Análises de área superficial específica deste material mostraram que a incorporação de titânio na... more Análises de área superficial específica deste material mostraram que a incorporação de titânio na sílica não afeta drasticamente as propriedades da sílica pura. A área superficial da sílica diminuiu em apenas 20%. Micrografias eletrônicas de varredura mostraram que, apesar ...
An endcapped stationary phase is prepared by thermal immobilization of poly(methyltetradecylsilox... more 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 a3.0% 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.
As separações por cromatografia líquida de alta eficiência (CLAE) são realizadas, na maioria das ... more As separações por cromatografia líquida de alta eficiência (CLAE) são realizadas, na maioria das vezes, em condições de fase reversa empregando fases estacionárias a base de sílica. Um problema sério com estas fases reversas é a tendência da sílica dissolver em soluções de pH elevado, frequentemente necessárias para separar compostos básicos. A literatura reporta várias maneiras diferentes de tentar solucionar este problema. Este artigo relata os resultados obtidos com fases estacionárias desenvolvidas no nosso laboratório, utilizando suportes de sílica, contendo uma camada de um óxido metálico (zircônia ou titânia) ligada à sua superfície, seguido por imobilização de um polissiloxano ou por organofuncionalização com um trimetóxialquilsilano. Testes de estabilidade, também desenvolvidos no nosso laboratório, indicaram que a camada de óxido metálico aumenta o tempo de vida da coluna cromatográfica, por tornar a fase estacionária menos susceptível à dissolução. 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.
To further extend our studies in the search for reversed phases with enhanced durability at high ... more 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.
A new material based on poly(methyltetradecylsiloxane) (PMTDS) thermally immobilized onto a silic... more A new material based on poly(methyltetradecylsiloxane) (PMTDS) thermally immobilized onto a silica support has been tested as a sorbent for the solid-phase extraction (SPE) from water of several pesticides used in soybean cultivation. The SPE methodology was developed and validated for six of these pesticides (imazethapyr, imazaquin, metsulfuron-methyl, bentazone, chlorimuron-ethyl and tebuconazole) according to the International Conference on Harmonization directives and the results were compared with those obtained with a commercial C18 SPE cartridge. The PMTDS-based sorbent gives results similar to the commercial sorbent with recoveries and precisions in agreement with directives for residue analysis. The quantification limits, after concentration, of all the pesticides evaluated were 1.0 g L −1 , below the levels imposed by the principal regulatory agencies. The PMTDS-based sorbent preparation is fast, easy and reproducible and the cartridges are less expensive than similar commercial SPE materials.
The preparation and characterization of zirconized silica has been investigated. The material was... more The preparation and characterization of zirconized silica has been investigated. The material was prepared via the reaction of silica with zirconium tetrabutoxide, optimized by a central composite design and response surface methodology. The new material was characterized by nitrogen adsorption-desorption investigations (BET/BJH) showing specific surface areas adequate for use as a chromatographic support. DRUVS, FTIR, XPS, XAS, XRF and SEM methods also were used to characterize the new material. It was shown that silica networks were not significantly modified with the introduction of zirconium. Surface analyses show that there is appreciable element enrichment at the surface, while significant changes in binding energies of Zr 3d, Si 2p, and O 1s have been detected. The above observations indicate that Si O Zr bonds were formed, with zirconium grafted onto the silica surface, yielding a support suitable for HPLC.
Kinetic adsorption isotherms were obtained by gravimetric determination of water adsorption into ... more 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.
