Evaluations of the BET, I-point, and α-plot procedures for the routine determination of external specific surface areas of highly dispersed and porous silicas.

Nitrogen adsorption isotherms of silicas and other oxidic materials are distorted by the presence of micropore adsorption and capillary condensation. This distortion affects the determination of the specific area of the material, depending on the chosen calculation procedure. Correction of the initial (total) isotherm for micropore capacity decreases or eliminates this source of error to give a useful estimate of the external surface area. In the present work, 26 silica-based adsorbent materials were studied to obtain total and external specific surface areas by the Brunauer-Emmett-Teller (BET), I-point, and α-plot procedures, using the micropore capacities from the α-plots to obtain the corrected (external) isotherms. Errors in the specific surface areas due to the presence of micropores are given by the equation ΔsA = 3.267 (m(2)/cm(3) STP) sV(mic), where sA is the specific surface area in m(2)/g and sV(mic) is the micropore capacity in cm(3) STP/g. A consistent set of conversion factors was obtained by which the external specific surface area obtained using one of these procedures can be converted, with part-per-thousand precision, to either of the others. Although the I-point procedure presents the advantage of not requiring a defined p/p(0) range, the α-plot procedure is recommended for routine determinations of external specific areas of silicas and other oxidic materials, except for cases in which the shapes of the adsorption isotherms of the sample and the reference differ significantly from one another in the p/p(0) range used for the determination.

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.

Stability studies of stationary phases from poly(methyltetradecylsiloxane) sorbed and immobilized onto metalized and unmodified silicas.

Stationary phases for RP-HPLC were prepared from metalized (titanized and zirconized) and unmodified silica particles using sorbed and immobilized poly(methyltetradecylsiloxane) (PMTDS). Different immobilization procedures, such as gamma irradiation and thermal treatments, were used for the preparation of the immobilized PMTDS phases. The stabilities of these stationary phases were evaluated by passing alkaline (pH 10) mobile phase through 60 mm x 3.9 mm columns of the different phases, with periodic tests to evaluate chromatographic performance. The results show that higher stabilities were obtained with stationary phases based on PMTDS immobilized on zirconized silica, these phases being 50% more stable than their titanized silica counterparts and 400% more stable than those based on unmodified silica. These supports provide higher chemical stability to the laboratory-made stationary phases, when compared with chemically bonded silica-based phases.

Preparation of stationary phases for reversed-phase high-performance liquid chromatography using thermal treatments at high temperature.

Batches of poly(methyloctylsiloxane) (PMOS)-loaded silica were prepared by deposition from a solution of PMOS into the pores of HPLC silica. Portions of PMOS-loaded silica were subjected to a thermal treatment at 100 degrees C for 24h (condition 1) in a tube furnace under a nitrogen atmosphere. After that, the material was heated for 4h at higher temperatures (150-400 degrees C) (condition 2). Heating at higher temperatures produces polymer bilayers. Non-immobilized and thermally treated stationary phases were characterized by percent carbon, (29)Si cross-polarization magic angle spinning nuclear magnetic resonance spectroscopy and reversed-phase chromatographic performance. The results show that thermal treatment between 150 and 300 degrees C accelerates the immobilization process, possibly due to some bond breaking of the polysiloxane, with formation of strong linkages to the surface of the support, resulting in more complete coverage of the silica. The chromatographic results show an improvement of efficiency with the increase of the temperature of condition 2 up to 300 degrees C and an increase in the resolution of the components, mainly for the phase heated at 300 degrees C. Such results demonstrate that a two-step thermal treatment (100 degrees C then 150-300 degrees C) produces stationary phases with good properties for use in reversed-phase high-performance liquid chromatography.

Preparation and characterization of poly(methyltetradecylsiloxane) stationary phases immobilized by gamma radiation onto zirconized silica.

