Developments in the Preparation of Organic Polymer Monoliths for the Separation of Small Molecules

Jul 31, 2014

Organic polymer monoliths are mainly used to separate macromolecules in gradient elution liquid chromatography (LC) because of their favorable porous structure. The main reason for the poor behavior of organic polymer monoliths when separating small molecules isocratically was attributed to the lack of small pores with a stagnant mobile phase, and the resulting low surface area. Recent efforts have improved the separation power of organic polymer monoliths for small molecules, offering column efficiencies up to 50,000–80,000 plates/m. This review describes recent developments in the preparation of organic polymer monoliths suitable for the separation of small molecules in the isocratic mode, and discusses the main factors affecting the column efficiency.

It has been over 20 years since Hjertén published a paper describing the development and application of a "continuous polymer bed" (1). The "macroporous polymers membranes" described by Svec and Tennikova (2) followed and "continuous rods" were later introduced by Svec and Fréchet (3). These continuous materials initiated extensive research of the novel types of stationary phases that consisted of one piece of porous material filling a whole volume of a cylindrical column, later named "monolithic stationary phases" (4). Since then, monolithic stationary phases have been established as useful members of a family of stationary phases with inorganic silica-based (5) and organic polymer-based matrices (6–8).

The morphology of monolithic materials with an interconnected network of micrometer-sized flow-through pores enables high flow of the mobile phase at moderate back-pressures and fast separations. Silica-based monoliths are suitable for the separation of small molecules (5,9) while polymer monoliths have been successfully used for fast gradient separations of synthetic and natural polymers (10). The high temperature and chemical stability of organic polymer monoliths in the separation of low-molecular-weight compounds triggered significant efforts in tailored preparations of polymer-based, highly efficient monolithic stationary phases suitable for fast and efficient analysis of low-molecular-weight compounds. This topic has been recently reviewed by Svec (11) and Nischang (12).

This article attempts to summarize current progress in the preparation of organic polymer monoliths suitable for the separation of small molecules. Numerous protocols including adjusting the polymerization mixture composition, controlling the polymerization time and temperature, and post-polymerization surface modifications are discussed.