Recebido em 31/5/00; aceito em 21/9/00 CHEMOMETRIC METHODS APPLIED TO THE SPECIATION OF AQUEOUS C... more Recebido em 31/5/00; aceito em 21/9/00 CHEMOMETRIC METHODS APPLIED TO THE SPECIATION OF AQUEOUS Cr(VI) SOLUTIONS. The equilibria, the spectra and the identities of the species of Cr(VI) that are present in aqueous solution have long been an active subject of discussion in the literature. In this paper, three different chemometric methodologies are applied to sets of UV/Visible spectra of aqueous Cr(VI) solutions, in order to solve a chemical system where there is no available information concerning the composition of the samples nor spectral information about the pure species. Imbrie Q-mode factor analysis, followed by varimax rotation and Imbrie oblique projection, were used to estimate the composition of Cr(VI) equilibrium solutions and, by combining these results with the k-matrix method, to obtain the pure spectra of the species. Evolving factor analysis and self modeling curve resolution were used to confirm the number of the species and the resolution of the system, respectively. Sets of 3.3×10-4 and 3.3×10-5 mol L-1 Cr(VI) solutions, respectively, were analyzed in the pH range from 1 to 12. Two factors were identified, which were related to the chromate ion (CrO 4 2-) and bichromate ion (HCrO 4-). The pK of the equilibrium was estimated as 5.8.
Misturas de etanol:água foram reavaliadas para uso como fases móveis para Cromatografia Líquida d... more Misturas de etanol:água foram reavaliadas para uso como fases móveis para Cromatografia Líquida de Alta Eficiência no modo fase reversa (CLAE-FR). A otimização das condições cromatográficas para as colunas C8 e C18 foram realizadas através de um planejamento fatorial o qual avaliou temperatura, concentração de etanol e vazão da fase móvel. Com a fase estacionária C8, as assimetrias dos picos não alteraram nos diferentes pontos do fatorial, enquanto os fatores de retenção e as resoluções diminuíram com o aumento da temperatura, na qual a viscosidade do etanol é mais baixa. Observações similares foram obtidas com a fase estacionária C18. A eficiência cromatográfica obtida com a fase móvel etanol:água na separação de misturas contendo compostos neutros e básicos foi comparada com aquelas obtidas usando fases móveis de metanol:água e acetonitrila:água. O etanol mostrou ser um bom modificador orgânico para CLAE-FR apresentando boas propriedades cromatográficas. Assim, considerando a menor toxicidade do etanol, a facilidade do seu descarte e seu custo favorável, etanol:água pode ser uma das escolhas de fase móvel para as diversas aplicações de CLAE-FR. Ethanol:water mixtures have been reevaluated for use as reversed phase mobile phases. Optimization of the chromatographic conditions for both C8 and C18 columns was carried out through a factorial design which evaluates temperature, ethanol concentration and mobile phase flow rate. With the C8 stationary phase, peak asymmetries were not significantly altered at the different points in the factorial design while the retention factors and resolutions were somewhat lower at higher temperatures, where the viscosity of EtOH is lower. Similar observations were obtained with the C18 phase. The efficiency of the ethanol:water mobile phase for the separation of mixtures containing neutral and basic compounds was compared with those obtained using methanol:water and acetonitrile:water mobile phases. Ethanol was shown to be a good organic modifier for RP-HPLC, with good chromatographic properties. This, considering the much lower toxicity of ethanol, the facility of its disposal, and its favorable cost, should make ethanol:H 2 O the mobile phase of choice for many RP-HPLC applications.
Phases based on titanized silicas are an alternative in the search for HPLC stationary phases wit... more 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.
Page 1. J Mol Evol (1987) 25:12-14 Journal of Molecular Evolution (~) Springer-Verlag New York In... more Page 1. J Mol Evol (1987) 25:12-14 Journal of Molecular Evolution (~) Springer-Verlag New York Inc. 1987 Formation of Organic Products in Self-Radiolyzed Calcium Carbonate Guadalupe Albarrfin, 1 Kenneth E. Collins, 2 and Car01 H. Collins 2 ...
Stability studies of stationary phases from poly(methyltetradecylsiloxane) sorbed and immobilized... more 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 mm63.9 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.
