparison to the method published by the authors which makes use of an enolate of acetylacetone. This reagent proved to be highly reactive. New types of reversed phase materials were also synthesized by polymer coating applying crosslinking and chemical bonding to the surfaces of silica and alumina. In this work dicumyl peroxide and azo-t-butane were used for radical generation. The reversed phase materials obtained with alumina can be used for separations at P_H-values up to 12. Silica materials begin to already be deteriorated at a P_H-value of 8. Dedicated to Professor G. Wilke on the occasion of his 60th birthday. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Synthesis and characterization of silica-based hyper-crosslinked sulfonate-modified reversed stationary phases

A novel type of silica-based sulfonate-modified reversed phase ((-)SO3-HC-C8) has been synthesized; it is based on a newly developed acid stable "hyper-crosslinked" C8 derivatized reversed phase, denoted HC-C8. The (-)SO3-HC-C8 phases containing controlled amounts of sulfonyl groups were made by sulfonating the aromatic hyper-crosslinked network of the HC-C(8) phase at different temperatures. The (-)SO3-HC-C8 phases are only slightly less hydrophobic than the parent HC-C8 phase. The added sulfonyl groups provide a unique strong cation-exchange selectivity to the hydrophobic hyper-crosslinked substrate as indicated by the very large C coefficient as shown through Snyder's hydrophobic subtraction reversed-phase characterization method. This cation-exchange activity clearly distinguishes the sulfonated phase from all other reversed phases as confirmed by the very high values of Snyder's column comparison function F(s). In addition, as was found in previous studies of silica-based and zirconia-based reversed phases, a strong correlation between the cation-exchange interaction and hydrophobic interaction was observed for these sulfonated phases in studies of the retention of cationic solutes. The overall chromatographic selectivity of these (-)SO3-HC-C8 phases is greatly enhanced by its high hydrophobicity through a "hydrophobically assisted" ion-exchange retention process.     

New alumina-based stationary phases for high-performance liquid chromatography Synthesis by olefin hydrosilation on a silicon hydride-modified alumina intermediate

A silanization procedure is used to form a layer of silicon hydride on the surface of alumina. IR and NMR data confirm the presence of the hydride on the surface. Modification of the hydride intermediate is accomplished by reaction with a terminal olefin in the presence of a transition metal catalyst to form an alkyl-bonded material. Both IR and NMR confirm the bonding of the alkyl ligand to the hydride surface. Chromatographic tests indicate reversed-phase behavior. Exposure to high concentrations of phosphate solution for an extended period resulted in no significant deterioration of the bonded phase.     

Comparison of silanization/hydrosilation and organosilanization modification procedures on etched capillaries for electrokinetic chromatography

Etched capillaries for use in open tubular electrochromatography are modified by silanization/hydrosilation and organosilanization. The migration behavior of both types of capillaries is evaluated with small basic molecules, peptides and proteins. Comparisons of peak symmetry and efficiency are used to measure the effectiveness of the two methods for modifying the etched surface. From this information, the suitability of each method for use with etched capillaries can be determined.     

Synthesis and characterization of benzyl β-cyclodextrins used as GC stationary phases

Summary Heptakis(2,6-di-O-benzyl)-β-CD(I), heptakis(2,6-di-O-benzyl-3-O-pentyl)-β-CD(II), heptakis(2,6-di-O-benzyl-3-O-methyl)-β-CD(III) and heptakis(2,6-di-O-benzyl-3-O-acetyl)-β-CD(IV) derivatives were synthesized and identified. Their thermal stabilities were tested using PyGC. These CD derivatives are stable up to 300°C and suitable for use as gas chromatographic stationary phases. The fused silica capillary columns coated with heptakis(2,6-di-O-benzyl-3-O-methyl)-β-CD and heptakis(2,6-di-O-benzyl-3-O-pentyl)-β-CD showed excellent chromatographic properties in separating positional isomers.     

Synthesis and characterization of hypercrosslinked, surface-confined, ultra-stable silica-based stationary phases

