Page 53 - Tandem Techniques

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Each equation was tested against an extensive set of data by Katz et al. [30], who identified the Van
Deemter equation as the one that best fitted the experimental data. An example of a fit of their data to
the Van Deemter equations is shown in Figure 1.8. From the curve fit, the individual contributions to
the column variance per unit length depicted in the Van Deemter equation could be isolated. The
horizontal line, independent of mobile phase velocity, represents the Multipath Effect, the reciprocal
function, the Longitudinal Diffusion contribution and the linear curve the Resistance to Mass Transfer
effect. It is also seen that the index of determination is very close to unity, indicating very good
correlation between the experimental data and the Van Deemter equation. It should also be noted that in
LC, as the liquid mobile phase is virtually incompressible relative to that of a gas, the mean linear
velocity can be employed in the equation and a correction involving the inlet/outlet pressure ratio is
unnecessary.

The demand for high resolution from the column, and the need for low dispersion in the interface of a
tandem system, are in direct conflict. Low dispersion is required in the column to ensure adequate
resolution, which means the peaks must be narrow and their volume small. In contrast, the interface
requires the peaks to have as large a volume as possible, to ensure that the dispersion that must take
place in the interface has a minimum impact on the resolution. It follows that column and interface
design must always be a compromise. In some cases the ideal column that allows the most efficient use
of the tandem spectroscopic instrument cannot be employed. The design of interfaces will be discussed
in a subsequent chapter.

Capillary Electrophoresis

To date, capillary electrophoresis has not been used extensively as a separation method in conjunction
with spectroscopic or other identifying techniques, but a number of examples have been reported, so
this situation may well change in the future. For this reason capillary electrophoresis, the most likely
electrophoretic method that would be used in tandem systems, will be briefly described.