Packed drawbacks

Eor a long time there have been discussions about which type of sorbent is the best for SEC separations in various mobile phases. In principle, organic (copolymer) and inorganic packings can be used. Each type of packing has its benefits and drawbacks. Table 9.3 summarizes major sorbent properties and reveals some interesting aspects of SEC separations and its requirements on packings. 

SEC columns have become much more efficient since they were introduced in the late 1950s. The major factor for this has been the ability of synthetic polymer chemists to produce smaller particle sizes of column packing materials. The first sorbents were several 100 /mm wide in diameter (20), whereas modem columns are filled with particles in the range between 3 and 20 /mm, which caused an immense improvement in separation power. The major drawback... 

Nevertheless, the simple and reliable coil spring seal has proven itself in the pumping industry and is often selected for use despite its drawbacks. In regulated industries, this type of seal design far exceeds the capabilities of a compressed packing ring seal. 

Analytical (flow-pattern) characterization is more difflcult as the particle bed is not transparent and covers most of the flow-through chamber. Another drawback stems from the size distribution of the particles of the catalyst bed, giving interstices which vary in typical dimensions. Here, however, today s considerable efforts in nano- and micro-material research may provide regular, mono-sized particles in the near future which will allow one to create much improved micro flow-packed beds. 

Modem technologies provide many techniques for expanding the throughput of an analytical laboratory. The task that needs to be accomplished and the possible drawbacks should be carefully considered. Optimized LC equipment can utilize columns packed with much smaller stationary phase particles to achieve significant reductions in gradient time while still achieving the same or even better peak capacities than conventional methods. 

Reproducibility of monolithic columns has also been cited as a major concern because the monoliths are manufactured individually.34-35 An extensive study by Kele and Guiochon indicates that the reproducibility results of Chromolith columns were almost comparable to those from different batches of particle-packed columns.37 Other drawbacks of monolithic columns include weak reten-tivity for polar analytes,38 efficiency loss at high flow rates for larger (800 MW) molecules,39 and peak tailing, even for neutral non-ionizable compounds.36-38-40 Furthermore, silica-based monolithic... 

Modified silica with a C18 reversed-phase sorbent has historically been the most popular packing material, owing to its greater capacity compared to other bonded silicas, such as the C8 or CN types [22]. Applications of C18 sorbents include the isolation of hydrophobic species from aqueous solutions. The mechanism of interaction with such sorbents depends on van der Waals forces, and secondary interactions such as hydrogen bonding and dipole-dipole interactions. Nevertheless, the main drawbacks of such sorbents are their limited breakthrough volumes for polar analytes, and their narrow pH stability range. For these reasons, reversed-phase polymeric sorbents are also used frequently in environmental applications for the trace enrichment of soluble molecules that are not isolated by reversed-phase sorbents such as C18. 

A continuous-bed or monolith is a capillary containing a wall-supported porous continuous bed that is formed in situ. These columns have been developed for CEC use in recent years.The surface chemistry can be functionalized to convert it into a phase with the desired chromatographic properties. Monolithic columns are stable and have shown great potential for CEC due to the absence of a requirement for retaining frits, thereby eliminating the drawbacks in OT-CEC and packed columns. 

---