Page 15 - Multidimensional Chromatography

4 Multidimensional Chromatography

demonstrated. None the less, practical difficulties associated with capillary column
technology generally restricted open tubular GC to a minority of applications until
the fused silica column revolution in 1979. Dandeneau and Zerenner realized (11)
that manufacturing methods for fibre-optic cables could be applied to make robust
and durable capillary tubes with inactive inner surfaces. Lee et al. then delineated
(12) the chemistry underlying the coating of such capillaries with a variety of sta-
tionary phases, and the age of modern high-resolution GC was born. Small diameter
fused-silica capillaries were also found by Jorgenson and Lukacs (13) to be suitable
for electrodriven separations since the heat generated could be readily dissipated
because of the high surface-area-to-volume ratio. The invention of capillary super-
critical fluid chromatography (SFC) in 1981 by Lee, Novotny and, co-workers (14)
also depended on the availability of fused-silica capillary columns.
Liquid chromatography, however, took a different course, largely because slow
diffusion in liquids meant that separations in open tubes necessitated inner diameters
which were too small to make this approach practical. On the other hand, greatly
increased efficiencies could be achieved on columns packed with small silica parti-
cles with bonded organic groups, and the technology for such columns was made
available following the pioneering work of Horvath et al. (15) and Kirkland (16),
thus giving rise to high performance liquid chromatography (HPLC). Even so, the
available theoretical plate numbers (N) are limited in HPLC at normally accessible
pressures and a different separation principle is therefore made use of. Since the res-
olution, R, for the separation of two compounds with retention factors k 1 and k 2 is
given by:
√N 1 k 2
R (1.1)
4 1 k 2
where , the selectivity, is k 2 /k 1 , it follows that increased resolution based on column
efficiency can only be achieved by very large increases in column length, because of
the square-root dependence of R on N. However, a small increase in has a major
influence on R, and selectivity is therefore the principal means of achieving separa-
tion in HPLC through the tremendous variety of differently bonded stationary phase
groups.

1.2 PACKED CAPILLARY COLUMN AND
UNIFIED CHROMATOGRAPHY

Small-diameter packed columns offer (17) the substantial advantages of small volu-
1
metric flow rates (1–20 ( L min )), which have environmental advantages, as well
as permitting the use of ‘exotic’ or expensive mobile phases. Peak volumes are
reduced (see Table 1.1), driven by the necessity of analysing the very small (pico-
mole) amounts of substance available, for example, in small volumes of body fluids,
or in the products of single-bead combinatorial chemistry.
The increasing use of microcolumns has moved chromatography towards unifica-
tion. Giddings was the first to point out (18) that there was no distinction between