Porous Silica Microspheres Research Articles

Fiber-Optic Microsensors Using Porous, Dye-Doped Silica Gel Microspheres

AbstractDye-doped porous silica microspheres can be prepared from liquid solution at temperatures near ambient. Microsphere diameter can be controlled between approximately 5.0 microns to in excess of a millimeter. The resulting microspheres can be attached to the distal end of an optical fiber in which the proximal end is attached to a spectrophotometer. Depending upon the organic species doped into the microsphere, a wide variety of sensing functions are possible. In this paper, the use of microsensors for measuring pH, temperature, and solvent content of aqueous solutions is demonstrated. Potential utility of this type of sensor to heavy metals detection and biomedical diagnostics is also discussed.

HPLC Method Development: Practical Aspects of Increasing Analysis Speed While Maintaining Separation Resolution

A practical composite of favorable features in column design permits typical separations in one-half the time without loss in resolution compared with commonly used columns. A highly purified, low-acidity 3.5-µm porous silica microsphere support combined with sterically protected, monomeric silane stationary phases provides the basis for the new column type. These units show considerable promise for developing fast, rugged, reproducible methods. Carefully configured columns packed with 3.5-µm particles with a narrow particle size distribution resist the pluggage usually associated with columns packed with traditional 3-µm particles. Extra-column band-broadening and back pressure effects are also minimized so that state-of-the-art HPLC equipment can be used. Columns of highly stable stationary phases have higher operating temperatures that are useful for fast separations, particularly for macromolecules.

Normal-phase high-performance liquid chromatography with highly purified porous silica microspheres

Column reproducibility and peak shapes are improved for polar and basic solutes in normal-phase chromatography using new highly purified, low-acidity unmodified porous silica microspheres. The chromatographic characteristics of this new type B silica are compared with a conventional type A silica to determine the type and level of mobile phase modifiers needed for good results. The effect of sample type and sample loading on retention, column efficiency and peak shape also was measured. Critical normal-phase tests compared this new silica with several commercial silicas. The new silica shows special promise for developing rugged separation methods to analyze polar and basic compounds that traditionally are handled by reversed-phase chromatography. Normal-phase chromatography is well-suited for highly organic-soluble compounds and certain structures such as positional isomers.

Superficially porous silica microspheres for the fast high-performance liquid chromatography of macromolecules

New superficially porous «Poroshell» particles have been synthesized and tested for HPLC separations of macromolecules. Spray-drying techniques were used to prepare silica microspheres that typically have a 5-μm solid core and a ∼1 μm-thick outer shell with 300-A pores. Compared to conventional totally porous particles of the same size, Poroshell particles show improved mass-transfer kinetic properties that permit more rapid separations of macromolecules. The utility of the new Poroshell column packing is demonstrated with separations of proteins and synthetic polymers.

Preparation of silica glass microspheres by sol-gel processing

A method for making glass microspheres by the sol-gel process has been developed. Porous silica microspheres were produced at temperatures as low as 500 °C and densified silica microspheres were prepared at 800 °C. The size of the microspheres was controlled by adjusting the feed rate of the raw materials and the frequency of the droplet generator so that uniform spheres were obtained. Glass microspheres prepared by this method were characterized by Nuclear Magnetic Resonance (NMR), Differential Thermal Analysis (DTA), Thermogravimetric Analysis (TGA), and optical microscopy.

Surface treatment and porosity control of porous silica microspheres

Porous silica microspheres (PSM) have been treated with ammonium bifluoride to adjust porosity, pore size, remove surface impurities, and minimize surface acidity. The porosities of four silicas having mean pore diameters ranging from 150 to 750 A have been altered from initial values to the point at which the mechanical strength is insufficient to allow packed columns with acceptable performance. It is shown that a linear relationship exists between a change in porosity and the relative amount of ammonium bifluoride used to treat the silica. This reagent removes silica homogeneously from all pores in a given microsphere in a controllable and predictable manner. This treatment increases the peak capacity and improves chromatographic performance. The surfaces of treated silicas were probed with thiamine in the ion-exchange chromatographic mode. The slopes and intercepts of plots in which retention is plotted against the reciprocal of buffer concentration were both significantly reduced indicating that surface acidity is minimized by this treatment.

