Kenneth E. Collins

Speciation of aqueous chromium(VI) solutions with the aid of Q-mode factor analysis followed by oblique projection

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 3.3x10(-4) and 3.3x10(-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.

Reevaluation of ethanol as organic modifier for use in HPLC-RP mobile phases

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.

Stability of high-performance liquid chromatography columns packed with poly(methyloctylsiloxane) sorbed and radiation-immobilized onto porous silica and zirconized silica.

Reversed-phase packing materials were prepared from HPLC silica and from zirconized HPLC silica support particles having sorbed poly(methyloctylsiloxane) (PMOS) as the stationary phase. Portions of zirconized material were subjected to 80 kGy of ionizing radiation. Columns prepared from these packing materials were subjected to 5000 column volumes each of neutral and alkaline (pH 10) mobile phases, with periodic tests to evaluate chromatographic performance. It was shown that the PMOS stationary phase sorbed onto zirconized silica requires an immobilization treatment (such as gamma irradiation) for long term stability while prior surface zirconization of the silica support surface greatly improves the chromatographic stability of the stationary phase when using alkaline mobile phases.

Radiation immobilization of poly(methyloctylsiloxane) on silica for use in HPLC: a uniform layer model.

Poly(methyloctylsiloxane) (PMOS) was sorbed into the pores of HPLC silica by a solvent evaporation procedure, then irradiated with gamma rays from a cobalt-60 source to absorbed doses in the range from 0 to 200 kGy (1Gy = 1J kg-1). Non-irradiated and irradiated samples were characterized by solvent extraction, specific surface area determination, infrared spectroscopy and reversed-phase column performance. Solvent extraction data reveal that about 40% of the PMOS is not extractable prior to irradiation and this increases to about 75% with radiation doses of 50 kGy or higher. Column performance was improved by the radiation treatment, reaching a maximum efficiency in the dose range of 80-140 kGy while the peak symmetry changed from As = 1.7 to 1.1. The improvement is attributed to the increased mass of polymer immobilized by the radiation treatment and to a more uniform distribution of the immobilized polymer in the silica pore system. A multi-layer stationary-phase model is presented in which the first layer consists of an adsorbed monolayer of PMOS and the second layer is immobilized by gamma radiation.

Fases estacionárias para cromatografia líquida de alta eficiência em fase reversa (CLAE-FR) baseadas em superfícies de óxidos inorgânicos funcionalizados

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.

Poly(alkylmethylsiloxanes) thermally immobilized on silica as stationary phases for high-performance liquid chromatography.

Poly(methyloctylsiloxane) (PMOS) and poly(methyloctadecylsiloxane) (PMODS) were sorbed onto porous HPLC silica and thermally immobilized, in the absence of radical initiators, at temperatures in the range of 80 to 180 degrees C. Following extraction of non-immobilized polymer the materials were packed into columns and their chromatographic properties evaluated. The shorter chain (PMOS) stationary phase showed good HPLC characteristics after thermal immobilizations up to 120 degrees C while the longer chain (PMODS) phase gave satisfactory HPLC phases following thermal immobilizations at 80 and 100 degrees C. Stability evaluation for the PMOS and PMODS columns immobilized at 100 degrees C required 250 ml of pH 8.5 mobile phase at 60 degrees C to significantly decrease efficiency, suggesting a long useful life time at neutral pH and ambient temperature.

Liquid chromatographic separation of aqueous species of Cr(VI) and Cr(III)

Although chromium can be present in aqueous solutions as Cr(VI) or as any of various kinetically stable forms of Cr(III), whose distribution depends on the chemical history of the aqueous sample, most chromium speciation procedures in the literature consider only Cr(VI) and Cr(III) (as the hexaaquo species). The present paper gives examples of characterizations involving Cr(VI) and several typical forms of Cr(III) using a cation-HPLC separation scheme with spectrophotometric or radiometric detection. These results are compared with those obtained with open column cation-exchange chromatography using radiometric detection.

High-performance liquid chromatographic stationary phases based on poly(methyloctylsiloxane) immobilized on silica: I. Physical and chemical characterizations

Five different reversed-phase materials for high-performance liquid chromatography were obtained by deposition of poly(methyloctylsiloxane) in HPLC silica particles, followed by immobilization using different processes: thermal treatment (120 or 220 degrees C for 4 h), irradiation with microwaves (495 W for 15 min), gamma radiation (dose of 80 kGy) and self-immobilization. These phases were characterized by gel permeation chromatography, percent carbon, 13C and 29Si solid-state nuclear magnetic resonance spectroscopy, infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy. The results show that the different immobilization processes produce different physical characteristics in the prepared phases.

Effects in high-performance liquid chromatography of a high pH in the mobile phase on poly(methyloctylsiloxane) immobilized by γ-radiation on titanium-grafted silica

Effects of high-pH environments on a stationary phase prepared by gamma-radiation immobilization of poly(methyloctylsiloxane) on titanium-grafted silica were investigated by HPLC testing with standard sample mixtures. The HPLC parameters indicate good stationary phase stability to 10000 column volumes each of mobile phases with pH of 7, 9 and 12. At pH 13, the efficiency decreases slowly, although reasonably good separations are still possible until increasing flow resistance no longer allows easy passage of the mobile phase.

High-performance liquid chromatographic stationary phases based on poly(methyloctylsiloxane) immobilized on silica II. Chromatographic evaluation

This work describes the chromatographic characterization of stationary phases prepared by deposition of poly(methyloctylsiloxane) (PMOS) on silica followed by immobilization using one of several different processes: thermal treatments (120 or 220 degrees C for 4 h), microwave irradiation (495 W for 15 min), gamma radiation (dose of 80 kGy) or self-immobilization. This evaluation was based on the chromatographic parameters of several test solutes. The stationary phases immobilized at 220 degrees C and which underwent self-immobilization were not appropriate for chromatographic use but the other immobilized phases presented chromatographic performances similar in most respects to a commercial phase (Rainin C8) while the peak characteristics of the basic probe were significantly better with these phases.