César R. Silva

New generation of sterically protected C18 stationary phases containing embedded urea groups for use in high-performance liquid chromatography

New monofunctional C18 urea stationary phases with sterically protecting dimethyl and diisopropyl groups were prepared by a single step modification process. ProntoSil spherical silica (3 microm) was chemically modified with the monofunctional ethoxysilanes, [(3-octadecylurea)propyl]dimethyl and [(3-octadecylurea)propyl]diisopropyl ethoxysilanes. The phases were characterized by elemental analysis, infrared and solid-state 29Si and 13C NMR spectroscopies. Chromatographic characterizations of the new phases in 50x3.9 mm HPLC columns were performed by the separation of two different test mixtures, containing nonpolar, polar and highly basic compounds.

Development of new urea-functionalized silica stationary phases: Characterization and chromatographic performance

New urea-functionalized silica stationary phases were prepared by a single-step surface modification through reaction of LiChrosorb Si100 (5 microm particle size) with a homologous series of alkoxysilanes, synthesized in our laboratory, with the general formula (CH3CH2O)3Si(CH2)3NHC(O)NH(CH2)nCH3, where n=4, 6 and 11. The modified silicas were characterized by elemental analysis of carbon and nitrogen, solid-state 29Si- and 13C-cross polarization magic angle spinning nuclear magnetic resonance and nitrogen adsorption isotherms at 77 K. Chromatographic evaluation of the three urea-functionalized silicas in 150x3.9 mm I.D. HPLC columns was carried out by the separation of a test mixture composed of uracil, acetophenone, benzene, toluene and naphthalene, using acetonitrile-water as mobile phase. These new stationary phases, with embedded polar urea groups, are very promising when compared with amide phases prepared by the conventional two-step modification process. A single-step reaction process silica modification is better for obtaining a well-characterized and homogeneous modified surface.

Preparation of a new C18 stationary phase containing embedded urea groups for use in high-performance liquid chromatography

A new C18 urea stationary phase was prepared by a single-step modification process through the reaction of ProntoSil spherical silica (3 microm, Bischoff) with the trifunctional alkoxysilane (CH3CH2O)3-Si-(CH2)3-NH-C(O)-NH-(CH2)17-CH3, prepared in our laboratory. The phase was characterized by elemental analysis and solid-state 29Si and 13C nuclear mangnetic resonance spectrometry. Chromatographic evaluations of the new phase in 150 x 3.9 mm HPLC columns involving the separation of different test mixtures, indicate good performance for both polar and basic mixtures and also show promising results for further applications.

Evaluation of the applicability and the stability of a C18 stationary phase containing embedded urea groups.

Chromatographic evaluations of a new C18 urea phase in 150x3.9 mm HPLC columns, involving the separation of different test mixtures, indicate good performance for both polar and basic compounds when compared with a commercial C18 reversed phase and also show promising results for the separation of some herbicides. An aging study was performed by passing a potassium phosphate mobile phase buffered at pH 7 through 50x3.9 mm HPLC columns. The column stability was evaluated by means of the chromatographic parameters obtained for the separation of some compounds of the Neue test mixture, containing apolar, polar and highly basic analytes. The applicability of the new C18 urea phase was evaluated with a herbicide mixture.

New stationary phase prepared by immobilization of a copper-amine complex on silica and its use for high performance liquid chromatography

Chromatographic silica (10 μm) was chemically modified with the silylating agent: [3-(2-aminoethyl)aminopropyl]trimethoxysilane (AEAPTS). The reaction product was characterized by elemental analysis and infrared and 13C and 29Si NMR spectra. The chemically modified silica was treated with Cu(II) in methanol medium. This cation was strongly adsorbed through complexation by the pendant ethylenediamine groups attached to the silica surface. The complex formed on the silica surface was shown to be stable in both aqueous and non-aqueous media. The aim of Cu(II) immobilization is to use this new material as a stationary phase in High Performance Liquid Chromatography (HPLC). Separations of synthetic mixtures of aromatic amines and of polyaromatic hydrocarbons were undertaken using 150×3.9 mm HPLC columns packed with the modified silica, with and without copper ions, to follow the influence of the cation on the chromatographic separation and to verify the efficiency of the new stationary phase for HPLC.

