Permeability of Monolithic Stationary Phases

The absolute permeability (8 x 10 ° cm ) of currently available silica monoliths (Chromolith, Merck) is similar to that of columns packed with 9 jm particles [213], To increase efficiency, columns are usually packed with particles of 2, 3, and 5 jm, which possess absolute permeabilities of approximately 4x10 , 9x10 , and 2.5xl0 °cm, respectively. If we try to compare columns packed with 5 pm particles and monolithic columns with through-pores of an average size of 1.5 pm, we observe that the permeability of the monolithic columns is around three to four times larger than that of packed columns. 

A study of the influence of the average sizes of the through-pores on the performance of a series of monolithic columns was carried out by Motokawa et al. [214]. This group found permeabilities between 8 x 10 ° and 1.3 x 10 cm.  

A comprehensive survey of monolithic stationary phases that have been inttoduced in literature together with their key applications in separation science can be found in Table 1.1. 

Styrene monoliths have been prepared by thermally (AIBN or benzoyl peroxide) initiated copolymerization of styrene and divinylbenzene to result mechanically stable, hydrophobic column supports for RPC as well as IP-RP-HPLC and CEC application [24,49,134-140]. 

Methacrylate monoliths have been fabricated by free radical polymerization of a number of different methacrylate monomers and cross-linkers [107,141-163], whose combination allowed the creation of monolithic columns with different chemical properties (RP [149-154], HIC [158], and HILIC [163]) and functionalities (lEX [141-153,161,162], IMAC [143], and bioreactors [159,160]). Unlike the fabrication of styrene monoliths, the copolymerization of methacrylate building blocks can be accomplished by thermal [141-148], photochemical [149-151,155,156], as well as chemical [154] initiation. In addition to HPLC, monolithic methacrylate supports have been subjected to numerous CEC applications [146-148,151]. Acrylate monoliths have been prepared by free radical polymerization of various acrylate monomers and cross-linkers [164-172]. Comparable to monolithic methacrylate supports, chemical [170], photochemical [164,169], as well as thermal [165-168,171,172] initiation techniques have been employed for fabrication. The application of acrylate polymer columns, however, is more focused on CEC than HPLC. 

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