Column bed structure

The column bed structure, in the context of column performance and column stability, has been subject to intensive studies over the years, mostly by Cherrak et al. (2002). Attempts have also been made to model the column filling mechanisms and to explain the specific feature of the column bed structure at analytical and prepara-tive/process columns. Table 3.14 lists the studies characterizing the column bed structure. 

The Waters system uses a plastic cartridge which is inserted into a device (the Z-module) that subjects the column to radial compression, ie pressure is applied along the radial axis of the column tube. The flexible wall of the column then moulds itself into the voids that are present in the wall regions of the column. This method is claimed to produce an improvement in the packed bed structure, better column performance and longer useful column life. 

The effect that the quality of the bed structure has on the chromatographic properties of columns packed with particles has been well known for a long time [1]. Similarly, the efficiency of capillary electrophoretic separations reaches its maximum for a specific capillary diameter, and then decreases steeply for both larger and smaller size [ 117]. Therefore, any improvement in the efficiency of the polymeric monolithic columns for the isocratic separations of small molecules is likely to be achieved through the optimization of their porous structure rather than their chemistry. 

Several protocols can be used to fabricate packed bed structures for use in CEC. In this chapter, we will discuss the packing techniques and column fabrication protocols that have been used for packing particulate material. We concentrate, therefore, on the different approaches used to deliver chromatographic particles into the capillary column. We present an overview of the different packing protocols available to the practitioner, as well as of the CEC column fabrication method, as performed in our laboratory. Our own experiences, practices, and views regarding packing procedures are also provided, when appropriate. 

In CEC, the EOF depends on the column packing structure and pore size of the packing material [51,111-116]. In a packed bed, there are many interconnected channels between particles, which leads to a porous packed structure. The porosity of the packed bed dictates the permeability through the column. The average channel size between particles in a CEC column can be estimated if the packed bed is assumed a... 

The A term of the van Deemter equation is independent of the mobile-phase linear velocity and describes the broadening that occurs due to the multiple flow paths present within the column. Since these paths are of different lengths, molecules will travel different distances depending on what flow paths they experience. For a column bed of randomly packed particles, the A term is proportional to the particle diameter, dp, and to a factor X related to the packing structure ... 

The results of examining the bed structure of a dc = 50 mm inner diameter silica column with Kromasil and LiChrospher silicas were (Marme, 1991) ... 

Bed structure was less dense at the wall regimes than at the core of the column... 

Simulations of an industrial scale column with structured packing have been reported by Taylor et al. (1992). They modeled a packed C4 splitter that had an internal diameter of about 2.5 m and five beds of structured packing with a total height of approximately 37 m as shown in Figure 14.33. The feed, which contains predominantly isobutane and n-butane... 

One can get that better resolution for a particular particle size without the increased pressure through radial compression technology. The need to improve resolution as in the analytical world is there, but in the preparative world there is also the need to control the bed structure over long periods of time. The use of massive overload and massive overworking of the column to serve production purposes, demands a more rigid control over the... 

If one does not control mixing volumes adequately, there will be an automatic increase in the volume of the product. If one utilizes a large volume taper at the end of the column to control fluid velocity, as has been done in the laboratory column technologies, one can get a smooth addition of sample onto the column at low linear velocities. However, in a production environment where one is going to try to optimally pump that bed structure, one can see from the Van Deemeter plot comparison to the fluid velocity profiles within this schematic of a column (Figure 3) that one will be operating at different linear velocities within that distribution oriface. This adds volume to the product, and consequently, there s a loss of resolving power within the system. 

Schematically, one can get a variety of band profiles from column design and bed structure parameters. On the top left of Figure 4 is one profile from a high linear velocity flow of the column. The center profile shows the effect of wall effect and point source distribution of sample and solvent at high linear velocities. If one eliminates the wall effect by utilizing very wide diameter columns and smaller particle packing, but still has a point source distribution of sample and solvent, one will get band distortion in the center of the band at higher fluid velocities.

If one is going to control column sequence, this allows one to utilize complex solvent sequences as well (Figure 7). For example, for certain samples one will have to put on a sample after that separation is complete, one will have to flush that bed structure and then one will have to re-equilibrate that bed structure before the next sample can be added. If one uses a segmented column system, one can operate each column independently in terms of the operation sequence, and optimally get the downstream side of the system functioning at its maximum because, at each step, there will be a product coming out of the bed. 

Time 30-45 min/m at 600 atm using a gas amplification piston pump Column fused silica 50-150 irm I. D. at least 10 cm longer than the required column length Once the column is packed the pump is switched off and the column allowed to slowly return to atmospheric pressure. An optical microscope is used to check the homogeneity of the bed structure. 

Shelly, D. C., V. L. Antonuod, T. J. Edkins, and T. J. Dalton, Insights into the Slurry Packing and Bed Structure of Capillary Liquid Chromatograhy Columns, J. Chromatogr. 458, 267-279 (1989). 

Bed - The principle base of the machine tool supporting the other parts of the machine. Column - The structural part of the machine tool support axes on the spindle side. 

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