  Isotachophoresis (ITP) is a versatile electrophoretic technique which can be
used for sample preconcentration, separation, purification, mixing, and control
and acceleration of chemical reactions. Although the basic technique is nearly
a century old and widely used, there has been a persistent need for an easily
approachable, succinct, and rigorous review of ITP theory and analysis. This is
important as interest and adoption of the technique has grown over the last two
decades, especially because of its implementation into microfluidics and
integration with on-chip chemical and biochemical assays. We here provide a
review of ITP theory with a strong emphasis on steady and unsteady transport
starting from physicochemical first principles including conservation of
species, conservation of current, the approximation of charge neutrality, pH
equilibrium of weak electrolytes, and so-called regulating functions governing
transport dynamics. We combine these generally applicable (to all types of ITP)
theoretical discussions with applications of ITP in the field of microfluidic
systems, particularly on-chip biochemical analyses. Our discussion includes
principles governing ITP focusing of weak and strong electrolytes, ITP dynamics
in peak and plateau modes, review of simulation tools, experimental tools and
detection methods, applications of ITP for on-chip separations and trace
analyte manipulation, and design considerations and challenges for microfluidic
ITP systems. We conclude with remarks on possible future research directions.
The intent of this review is to help make ITP analysis and design principles
more accessible to the scientific and engineering communities, and to provide a
rigorous basis for increased adoption of ITP into microfluidics.
