Isotachophoresis (ITP) is a technique in

analytical chemistry Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...

used for selective separation and concentration of ionic

analyte An analyte, component (in clinical chemistry), or chemical species is a substance or chemical constituent that is of interest in an analytical procedure. The purest substances are referred to as analytes, such as 24 karat gold, NaCl, water, etc. ...

s. It is a form of

electrophoresis Electrophoresis, from Ancient Greek ἤλεκτρον (ḗlektron, "amber") and φόρησις (phórēsis, "the act of bearing"), is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric fie ...

; charged analytes are separated based on

ionic mobility Electrical mobility is the ability of charged particles (such as electrons or protons) to move through a medium in response to an electric field that is pulling them. The separation of ions according to their mobility in gas phase is called ion ...

, a quantity which tells how fast an

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

migrates through an

electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...

Overview

In conventional ITP separations, a discontinuous buffer system is used. The sample is introduced between a zone of fast leading

electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...

(LE) and a zone of slow terminating (or: trailing) electrolyte (TE). Usually, the LE and the TE have a common

counterion 160px, Polystyrene sulfonate, a cation-exchange resin, is typically supplied with as the counterion.">cation-exchange_resin.html" ;"title="Polystyrene sulfonate, a cation-exchange resin">Polystyrene sulfonate, a cation-exchange resin, is typical ...

, but the co-ions (having charges with the same sign as the analytes of interest) are different: the LE is defined by co-ions with high

, while the TE is defined by co-ions with low ionic mobility. The analytes of interest have intermediate ionic mobility. Application of an electric potential results in a low

electrical field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...

in the leading electrolyte and a high electrical field in the terminating electrolyte. Analyte ions situated in the TE zone will migrate faster than the surrounding TE co-ions, while analyte ions situated in the LE will migrate slower; the result is that analytes are focused at the LE/TE interface. ITP is a displacement method: focusing ions of a certain kind displace other ions. If present in sufficient amounts, focusing analyte ions can displace all electrolyte co-ions, reaching a plateau

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', ''number concentration'', an ...

. Multiple analytes with sufficiently different ionic mobilities will form multiple plateau zones. Indeed, plateau mode ITP separations are readily recognized by stairlike profiles, each plateau of the stair representing an electrolyte or analyte zone having (from LE to TE) increasing electric fields and decreasing conductivities. In peak mode ITP, analytes amounts are insufficient to reach plateau concentrations, such analytes will focus in sharp

Gaussian Carl Friedrich Gauss (1777–1855) is the eponym of all of the topics listed below. There are over 100 topics all named after this German mathematician and scientist, all in the fields of mathematics, physics, and astronomy. The English eponymo ...

-like peaks. In peak mode ITP, analyte peaks will strongly overlap, unless so-called spacer compounds are added with intermediate ionic mobilities between those of the analytes; such spacer compounds are able to segregate adjacent analyte zones. A completed ITP separation is characterized by a

dynamic equilibrium In chemistry, a dynamic equilibrium exists once a reversible reaction occurs. Substances transition between the reactants and products at equal rates, meaning there is no net change. Reactants and products are formed at such a rate that the conc ...

in which all coionic zones migrate with equal velocities. From this phenomenon ITP has obtained its name: iso = equal, ''tachos'' = speed, ''phoresis'' = migration. Isotachophoresis is exactly equal to the steady-state-stacking step in

Discontinuous electrophoresis Discontinuous electrophoresis (colloquially ''disc electrophoresis'') is a type of polyacrylamide gel electrophoresis. It was developed by Ornstein and Davis. This method produces high resolution and good band definition. It is widely used techn ...

.(1959–1964) see Ornstein*

Transient ITP

A popular form of ITP is transient ITP (tITP). It alleviates the limitation of conventional ITP that it has limited separation capacity because of analyte zone overlap. In transient ITP, analytes are first concentrated by ITP, and then can be baseline separated by zone electrophoresis. Transient ITP is usually accomplished by dissolving the sample in the TE and sandwiching the sample/TE plug between LE zones - or vice versa: a sample/LE plug can also be sandwiched between TE zones. In the first case, analytes are focused at the front TE/LE interface. Meanwhile, the back of the TE plug becomes dissolved in the LE because the faster LE ions overcome the TE ions. When all of the TE ions are dissolved, the focusing process ceases and the analytes are separated according to the principles of zone electrophoresis. tITP is nowadays more widespread than conventional ITP because it is easily implemented in

capillary electrophoresis Capillary electrophoresis (CE) is a family of electrokinetic separation methods performed in submillimeter diameter capillaries and in micro- and nanofluidic channels. Very often, CE refers to capillary zone electrophoresis (CZE), but other electr ...

(CE) separations as a preconcentration step, making CE more sensitive while profiting from its powerful separation capacities.

References

*{{cite book , title=Biochemical and biological applications of isotachophoresis , first1=Albert , last1=Adam , first2=Carlo , last2=Schots , isbn= 0-444-41891-1 , publisher=Elsevier Scientific Pub. Co , year=1980 Electrophoresis