text: In analytical chromatography, the goal is to separate and uniquely identify each of the compounds in a substance. Alternatively, preparative scale chromatography is a method of purification of large batches of material in a production environment. The basic methods of separation in HPLC rely on a mobile phase (water, organic solvents, etc.) being passed through a stationary phase (particulate silica packings, monoliths, etc.) in a closed environment (column); the differences in reactivity among the solvent of interest and the mobile and stationary phases distinguish compounds from one another in a series of adsorption and desorption phenomena. The results are then visually displayed in a resulting chromatogram. Stationary phases are available in many varieties of packing styles as well as chemical structures and can be functionalized for added specificity. Monolithic-style columns, or monoliths, are one of many types of stationary phase structure.
  Monoliths, in chromatographic terms, are porous rod structures characterized by mesopores and macropores. These pores provide monoliths with high permeability, a large number of channels, and a high surface area available for reactivity. The backbone of a monolithic column is composed of either an organic or inorganic substrate in, and can easily be chemically altered for specific applications. Their unique structure gives them several physico-mechanical properties that enable them to perform competitively against traditionally packed columns.
  Historically, the typical HPLC column consists of high-purity particulate silica compressed into stainless steel tubing. To decrease run times and increase selectivity, smaller diffusion distances have been pursued. To achieve smaller diffusion distances there has been a decrease in the particle sizes. However, as the particle size decreases, the backpressure (for a given column diameter and a given volumetric flow) increases proportionally. Pressure is inversely proportional to the square of the particle size; i.e., when particle size is halved, pressure increases by a factor of four. This is because as the particle sizes get smaller, the interstitial voids (the spaces between the particles) do as well, and it is harder to push the compounds through the smaller spaces.  Modern HPLC systems are generally designed to withstand about  of backpressure in order to deal with this problem.
  Monoliths also have very short diffusion distances, while also providing multiple pathways for solute dispersion. Packed particle columns have pore connectivity values of about 1.5, while monoliths have values ranging from 6 to greater than 10. This means that, in a particulate column, a given analyte may diffuse into and out of the same pore, or enter through one pore and exit through a connected pore. By contrast, an analyte in a monolith is able to enter one channel and exit through any of 6 or more different venues. Little of the surface area in a monolith is inaccessible to compounds in the mobile phase. The high degree of interconnectivity in monoliths confers an advantage seen in the low backpressures and readily achievable high flow rates.
  Monoliths are ideally suited for large molecules; although the purification of larger molecules can be very time-consuming. As mentioned previously, particle sizes are decreasing in an attempt to achieve higher resolution and faster separations, which led to higher backpressures. When the smaller particle sizes are used to separate biomolecules, backpressures increase further because of the large molecule size. In monoliths, where backpressures are low and channel sizes are large, small molecule separations are less efficient. This is demonstrated by the dynamic binding capacities, a measure of how much sample can bind to the surface of the stationary phase. Dynamic binding capacities of monoliths for large molecules can be an order of ten times greater than that for particulate packings.
  Monoliths exhibit no shear forces or eddying effects. High interconnectivity of the mesopores allows for multiple avenues of convective flow through the column. Mass transport of solutes through the column is relatively unaffected by flow rate. This is completely at odds to traditional particulate packings, whereby eddy effects and shear forces contribute greatly to the loss of resolution and capacity, as seen in the vanDeemter curve. Monoliths can, however, suffer from a different flow disadvantage: wall effects. Silica monoliths, especially, have a tendency to pull away from the sides of their column encasing. When this happens, the flow of the mobile phase occurs around the stationary phase as well as through it, decreasing resolution. Wall effects have been reduced greatly by advances in column construction.
  Other advantages of monoliths conferred by their individual construction include greater column to column and batch to batch reproducibility. One technique of creating monolith columns is to polymerize the structure in situ. This involves filling the mold or column tubing with a mixture of monomers, a cross-linking agent, a free-radical initiator, and a porogenic solvent, then initiating the polymerization process under carefully controlled thermal or irradiating conditions. Monolithic in situ polymerization avoids the primary source of column to column variability, which is the packing procedure.
  Additionally, packed particle columns must be maintained in a solvent environment and cannot be exposed to air during or after the packing procedure. If exposed to air, the pores dry out and no longer provide adequate surface area for reactivity; the column must be repacked or discarded. Further, because particle compression and packing uniformity are not relevant to monoliths, they exhibit greater mechanical robustness; if particulate columns are dropped, for example, the integrity of the column may be corrupted. Monolithic columns are more physically stable than their particulate counterparts.