## 1 Introduction
### Increase in the Mass Transfer by Convection  
The use of solid catalysts with large pores has shown that mass transfer is enhanced when it occurs by convection [11]. Here, the mobile phase flows to increase the mass transfer of solutes. Therefore, polymeric supports with large pores would be necessary for high-molecular-weight solutes to be transported by convection. However, most polymers in particle form present pores up to 100 nm, not being adequate to achieve a convective transport. Therefore, particulate media with larger pores began to develop [9]. Porous poly(styrene-co-divinylbenzene) [poly(Sty-co-DVB)] particles with pores of approximately 400 nm were used for separation of biopolymers. Even with this porous size, the mobile phase flows mainly between the inter particle voids since the transport by convection in the above example [9, 12] is only 2% of the total; yet this low percentage was sufficient to improve the separation of biopolymers. Theoretical studies indicate that the maximum effect by convective transport could be achieved if the mobile phase is forced to go through the porous medium [13]. As mentioned above, particulate systems always have an important inter particle volume through which the mobile phase flows preferentially. Thus, new materials such as \"continuous\" macroporous polymeric systems were designed to reduce or avoid discontinuity and to solve the problem of flow through inter particle spaces. The major advantage of these new \"monolithic materials\" has been the increase in the mass transfer by convective transport, since in this case the mobile phase is forced to cross the whole separation medium [10]. Therefore, resolution, efficiency and the dynamic binding capacity are independent of flow rate. This variation in the passage of the mobile phase is shown in Figure 1.  
Monolithic columns create a \"single large particle\" that fills the column entirely, allowing no void. Within this monolith, a series of connected pores forms a continuous skeleton, filled with interconnected pores that form flow channels of a consistent size [14]. The monolithic column developes a network of channels in the continuous phase of a porous material that shows high axial permeability, a large internal pore surface area and less back pressure than that of conventional packed columns. Therefore, these monoliths allow performing separation processes at high flow rates and low back-pressures. The channels allow better contact between the analyte and the active sites of the stationary phase [8,10,14]. In addition, differences are found in the hydrodynamic properties. Whereas the pores are only partially used and diffusion is the major limitation in particle columns, the interphase mass transfer in monoliths is governed by convection, and the total pore volume is used. Monoliths contain micrometer-sized pores responsible for convective flow, and smaller pores in the nanometer size range representing an important factor for the sorption capacity for small solutes in the total surface area. Monolithic compressed soft gels based on poly(acrylamide) [polyAAm] were developed in 1989 by [PERSON] to separate proteins [15]. Then, monolithic materials were prepared by [PERSON] and [PERSON] [16] from organic monomers, in a tubular mold. The polymeric materials were removed from the molds and sliced to obtain disks, which were placed in a cartridge and used to separate high-molecular-weight substances. Furthering research, [PERSON] and [PERSON] [17] developed monolithic materials in a column format. These monoliths were prepared by _in situ_ polymerization into the chromatographic tube. On the other hand, the use of sol-gel chemistry to develop porous silica monoliths was introduced by [PERSON] [18,19] and [PERSON] [20]. Now then, the processes developed to improve the characteristics and performance of monolithic materials has developed into versatile and efficient alternatives to polymeric beads for a wide range of applications.