Porous-silica particles

In liquid-liquid chromatography the stationary phase is a liquid film coated on a packing material consisting of 3-10 pm porous silica particles. The stationary phase may be partially soluble in the mobile phase, causing it to bleed from the column... 

Modern SEC columns are packed with material other than polystyrene gels, such as porous silica particles or highly cross-linked styrene-divinylbenzene copolymers. Because of improvements in speed and resolution, the term SEC is sometimes replaced by the term high-performance size-exclusion chromatography (HPSEC). 

In 1972, Kirkland at E. I. du Pont de Nemours patented porous silica microspheres (PSM) specifically for high-performance liquid chromatography (HPLC) applications (3). Prior to this development, silica particles used for chromatographic applications were simply adapted from some other use. In the 1970s, Kirkland showed that porous silica particles could be used for size-... 

Nevertheless, this method was successfully applied by Gulyaeva et al. for the log P and log D determination of 15 P-sympatholytic drugs [56]. Another study by Welerowicz and Buszewski compared the HpophiHcity values of P-blockers obtained with a column made of a monoHthic-silica Cjg with a conventional porous silica particles Cjg as reference material [27]. A modified method was used for evaluating logP with two main differences (i) logfeg was considered rather than retention times, and (ii) benzene and butyl-benzene were used as calibration compounds. 

Graham (10) flocculated porous silica particles, diameter 7.6 pm, in a paddle-stirred vessel with an average shear rate of... 

Non porous silica particles (NPS) with diameters of around 1.5 jm are used in high-speed separations [11]. Retention takes place here only at the geometrical surface, which amounts to 3 m /g. By the preparation process, uniform spherical particles with narrow size distribution are obtained. 

An interesting modification of the Stober silica process has been described by Unger et al. (50). By using a mixture of TEOS and an alkyltriethoxysilane they were able to synthesize monodispersed porous silica particles. The porosity is created by the alkyl groups, which act like space holder. After calcination/burnout of the organics, a well-defined porosity is left behind in the silica particles. The materials are used for very fast high-pressure liquid chromatography. 

Benzalkonium chloride ( ) is used as an antimicrobial preservative in Nasonex. It is quantifiable using a HPLC method with a column that has a cyano group chemically bonded to porous silica particles. The HPLC was equipped with a UV-Vis detector, a 150 mm x 4.6 mm, 3- jm Spherisorb S3 CN column. The mobile phase consisted of 45% acetonitrile in 0.05 M phosphate buffer (pH 6.0). The flow rate was set at 1 mL/min. The following procedure was used for sample and standard preparations. About 2.5 g of Nasonex was transferred into a 25-mL volumetric flask. This was diluted to volume with acetonitrile and... 

Porous silica particles can be used to support alkylphenyl chains substituted by sulfonated groups or quaternary ammonium groups using covalent bonding (silica — R — NR3)+OH . This approach is similar to that used to obtain the bonded silica phases used in HPLC. Certain phases compound the particularities of each technique. In such cases, separation depends on ionic coefficients as well as the partition coefficient. 

Chromatographic System The HPLC system is equipped with a 240-nm detector and a 4.6-mm 25-cm column that contains packing LI (octadecyl silane bound to porous silica particles). The flow rate is about 0.8 mL/min. System Suitability Chromatograph the Standard Preparation, and record the peak responses as directed in the Procedure section. The relative retention times are about 0.8 for benzoylformic acid, 1.0 for mandelic acid, 2.5 for benzoic acid, 2.8 for benzaldehyde, and 3.7 for acetophenone. The tailing factor for each peak is not more than 2.0. The resolution between the benzoylformic acid peak and the mandelic acid peak, and between the benzoic acid peak and the benzaldehyde peak, is not less than 3.0. The relative standard deviation for replicate injections is not more than 1% for the mandelic acid peak. 