Poly(methyloctylsiloxane) (PMOS) was deposited on HPLC silica by a solvent evaporation procedure ... more Poly(methyloctylsiloxane) (PMOS) was deposited on HPLC silica by a solvent evaporation procedure and this material was then extracted, using a good solvent for the PMOS, after different time periods, to remove unretained liquid polymer. Solvent extraction data reveal changes which occur at ambient temperature as a function of the time interval between particle loading and extraction. The quantity of PMOS remaining on the silica after extraction, as determined by elemental analysis for carbon, is attributed to strongly adsorbed polymer. This phenomenon is termed self-immobilization. Solid-state 29Si NMR spectra indicate the formation of a silicon species with a different chemical shift than the original PMOS. These new signals are attributed to a combination of different adsorbed and chemically bonded groups.
A new stationary phase containing embedded urea groups (NH C(O) NH) was prepared by a procedure b... more A new stationary phase containing embedded urea groups (NH C(O) NH) was prepared by a procedure based on the synthesis of a trifunctional C 18 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 C 18 titanized silica phases. Infrared, 13 C and 29 Si spectroscopies were employed to characterize the C 18-urea titanized silica phase. Chromatographic evaluations used hydrophobic, polar and basic compounds to verify the effects of the polar urea groups embedded in the C 18 urea phase. The chromatographic parameters, especially for the separation of basic compounds, compare favorably with those obtained on a C 18 titanized silica stationary phase, prepared by silanization of titanized silica with octadecyltrimethoxysilane.
Batches of poly(methyloctylsiloxane) (PMOS)-loaded silica were prepared by the deposition of PMOS... more 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 • C for 4 h) after 1, 2, 5, 7, 9, 15, 20, 25, 70, 111 and 184 days. Self-immobilized and thermally treated samples were characterized by % C, 29 Si 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 (∼100 days for PMOS on Kromasil silica), the thermal treatment does not alter this configuration and, thus, does not change the resulting chromatographic parameters.
To extend pH stability, protective metal oxide layers, such as titanium oxide, that are more stab... more 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 C 18 phase onto a doubly-titanized silica support and comparing its chromatographic performance with a C 18 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 • C using aggressive phosphate mobile phases at pH 10.
The identities of the species of chromium(VI) that are present in aqueous solution, their spectra... more The identities of the species of chromium(VI) that are present in aqueous solution, their spectra and their equilibria, continue to be a subject of discussion in the literature. In this paper, the composition of the Cr(VI) equilibria was estimated from the UV-vis spectra of dilute potassium dichromate solutions, without any prior knowledge of the quantities of pure components, with the aid of Imbrie Q-mode factor analysis (Q-mode FA) followed by Varimax rotation and Imbrie oblique projection. Combining these results with the k-matrix method, it was possible to obtain the spectra of the individual Cr(VI) species. Sets of 3.3× 10 − 4 and 3.3× 10 − 5 mol l − 1 Cr(VI) solutions were studied. In the pH range from 1 to 12, two factors were identified, which were related to the two species, chromate ion (CrO 4 2 −) and bichromate ion (HCrO 4 −). When the analysis was extended to concentrated acid media, another factor appeared, which was related to chromic acid (H 2 CrO 4). No evidence for the dichromate ion (Cr 2 O 7 2 −) was seen at the Cr(VI) concentrations used. The spectra of the pure components were obtained and pK values for the first and second chromic acid dissociations were estimated as − 0.54 and 5.8, respectively.
... In environmental analysis, some theoretical models [3] and [4] predict, from both thermodynam... more ... In environmental analysis, some theoretical models [3] and [4] predict, from both thermodynamic and kinetic considerations, that chromium(III) as [Cr(H 2 O) 4 (OH) 2 ] + and chromium(VI) as CrO 4 2− are the dominant species found in natural waters. ...
A procedure is presented for the rapid evaluation of HPLC stationary phase stability at pH 8.4 or... more 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 • C. Mobile phase (MeOH-0.1 mol l −1 aqueous NaHCO 3 , 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.
