ChromFAQ:MonolithicColumn

From ChromFAQ

What is a "monolithic" column?



For over a century, liquid chromatography columns have been made by packing an appropriate powder into a tube. The resulting column has come to seem so natural that we oten refer to the stationary phase as the "column packing". An idealized sketch of a the inside of a packed column is shown at right. The stationary phase particles themselves are usually porous (this sketch is not drawn to scale, the pores are much smaller than we can display!). In general, smaller particles provide better results: because they allow faster equilibration of analyte molecules between the mobile phase and the stationary phase, small-particle packings can be used at higher mobile phase flow velocity without loss of efficiency. The catch is that the spaces between the packing particles are also small, which means a high flow resistance and high operating pressures.


An alternative to packing a particulate stationary phase into a column is to generate a column of porous stationary phase in situ. This involves some tricky chemistry because the resulting rod or monolith must have a bimodal pore size distribution with macro (large) pores interconnecting to allow liquid flow through the monolith and meso (smaller) pores, comparable to the pores in conventional packings. An idealized sketch of a monolith structure is shown at the right (again, the pores are not drawn to scale). The advantage is that the macropores can be made much larger than the inter-particle distance in a conventional packed column. That translates to lower flow resistance and a lower operating pressure for a given flow rate. Alternatively, a longer column can be used at a given flow rate and operating pressure.


Although work on monoliths goes back a decade or so, they still represent a small fraction of LC columns. Some of the drawbacks are: - monoliths are made one at a time, rather than in large batches like conventional packings, thus raising a greater risk of column-to-column variability - "standard" diameter columns (in the range of 4 mm i.d.) are not commercially available in lengths greater than 100 mm. This is arguably the result of manufacturing limitations; stationary phase polymerization is exothermic (heat dissipation can be an issue) and is accompanied by shrinkage (the fragile monolith must be able to support itself at times during the process). Capillary monoliths are easer to make in this respect and are available in longer lengths. As with capillary columns generally, however, they are not "plug and play" with older instruments designed for standard columns. - patent and technology issues limit the number of suppliers, so a more limited range of stationary phase chemistry is available.

More detailed information on monolithic columns can be found in the following LC-GC Magazine (http://www.lcgcmag.com/lcgc/) articles:


Comments on monoliths vs. packed columns also came up in a wide-ranging thread on Chromatography Forum in November, 2004 (http://www.sepsci.com/chromforum/viewtopic.php?t=977)

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