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R Stol

Publications and source records attributed to R Stol.

7 recordsLinked to original sources
PubMed2001

Determination of the molecular mass distribution of synthetic polymers by size-exclusion electrochromatography.

The performance of size-exclusion electrochromatography (SEEC) for the mass distribution analysis of synthetic polymers was studied and compared to conventional, pressure-driven size-exclusion chromatography (SEC). Electroosmotic flow control, within-day, day-to-day and column-to-column repeatability were determined for SEEC with respect to retention and separation efficiency. It was shown that by using the retention ratio instead of the migration time, the precision of the mass distribution calculations is sufficiently high, and that similar distributions were obtained for a sample analyzed by pressure-driven SEC and by SEEC. Furthermore, hexafluoroisopropanol was demonstrated to be a new and potent solvent for SEEC. It was used for the separation of narrow polymethylmethacrylate standards and several commercially important polymers such as polycarbonate, polycaprolactam and poly(ethylene terephthalate), using UV detection in the deep UV region (195-230 nm).

Chromatography, Gel↗
PubMed2001

Effects of pore flow on the separation efficiency in capillary electrochromatography with porous particles.

The effect of pore flow on the separation efficiency of capillary electrochromatography (CEC) has been studied using columns packed with particles with different pore sizes. A previously developed model was used to predict the (relative) pore flow velocity in these columns under various experimental conditions. Equations are derived describing the effect of pore flow on peak broadening in CEC. The theory has been compared with practice in the reversed-phase CEC separation of various polyaromatic hydrocarbons. It is shown, by theory and experimentally, that the mass-transfer resistance contribution to peak dispersion can be effectively eliminated when using porous particles with a high (> or =50 nm) average pore diameter. Moreover, at high pore-to-interstitial flow ratios the flow inhomogeneity contribution (the A term in the plate height equation) is also shown to decrease. Under optimal conditions, a reduced plate height of 0.3 for the nonretained compound could be obtained. It is argued that fully perfusive porous particles can be a more efficient separation medium in CEC than nonporous particles.

Journal Article↗
PubMed2001

Size-exclusion electrochromatography with controlled pore flow.

In size-exclusion electrochromatography (SEEC) there exists an optimum in pore-to-interstitial flow ratio with respect to the resolution. Two methods for finding and controlling the optimal pore-to-interstitial flow ratio in SEEC have been studied: (i) varying the ionic strength of the mobile phase and (ii) the application of a hydrodynamic flow in addition to the electrco-driven flow. Both methodologies have been evaluated in terms of efficiency and applicability with columns packed with silica particles containing pores of either 10 or 50 nm in diameter, and with different ionic strength mobile phases. Using the first method with the 10-nm pore particles, the flow ratio could be adjusted within an appropriate range. However, with the wide-pore (50 nm) particles it appeared that the pore-to-interstitial flow ratio was too high at all conditions tested to obtain proper selectivity. In the second approach, the desired pore flow was generated by the electric field and the pore-to-interstitial flow ratio could then be adjusted by an applied pressure over the column. This method was applicable with both particle types studied. The application of a (low) voltage gradient in addition to a pressure-driven flow resulted in a sharply improved separation efficiency as a result of a strongly improved mass transfer due to intra-particle electroosmotic flow. When optimized, pressurized SEEC generates identical separation efficiencies for polystyrene standards as are obtained with pure SEEC, while the reduction in selectivity, in comparison to pressure-driven SEC, is kept minimal.

Chromatography, Gel↗
PubMed2000

Modelling of the pore flow in capillary electrochromatography.

Pore flow in capillary electrochromatography (CEC) on porous silica particles has been investigated. To that end the migration behaviour of narrow polystyrene (PS) standards dissolved in di-methylformamide (DMF) with lithium chloride in 1 and 10 mmol/l concentration has been measured. These data have been compared to theoretical predictions. The latter were based on a model comprising cylindrical pores of varying diameter as measured experimentally by porosimetry, while the flow in each set of pores was calculated with the expression given by Rice and Whitehead. A reasonable to good agreement between experimental and predicted data was observed, provided it was assumed that pores of differing diameter occur in series. It was found that the flow in pores with a nominal size of 100 A can be considerable compared to the interstitial flow, especially at 10 mmol/l ionic strength. It is concluded that pore flow within porous particles in CEC, of great importance for improved efficiency in both interactive and exclusion type CEC, can be predicted fairly reliably by means of the Rice and Whitehead expression.

Dimethylformamide↗
PubMed2000

Pseudo-electrokinetic packing of high efficiency columns for capillary electrochromatography.

An improved and easy electrokinetic packing procedure is presented for the production of stable capillary columns suitable for capillary electrochromatography (CEC). In pseudo-electrokinetic packing a high electric field is used in conjunction with a hydrodynamic flow. The packing of silica-based reversed-phase columns can be achieved with basic, commercially available capillary electrophoresis (CE) equipment in approximately 15 min. The procedure is robust and a high success rate is achieved. No steps which might damage the stationary phase are involved and only a minimum amount of packing material is required. Columns packed according to the developed procedure are operated at high electric field strengths during the CEC separation, without the application of a stabilising pressure. Columns are stable for at least hundred runs and were tested using mixtures of polycyclic aromatic hydrocarbons and positively charged drugs. Separations were performed in a relatively high conducting ammonium acetate buffer, with efficiencies of up to 283000 plates/m.

Chromatography, Micellar Electrokinetic Capillary↗
PubMed1999

Capillary electrochromatography with macroporous particles.

The performance of macro-porous particles in capillary electrochromatography is studied. Three reversed-phase stationary phases with pore diameters between 500 A and 4000 A have been tested for separation efficiency and mobile phase velocity. With these stationary phases, a large portion of the total flow appears to be through the pores of particles, thereby increasing the separation efficiency through a further decrease of the flow inhomogeneity and through enhancement of the mass transfer kinetics. The effects of pore size and mobile phase composition on the plate height and mobile phase velocity have been studied. With increasing buffer concentrations and larger pore diameters, higher mobile phase velocities and higher separation efficiencies have been obtained. Columns packed with 7 microns particles containing pores with a diameter of 4000 A generated up to 430,000 theoretical plates/m for retained compounds. Reduced plate heights as low as 0.34 have been observed, clearly demonstrating that a significant portion of the flow is through the pores. For the particles containing 4000 A pores no minimum was observed in the H-u plot up to linear velocities of 3.3 mm/s, suggesting that the separation efficiency is dominated by axial diffusion. On relatively long (72 cm) columns, efficiencies of up to 230,000 theoretical plates/column have been obtained under non-optimal running conditions. On short (8.3 cm) columns fast separations could be performed with approximately 15,000 theoretical plates generated in less than 30 s.

Chromatography↗