Análises de área superficial específica deste material mostraram que a incorporação de titânio na... more Análises de área superficial específica deste material mostraram que a incorporação de titânio na sílica não afeta drasticamente as propriedades da sílica pura. A área superficial da sílica diminuiu em apenas 20%. Micrografias eletrônicas de varredura mostraram que, apesar ...
An endcapped stationary phase is prepared by thermal immobilization of poly(methyltetradecylsilox... more 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 a3.0% 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.
As separações por cromatografia líquida de alta eficiência (CLAE) são realizadas, na maioria das ... more As separações por cromatografia líquida de alta eficiência (CLAE) são realizadas, na maioria das vezes, em condições de fase reversa empregando fases estacionárias a base de sílica. Um problema sério com estas fases reversas é a tendência da sílica dissolver em soluções de pH elevado, frequentemente necessárias para separar compostos básicos. A literatura reporta várias maneiras diferentes de tentar solucionar este problema. Este artigo relata os resultados obtidos com fases estacionárias desenvolvidas no nosso laboratório, utilizando suportes de sílica, contendo uma camada de um óxido metálico (zircônia ou titânia) ligada à sua superfície, seguido por imobilização de um polissiloxano ou por organofuncionalização com um trimetóxialquilsilano. Testes de estabilidade, também desenvolvidos no nosso laboratório, indicaram que a camada de óxido metálico aumenta o tempo de vida da coluna cromatográfica, por tornar a fase estacionária menos susceptível à dissolução. 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.
To further extend our studies in the search for reversed phases with enhanced durability at high ... more 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.
A new material based on poly(methyltetradecylsiloxane) (PMTDS) thermally immobilized onto a silic... more A new material based on poly(methyltetradecylsiloxane) (PMTDS) thermally immobilized onto a silica support has been tested as a sorbent for the solid-phase extraction (SPE) from water of several pesticides used in soybean cultivation. The SPE methodology was developed and validated for six of these pesticides (imazethapyr, imazaquin, metsulfuron-methyl, bentazone, chlorimuron-ethyl and tebuconazole) according to the International Conference on Harmonization directives and the results were compared with those obtained with a commercial C18 SPE cartridge. The PMTDS-based sorbent gives results similar to the commercial sorbent with recoveries and precisions in agreement with directives for residue analysis. The quantification limits, after concentration, of all the pesticides evaluated were 1.0 g L −1 , below the levels imposed by the principal regulatory agencies. The PMTDS-based sorbent preparation is fast, easy and reproducible and the cartridges are less expensive than similar commercial SPE materials.
The preparation and characterization of zirconized silica has been investigated. The material was... more The preparation and characterization of zirconized silica has been investigated. The material was prepared via the reaction of silica with zirconium tetrabutoxide, optimized by a central composite design and response surface methodology. The new material was characterized by nitrogen adsorption-desorption investigations (BET/BJH) showing specific surface areas adequate for use as a chromatographic support. DRUVS, FTIR, XPS, XAS, XRF and SEM methods also were used to characterize the new material. It was shown that silica networks were not significantly modified with the introduction of zirconium. Surface analyses show that there is appreciable element enrichment at the surface, while significant changes in binding energies of Zr 3d, Si 2p, and O 1s have been detected. The above observations indicate that Si O Zr bonds were formed, with zirconium grafted onto the silica surface, yielding a support suitable for HPLC.
Kinetic adsorption isotherms were obtained by gravimetric determination of water adsorption into ... more 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.