The synthesis and chromatographic characterization of a highly crosslinked self-assembled monolayer (SAM) stationary phase whose acid and thermal stability were significantly improved relative to a sterically protected octadecylsilane (ODS) stationary phase were recently described [B.C. Trammell, L. Ma, H. Luo, D. Jin, M.A. Hillmyer, P.W. Carr, Anal. Chem. 74 (2002) 4634]. Unfortunately, this highly crosslinked SAM phase is much more silanophilic than a conventional sterically protected octadecyl silane phase. 29Si CP-MAS NMR analysis shows that the high concentration of silanol groups in the self-assembled monolayer causes the increased retention and poor peak shape of basic solutes. In this work dimethyl-chloromethyl-phenylethylchlorosilane (DM-CMPES), a silane with only a single reactive silyl chloride group was tested as an alternative to chloromethyl-phenyethyltrichlorosilane (CMPES) as the basis for forming the starting phase. Most importantly this "conventional" silanization step (i.e., a non-SAM silanization) was followed by a Friedel-Crafts reaction using aluminum chloride as the catalyst and styrene heptamer as the multi-valent crosslinker to form the surface DM-CMPES groups into a network polymer which is fully confined and attached to the surface. An octyl (C8) derivative of the hypercrosslinked (HC) dimethyl-chloromethyl-phenylethyl (DM-CMPES) surface-confined stationary phase was synthesized to demonstrate the potential of a Friedel-Crafts based approach to making high efficiency, acid and thermally stable polymerized phases on silica with selectivity closer to conventional aliphatic phases. The stability of the retention factors of these phases under very aggressive conditions (5%, (v/v) trifluoroacetic acid and 150 degrees C) are compared to that of a sterically protected octadecylsilane (ODS) phase. The comparisons show that the long term stability of highly crosslinked DM-CMPES phases in acid is superior to the conventional phase. The HC-C8 phase is even more stable in acid than the HC-styrene heptamer DM-CMPES phase on which it is based. Additionally, the efficiency and peak shape of several prototypical bases under acidic (0.1% TFA, pH 2.0) elution conditions are discussed. The column dynamics and thermodynamic characteristics of the HC-C8 phase were investigated to demonstrate the chromatographic utility of this ultra-stable phase. Inverse size exclusion chromatography and flow studies of the HC-C8 and the sterically protected C18 stationary phases indicate the absence of pore plugging and quite good (nearly 100,000 plates/m) chromatographic efficiency. Further chromatographic investigations show that the HC-C8 stationary phase behaves as a typical reversed phase material. The HC-C8 stationary phase offers unique chromatographic selectivity for certain classes of analytes compared to both alkyl and phenyl bonded phases.     

Synthesis of cation-exchange stationary phases using an adsorbed polymeric coating

We have prepared several silica-based cation-exchange materials that were suitable for the high-performance liquid chromatography of basic proteins. Two synthetic routes were examined. Central to both procedures was the adsorption of a low molecular weight polyamine. One method crosslinks the adsorbed polyamine with a multifunctional oxirane, which is then extensively derivatized with a monomeric cyclic anhydride. The second involves an adsorbed uncrosslinked polyethyleneimine layer which is reacted with polyacrylic anhydride, thereby crosslinking and imparting anionic character simultaneously. The resulting media prepared by either of these methods bound more than 40 mg of hemoglobin per gram of support depending on the reaction conditions. These cation-exchange stationary phases also exhibited good chromatographic performance, successfully resolving (horse heart) cytochrome c and lysozyme. Two of the more promising support materials were effectively used to isolate cytochrome c553 from a crude extract of cyanobacteria.     

Preparation of stationary phases for open-tubular capillary electrochromatography using the sol-gel method

Capillary electrochromatography requires the deposition of a stationary phase inside the capillary. In this paper the sol-gel method is proposed for this purpose. The gels were prepared externally and injected into a fused-silica capillary, where anchorage to the capillary wall was possible through condensation reactions between the silanol groups of the capillary wall and the residual silanol groups the gel. Contrary to a commonly used practice, alkaline pretreatment of the inner capillary wall prior to the introduction and anchoring of the gel was found to be only marginally effective in improving the mechanical stability of the column. The influence of various parameters, such as the pH, the water content, the presence of alcohol (ethanol) on the formation of tetraethoxysilane (TEOS)-n-octyltriethoxysilane (C8-TEOS) hybrid gels of varied composition is discussed. The pH and the amount of water present were found to be the determining factors in the preparation of a stable gel with the desired mechanical and chromatographic properties. By carrying out the gel formation at 80 degrees C, capillary columns could be produced in 2.5 h. While an acidic pH was required during (external) gel formation, subsequent treatment of the gel inside the capillary with an alkaline solution ('aging') was found to improve separation and stationary phase capacity significantly. The capillary columns were subsequently used to separate a mixture of polycyclic aromatic hydrocarbons in less than 3 min.     

Synthesis and evaluation of silica hydride‐based fluorinated stationary phases

Two novel silica hydride‐based fluorinated bonded phases have been synthesized using a hydrosilation procedure to test combined fluorine and hydride selectivity. The bonded moieties were characterized by elemental and spectral analysis. Chromatographic testing was done using hydrophilic analytes in the aqueous normal phase mode. At higher amounts of the nonpolar solvent in the mobile phase, there should be increased retention for solutes such as acids, bases and other polar compounds, whereas nonpolar solutes can be retained when water is increased as in RP chromatography. The synergistic effects of the fluorinated phase selectivity and aqueous normal phase retention on a hydride surface have been explored for small polar molecules. The stability and repeatability of the hydride‐based fluorinated stationary phases were evaluated. The use of acetone as the organic component in the mobile phase was also tested.     

