The concept of “stagnant pools” of mobile phase being trapped within chromatographic packings is one
of the most useful when attempting to explain the characteristics and developments in LC column
technology. To overcome inefficiencies produced by the mobile phase mass transfer phenomenon it is
necessary to minimize the pores or sites where mobile phase is able to stagnate. In the following
sections, it will become apparent that this effect can be minimized by either making the internal pore
structure impervious, reducing the overall diameter of the column packing material or preparing
supports with very wide pores so that liquid can flow easily in and out or even through the particles. The
ultimate aim in the development is to achieve a high inherent efficiency, i.e., low HETP value, which
remains essentially unchanged by the mobile phase velocity. In Fig. 3.3 such a performance might be
indicated by a straight-line plot of HETP versus velocity parallel with, and close to, the horizontal axis.
Having achieved such a performance it would be reasonable to suppose the velocity of the mobile phase
could be increased indefinitely to achieve faster and faster analyses. Understandably there is a limit to
this supposition, usually measured in terms of the capabilities of the chromatograph being used. These
limitations will become apparent in forthcoming paragraphs and in the chapters dealing with
chromatographic instrumentation.