Monoliths and small particle-packed columns for liquid chromatography
Advances in chromatography have led to two recent active areas of study: the reduction of particle size for column packing and development of monolithic materials. Reducing the size of the packing material leads to an increase in chromatographic efficiency and a decrease in the analysis time. The monolithic column is composed of one single piece of material with through-pores that offers very high permeability, thereby, allowing the column can offer a variable external porosity and operation at high linear velocity with low pressure requirements. The performance of commercially available columns, a monolith and a sub-2 gm particle packed column was examined. Higher efficiencies were obtained with the sub-2 gm particle packed column. Very high pressures 14,000 psi were required to operated the sub-2 gm packed column to obtain a linear velocity of 6.4 mm/sec, while a pressure of 1,600 psi was required to achieve a linear velocity of 12 mm/sec with the monolithic column. Examination by the van Deemter model shows that the monolithic column behaves like a 3 gm packed column. A new approach to synthesize monolithic columns for capillary LC and capillary electrochromatography (CEC) was also examined. A silica based hybrid monolithic material was synthesized and characterized. The material fabricated inside capillary columns consisted of an allyl-functionalized monolith, which was prepared in a one-pot reaction. The material was characterized by SEM, nitrogen absorption (BET method), and chromatographically by CEC and CLC. The chromatographically tested column with best performance had a surface area of about 100 m 2/g with through pores 7 gm and 6 nm mesopores. A stationary phase for CEC was synthesized by polymerizing n¬isopropylacrylamide on aminated silica particles. The poly-n-isopropylacrylamide was characterized by IR spectroscopy and thermogravimetric analysis. Both IR spectroscopy and thermogravimetric thermograms showed the grafting of the poly-n¬isopropylacrylamide on the aminated silica. Tested under CEC conditions, using a series of neutral and acidic compounds, the stationary phase showed a mixed mode separation mechanism with both hydrophobicity and hydrophilicity contributing to the separation.