The Search for More pH Stable Stationary Phases for High Performance Liquid Chromatography

High performance liquid chromatographic (HPLC) separations are largely carried out using reversed phase conditions with stationary phases based on silica. A serious problem with these stationary phases is the tendency of silica to dissolve in high pH solutions often needed to separate basic compounds. The literature reports many different ways that have been tried to resolve this problem. This paper reports the results obtained in our laboratory with stationary phases prepared using silica supports having a layer of a metal oxide (zirconia or titania) attached on their surfaces, followed by immobilization of a polysiloxane or by organofunctionalization with a trimethoxyalkylsilane. Stability tests, also developed in our laboratory, indicate that the metal oxide layer increases the HPLC column lifetimes by making the stationary phase less susceptible to dissolution.

Anion separations for liquid chromatography using propylpyridinium silica as the stationary phase.

This work describes the characterization and potential applications of a silica-based anion-exchange phase prepared by a two-step modification process that incorporates a propylpyridinium group. The effects of pH and eluent concentration on anion separation were examined using 150 mm × 3.9 mm HPLC columns packed with the new phase. The mobile phase pH values ranged from 3.8 to 6.6 using phthalic acid/Tris solutions. The best separation was achieved using 2.5 mmol L(-1) phthalate/2.4 mmol L(-1) Tris solution at pH 4.2 as mobile phase with non-suppressed conductivity detection. The new stationary phase was used for the separation of some inorganic and organic anions showing good resolution. The stability of the silica-based anion exchange phase was also evaluated. Analytical curves, for concentrations ranging from 0.25 to 10 mg L(-1) for the inorganic anions chloride, nitrite, bromide and nitrate, showed good linear correlations (r>0.998). The method was tested with certified rainwater samples. The measured and certified values were in good agreement, indicating that the new phase holds significant promise for the analysis of these anions in environmental samples.

Novas fases estacionárias à base de sílica para cromatografia líquida de alta eficiência

The present work reviews recent advances in the preparation of new reversed phase packing materials such as sterically protected, bidentate, hybrid organic-inorganic and monolithic phases and phases containing embedded polar groups. The bonding chemistry involved in the preparation of these phases as well as their advantages over conventional C8 and C18 reversed phases are discussed. Understanding the reasons behind the development of these newer column packings helps analysts select the best stationary phase for a given application.

A new generation of more pH stable reversed phases prepared by silanization of zirconized silica

To further extend our studies in the search for reversed phases with enhanced durability at high pH, zirconized silica has now been explored as an alternative support. The synthesis of the new stationary phases involves silanization of a zirconium-modified silica support with a C(18) trifunctional silane, followed by endcapping. The chromatographic properties of the C(18) phases based on zirconized silica are similar to their titanized silica counterparts. Accelerated high pH stability tests, using phosphate mobile phases and elevated temperature, have shown that the zirconized silica phases have promising advantages not only over similarly prepared non-metalized phases but also over titanized silica C(18) phases.

Development of a polymer-coated stationary phase with improved chemical stability in alkaline mobile phases.

An endcapped stationary phase is prepared by thermal immobilization of poly(methyltetradecylsiloxane) (PMTDS) onto a doubly zirconized silica support followed by endcapping using a mixture of hexamethyldisilazane and trimethylchlorosilane. The preparation of the Si-Zr(PMTDS)ec phase shows good repeatability with RSD <3.0% for carbon loadings and column efficiency. This new stationary phase has a lower density of residual hydroxyl groups, according to spectroscopic methods while basic compounds from the Tanaka and Engelhardt test mixtures are eluted with essentially symmetric peaks. Furthermore, the stability of the Si-Zr(PMTDS)ec stationary phase, measured using an accelerated aging test, is twice as great as the stability of a similar nonendcapped phase. The new phase shows promise for the separation of basic pharmaceuticals.