Amino Acid Measurements from a High Conductivity Matrix by Gradient Elution Isotachophoresis

Isotachophoresis: Theory and Microfluidic Applications

Isotachophoresis (ITP) is a versatile electrophoretic technique that can be used for sample preconcentration, separation, purification, and mixing, and to control and accelerate chemical reactions. Although the basic technique is nearly a century old and widely used, there is a persistent need for an easily approachable, succinct, and rigorous review of ITP theory and analysis. This is important because the interest and adoption of the technique has grown over the last two decades, especially with its implementation in microfluidics and integration with on-chip chemical and biochemical assays. We here provide a review of ITP theory starting from physicochemical first-principles, including conservation of species, conservation of current, approximation of charge neutrality, pH equilibrium of weak electrolytes, and so-called regulating functions that govern transport dynamics, with a strong emphasis on steady and unsteady transport. 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 that govern the ITP focusing of weak and strong electrolytes; ITP dynamics in peak and plateau modes; a 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 the increased adoption of ITP in microfluidics.

Gradient counterflow electrophoresis methods for bioanalysis

CE has evolved as one of the most efficient separation techniques for a wide range of analytes, from small molecules to large proteins. Modern microdevices facilitate integration of multiple sample-handling steps, from preparation to separation and detection, and often rely on CE for separations. However, CE frequently requires complex geometries for performing sample injections and maintaining zone profiles across long separation lengths in microdevices. Two novel methods for performing electrophoretic separations, gradient elution moving boundary electrophoresis (GEMBE) and gradient elution isotachophoresis (GEITP), have been developed to simplify microcolumn operations. Both techniques use variable hydrodynamic counterflow and continuous sample injection to perform analyses in short, simple microcolumns. These properties result in instruments and microdevices that have minimal ‘real-world’ interfaces and reduced footprints. Additionally, GEITP is a rapid enrichment technique that addresses sensitivity issues in CE and microchips.