A new, stabilized, hydrophilic silica packing for the high-performance gel chromatography of macromolecules

A new approach for the stabilization of bonded-phase silica packings for the gel chromatography of proteins has been examined. Porous silica microspheres were stabilized by a zirconium salt treatment and then covalently bonded with a hydrophilic organo-silane. The resultant product (Zorbax® Bio Series GF-250) is a high-resolution, high-speed chromatographic packing with enhanced bonded-phase stability in aqueous buffered mobile phases. Packed columns of this material fractionate proteins roughly by molecular weight (MW) in the range of from 12 000 to 120 000 Daltons with approximately log MW-elution volume linearity. The zirconium-treated surface presents no unusual restraints in operating conditions and even permits short-term use of buffered eluents at pH 9. The useful operating lifetime of this packing has been shown to surpass some other currently available products.

Separation of Aflatoxins from Filter Cake

Size-exclusion chromatography using silianized porous silica microspheres is used to clean up an environmental sample prior to aflatoxin analysis. B/sub 1/ and B/sub 2/ aflatoxins were found in an anaerobic digestor filter cake sample at concentrations of 1 ppb.

High-performance size-exclusion liquid chromatography of inorganic colloids

High-resolution separations of inorganic colloids in the 1–50 nm range can be rapidly carried out by size-exclusion chromatography using columns of < 10 μm porous silica microspheres (PSM). The high efficiency of the PSM columns used in this application is the result of the rapid equilibration of slowly diffusing colloids with the porous structure of these very small particles. Subtle differences in the size characteristics of aluminosilicate sols have been measured in a few minutes with columns of ≈7 μm PSM particles. The mass transfer properties of superficially porous particles (solid core, porous crust) suggest that this type of packing should also be useful for the rapid separation of slowly diffusing colloids. Size-exclusion chromatography appears to be a useful technique for characterizing a variety of both inorganic and organic colloids.

Radiochemical Separations by Liquid-Liquid Chromatography Using PSM Support

Radiochemical separations of metal lons by liquid-liquid chromatography were carried out on 2-cm long minicolumns packed with 4 to 6 ..mu..m porous silica microspheres containing 25 to 30 weight percent of 1.5 F di(2-ethylhexyl) phosphoric acid in dodecane. Column performance was evaluated using the elution of both tracer and macro quantities of Eu(III). Plate heights for tracer scale elutions were 0.015 to 0.025 mm and 0.05 to 0.07 mm at v = 2 and 30 cm/min, respectively. The separation of certain dipositive metal ions, tripositive lanthanide lons, and tripositive transplutonium lons were studied using the high efficiency minicolumns.

Porous silica microspheres for high-performance size exclusion chromatography

Rapid and precise high-performance gel permeation chromatography of polymers has been accomplished with columns of 6–9-μm trimethylsilyl-modified porous silica microspheres with narrow pore-size distributions over the range 60–3500 Å. A 60-cm, five-column combination of this pore-size range exhibited a plate count of 2.5-10 4 for a totally permeating solute (toluene) and 10 4 plates for a 10 5 molecular weight polystyrene. This column set has an overall fractionation range of 10 2->7·10 6 for polystyrene, and its high resolution permits the determination of molecular weights in a 15-min analysis with an accuracy of about ± 2%, without peak dispersion corrections. The high resolving power of the microspheres, coupled with excellent mechanical strength and high purity, provides superior systems for rapidly characterizing both high-molecular-weight polymers and mixtures of small molecules by the size exclusion method.

