Page 160 - Multidimensional Chromatography

152 Multidimensional Chromatography

Why are these techniques so different, and how did this division of practice arise?
Helium and hydrogen are used in GC in preference to other mobile-phase gases
because of diffusion. Since there are no interactions involving the mobile phase,
gases with the fastest diffusion rates produce analyses in the shortest times. Tswett
and the other pioneers of LC chose from among a few convenient liquids to use as
mobile phases. These liquids had to have several important features. They could not
be so viscous that they would not flow through packed columns by gravity or with
the application of modest pressure. They could not be so volatile that they would
evaporate before reaching the column outlet. In short, they had to be well-behaved
liquids.
This thinking has carried through to the present day and is reflected in our choices
of mobile-phase fluids in LC: water, acetonitrile, methanol, tetrahydrofuran, hexane,
etc., are still among our popular choices. However, these particular materials are
completely dependent on the conditions of column temperature and outlet pressure.
Tswett’s original conditions at his column outlet, actually the earth-bound defaults
we call ambient temperature and pressure, determined his solvent choices and con-
tinue to dominate our thinking today.
If our ambient conditions were different from what they actually are, then we
would surely have a much different list of our favorite liquid solvents. Unified chro-
matography begins with the simple notion of rejecting ambient temperature and out-
let pressure as being necessarily correct. Instead, we simply include these
parameters with the other chromatographic variables and find the best values of all
the parameters, taken together, to accomplish our separation goals. This simple
notion leads to a greatly expanded list of possible mobile phases. It also leads to new
and different chromatographic performance characteristics that may be of great util-
ity in trying to achieve orthogonality in multidimensional separations.
The history of unified thinking has been driven by a few pioneers. Martire was
among the first to address theoretical aspects (2–7). His unified theory of chro-
matography spans a variety of mobile-phase conditions from a thermodynamics and
statistical mechanics standpoint. Ishii and Takeuchi built instruments that could per-
form GC, supercritical fluid chromatography (SFC), and LC, usually in discontinu-
ous steps involving a change in mobile phase (8, 9). From this came the concept Ishii
called Troika, after the Russian carriage drawn by three horses (GC, SFC, LC).
Bartle’s group also contributed developments in instrumentation aimed primarily at
performing multiple separation modes sequentially (10–15). Beginning in the
1960s, Giddings and co-workers did a great deal of work using dense-gas and super-
critical-fluid mobile phases. Although Giddings’ research focus turned to field flow
fractionation later in his career, his philosophical influence toward thinking about the
common or unified traits of separation processes, using a variety of fluids, fluid
propulsion means, and separation mechanisms, was immense. This is best exempli-
fied by his 1991 book, Unified Separation Science (16).