Recebido em 31/5/00; aceito em 21/9/00 CHEMOMETRIC METHODS APPLIED TO THE SPECIATION OF AQUEOUS C... more Recebido em 31/5/00; aceito em 21/9/00 CHEMOMETRIC METHODS APPLIED TO THE SPECIATION OF AQUEOUS Cr(VI) SOLUTIONS. The equilibria, the spectra and the identities of the species of Cr(VI) that are present in aqueous solution have long been an active subject of discussion in the literature. In this paper, three different chemometric methodologies are applied to sets of UV/Visible spectra of aqueous Cr(VI) solutions, in order to solve a chemical system where there is no available information concerning the composition of the samples nor spectral information about the pure species. Imbrie Q-mode factor analysis, followed by varimax rotation and Imbrie oblique projection, were used to estimate the composition of Cr(VI) equilibrium solutions and, by combining these results with the k-matrix method, to obtain the pure spectra of the species. Evolving factor analysis and self modeling curve resolution were used to confirm the number of the species and the resolution of the system, respectively. Sets of 3.3×10-4 and 3.3×10-5 mol L-1 Cr(VI) solutions, respectively, were analyzed in the pH range from 1 to 12. Two factors were identified, which were related to the chromate ion (CrO 4 2-) and bichromate ion (HCrO 4-). The pK of the equilibrium was estimated as 5.8.
Misturas de etanol:água foram reavaliadas para uso como fases móveis para Cromatografia Líquida d... more Misturas de etanol:água foram reavaliadas para uso como fases móveis para Cromatografia Líquida de Alta Eficiência no modo fase reversa (CLAE-FR). A otimização das condições cromatográficas para as colunas C8 e C18 foram realizadas através de um planejamento fatorial o qual avaliou temperatura, concentração de etanol e vazão da fase móvel. Com a fase estacionária C8, as assimetrias dos picos não alteraram nos diferentes pontos do fatorial, enquanto os fatores de retenção e as resoluções diminuíram com o aumento da temperatura, na qual a viscosidade do etanol é mais baixa. Observações similares foram obtidas com a fase estacionária C18. A eficiência cromatográfica obtida com a fase móvel etanol:água na separação de misturas contendo compostos neutros e básicos foi comparada com aquelas obtidas usando fases móveis de metanol:água e acetonitrila:água. O etanol mostrou ser um bom modificador orgânico para CLAE-FR apresentando boas propriedades cromatográficas. Assim, considerando a menor toxicidade do etanol, a facilidade do seu descarte e seu custo favorável, etanol:água pode ser uma das escolhas de fase móvel para as diversas aplicações de CLAE-FR. Ethanol:water mixtures have been reevaluated for use as reversed phase mobile phases. Optimization of the chromatographic conditions for both C8 and C18 columns was carried out through a factorial design which evaluates temperature, ethanol concentration and mobile phase flow rate. With the C8 stationary phase, peak asymmetries were not significantly altered at the different points in the factorial design while the retention factors and resolutions were somewhat lower at higher temperatures, where the viscosity of EtOH is lower. Similar observations were obtained with the C18 phase. The efficiency of the ethanol:water mobile phase for the separation of mixtures containing neutral and basic compounds was compared with those obtained using methanol:water and acetonitrile:water mobile phases. Ethanol was shown to be a good organic modifier for RP-HPLC, with good chromatographic properties. This, considering the much lower toxicity of ethanol, the facility of its disposal, and its favorable cost, should make ethanol:H 2 O the mobile phase of choice for many RP-HPLC applications.
Phases based on titanized silicas are an alternative in the search for HPLC stationary phases wit... more 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.
Page 1. J Mol Evol (1987) 25:12-14 Journal of Molecular Evolution (~) Springer-Verlag New York In... more Page 1. J Mol Evol (1987) 25:12-14 Journal of Molecular Evolution (~) Springer-Verlag New York Inc. 1987 Formation of Organic Products in Self-Radiolyzed Calcium Carbonate Guadalupe Albarrfin, 1 Kenneth E. Collins, 2 and Car01 H. Collins 2 ...
Stability studies of stationary phases from poly(methyltetradecylsiloxane) sorbed and immobilized... more 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 mm63.9 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.