Radiochemical and isotope separations by high-efficiency liquid-liquid chromatography

A very-high-efficiency liquid-liquid chromatographic (LLC) system based on 5- and 20-μm diameter porous silica microspheres was used to effect radiochemical separations of metal ions that required very large decontamination factors and/or high speeds. The stationary phase was 25–30% (w/w) of di(2-ethylhexyl) orthophosphoric acid (HDEHP) in dodecane. Nitric acid was used as the mobile phase. The mass transfer coefficient term, C, was determined from height equivalent to a theoretical plate vs. interstitial linear velocity data for the elution of Ca 2+ and compared with values calculated from the interfacial mass transfer coefficients measured in the corresponding liquid-liquid extraction system. The HDEHP high-efficiency LLC system was also investigated for possible usein enriching the naturally occurring isotopes of calcium. Isotopic exchange constants for 40Ca or 42Ca and 48Ca were measured chromatographically as a function of temperature. The enrichment of calcium isotopes was attempted on coupled columns at low flow velocities.

Microparticles with bonded hydrocarbon phases for high-performance reverse-phase liquid chromatography

Porous silica microspheres with bonded hydrocarbon phases have been prepared for high-performance reverse-phase liquid chromatography. These bonded-phase particles exhibit high efficiency because of their small size (∼7 μm), narrow particle size distribution, and excellent solute mass transfer characteristics. Slurry-packed 25 cm×0.32 cm i. d. columns of 7 μm particles modified with octadecyl groups (18% C) exhibit plate height of <0.005 cm with water/methanol mobile phases at 50°C. The high concentration of octadecyl, dimethylbenzyl and trimethyl groups that are introduced by reaction with chlorosilanes minimizes band tailing due to solute adsorption by residual silanol groups on the support surface. These small particles are prepared with different size pores, which determine the level of organic stationary phase and resultant chromatographic retention. These particles with various pore sizes permit optimum reverse-phase interaction with solutes of different molecular weight.

High Performance Liquid-Liquid Partition Chromatography on Porous Silica Microspheres

High Performance Liquid-Liquid Partition Chromatography on Porous Silica Microspheres

Porous silica microsphere column packings for high-speed liquid—liquid chromatography

A new column packing for high-performance liquid chromatography, porous microspheres of silica produced by the agglutination of colloidal silica particles, has recently been introduced for use in adsorption chromatography. The narrow-size range, relatively homogeneous pore structure and short diffusion path lengths of these <10-μ particles result in very high column efficiencies, and the relatively large, highly available surface area provides for high sample capacity. The microsphere packing displays retention and efficiency characteristics which are less dependent on water content than wide-pore silica gel. Columns of the microspheres may be prepared which are reproducible in chromatographic performance, using a simple high-pressure slurry-packing procedure. More than 10,000 theoretical plates have been obtained on a single 25-cm-long column of 5-μ microspheres at carrier velocities of about 0.7 cm/sec. Plate heights of about five particle diameters and more than thirty-six effective plates/sec have been demonstrated for solutes with capacity factors ( k′) in the 2–5 range. These columns may be connected in series using low-volume fittings with little loss in efficiency. Columns of the 5-μ particles appear to be limited by mobile phase mass transfer effects, contrasted to the stagnant mobile phase mass transfer limitations exhibited by similar 8- to 9-μ particles.

High-Performance Liquid Chromatography With Porous Silica Microspheres

New totally porous, spherical silica particles of <10 µ are shown to be superior column packings for high-performance liquid chromatography (LC). These porous microspheres are prepared in narrow particle size ranges and have been used for liquid-liquid, liquid-solid and exclusion chromatography since both the overall size and internal porosity can be selected and closely controlled. High-efficient columns of these particles are conveniently prepared by a high-pressure slurry-packing technique using stabilized aqueous suspensions. Performance of columns of these microspheres are the highest yet reported, with 5–6 µ, particles producing 23 effective plates per second for solutes with capacity factors, k′ = 12. The performance of the microsphere columns is more than 1000 times that of classical LC systems and equals that of open tubular (capillary) GC columns.