Anthracene silica gel,a new polycyclic-aromatic-bonded stationary phase for HPLC

Summary N(9-methylanthracene)aminopropyl silica gel is introduced as a new reversed-phase packing material. It has specific selectivity for some aromatic compounds. In this paper the anthracene phase is compared with commercial octadecyl-, phenyl- and aminopropyl-bonded silica gels.     

CP/MAS NMR investigations of silica gel surfaces modified with aminopropylsilane

Summary Aminopropyl chemically bonded phases for high performance liquid chromatography (HPLC) have been prepared using mono- and trifunctional methoxyor ethoxysilanes. Three types of silica gel with different surface characteristics were used as support for the chemically bonded phases (CBPs). Surface characteristics of the packings before and after chemical modification were determined by porosity parameters, elemental analysis and CP/MAS NMR spectroscopy.²⁹Si and¹³C CP/MAS NMR investigations gave informations about different interactions between aminosilyl ligands and/or these ligands and/or water molecules condensed in the pores of the silica gel surface. With decreasing pore diameter of the silica gel the proportion of protonated aminopropyl ligand increases.     

Behaviour of antiepileptic drugs in high-performance liquid chromatography on six kinds of silica gel modified with four types of phenyl groups

Summary The retention of and selectivity for some antiepileptic drugs were studied by high-performance liquid chromatography on 24 kinds of phenyl-modified silica gel, prepared with xylene solutions of phenyldimethylchlorosilane, diphenylmethylchlorosilane, triphenylchlorosilane or benzyldimethylchlorosilane, using six different silica gels with various mean pore diameters and specific surfaces. Using various CH₃CN — 0.01M KH₂PO₄ mixtures as eluents, the antipileptic drugs were separated on all the gels studied, but with different degrees of resolution. With increase in specific surface, the k’ values of some antiepileptic drugs increased. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Chromatographic properties of mixed cyano/ODS chemically bonded phases for HPLC

Summary A number of different cyano/ODS polymeric modified silica gel phases were prepared by simultaneous bonding of cyanoalkylsilanes and octadecylsilane. Cyano contents between 10–80% were obtained. High surface coverage (above 4 μmole/m²), as well as low hydroxyl content, characterised all phases. The modified silica was suitable for separations in both straight and reversed phase mode. Significantly reduced retention times were obtained in the reversed phase mode for high mole-cular weight polyaromatic compounds (Mw above 300).     

High-performance liquid chromatographic resolution of enantiomers on chiral amine-bonded silica gel

Summary A chiral stationary phase with an immobilized, optically active diamine was prepared for the separation of enantiomers. The synthesis of the phase was carried out by bonding (–)trans-1,2-cyclohexanediamine to microparticulate silica gel through the coupling agent 3-glycidoxypropylsilane. The resolution of the racemic compounds catechin, 2,2-dihydroxy-1,1binaphthyl and trans-1,2-cyclohexandiol, is reported.     

Retention characteristics of octadecylsilyl silica,trimethylsilyl silica and phenyldimethylsilyl silica in reversed-phase liquid chromatography

Summary The hydrophobic retention characteristics of stationary ligands for reversed-phase high-performance liquid chromatography have been evaluated from the slope (r-value) of the plots relating the capacity factors (log k) of selected aliphatic and aromatic compounds with the reciprocal of methanol concentration (log (1/[MeOH])) in aqueous mobile phase. Octadecylsilyl (ODS), trimethylsilyl (TMS) and phenyldimethylsilyl (phenyl) groups were selected as the stationary ligands bonded to silica support.On ODS or TMS silicas, unlike on phenyl silica, aliphatic compounds gave slightly larger r-values than aromatic compounds, indicating that the shape of the ligand recognizes the hydrophobic surfaces of aliphatic and aromatic solute molecules. On TMS and phenyl silicas, the degree of solute hydrophobicity contributing to its retention is about 90% and 85% of that on ODS silica, respectively. On the other hand, on TMS and phenyl silicas, the polar functional group on the solute molecule brought about a smaller decrease in retention than on ODS silica.     

