RESUMO

Solar driven semiconductor-based photoreforming of biomass derivatives, such as glycerol is a sustainable alternative towards green hydrogen evolution concerted with production of chemical feedstocks. In this work, we have investigated the influence of the pH of the hydrothermal treatment on the efficiency of Bi2WO6 as photocatalyst in the glycerol photoreforming. Bi2WO6 is pointed as a promising material for this application due its adequate band gap and the ability to promote hole transfer directly to glycerol without formation of non-selective ⋅OH radicals. Samples prepared at neutral to moderate alkaline conditions (pH = 7-9) are highly crystalline, while those prepared in acidic media (pH = 0-2) exhibit higher concentrations of oxygen vacancies. At pH = 13, the non-stoichiometric Bi(III)-rich phase Bi3.84W0.16O6.24 is formed. All samples were fully characterized towards their optical and morphological properties. UV-Vis irradiation of the photocatalysts modified with 1% m/m Pt and in the presence of 5% v/v aqueous glycerol solution leads to H2 evolution and glycerol oxidation. The sample prepared at pH = 0 exhibited the highest photonic efficiency (ξ) for H2 evolution (1.4 ± 0.1%) among the investigated samples with 99% selectivity for simultaneous formic acid formation. Similar performance was observed for the non-stoichiometric Bi3.84W0.16O6.24 sample (ξ = 1.2 ± 0.1% and 88% selectivity for formic acid), whereas the more crystalline sample prepared at pH = 9 was less active (ξ = 0.9 ± 0.1%) and leads to multiple oxidation products. The different behaviors were rationalized based on the role of oxygen vacancies as active adsorption and redox sites at the semiconductor surface, stablishing clear relationships between the semiconductor structure and its photocatalytic performance. The present work contributes for the rational development of specific photocatalysts for glycerol photoreforming.

Assuntos

RESUMO

The chemical agent free preparation of a stationary phase using a natural macromolecule was the focus of this paper. Thermal immobilization of cellulose dodecanoate on silica particles was used for the preparation of a stationary phase without the use of chemical reagents. Cellulose modification was performed to produce a hydrophobic macromolecule with solubility in common organic solvents. The new stationary phase was characterized morphologically and physico-chemically, presenting as spherical particles immobilized with a thin cellulose dodecanoate layer. The degree of substitution of cellulose dodecanoate was 1.7, which resulted in a separation mechanism in reversed phase mode, but with lower hydrophobicity and higher steric selectivity, which are properties from cellulose. These characteristics resulted in a stationary phase with intrinsic selectivity that was able to separate mixtures of polar drugs, homologs of an anionic surfactant and omeprazole isomers, which are not well resolved in typical C18 phases. Considering that cellulose is a natural polymer and the preparation method of stationary phase involves only physical processes of silica modification, the final material presents as a stationary phase with specific retention properties coming from both dodecanoate and cellulose.

Assuntos

RESUMO

A method for the determination of artemether (ART) and its main metabolite dihydroartemisinin (DHA) in plasma employing liquid-phase microextraction (LPME) for sample preparation prior to liquid chromatography-tandem mass spectrometry (LC-MS-MS) was developed. The analytes were extracted from 1mL of plasma utilizing a two-phase LPME procedure with artemisinin as internal standard. Using the optimized LPME conditions, mean absolute recovery rates of 25 and 32% for DHA and ART, respectively, were achieved using toluene-n-octanol (1:1, v/v) as organic phase with an extraction time of 30min. After extraction, the analytes were resolved within 5min using a mobile phase consisting of methanol-ammonium acetate (10mmolL(-1), pH 5.0, 80:20, v/v) on a laboratory-made column based on poly(methyltetradecylsiloxane) attached to a zirconized-silica support. MS-MS detection was employed using an electrospray interface in the positive ion mode. The method developed was linear over the range of 5-1000ngmL(-1) for both analytes. Precision and accuracy were within acceptable levels of confidence (<15%). The assay was applied to the determination of these analytes in plasma from rats treated with ART. The two-phase LPME procedure is affordable and the solvent consumption was very low compared to the traditional methods of sample preparation.

Assuntos

RESUMO

Many chromatographic separations require high pH mobile phases for the elution of basic compounds with symmetrical peaks appropriate for quantitation. Phosphate and carbonate salts are the most common additives used to achieve these conditions. However, they act in different ways on the stability of silica-based stationary phases. To provide a faster evaluation of the effects of these high pH solutions on the column lifetime of new immobilized-polymer metalized-silica-based phases prepared in our laboratory, an accelerated stability test at a nominal pH of 10 and at 50 degrees C was developed and applied with monitoring of the retention factor, column efficiency, and asymmetry as functions of the volume of high pH mobile phase. The laboratory-made stationary phases presented chemical stabilities in phosphate solution more than three times better their chemical stabilities in carbonate buffer.

RESUMO

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.

RESUMO

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.

Assuntos

RESUMO

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 microg 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.

Assuntos

RESUMO

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.

Assuntos

RESUMO

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.

RESUMO

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.

Assuntos