Handbook of Size Exclusion Chromatography and Related ...

polymers. References describing the use of controlled pore glass in
chromatography have been compiled in a commercial bibliography (21).
This chapter first discusses the characteristics of silica as it pertains to size
exclusion chromatography. Next, several methods for molecular weight calibration
in SEC are examined and the effects of secondary retention discussed. The chapter
concludes with an overview of practical aspects associated with the application of
size exclusion chromatography.
2 PROPERTIES OF SILICA
Silicon dioxide (SiO2), silica gel, or silica is the most abundant compound in the
Earth’s crust. Many industries depend on it being readily and abundantly available
in relatively pure form. Traditionally, silica has been an important natural resource
for the glass industry. More recently, ultrapure silica particles have become the raw
material for manufacturing computer chips. Other common applications of silica
include its widespread use as a drying agent, food ingredient, and its incorporation
in floor waxes to impart nonskid properties (22). The properties of porous silica
and its use as a support in column liquid chromatography (LC) were described in a
book by Unger (23). The chemistry of silica is the topic of a comprehensive book
by Iler (24). Silica as a backbone of LC column packings was recently reviewed by
Berthod (25). Henry discussed the design requirements of silica-based matrices for
biopolymer chromatography, including their use in SEC (26).
2.1 Structure, Synthesis, and Purity
Silica gel has an amorphous structure, is highly porous, and exhibits a very large
surface area, most of which is located in the pores. It consists of a threedimensional
network of SiO2 repeating units with siloxane and silanol terminal
units on the surface. Silica gel can be synthesized into particles ranging in
diameter from millimeters to micrometers; the particle size of silica sols (colloids
consisting of discrete silica particles—nonporous, spherical, and amorphous) is in
the nanometer range. Refer to Refs 22–24 for thorough treatments of the synthesis
of silica gel particles for use in chromatography.
The purity of silica has been a topic of debate among those studying
interactive modes of liquid chromatography. The effect of metal ion impurities on
the retention of basic solutes and chelating compounds was first addressed by
Verzele et al. (27). Depending on the manufacturing process, chromatographic
silica gel contains impurities in concentrations ranging from low to high parts per
million. Although to the knowledge of the authors this issue has not yet been
discussed in the context of silica-based size exclusion chromatography, it is
expected that the use of high-purity silica gels can lead to further improvements
in obtaining true SEC retention behavior, as well as improved recovery of
© 2004 by Marcel Dekker, Inc.