Preparation of a polyglycol-C8 bonded phase and its characteristics

Summary A new bonded phase containing both non-polar C₈ and polar polyglycol chains was synthesized in two steps. Silica was first silanized with octyltrimethoxysilane and then the intermediate product was reacted with tetraethylene glycol to introduce the polar group. The bonded phase exhibits a selectivity different from that of C₈ or C₁₈ reversed phases with a mixed-mode retention mechanism. The silanol effect on the polyglycol-C₈ bonded phase was examined with the test mixture proposed by Engelhardt and co-workers. The experimental results indicate that the new bonded phase displays less silanol effect. The observed tailing of some ionizable species resulted mainly from their partial dissociation at the pH values of eluents close to thepK _a values of the solutes. Some potential applications of the bonded phase are also given.     

A comparison of temperature and mobile phase composition effects for bonded liquid crystal and polymeric stationary phases in HPLC

Summary Using two polycyclyic aromatic hydrocarbons as solutes, a comparison is made between a bonded liquid crystal stationary phase and a conventional polymeric C-18 phase. The bonded nematic liquid crystal phase was the silanized form of 4-[4-(allyloxy)benzoyl-oxy]biphenyl and the polymeric phase was Vydac 201TP. Both phases display shape and planarity selectivity as indicated by the results of the variable temperature and mobile phase composition studies. The slot theory of retention can be used to explain these results. However, the liquid crystal phase is more sensitive to molecular geometry, probably due to its more ordered structure on the surface. Variable temperature experiments which compare retention during both heating and cooling provides additional support for this conclusion. With the polymeric bonded C-18 phase, each solute had identical retention at the same temperature during both the heating and cooling cycles. On the bonded liquid crystal phase, measurable differences in retention were observed at identical temperatures depending on whether the column was heated or cooled. This effect is attributed to a degree of partially reversible disordering which occurs as the column temperature was increased. However, conditioning with the appropriate mobile phase can restore the original retention characteristics of the bonded liquid crystal phase.     

Dependence of the methylene selectivity on the composition of hydro-organic eluents for reversed-phase liquid chromatographic systems with alkyl bonded phases

Summary A theoretical treatment is presented which considers differences between the composition of the mobile phase and solvents that are incorporated into the bonded phase via preferential sorption. Equations are derived and used to analyze retention data for various homologs chromatographed under reversed-phase conditions using alkyl bonded phases and combinations of water-methanol, water-acetonitrile and watertetrahydrofuran as mobile phases. In the case of water-methanol the surface phase and bulk mobile phase compositions are similar. However, significant differences in composition between the two phases are observed when binary combinations of water-acetonitrile and water-tetrahydrofuran are used as the cluents.     

Retention behaviour in micro-HPLC on various types of glasses modified with phenyl groups

Summary The retention and selectivity behaviour of some antiepileptic drugs were studied by micro high-performance liquid chromatography on nine types of phenyl-modified glasses, prepared with xylene solution containing phenyldimethylchlorosilane (P), diphenylmethylchlorosilane (D) or triphenylchlorosilane (T), using three types of glass with various mean pore diameters and/or specific surface areas. From elemental analysis data for carbon, the maximum number of accessible phenyl surface groups per 100?² of glass (mean pore diameter: 335?, specific surface area: 69m²/g) in P, D and T gel was calculated to be 2.38, 1.58 and 0.76, respectively. Using various CH₃CN−0.01 M KH₂PO₄ mixtures as eluents, the antiepileptic drugs were separated on all the glasses studied, but with different degrees of resolution. With an increase in the specific surface area, the k′ values of some antiepileptic drugs also increased.     

Chiral bonded packings for liquid chromatography: Preparation and evaluation of N-acylated aminoacid packings based on aminopropylated silica gel

Summary N-Protected single amino acid residues have been chemically bound to aminopropylated silica gel to give chiral HPLC column packings. Surface coverages have been determined by both elemental and aminoacid analyses. In addition, the nature of the surface-bound groups has been studied by reflectance fourier transform infrared (FTIR) spectroscopy. The resolution of a range of racemic mixtures has been achieved using the column packings.     

Preparation and evaluation of phenylpropyl-treated porous glasses for high performance liquid chromatography

The retention and selectivity behavior of some aromatic hydrocarbons and alkylbenzenes on phenyl- or phenylpropyl-modified glasses have been studied by high performance liquid chromatography. From elemental analysis data for carbon, the maximum number of accessible phenyl or phenylpropyl surface groups per 100 Ų of glass (mean pore diameter, 157 Å; specific surface area, 213 m²/g) in the phenyl and phenylpropyl modified glasses were calculated to be 1.93 and 1.97, respectively. Using 60:40 methanol-water mixtures as eluents, the aromatic hydrocarbons or alkylbenzenes were separated on the glasses studied, but with different degrees of resolution.     