The preparation of stationary phases with enhanced chemical stability in alkaline eluents has been the principal objective of many chromatographers. New and improved silica substrates and advanced chemical modification methods are among the possibilities being investigated to reach this objective. The present work has evaluated these two possibilities for new stationary phases. First, the silica surface was modified by reaction with zirconium tetrabutoxide to produce zirconized silica particles having about 21% (w/w) of zirconium. Then poly(methyltetradecylsiloxane) (PMTDS) was immobilized onto this surface using different doses (50-120 kGy) of gamma radiation. These new phases were characterized using elemental analysis and infrared and solid-state (29)Si-nuclear magnetic resonance (NMR) spectroscopies. These new stationary phases presented column efficiencies of about 68,000 plates m(-1), symmetric peaks for apolar compounds and retention factors that depend on the irradiation dose and show improved stability in high pH mobile phases. The separation of several pharmaceuticals at pH 11 is presented.

An overview of the chromatographic properties and stability of C18 titanized phases.

Phases based on titanized silicas are an alternative in the search for HPLC stationary phases with enhanced pH stability. This technology explores the chemical modification of the bare silica surface by grafting a titanium oxide layer with the objective of retarding the dissolution of the support in alkaline mobile phases, followed by C18 silanization. The present manuscript describes recent work on the development of chemically bonded titanized phases, including phases containing embedded urea groups, and phases prepared both in the absence and in the presence of a monolayer of water preadsorbed onto the bare silica. The advantages and disadvantages of these alternative C18 titanized phases are discussed, taking into account their chromatographic properties accessed by some common test procedures. Column lifetimes, measured by accelerated aging tests using aggressive conditions, such as high-pH phosphate mobile phases and elevated temperature, are also discussed.

Immobilized polymeric stationary phases using metalized silica supports.

Immobilized presynthesized polymers on porous metalized (zirconized or titanized) silica particles as new stationary phases with improved chemical stability for RP-HPLC are reviewed. The preparations using different polymers, such as poly(methyloctylsiloxane), poly(methyltetradecylsiloxane), and poly(butadiene), different immobilization steps (gamma radiation, thermal treatment, and microwave radiation), and the chromatographic performances of these phases for polar, apolar, acidic, and basic compounds are discussed. The stability of some of these stationary phases using alkaline mobile phases is also presented.

New stationary phases for high-performance liquid chromatography based on poly(methyltetradecylsiloxane) thermally immobilized onto zirconized silica.

The modification of silica with zirconium oxide followed by sorption and thermal immobilization of poly(methyltetradecylsiloxane) (PMTDS) is used to prepare a reversed stationary phase for high-performance liquid chromatography. The thermal immobilization of PMTDS on zirconized silica was optimized using a central composite design. The new stationary phase was characterized by spectroscopic and chromatographic methods. Stationary phases with good reproducibility and good chromatographic performance for various compounds were obtained. PMTDS thermally immobilized on zirconized silica presented quite significant chemical stability at pH 10 and 50 degrees C.

Influence of the TiO2 content on the chromatographic performance and high pH stability of C18 titanized phases.

To extend pH stability, protective metal oxide layers, such as titanium oxide, that are more stable in alkaline medium, can be chemically bonded to the chromatographic silica surface prior to reaction with silanes. In the present work, the influence of the titanium oxide content on the chromatographic performance was investigated by synthesizing a C18 phase onto a doubly-titanized silica support and comparing its chromatographic performance with a C18 phase on singly-titanized silica. The Engelhardt and Tanaka test mixtures were used for chromatographic characterizations using short HPLC columns. The column lifetimes of these titanized phases were also compared by performing accelerated aging tests at 50 degrees C using aggressive phosphate mobile phases at pH 10.

High-performance liquid chromatographic stationary phases based on polysiloxanes with different chain lengths thermally immobilized on silica supports.

Reversed phases for high-performance liquid chromatography (RP-HPLC) were obtained by thermal immobilization of polysiloxanes having different length chains (C1, C8 and C14) onto HPLC silica particles. The importance both of percent loading of the stationary phase promoted by each immobilization procedure and of the length of the lateral chain of the polymer on the chromatographic performances of the phases obtained is compared and discussed.

Preparation and characterization of a new C18 urea phase based on titanized silica.

