Open Channel Columns

While open channels are not used frequently for the flow of material other than cooling water between plant units, a weir is frequently installed for controlling the flow within the unit itself, for instance in a distillation column or reactor. 

The flow of thin liquid films in channels and columns has also served as the basis of fundamental studies of wave motion (M7), the effects of wall roughness in open-channel flow (R4), the effects of surface-active materials (T9-T12), and the like. 

If there are n0 open channels at energy E, there are n linearly independent degenerate solutions of the Schrodinger equation. Each solution is characterized by a vector of coefficients aips, for i = 0,1, defined by the asymptotic form of the multichannel wave function in Eq. (8.1). The rectangular column matrix a consists of the two n0 x n0 coefficient matrices ao, < i Any nonsingular linear combination of the column vectors of a produces a physically equivalent set of solutions. When multiplied on the right by the inverse of the original matrix a0, the transformed a-matrix takes the canonical form... 

Initially modeled as flow between infinite plates, Giddings et al. [36] showed that the performance of rectangular geometry for open channel LC columns... 

In contrast to columns, the microfabricated devices feature a network of microchannels etched in glass or imprinted in a polymer plate that are designed to enable much smaller sample volumes to be analyzed at an increased speed and permitting a large number of analysis to be performed simultaneously, thus increasing the overall throughput. Microfluidic analytical devices with open channel geometry are currently most frequently used to achieve the desired functions. Open channels are best suited for separation systems in which no interactions with functionalities located at the solid surface are required or are even undesirable, such as in electrophoresis. 

The macroporous monolith approach, introduced by Frechet and Svec, seemed to address many of the problems associated with open-tubular and particle packed columns. First, the adsorptive capacity of capillary monoliths has been found to be 3-5 orders of magnitude larger than that of both open channel and bead-packed columns [32]. Next, since the polymerization takes place within the fused-silica capillary, the tedious process of packing the capillary columns may be avoided. Furthermore, the limitations in chromatographic efficiency caused by irregularities in particle packing and by the nonuniformity of particle sizes are eliminated. 

Packed column beds, packed channels, or open channels can be constructed on microchips made from glass or organic polymers. The channels in glass chips are made with etching techniques. The mobile phase is delivered by a pump or by an electric potential. The chip connects either to a fluorescence detector or to a mass spectrometer with a docking station. The inner diameters of the channels/ columns are currently approximately 50-75 pm and the lengths are 5-15 cm. Specially made valves result in very low dead volumes. The chip mode was initially thought to be able to handle disposable chips, but so far the commercial products are made for multiple use and are priced at the same level as the conventional columns. 

The column itself can be produced as a single open channel or as a branched system of channels starting from one microchannel and ending in another microchannel. The latter type is often named as COMOSS (collocated monolith support structure). With some of the materials mentioned, the channels can serve as stationary phase itself or can be functionalized by coating, packing, or incolumn polymerization with appropriate chromatographic phases. 

Typical dimensions for an open channel column are 20 pm x 60 pm x 10-15 cm. The disadvantage with open channel columns is that there is only a limited sized surface on which chromatographic interactions can take place, and which in its turn limits the capacity of the column. This can partly be circumvented by coating the walls with porous particles. 

Packing the open channels with functionalized particles, as in conventional liquid chromatography, is challenging due to reproducibihty issues and difficult immobilization of the particles. However, at present, there are excellent commercially available chromatography on a chip devices with columns slurry-packed with a great variety of stationary phases. 

By performing in situ polymerization, monolithic columns can also be produced. This also greatly improves the surface area compared to open channel columns. These monoliths can be functionalized during the polymerization or afterward. 

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