A supported nickel catalyst (containing 20 to 25 weight percent Ni on a porous silica particle) is typically used. The pores allow access of the reactants to the extended pore surface, which is in the range of 200 to 600 m2/g (977 x 103 to 2931 x 103 ft2/lbm) of which 20 to 30 percent is catalytically active. The concentration of catalyst in the slurry can vary over a wide range but is usually under 0.1% Ni. After the reaction is complete, the catalyst can be easily separated from the product. Catalysts are subject to degradation and poisoning particularly by sulfur compounds. Accordingly, 10 to 20 percent of the recovered catalyst is replaced by fresh catalyst before reuse. Other catalysts are applied in... 

The juBondapak C 8 is fully porous silica particles that have the surface coated with permanently bonded C)8 functional groups and is endcapped. Resolve C]8 is fully coated with the C 8 functional group and is not endcapped. 

Since the pioneering work of Knox et al. on CEC [9,10], porous silica particles have been used as the column packing material in the majority of research studies and applications. Porous silica has a number of characteristics that make it suitable for use in CEC. These are a large surface area, a high surface potential at moderate pH values, which allows the generation of a high EOF, and the commercial availability of materials with various surface chemistries. However, other support materials, such as polymeric phases [11] and alternative inorganic base materials [12], are also applicable in CEC. 

Venema et al. studied SEEC with porous silica particles [14,15]. They separated narrow polystyrene standards on columns packed with particles of different pore size, and observed a significant improvement of the efficiency in SEEC over that in pressure-driven size-exclusion chromatography. Also, they observed that the efficiency improvement was more significant for large-pore particles and related this to a higher pore flow. 

Another approach is the use of monolithic columns consisting of silica based rods of bimodal pore structure. They contain macropores (-1-2 pm) and smaller mesopores ( 10-20nm) [38]. The macropores allow for low backpressure at high flow rates. The mesopores provide the needed surface area for interactions between the solute and stationary phase. The macropores result in higher total porosity as compared to porous silica particles. Flow rates of 5 mL/min can be tolerated on a 10-cm column without an appreciable loss in... 

The solid phase is a porous silica particle with a pore diameter of 80 A. The internal surface of the pores are coated with reversed-phase moieties, while the exterior of the particle is coated with a polar group. Interferences such as proteins cannot pass through the small pores, do not interact with the polar exterior of the particles, and pass through the sorbent nonretained. Smaller analytes of interest enter the interior of the particle where they are sorbed by the reversed-phase sorbent (Fig. 12.6). 

Eklund, T. et al., Investigation of the adsorption of mono- and bifunctional silanes from toluene onto porous silica particles and from aqueous solutions onto E-glass fibers, in Silanes and Other Coupling Agents, Vol. 2, K.L. Mittal, Ed., VSP BV, Zeist, 2000, 55. 

Liquid chromatography (LC) is a physical separation technique conducted in the liquid phase. A sample is separated into its constituent components (or analytes) by distributing between the mobile phase (a flowing liquid) and a stationary phase (sorbents packed inside a column). For example, the flowing liquid can be an organic solvent such as hexane and the stationary phase can be porous silica particles packed in a column. HPLC is a modem form of LC that uses small-particle columns through which the mobile phase is pumped at high pressure. 

MIPs can be synthesized in the pores and on the surface of pre-made porous particles. Porous silica particles have been applied for this purpose. To ensure that the imprinted polymer is attached firmly to the particle, the particles are often chemically modified by coupling of polymerizable groups or initiator molecules to the particle surface prior to the MIP polymerization [100-102]. The use of immobilized initiators is often referred to as the iniferter (initiator-transfer agent-terminator) approach [103]. The method has been applied to the imprinting of a range of templates [104—107]. 

The silica samples examined by different FFF subtechniques and systems are shown in Table II. These samples have particles that fall in three categories nonporous colloidal microspheres, fumed silicas having a chainlike structure, and larger porous silica particles typical of chromatographic supports. The carriers and sample amounts injected for each type of silica analyzed by different FFF subtechniques are listed in Table III. 

For high-performance analytical applications, small polystyrene or micro-porous silica particles of 5- to 10-/xm diameter are used, with pore sizes of a few nanometers to several hundred nanometers. The controlled pore silica particles are coated with a hydrophilic phase to reduce solute adsorption. The polymeric particles can be used over a wider pH range (2 to 12) since silica is limited to pH 2 to 7. 

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