ABSTRACT
The first generation of silica monoliths for chromatographic separations was introduced almost 20 years ago by Nakanishi, Soga, and Tanaka [1-3] and was commercialized by Merck a few years later [4]. At that time, the search for faster and more efficient separations had resulted in efforts to reduce the diffusion path length by decreasing the particle size. The faster mass transfer obtained by the smaller particle size led to a dramatic increase in column back pressure making high operating pressures or high temperatures mandatory to obtain acceptable separations. As a possible solution for this problem, attempts were made to reduce the diffusion path length, while maintaining the size of the convection path. This resulted in the introduction of continuous silica gel monoliths with a broad range of well-defined, controllable pores in the micrometer range (throughpores) as well as nanometer range mesopores. In contrast with particle-packed columns, the size of the monolith silica skeleton could be controlled independently from the size of the interstitial voids within certain limitations. This led to an initial introduction of silica monoliths with skeleton sizes of ∼1 µm, throughpores of ∼2 µm, and mesopores of 10-25 nm in the silica skeleton. The ratio of throughpore-to-skeleton size larger than unity resulted in porosities that were more than 20% higher than those of particle-packed columns. This gave rise to large permeability values allowing monolithic columns to be used at high linear velocities or in long column lengths leading to unprecedented efficiencies [5]. The small size of the silica skeleton branches on the other hand resulted in efficiencies that were comparable or even better than those obtained on 5 µm particle-packed columns, especially when the columns were operated at high flow rates. For high molecular weight solutes, the silica monoliths performed better than their 5 µm particle-packed counterparts due to the shorter diffusion path lengths of the silica monoliths.
