Page 40 - Tandem Techniques

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maximum available pressure is employed, then time must be sacrificed if very high resolution is
required.

The commonly used stationary phases in LC are silica, bonded silica's (e.g. the reverse phases) and
micro-reticulated cross-linked polystyrene resins. Silica is manufactured in a wide variety of pore sizes,
the magnitude of which can significantly affect its chromatographic performance and the properties of
any bonded phases made from it. A graph showing the distribution of pore size with pore volume is
shown in Figure 1.7. The curves show that the distribution of pore diameters can differ greatly between
different silica gels. In fact, silica gel can be used as an exclusion medium for separation on the basis of
molecular size [15].

Figure 1.7
Graph of Pore Volume per Gram of
Silica Gel against Log Pore Diameter

The range of pore sizes that can occur in a silica gel, including bonded silica, render LC a poor
technique for thermodynamic measurements and solute identification from retention data. This is
because each solute has a unique dead volume and, depending on its molecular size, interacts with a
unique fraction of the available stationary phase 115]. This molecular size dependence has a very strong
effect on the capacity ratio (k') of a solute which is inversely proportional to the dead volume of the
solute. For the want of a better chromatographic measurement (k') values are often