Microparticulate materials

In section 9.2.1 we discuss the special circumstances under which microparticulate materials can be taken up by specialised cells in the gut and, by way of the lymphatic circulation, reach the Uver and blood and other organs. It may be that very insoluble colloidal dmg suspensions are absorbed by this route also. 

Where impurities are present as microparticulate material filtration affords a convenient technique for solvent purification. The mobile phase containing added buffers or reagents may be filtered through a 0.5 pm or smaller filter to remove particulate matter that can damage the analytical system. The equipment for filtration is simple. Usually, it consists of an Elenmayer flask connected to vacuum and a reservoir in which a porous filter disk or membrane is placed. The porous disk is usually made from nonporous spherical glass beads (1-2 pm) and/or polytetrafluoroethylene (PTEE). Membrane materials are usually made from PTEE, cellulose, or nylon. To improve the efficiency of the separation process, the surface of the filter disks or membrane surface are often modified chemically, similar to that used for chemically bonded packing materials in RP-HPLC and/or SPE. In this case, the surface properties (hydrophobic or hydrophilic) of filters and/or membranes determine the extent of purification possible. 

Analytical hplc these days is nearly always done with microparticulate column packings, which are small porous particles, usually spherical or irregular silica, with nominal diameters of 3,5 or 10 fxm. They combine the best features of the other two types, having high efficiency as well as a large surface area. In bulk, the appearance of a microparticulate silica resembles that of a fine talcum powder. With microparticulates, dry packing methods result in column beds that are unstable under pressure, so they are packed into columns using a slurry of the material in a suitable solvent and under considerable pressure. 

The particle size of the stationary phase material plays a very vital and crucial role in HPLC. In fact, high-elficiency-stationary-phase materials have been researched and developed exclusively for HPLC having progressively smaller particle size termed as microparticulate column packings. These silica particles are mostly uniform, porous, with spherical or irregular shape, and having diameter ranging from 3.5 to 10 pm. 

The simplest way to eliminate the problems of porous particles is to eliminate the pores. Nonporous particles of 10 pm diameter with a thin layer of stationary phase on their surfaces were in fact introduced early in the evolution of HPLC packings as a solution to the pore problem, but achieved little popularity because of their low capacity. Recently, nonporous materials have returned to the marketplace in the form of very small particles with diameters of 1.5 to 2.5 pm. The use of small particles compensates to some extent for the loss in capacity.11 However, because of the high flow resistance of microparticulate nonporous packings, they are generally packed in short lengths and often operated at elevated temperatures. 

Currently, almost all the available SEC columns are packed with the high efficiency, microparticulate packings (<10u). Recent state-of-the-art developments on column packings have been described by Majors (10). A listing of such type of packing materials is shown in Table 1. 

Production of materials in which the daughter polymer and the template together form a final product seems to be the most promising application of template polymerization because the template synthesis of polymers requiring further separation of the product from the template is not acceptable for industry at the present stage. Possible method of production of commonly known polymers by template polymerization can be based on a template covalently bonded to a support and used as a stationary phase in columns. Preparation of such columns with isotactic poly(methyl methacrylate) covalently bonded to the microparticulate silica was suggested by Schomaker. The template process can be applied in order to produce a set of new materials having ladder-type structure, properties of which are not yet well known. A similar method can be applied to synthesis of copolymers with unconventional structure. 

Some progress along these lines has recently been made90) in a comparative study of some of the principal formulae quoted by Barrer 88). Most of these refer to binary composite materials consisting of phase A dispersed in microparticulate form in a continuous matrix of B. In the study in question 90), attention was first drawn to the upper and lower bounds of Eqs. (28) and (29) already mentioned, in place of the more... 

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