HPLC evaluation of mono-ol,butylphenyl and perfluorinated columns prepared via olefin hydrosilation on a silica hydride intermediate

Summary Three columns, mono-ol, butylphenyl and perfluorinated synthesized through the reaction of an olefin with a silica hydride intermediate, are evaluated for the separation of various model compounds. Two of the columns, mono-ol and perfluorinated, can operate in both the normal phase and reversed phase modes. Some effects of the hydride surface are detected, particularly for the perfluorinated phase. A four-component steroid mixture is tested on both the mono-ol and perfluorinated phases in the normal mode.     

Selectivity studies on branched and linear alkyl bonded stationary phases in reversed-phase liquid chromatography

Summary Reversed-phase liquid chromatographic retention characteristics for the sixteen acyclic C₁−C₅ N-alkylbenzamide congeners were measured on various branched and linear, alkyl bonded hydrocarbon stationary phases. Retention factors, k′, were determined in acetonitrile-water mobile phase compositions on ethyl, n-octyl, n-dodecyl, n-octadecyl, 1-ethyladamantyl, 4-butyloctyl, and 2,4,4-trimethylpentyl stationary phases. Statistical analysis of the two main effects investigated — type of stationary phase and percentage of organic modifier (acetonitrile) in the mobile phase — described greater than ninety percent of the variability in the data for most of the comparisons. Selectivity effects due to variation in the mobile phase dominated the results.     

Correlation between retention and the C18 silica surface coverage in reversed-phase liquid chromatography

Summary Members of three homologous series and five non-homologous solutes with various functional groups were chromatographed on a series of well-characterized C₁₈ reversed stationary phases with a range of methanol-water mobile phases. Measured capacity factors of solutes were correlated with the concentration of C₁₈ ligands on the stationary phase. A linear relationship with the slope depending on a solute molecular structure and the volume fraction of methanol in the mobile phase was obtained. A method for the evaluation of phase ratio is also proposed. Presented at the 17th International Symposium on Chromatography, September 25–30, 1988, Vienna, Austria.     

New polymer coated anion-exchange HPLC-phases: Immobilization of poly (2-hydroxy, 3N-ethylenediamino) butadiene on silica and alumina

Summary PHEB=POLY(2-hydroxy, 3N-ethylenediamino)butadiene has been synthesized. This unsaturated pre-polymer can be immobilized on different, preferably inorganic supports of suitable porosity such as silica or alumina by cross-linking. Weak anion-exchange phase are obtained. Such phases do not have the disadvantages of organic polymeric phases, regarding pressure stability and swelling by solvent influence. The ion-exchange capacity and the retentivity of IE-separations can deliberately be varied via the thickness of the cross-linked polymer layer. Alumina can also be coated with PHEB applying the same cross-linking procedure. Such phases exhibit an exceptional chemical stability even when operated with mobile phases at very high pH-values. Separations of excellent selectivity are achieved for inorganic anions, as well as with mixtures of aliphatic and aromatic free acids also including hydroxy- (mono- or poly)carboxylic acids.     

Effect of temperature on the mechanism of retention of fullerenes in liquid chromatography using various alkyl bonded stationary phases

Summary The temperature-dependency of the separation of fullerenes in liquid chromatography (LC) has been examined using various alkyl bonded stationary phases. It has been found that a maximum retention temperature exists with long alkyl bonded stationary phases, whereas there is no similar effect with the newly synthesized alkyl bonded phases which have two phenyl groups at the base of the bonded phase. The interpretation of the retention behavior of fullerenes in the low temperature region on alkyl bonded stationary phases is discussed using information obtained by CP-MAS solid-state NMR spectroscopy and LC.     

Influence of silica gel pretreatment and bonding technique on PAH selectivity of octadecyl bonded phases

Summary The effect of silica gel pretreatment and of different bonding techniques, on the chromatographic properties of octadecyl bonded phases is described. The experiments show that a monomeric octadecyl bonded phase, prepared from silica gel that has not been treated before bonding, has a poor selectivity towards polycyclic aromatic hydrocarbons (PAHs) and shows high adsorption of basic components. Silica gel treatment before the bonding of the monofunctional silane does not give a substantial improvement. Polymeric phases on the other hand show a much better PAH selectivity and a lower adsorptivity towards basic components. Further improvement can be achieved with the polymeric modification if silica gel treatment is carried out before bonding.