As I mentioned in the last post from this series, capillary columns of 10–100 μm inner diameter are commonly used in nano-liquid chromatography setups. They are typically made either from fused silica or PEEK materials currently found in your everyday Gas Chromatography or HPLC, respectively.
There are three basic types of capillary columns used in nano-liquid chromatography: packed, monolithic, and open tubular.
Packed Capillary Column
Packed columns are made by “stuffing” the capillary with silica-modified particles of 3–5 μm. Though recently, particles of even smaller sizes 1.5–1.8 μm were successfully employed in ultra performance LC (UPLC).
Due to the high cost and limited types of stationary phases available, many research labs pack the columns “in-house”. But it is a difficult and skill-demanding process. The trick is to make the particles of the same diameter and to avoid undesirable void volumes.
So far, the application of packed capillary columns is the most explored option in nano-liquid chromatography.
Monolithic Capillary Column
Monoliths are a block of continuous materials made of highly porous rods with two types of pore structures (macropores and mesopores of different sizes), which allow the use of higher flow rates and thus reduces the analysis time.
Monolithic polymer columns were first used in the late 1980s, but monolithic silica columns did not become commercially available until 2000. Presently four types of monolithic capillary columns can be found: particle fixed, silica based, polymer based, and molecular imprinted monolith.
Up-to-date there is not much research information on application of monolithic capillary in nano-LC.
Open Tubular (OT) Capillary Column
In open tubular liquid chromatography column, the capillary wall is coated with highly permeable porous material that serves as the stationary phase.
The OT capillary has lower sample loading capacity of the column, because only a small surface area is available for analyte interaction that can result in column overloading causing peak asymmetry and poor efficiency.