Packing Material

Overview

Here we are discussing mainly reversed-phase packings since about 80% of all HPLC separations are done on this mode.

Reversed-phase separations employ polar eluent and nonpolar (hydrophobic) stationary phase. This hydrophobic layer ("phase") has to be bonded on some rigid support to be able to withstand the high pressure in HPLC column.

The most commonly used support material is silica. Chemistry of silica is well known it could be manufactured in many different forms. Spherical porous particles with relatively narrow particle size distribution are pretty much the standard type of the support. One could find them in a big variety of different pore sizes with different surface areas. The question of the selection of the proper pore size and surface area we will discuss later in this chapter.

The only drawback of the silica based packing material is that silica readily soluble in aqueous solutions with pH higher then 7.5. If you would pump eluent with pH 9 for more than an hour you have a good chance to end up with empty column (packing material will be washed out). Chemical modification shield silica surface, and the degree of shielding significantly depends on the bonding density.

Another silica based new material is nonporous spherical silica particles of very small particle size, 1.5 m. Their unique feature is that this material has extremely narrow particle size distribution, less than 1%. But it also has very low surface area, around 2 m2/g, this significantly restrict sample loading.

Third type of support is zirconia based porous particles. Their structure pretty much similar to the silica structure. And their main advantage over silica is that it is stable in the very vide range of the eluent pH.

Forth type of the support are the polymer-based materials.

Highly crosslinked porous polymer, usually styrene-divinilbenzene is the basis for that material. It is usually formed in the porous particles. This material is stable in a very broad range of the eluent pH. It is also does not have active polar groups, but there are also some drawbacks. Depends on the degree of crosslinkage it can shrink or swell in different solvents even different solvent composition may cause it swelling. Swelling may significantly alter polymer porous structure and even block some pores causing analyte trapping.

It also may show certain specificity due to the presence of polarizable benzene rings in the polymer structure.

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