Implementation of high slurry concentration and sonication to pack high-efficiency, meter-long capillary ultrahigh pressure liquid chromatography columns

Slurry packing capillary columns for ultrahigh pressure liquid chromatography is complicated by many interdependent experimental variables. Previous results have suggested that combination of high slurry concentration and sonication during packing would create homogeneous bed microstructures and yield highly efficient capillary columns. Herein, the effect of sonication while packing very high slurry concentrations is presented. A series of six, 1 m × 75 μm internal diameter columns were packed with 200 mg/mL slurries of 2.02 μm bridged-ethyl hybrid silica particles. Three of the columns underwent sonication during packing and yielded highly efficient separations with reduced plate heights as low as 1.05

Mesopore etching under supercritical conditions – A shortcut to hierarchically porous silica monoliths

Hierarchically porous silica monoliths are obtained in the two-step Nakanishi process, where formation of a macro microporous silica gel is followed by widening micropores to mesopores through surface etching. The latter step is carried out through hydrothermal treatment of the gel in alkaline solution and necessitates a lengthy solvent exchange of the aqueous pore fluid before the ripened gel can be dried and calcined into a mechanically stable macro mesoporous monolith. We show that using an ethanol water (95.6/4.4, v/v) azeotrope as supercritical fluid for mesopore etching eliminates the solvent exchange, ripening, and drying steps of the classic route and delivers silica monoliths that can withstand fast heating rates for calcination. The proposed shortcut decreases the overall preparation time from ca. one week to ca. one day. Porosity data show that the alkaline conditions for mesopore etching are crucial to obtain crack-free samples with a narrow mesopore size distribution. Physical reconstruction of selected samples by confocal laser scanning microscopy and subsequent morphological analysis confirms that monoliths prepared via the proposed shortcut possess the high homogeneity of silica skeleton and macropore space that is desirable in adsorbents for flow-through applications

Electroosmotic flow phenomena in packed capillaries: From the interstitial velocities to intraparticle and boundary layer mass transfer

Pulsed field gradient nuclear magnetic resonance studies of electrokinetic flow through a 250 m i.d. cylindrical fused-silica capillary packed with spherical porous particles (dp = 41 m) have revealed the following phenomena and parameters: (i) An electrokinetic wall effect exists due to a mismatch of zeta-potentials associated with the capillary inner wall and the particles surface. It results in a transcolumn velocity profile which depends on the column-to-particle diameter ratio and causes additional longitudinal dispersion. (ii) Compared to the pressure-driven flow through the porous medium, the intraparticle mass transfer rate constant is significantly increased under the influence of a potential gradient. This increase also depends on the buffer concentration via electric double layer overlap. (iii) Fluid molecules in the porous particles remain diffusion-limited in the presence of a pressure gradient. By contrast, intraparticle Peclet numbers above unity have been measured for electroosmotic flow and were found to increase with the applied potential difference. (iv) Interparticle resistance to mass transfer appears to vanish on the pore scale when electric double layers are small compared to the relevant pore dimensio

Electroosmotic and Pressure-Driven Flow in Open and Packed Capillaries : Velocity Distributions and Fluid Dispersion

The flow field dynamics in open and packed segments of capillary columns has been studied by a direct motion encoding of the fluid molecules using pulsed magnetic field gradient nuclear magnetic resonance. This noninvasive method operates within a time window that allows a quantitative discrimination of electroosmotic against pressure-driven flow behavior. The inherent axial fluid flow field dispersion and characteristic length scales of either transport mode are addressed, and the results demonstrate a significant performance advantage of an electrokinetically driven mobile phase in both open-tubular and packed-bed geometries. In contrast to the parabolic velocity profile and its impact on axial dispersion characterizing laminar flow through an open cylindrical capillary, a pluglike velocity distribution of the electroosmotic flow field is revealed in capillary electrophoresis. Here, the variance of the radially averaged, axial displacement probability distributions is quantitatively explained by longitudinal molecular diffusion at the actual buffer temperature, while for Poiseuille flow, the preasymptotic regime to Taylor-Aris dispersion can be shown. Compared to creeping laminar flow through a packed bed, the increased efficiency observed in capillary electrochromatography is related to the superior characteristics of the electroosmotic flow profile over any length scale in the interstitial pore space and to the origin, spatial dimension, and hydrodynamics of the stagnant fluid on the support particles' external surface. Using the Knox equation to analyze the axial plate height data, an eddy dispersion term smaller by a factor of almost 2.5 than in capillary high-performance liquid chromatography is revealed for the electroosmotic flow field in the same colum

Le Figuier de Barbarie (Étude botanique d'Opuntia ficus-indica (L.) Mill.. Exploitation et utilisations actuelles)

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