A new stationary phase containing embedded urea groups (-NH-C(O)-NH-) was prepared by a procedure based on the synthesis of a trifunctional C18 urea-alkoxysilane, followed by modification of titanized silica and further endcapping to evaluate if the embedded group would minimize the higher retention and tailing for basic compounds seen with C18 titanized silica phases. Infrared, 13C and 29Si spectroscopies were employed to characterize the C18-urea titanized silica phase. Chromatographic evaluations used hydrophobic, polar and basic compounds to verify the effects of the polar urea groups embedded in the C18 urea phase. The chromatographic parameters, especially for the separation of basic compounds, compare favorably with those obtained on a C18 titanized silica stationary phase, prepared by silanization of titanized silica with octadecyltrimethoxysilane.

Physisorbed water layer formation on fully hydroxylated mesoporous silicas.

Kinetic adsorption isotherms were obtained by gravimetric determination of water adsorption into fully hydroxylated mesoporous silicas using samples exposed to controlled humidity air at 22+/-2 degrees C. Twenty kinetic isotherms at several relative humidities (11, 33, 43, 51, 75, and 85%) were obtained with 11 different batches of silica using this simple procedure to obtain quantitative information on the formation of H2O adsorbates. The H2O surface concentrations obtained from the plateau data of individual kinetic adsorption isotherms at 43 and 51% RH, typically precise to about +/-1%, show that a complete monolayer is formed with negligible second-layer adsorption at these relative humidities. This monolayer has a surface concentration of 7.68+/-0.30 micromol H2O/m2, which is lower than the quasi-equilibrium concentration at these relative humidities obtained by the conventional equilibrium-isotherm procedure. Comparison with the Kiselev-Zhuravlev concentration of silanol groups on fully hydroxylated silicas (7.6+/-0.8 micromol SiOH/m2) confirms 1:1 H2O:SiOH stoichiometry of this monolayer. The presence of partial-layer structures at 2.85+/-0.1 and 5.7+/-0.1 micromol H2O/m2 is suggested by isotherms at 11 and 33% RH, respectively, while a bilayer at approximately 14+/-1 micromol H2O/m2 is suggested by kinetic isotherms at 75 and 85% RH.

High-performance liquid chromatographic stationary phases based on poly(dimethylsiloxane) immobilized on silica.

This work describes the preparation and characterization of six stationary phases for high-performance liquid chromatography (HPLC) obtained by deposition of poly(dimethylsiloxane) (PDMS) in HPLC silica particles, followed by immobilization using different processes (thermal treatments, thermal treatment + microwave irradiation, self-immobilization + gamma irradiation and self-immobilization + microwave irradiation). The chromatographic parameters of all the phases were evaluated with a mixture of test compounds having varied natures (acid, basic and neutral). The stability of one of these phases was evaluated in both a neutral mobile phase and a higher pH mobile phase used at an elevated temperature, with promising results.

New stationary phase for anion-exchange chromatography.

This work describes the preparation of an anion-exchange phase based on silica, using a two-step modification process. First, 10 microm Davisil silica particles were silanized with chloropropyltrimethoxysilane to yield chloropropyl silica. The modified silica was then reacted with pyridine to produce positively charged propylpyridinium groups on the surface, the anion-exchange sites. The phase was characterized by thermogravimetric analysis and infrared and solid state 13C and 29Si NMR spectroscopies. HPLC separations of common inorganic anions, including chloride, nitrite, bromide and nitrate, were performed using 150 x 3.9 HPLC columns packed with the phase, using a phthalate buffer solution as mobile phase with non-suppressed conductivity detection. Efficiency and resolution were calculated and the results show that the new phase has significant promise for the analysis of these anions in environmental samples.

Titanized silicas, modified by C18, as promising stationary phases for high pH separations.

To enhance the high pH stability of silica based reversed phases, chemically bonded octadecyl phases were prepared through silanization of titanized silica particles containing approximately 14% titanium oxide on the surface. The present work describes some spectroscopic characterizations using infrared, solid-state 13C and 29Si nuclear magnetic resonance (NMR) and X-ray absorption spectroscopy (XAS). Chromatographic characterizations for the titanized phase as well as for a conventional C18 phase, based on the same silica support without titanization, are also described using three different test mixtures containing neutral, polar and basic compounds. After an artificial stability test at pH 10, the titanized phase was again characterized by elemental and X-ray fluorescence analyses to determine the remaining carbon and titanium contents. As an application to real world samples, the separation of some herbicides and highly basic drugs using buffered mobile phases are also shown.