Poly(methyloctylsiloxane) (PMOS) was deposited on HPLC silica by a solvent evaporation procedure ... more Poly(methyloctylsiloxane) (PMOS) was deposited on HPLC silica by a solvent evaporation procedure and this material was then extracted, using a good solvent for the PMOS, after different time periods, to remove unretained liquid polymer. Solvent extraction data reveal changes which occur at ambient temperature as a function of the time interval between particle loading and extraction. The quantity of PMOS remaining on the silica after extraction, as determined by elemental analysis for carbon, is attributed to strongly adsorbed polymer. This phenomenon is termed self-immobilization. Solid-state 29Si NMR spectra indicate the formation of a silicon species with a different chemical shift than the original PMOS. These new signals are attributed to a combination of different adsorbed and chemically bonded groups.
A new stationary phase containing embedded urea groups (NH C(O) NH) was prepared by a procedure b... more A new stationary phase containing embedded urea groups (NH C(O) NH) was prepared by a procedure based on the synthesis of a trifunctional C 18 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 C 18 titanized silica phases. Infrared, 13 C and 29 Si spectroscopies were employed to characterize the C 18-urea titanized silica phase. Chromatographic evaluations used hydrophobic, polar and basic compounds to verify the effects of the polar urea groups embedded in the C 18 urea phase. The chromatographic parameters, especially for the separation of basic compounds, compare favorably with those obtained on a C 18 titanized silica stationary phase, prepared by silanization of titanized silica with octadecyltrimethoxysilane.
Batches of poly(methyloctylsiloxane) (PMOS)-loaded silica were prepared by the deposition of PMOS... more 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 • C for 4 h) after 1, 2, 5, 7, 9, 15, 20, 25, 70, 111 and 184 days. Self-immobilized and thermally treated samples were characterized by % C, 29 Si 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 (∼100 days for PMOS on Kromasil silica), the thermal treatment does not alter this configuration and, thus, does not change the resulting chromatographic parameters.
To extend pH stability, protective metal oxide layers, such as titanium oxide, that are more stab... more 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 C 18 phase onto a doubly-titanized silica support and comparing its chromatographic performance with a C 18 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 • C using aggressive phosphate mobile phases at pH 10.
The identities of the species of chromium(VI) that are present in aqueous solution, their spectra... more The identities of the species of chromium(VI) that are present in aqueous solution, their spectra and their equilibria, continue to be a subject of discussion in the literature. In this paper, the composition of the Cr(VI) equilibria was estimated from the UV-vis spectra of dilute potassium dichromate solutions, without any prior knowledge of the quantities of pure components, with the aid of Imbrie Q-mode factor analysis (Q-mode FA) followed by Varimax rotation and Imbrie oblique projection. Combining these results with the k-matrix method, it was possible to obtain the spectra of the individual Cr(VI) species. Sets of 3.3× 10 − 4 and 3.3× 10 − 5 mol l − 1 Cr(VI) solutions were studied. In the pH range from 1 to 12, two factors were identified, which were related to the two species, chromate ion (CrO 4 2 −) and bichromate ion (HCrO 4 −). When the analysis was extended to concentrated acid media, another factor appeared, which was related to chromic acid (H 2 CrO 4). No evidence for the dichromate ion (Cr 2 O 7 2 −) was seen at the Cr(VI) concentrations used. The spectra of the pure components were obtained and pK values for the first and second chromic acid dissociations were estimated as − 0.54 and 5.8, respectively.
... In environmental analysis, some theoretical models [3] and [4] predict, from both thermodynam... more ... In environmental analysis, some theoretical models [3] and [4] predict, from both thermodynamic and kinetic considerations, that chromium(III) as [Cr(H 2 O) 4 (OH) 2 ] + and chromium(VI) as CrO 4 2− are the dominant species found in natural waters. ...
A procedure is presented for the rapid evaluation of HPLC stationary phase stability at pH 8.4 or... more 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 • C. Mobile phase (MeOH-0.1 mol l −1 aqueous NaHCO 3 , 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.
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