Packing efficiencies, typical

Previously known as cyclopolyphosphates, these rings may contain up to 12 tetrahedra, but those with three, four, and six units are most common (see Table 1). The cyclotri- and cyclotetraphosphate rings adopt puckered geometries typical of saturated six and eight atom rings. The predominance of even membered cyciophosphates reflects their ability to pack efficiently in the solid state, rather than any inherent stability over odd membered ones. This is often reflected by a high internal symmetry in the crystalline state an analysis of thirty reliably determined cyclohexaphosphate structures shows that 18 have inversion symmetry and a further seven have threefold (Dsd) internal symmetry. ... 

Water has a typical surface tension of 72mN/m and, as can be seen fi om the above table, all the surfactants tested reduced the surface tension of the s em, and as a result the aqueous medium wets more efficiently. For the silicone surfactants the best results were achieved with product A, a low molecular weight material. Trisiloxane A gives a very low figure that is only improved upon by the fluorosurfactant. The Critical Micelle Concentration (CMC) is the required level of product to initiate the formation of micelles in the bulk of the liquid. Up to this point the surfactant added to die water migrates to the liquid/air interface to form a film which reduces the surface tension. The low CMC for A shows its high packing efficiency at the intoface, in the much lower level required in comparison to the other products. 

Packed cartridges and mini-columns are often used for SPE. A typical cartridge includes a plastic or glass tube with porous metal or plastic frits at both ends. It is filled with 100-500 mg of 40 pm particles. Although SPE cartridges are popular and easy to use, the relatively large particle size of the sorbent and mediocre packing efficiency necessitate the use of a relatively slow flow. 

Capillary columns may be coated with a thin, uniform liquid phase because of fused silica s smooth, inert surface, which generates a high efficiency, typically 3,000 to 5,000 theoretical plates per meter. Packed columns, on the other hand, have thicker, often nonuniform films, and generate only 2,000 plates per meter. Thus, total plates available in long capillary columns range from 180,000 to 300,000, while packed columns typically generate only 4,000 plates and show much lower resolution. 

The tower packing efficiency in vacuum distillation need not be less than in distillation at atmospheric pressure. Typical values of HETP for IMTP packing in atmospheric pressure distillations are given in Table 7-4 (see Chapter 7). Diffusion in the vapor phase is quite rapid, as long as this phase consists of the same components as the liquid phase. Due to the reduced colunm pressure, the boiling temperature of the liquid phase is lower than at atmospheric pressure. This may change the liquid physical properties significantly. In vacuum distillation, therefore, the liquid phase usually offers a substantial resistance to mass transfer. 

To minimize the multiple path and mass transfer contributions to plate height (equations 12.23 and 12.26), the packing material should be of as small a diameter as is practical and loaded with a thin film of stationary phase (equation 12.25). Compared with capillary columns, which are discussed in the next section, packed columns can handle larger amounts of sample. Samples of 0.1-10 )J,L are routinely analyzed with a packed column. Column efficiencies are typically several hundred to 2000 plates/m, providing columns with 3000-10,000 theoretical plates. Assuming Wiax/Wiin is approximately 50, a packed column with 10,000 theoretical plates has a peak capacity (equation 12.18) of... 

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