Rapid method for evaluating reversed-phase high-performance liquid chromatography column stability.

A procedure is presented for the rapid evaluation of HPLC stationary phase stability at pH 8.4 or 10.1 using a temperature of 60 degrees C. Mobile phase (MeOH-0.1 mol l(-1) aqueous NaHCO3, 50:50, v/v) is continuously passed through the column with periodic injections of a test solution until the several chromatographic parameters of the resulting chromatograms are degraded. The tests were applied to several commercial and laboratory-made stationary phases. After degradation two of these phases, one commercial and one laboratory-made, were examined by elemental analysis and scanning electron microscopy to elucidate the degradation process.

Titanized silica-based stationary phases prepared with thermally and microwave-immobilized poly(methyloctylsiloxane).

Silica supports having their surface modified with titanium oxide were prepared and coated with poly(methyloctylsiloxane) (PMOS). Subsequently, immobilization of the polysiloxane was induced by thermal treatment or microwave radiation. The thermal treatment was carried out for different times (4, 8, 16 and 24 h) at temperatures ranging between 100 and 220 degrees C. For PMOS immobilization by microwave radiation, 452, 520 and 586 W power levels and exposure times of 5, 15 and 30 min were used. After extraction of non-immobilized polymer, the chromatographic properties of the phases were evaluated. The phase immobilized at 120 degrees C for 8 h presented the best chromatographic parameters, suggesting that the quantity of acidic hydroxyl groups on the support surface was reduced, resulting in fewer undesirable interactions of a basic solute with the silanols not removed or covered on the support surface.

Self-immobilization and/or thermal treatment for preparing silica-poly(methyloctylsiloxane) stationary phases.

Batches of poly(methyloctylsiloxane) (PMOS)-loaded silica were prepared by the deposition of PMOS, into the pores of HPLC silica. Portions of PMOS-loaded silica were allowed to remain at ambient temperature, without further treatment for 2, 9, 20, 31, 51, 105 and 184 days after preparation to undergo self-immobilization (irreversible adsorption of a layer of polymer on silica at ambient temperature in the absence of initiators). Other portions were subjected to a thermal treatment (100 degrees C for 4h) after 1, 2, 5, 7, 9, 15, 20, 25, 70, 111 and 184 days. Self-immobilized and thermally treated samples were characterized by % C, 29Si cross-polarization magic angle spinning (CP/MAS) NMR spectroscopy and reversed-phase column performance. The results show that thermal immobilization accelerates the distribution and rearrangement of the polymer on the silica surface. However, from the time that a monolayer has been formed by self-immobilization (approximately 100 days for PMOS on Kromasil silica), the thermal treatment does not alter this configuration and, thus, does not change the resulting chromatographic parameters.

Microwave-immobilized polybutadiene stationary phase for reversed-phase high-performance liquid chromatography.

Polybutadiene (PBD) has been immobilized on high-performance liquid chromatography (HPLC) silica by microwave radiation at various power levels (52-663 W) and actuation times (3-60 min). Columns prepared from these reversed-phase HPLC materials, as well as from similar non-irradiated materials, were tested with standard sample mixtures and characterized by elemental analysis (%C) and infrared spectroscopy. A microwave irradiation of 20 min at 663 W gives a layer of immobilized PBD that presented good performance. Longer irradiation times give thicker immobilized layers having less favorable chromatographic properties.

Influence of air on polybutadiene used in the preparation of stationary phases for high-performance liquid chromatography.

A 100 ml bottle of polybutadiene, PBD, was repeatedly exposed to air over a period of 6 months. Samples were taken at time zero (PBD-0), after 3 months (PBD-3) and 6 months (PBD-6). These samples were sorbed onto HPLC silica by an open-air solution-evaporation procedure, which involved exposure to the atmosphere for 6 days. Portions of the three sets of samples were used to compare self-immobilization and the effects of 100 degrees C thermal treatments in air or nitrogen on HPLC performance of the resulting phases. It is concluded that self-immobilization is enhanced by prior exposure of sorbed PBD to air and subsequent heating at 100 degrees C further enhances column performance. The best performance (10(5) plates m(-1)) resulted from 4 h heating of PBD-6 material in nitrogen.