This work describes three C8-stationary phases for high performance liquid chromatography based on silica metallized with ZrO2, TiO2 or Al2O3 layers, having poly(methyloctylsiloxane) immobilized onto their surfaces. The stationary phases were characterized using XRF, XAS, FTIR, SEM and elemental analysis to determine the physical characteristics of the oxide and polysiloxane layers formed on the surfaces and chromatographically to evaluate the separation parameters. The results show the changes on the silica surface and allowed proposing a structure for the oxide layer, being observed tetrahedral and octahedral structures, what is completely new in the literature. The formation of a homogeneous layer of metallic oxide (TiO2 and ZrO2) was observed on the silica. The C8-titanized and C8-aluminized stationary phases presented good chromatographic performances, with good values of asymmetry and efficiency. All stationary phase presented few loss of the polymeric layer after the HPLC, indicating that this layer is well attached on the metalized support.

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.

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:H2O the mobile phase of choice for many RP-HPLC applications.

O comportamento químico de Cr(VI) a baixas concentrações (10-4 to 10-7 mol L-1) em vários ácidos fortes foi estudado utilizando-se 51Cr(VI) de alta atividade específica como marcador. A especiação dos produtos destes sistemas foi realizada por cromatografia de troca iônica com eluição em etapas. Os resultados mostram que traços de Cr(VI), monitorados por radiocromo (51Cr), são reduzidos na presença de ácidos minerais tais como perclórico, clorídrico, fluorídrico, sulfúrico, nítrico e trifluormetanosulfônico, mesmo na ausência de agentes redutores convencionais, produzindo várias espécies de Cr(III) diferentes, dependendo do ânion do ácido. Estudos detalhados com soluções de ácido nítrico mostram que a velocidade relativa da redução aumenta com o aumento da concentração do ácido ou com o decréscimo da concentração de Cr(VI).

The chemical behavior of Cr(VI) at low concentrations (10-4 to 10-7 mol L-1) in several strong acids was studied using high specific activity 51Cr(VI) as a tracer. The speciation of the products from these systems was carried out by ion exchange chromatography with stepwise elution. The results show that trace quantities of Cr(VI), monitored by means of radiochromium (51Cr), are reduced in the presence of mineral acids such as perchloric, hydrochloric, hydrofluoric, sulfuric, nitric and trifluoromethanesulfonic acids, even in the absence of conventional reducing agents, producing different measureable Cr(III) species, depending on the acid anion. Detailed studies of the reduction of low concentrations of Cr(VI) with nitric acid have shown that the relative rate of reduction increases as the concentration of the acid increases or as the concentration of the Cr(VI) decreases.

Dois tipos de fase estacionária, auto-imobilizada e imobilizada por radiação, foram preparadas com poli(metiloctilsiloxano) sobre oito suportes de sílica com tamanho e forma de partículas, e tamanho de poros diferentes. As colunas recheadas com as fases estacionárias preparadas através dos dois procedimentos de imobilização apresentaram eficiências, resoluções e fatores de separação similares, mas as colunas recheadas com a fase estacionária auto-imobilizada tiveram percentagens de carga menores e, portanto, fatores de retenção menores. Os resultados evidenciam diferenças estruturais entre as fases.

Self-immobilized and radiation-immobilized stationary phases were prepared with poly(methyloctylsiloxane) on eight silica supports having different particle sizes, particle shapes and pore sizes. Columns prepared by the two immobilization procedures had similar efficiencies, resolutions and separation factors but columns with self-immobilized stationary phases had lower percent loadings and, thus, lower retention factors. Results show structural differences between the phases.

Particles of porous silica or other solvent resistent inorganic oxides can be functionalized by aliphatic (e.g., C-8 or C-18) or other groups to give stationary phases for use in reversed phase HPLC. The functionalization can be done by bonding of individual groups to the surface of the support particles, by producing an organic polymeric film from pre-polymers, or by adsorbing/immobilizing pre-formed polymers on the surfaces. These three types of functionalization are reviewed.

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 (CrO4(2-)) and bichromate ion (HCrO4-). The pK of the equilibrium was estimated as 5.8.