Silica practical aspects

K. A. Loftman, Cabot Corporation, Boston, Paper on Theoretical and Practical Aspects of the Rheological Pro perties of Pyrogenic Silica in Aqueous and Non-Aqueous Media, April I960. 

This chapter first discusses the characteristics of silica as it pertains to size exclusion chromatography. Next, several methods for molecular weight calibration in SEC are examined and the effects of secondary retention discussed. The chapter concludes with an overview of practical aspects associated... 

Despite these difficulties (discussed in Section 5.3.3a) API sources are currently used widely in combination with a wide range of liquid chromatography methods, i.e. normal and reverse phase, isocratic and gradient, normal bore (4.6 mm i.d.) and capillary columns, as well as ESI with capillary electrophoresis and APCI with GC. In the context of trace level quantitation, API techniques are most often used in combination with reverse phase HPLC, and it is this combination that will be the main focus of discussions of matrix effects (Sections 5.1.1 and 5.3.6a) and of the more practical aspects in Sections 9.6 and 10.10.4.1d. In this regard it is worth noting here that in reverse phase chromatography using e.g., a Cjg-derivatized silica... 

Vail (1) has extensively reviewed the practical aspects of precipitated metal silicates. In the case of magnesium silicates, which are used as adsorbents, decolorizing agents, catalysts, and fillers in paints and also for treating stomach ulcers, the composition may be varied within wide limits. The strong tendency for magnesium hydroxide to react with silica (even in the monomeric state) is shown by the fact that the addition of 300 ppm Mg(OH)j to water will reduce the soluble silica content from 42 to O.rppm. 

Song BJ, Ahn JW, Cho KK, Roh JS, Lee DY, Yang YS, Lee JB, Hwang DY, Kim HS (2013) Electrical and mechanical properties as a processing condition in polyvinylchloride multi walled carbon nanotube composites. J Nanosci Nanotechnol 13(ll) 7723-7727 Speltini A, Merli D, Dondi D, Milanese C, Galinetto P, Bozzetti C, Profumo A (2013) Radiation-induced grafting of carbon nanotubes on HPLC silica microspheres theoretical and practical aspects. Analyst 138(13) 3778-3785... 

In 1958, the first on-line GC-MS instruments were introduced. By means of the jet separator, introduced in 1964 by Ryhage [7], the practical aspects of coupling GC and MS were greatly simplified. A major breakthrough in GC-MS was realized in 1968 with the introduction of the first data-processing units [8]. In 1975, the first commercial GC-MS instruments with capillary columns were introduced. In 1981, fused-silica capillary columns were applied for GC-MS [9]. 

Chapter 3 through Chapter 8 deal with the basic aspects of the practical uses of PLC. Chapter 3 describes sorbent materials and precoated layers for normal or straight phase (adsorption) chromatography (silica gel and aluminum oxide 60) and partition chromatography (silica gel, aluminum oxide 150, and cellulose), and precoated layers for reversed-phase chromatography (RP-18 or C-18). Properties of the bulk sorbents and precoated layers, a survey of commercial products, and examples of substance classes that can be separated are given. 

AMD is the only method with which reproducible gradient elution on silica gel is possible. In column chromatography, gradient elution can in practice only be used in the reverse phase silica gel would be irreversibly changed, which, with a technique involving frequent re-use of the stationary phase, is not acceptable. This aspect is not relevant in TLC. (Note that the TLC plate is a disposable item ) [159]. 

Values of the function /(2") for various groups k can be obtained directly from Tables 10-2 and 10-6. The a, values of localized sample groups on silica decrease with increasing adsorbent activity, until for a aa 1.0 values of a, are equal to the calculated values of Table 8-4. However, this is of limited importance in practical separations, which generally use water-deactivated silica. This and other aspects of solvent localization on silica and Florisil are further discussed elsewhere (22). 

In a preferred aspect of the process of this invention, the supplementary electrolyte of sodium sulfate is employed at a concentration of about 0.03-0.08 normal. Within this concentration range, the sol electrolyte has a sufficiently low electrical resistivity to permit the use of a practically low voltage on the cell, while at the same time the colloidal silica concentration can be greatly increased in the course of the process by the continued addition of concentrated sodium silicate solution, so that a silica sol containing as much as 35 g or more of Si02 per 100 ml, can be obtained without appreciable deposition of silica on the ion exchange membranes or formation of silica aggregates or gel. 

Microporous silica hydrogen permselective membranes have been extensively studied as a potentially more practical alternative to Pd membranes. Very recently, a comprehensive review was published, tackling various aspects of silica membrane synthesis, application and economics [63]. It was made evident that state-of-the-art silica membranes have good hydrogen flux and separation, as well as respectable thermal stability. However, the hydrothermal stability of a silica hydrogen permselective membrane is a key factor in determining its suitability for a commercial apphcation of membrane-assisted processes. 

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