Preface
Microthermal field-flow fractionation (micro-TFFF) is a new method for the
separation, analysis, and characterization of macromolecules and particles. The
range of applications includes not only synthetic polymers and polymer-based
colloidal particles but also biopolymers and a variety of particles of
different origin, natural inorganic as well as biological. There is no doubt
that the fundamental principle of micro-TFFF is the same as that of the
standard-size TFFF technique invented by J. Calvin Giddings, past Professor at
the
However, the apparent similarity between micro-TFFF and TFFF should not
conceal key differences. The miniaturization of the separation channel is not
only a mechanical reduction of the channel dimensions, but a rational,
theoretically justified choice of the optimal channel size and the accompanying
fundamental changes of the sample injection mode, the invention of the
hydrodynamic inlet splitting option, and many other modifications of the design
and construction details of a compact and versatile micro-TFFF separation unit.
The substantial reduction in heating power for a micro-TFFF channel compared
with standard-size TFFF channels, represents an economical gain. However, much
more important is the fact that the total heat flow produced on the hot side of
the channel must be evacuated from the cold wall. Consequently, an accurate
temperature of the cold wall is practically impossible to control with the use
of a standard-size channel, for which the electric power for heating is on the
order of kilowatts times more than for a micro-TFFF channel. The resulting
ability of micro-TFFF to control very precisely the temperatures of the hot as
well as the cold wall independently opened the opportunity to study the
behavior of retained species within a very large range of temperatures. This
fact has already contributed to a deeper understanding of the thermal diffusion
of macromolecules and particles, and opened the ways for future studies.
During the first 30 or so years of field-flow fractionation (FFF), the
research was oriented towards the exploration of various physical fields, thus
giving rise to a variety of FFF techniques, such as sedimentation FFF, flow
FFF, electric FFF. Nowadays, one of the trends is focused on a deeper
elaboration of asymmetrical-flow FFF combined with various detectors, such as
automatic pressure transducers, capillary viscometers and multi-angle
light-scattering detectors, and also combinations of the results from
asymmetrical-flow FFF and size exclusion chromatography. All these methods are
in current use in laboratories oriented towards polymer characterization. The
second trend is towards the miniaturization of analytical instrumentation with
the aim of constructing so-called lab-on-a-chip methods. Micro-TFFF is on this
path by exploiting recent inventions in micro-fluidic technologies but also by
contributing to the development in this field. One of the great advantages of
micro-TFFF, not yet fully explored, is that it is practically unlimited with
regard to the range of molar masses or particle sizes of the species that can
be fractionated and analyzed. Moreover, the experimental conditions are very
versatile because the range of temperatures, pH, etc., is very large and the
choice of solvents--often very corrosive ones--also practically without limit
due to the materials used for the construction of the micro-TFFF channel. The
cleaning of the channel is easy compared with the changing of membranes in
asymmetrical-flow FFF. On the other hand, the miniaturization of suitable
detectors has not yet followed the rapid growth of micro-TFFF channel
technology. Nevertheless, it is hoped that modern detector technologies will
lead to miniaturization.
The book is organized into five chapters. Chapter 1 is a historical
perspective on the invention and development of FFF in general. The basic
principles are briefly described. Chapter 2 is devoted to a detailed
description of the theory of FFF starting from the theory of retention, which
is related to the formation of various transverse (across-the-channel)
concentration distributions of the retained species and their coupling with the
flow-velocity profile established inside the channel in the same transverse
direction. Zone dispersion is also treated theoretically in detail. This is a
very important factor whenever macromolecules or particles are concerned,
because of their very low diffusion coefficients. Chapter 3 deals with the
theoretical bases that justify the advantages of miniaturization of the TFFF
method and thus the invention and development of micro-TFFF. The
instrumentation for micro-TFFF is described with regard to optimization of
different components within the micro-TFFF setup. Special attention is devoted
to practical experimental minimization of the zone broadening caused by the extra-channel
elements of micro-TFFF separation system as well as to optimized construction
of the channel. Chapter 4 presents a very detailed description of the primary
driving forces and separation mechanisms in micro-TFFF and of the secondary
effects that can often be exploited to increase the performances of this
method. It is shown that the contributions of secondary effects have often been
investigated with the use of complementary methods that have no direct relation
to micro-TFFF, but their use has been crucial for a complete, rigorous
understanding of secondary effects and their use. In Chapter 5, the analytical
methodology is analyzed to allow the full and accurate interpretation of
experimental data from micro-TFFF. The accuracy, precision, repeatability, and
reproducibility of micro-TFFF data are discussed from a theoretical point of
view, and the validity of theoretical predictions is justified by the
comparative experimental studies carried out in different laboratories. Some
typical examples of the methodological applications of micro-TFFF are described
at the end of the chapter. The book concludes with tables defining the symbols
and abbreviations used throughout.