Synthesis and characterization of new organometallic complexes bonded stationary phases for high-performance liquid chromatography

Summary The preparation, characterization and potential liquid chromatographic applications of various organometallic iron complexes silica stationary phases are presented. These new supports are synthesized by covalently linking ferrocene, as well as some of its cationic derivatives, to appropriately derivatized silica support matrices. These columns exhibit moderate to high selectivity towards the separation of polycyclic aromatic hydrocarbons (PAHs). A charge transfer retention mechanism has been proposed. A comparison with a reference stationary phase, 3,5-dinitrobenzamide (DNB), to quantify the acceptor power of the new stationary bonded phases, is also reported. Finally, the effect of varying the derivatives of the bonded metallocene on PAHs retention is discussed.     

Synthesis and chromatographic properties of HPLC chiral stationary phases based upon β-cyclodextrins

Chromatographia - β-cyclodextrin chiral stationary phases (CD-CSP) having different spacer lengths have been prepared and evaluated (retention, selectivity, resolution) for the liquid...     

Synthesis and characterization of oligocyclosiloxanes via the hydrosilation of vinylsilanes and vinylsiloxanes with heptamethylcyclotetrasiloxane

The preparation of oligocyclosiloxanes via hydrosilation reactions has been investigated. Hydrosilation reactions employing heptamethylcyclotetrasiloxane and vinyl-containing silanes and siloxanes yielded a variety of oligocyclosiloxanes with various numbers of cyclosiloxane rings connected through tri-, tetra-, penta-, hexa-, hepta-, or octafunctional junctions. The oligocyclosiloxanes were characterized by gas chromatography, IR spectroscopy, ²⁹Si-, ¹H-, and ¹³C-NMR spectroscopy, elemental analysis, and vapor phase osmometry. The polycyclics were redistributed with hexamethyldisiloxane and the resulting fragments analyzed by gas chromatography. All results were consistent with the formation of isomeric oligocyclosiloxanes due to a lack of absolute regioselectivity of the hydrosilation reaction. © 1993 John Wiley & Sons, Inc.     

Liquid chromatographic method for enantiopurity control of alaptide using polysaccharide stationary phases

Separation of veterinary drug alaptide ((S)-8-methyl-6,9-diazaspiro(4,5)decane-7,10-dione) from a chiral impurity (R-enantiomer) was developed. Five chiral columns (three amylose and two cellulose type) were evaluated in a reversed-phase system. Three of them offered satisfactory enantiomeric resolution. Finally, three methods were validated and proved to be applicable for the determination of a chiral impurity content below 0.1% (method A: 3-AmyCoat column, tris-[3,5-dimethylphenyl]carbamoyl amylose; mobile phase: water/methanol/propan-2-ol/butan-2-ol=75:20:3.5:1.5 v/v, flow rate: 0.40 mL/min; column temperature: 30 °C; method B: Chiralpak AS-3R, tris-[1-phenylethyl]carbamoyl amylose; water/acetonitrile=80:20 v/v, 0.40 mL/min; 40 °C; method C: Chiralcel OZ-3R, tris-[3-chloro-4-methylphenyl] carbamoyl cellulose; water/acetonitrile=80:20 v/v, 0.40 mL/min; 40 °C). Some decrease in efficiency with repeated sample injections was observed for the 3-AmyCoat column. The resistance to mass transfer in the stationary phase increased probably due to the change in chiral selector conformation. This effect was considerably suppressed by propan-2-ol or to a greater extent by butan-2-ol added to a mobile phase. Simple regeneration was also suggested to recover efficiency of the column.     

HPLC-MS/MS enantioseparation of triazole fungicides using polysaccharide-based stationary phases

The enantiomeric separation of 21 triazole fungicides was carried out on four polysaccharide-derived chiral stationary phases in the reversed phase separation mode using high performance liquid chromatography coupled with tandem mass spectrometry. All fungicides were detected in electrospray ionization (ESI) positive mode with selected reaction monitoring (SRM). Complete enantioseparation was achieved for 21 fungicides except for difenoconazole based on cellulose tris (3,5-dimethylphenylcarbamate) and cellulose tris (3-chloro-4-methylphenyl carbamate) columns by optimizing experimental conditions including mobile phase and column temperature. Mobile phase was 0.1% formic acid aqueous solution mixed with methanol or acetonitrile in different proportions. Among all the fungicides, 15 with two enantiomers and three with four stereoisomers (bitertanol, bromuconazole, and cyproconazole) were successfully separated at 25°C. Enantioseparation for the other three fungicides (propiconazole, triadimenol, and difenoconazole) with four stereoisomers could be achieved by changing the column temperature from 10 to 40°C. Propiconazole and triadimenol were enantioseparated on baseline at 40 and at 35°C, respectively, and difenoconazole was enantioseparated partially with the R(s) > 1.1 at 25°C. Moreover, linearities and limits of detection (LODs) of 21 fungicides except for difenoconazole were studied, showing coefficients of determination (R(2)) higher than 0.99 and LODs lower than 2.5 μg/L.