Drying in mobilized bed

In solid-supported LLE (SS-LLE) or liquid-liquid cartridge extraction, the aqueous sample is applied on to a dry bed of inert diatomaceous earth particles in a flow-through tube or in 96-well plate format. After a short equihbration time (3-5 nun), organic solvent is added. The organic eluate is collected, evaporated to dryness, and reconstituted in mobile phase. Compared to conventional LLE procedures, SS-LLE avoids the need for vortex-mixing, phase separation by centrifugation, and phase transfer by aqueous layer freezing. 

The idea of a mobilized bed is derived from the spout-fluid bed concept, which combines some characteristics of both fluidization and spouting techniques. The main part of the mobilized bed apparatus is the gas-distributor shown schematically in Figure 9.1. Since the drying chamber used in these experiments was square in cross-section, the distributor has a square configuration with the four walls inclined at 30° to the vertical. The lower section is... 

Comparison of the actual drying resuits with predictions from the models given by Eqs. (9.8) and (9.10) is shown in Figure 9.7. Clearly, the model based on moisture diffusion can be used to predict adequately drying kinetics of tobacco fibers in the mobilized bed dryer. 

Since the mobile phase is moving on a dry bed, several other undesirable effects occur. The adsorption of the first liquid (at the front) on the stationary phase is exothermic, causing the front to have a higher temperature than the rest of the system. Since the temperature of the system is not usually controlled but is allowed to assume the ambient value, some evaporation may occur at the solvent front. If the solvent is composed of a mixture of liquids, preferential evaporation of the most volatile one will cause a slight change in the solvent composition. In fact, the adsorption of a mixed mobile phase will probably also cause some changes in composition because the most polar component will be preferentially sorbed. The situation can become so severe that solvent demixing can occur. At best, a mixed solvent mobile phase is probably not uniform across the planar bed, and some temperature differentials probably exist as well. 

Two-dimensional development, in which a second mobile phase is run perpendicular to the first mobile phase flow on a square plate that is dried after the first development. The field has recently been reviewed by Guiochon and co-workers.24 A variation of this technique using a bed with two stationary phases was shown in Figure 10.4. 

Given its vapor pressure of 73 mmHg at 20°C, ethyl acetate will remain in the vapor phase if released to the atmosphere where it will react with photochem-ically produced hydroxyl radicals. It is expected to be quite mobile if released to soils (log Kqw = 0.73). Volatilization from both dry and moist soils is also expected. If released to water, ethyl acetate will not adsorb to suspended or bed sediments. Volatilization from water is anticipated to be an important loss process (Henry s law constant = 1.34 x 10atm m moH ). Biodegradation is also expected to be an important loss process in both soil and water. Ethyl acetate will not bioconcentrate in aquatic biota. 

If the sample is not soluble in the initial mobile phase, and the top of the column bed is accessible, one can dry load the column. This is most often used for silica gel columns where the starting mobile phase might be fairly nonpolar. In this procedure, the sample is dissolved in a solvent (for example, EtOAc or MeOH), and approx 10 times the weight of silica gel, or another inert carrier such as Celite, is added to this solution. Using a large flask (e.g., 20 times the volume of the silica gel that was added), the solvent is removed under reduced pressure using a rotary evaporator. This leaves behind silica gel on which the sample is now adsorbed. The diy powder can now be transferred to the top of the column bed and slurried with a little of the imtial mobile phase to remove air bubbles. 

To prevent diluting the sample in the mobile phase, the sample must be adsorbed on the silica. Using either gravity or a little nitrogen pressure, push the sample solution down into the sand bed without allowing the silica bed to dry out. 

Bed collapse may occur if the column has been shocked mechanically, for example, by an accidental drop on a hard floor. If a colunm has accidentally dried out, it can collapse on restart. Inuniscible solvents or the precipitation of a constituent of the sample or the mobile phase can cause a shift of the bed due to localized high-pressure drops. collapse can be caused by a continued pulsation of a malfunctioning pump, or by storing the column in the wrong solvent. For example, the bed of silica-based cyanopropyl packinp can collapse in solvents of intermediate polarity, such as acetonitrile or THF,... 

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