Trailing electrolyte

Isotachophoresis. In isotachophoresis (ITP), or displacement electrophoresis or multizonal electrophoresis, the sample is inserted between two different buffers (electrolytes) without electroosmotic flow. The electrolytes are chosen so that one (the leading electrolyte) has a higher mobility and the other (the trailing electrolyte) has a lower mobility than the sample ions. An electric field is applied and the ions start to migrate towards the anode (anions) or cathode (cations). The ions separate into zones (bands) determined by their mobilities, after which each band migrates at a steady-state velocity and steady-state stacking of bands is achieved. Note that in ITP, unlike ZE, there is no electroosmotic flow and cations and anions cannot be separated simultaneously. Reference 26 provides a recent example of capillary isotachophoresis/zone electrophoresis coupled with nanoflow ESI-MS. 

FIGURE 6.19 Fluorescence CCD images of tITP-ZE separation during injection (b) and after IIP concentration (c). (a) shows the general microfluidic channel configuration. Panels (b) and (c) show results obtained with a 250-pm injector. Conditions Sample was 1 mM fluorescein. Leading electrolyte 25 mM Tris+, 25 mM Cl. Trailing electrolyte 25 mM Tris+, 25 mM TAPS. Injection field 300 V/cm, current 18 pA. Separation field 200 V/cm, current 10 tl.A [634]. Reprinted with permission from the American Chemical Society. 

In the electrolyte system used in capillary isotachophoresis (cITP), the sample zone migrates between a leading electrolyte at the front and a different, trailing electrolyte at the end. The leading electrolyte contains a coion with mobility greater than that of any of the analyte ions. The trailing electrolyte contains a coion with mobility that is lower than that of any of the analyte ions. In isotachophoresis, it is possible to analyze for anions or cations, but not both simultaneously. Analyses are usually performed in the constant-current mode. 

In cITP, the sample zone migrates between a leading electrolyte and a trailing electrolyte. It is possible to analyze for anions or cations, but not for both simultaneously. 

However, there is a lack of experimental approaches to study real-time on-line events in cITP. Using the ability of NMR to track individual components in a complex mixture, sample stacking, the behavior of the leading and trailing electrolytes, and buffer components have been followed by using NMR-observ-able electrolytes [54],... 

Briefly, a solution of the polymer s uIlple is introduced at the interface between a leading electrolyte of high effective mobility and a trailing electrolyte of low effective mobility. Ssunple ions of different electrophoretic mobilities separate into individual zones with the zone order in direct relation to their effective mobilities. The zones migrate past an on-column potential-gradient or conductivity detector with a response that is proportional to the effective mobility and eunount of Ion present. 

In 1928 KendalP described experiments involving the separation of ions by means of the ionic migration technique . In 1953 Longsworth successfully separated alkaline earth ions, some amino acids and low-molecular organic acids by moving boimdary technique . He also used the names leading and trailing electrolytes for the first time. 

FIGURE 38.16 Schematic of the distribution of ions in the diffused interface between leading and trailing electrolyte in an ITP system. S is the characteristic length scale of this diffused boundary, obtained from the balance of electromigration and diffusion fluxes. 

The preconcentration factors or gains in signal that can be achieved with on-chip ITP can be quite spectacular, as will be discussed later. For a model FTP system, which consists of leading electrolyte ion (LE ), analyte ion (S ), trailing electrolyte ion (TE ), and a common leading counterion (LC "), an expression for the concentration adjustment between two zones in ITP can be derived as ... 

Isotachophoresis is a technique based on the principles of moving boundary electrophoresis. Two buffer systems are used a leading electrolyte and a trailing electrolyte. The leading electrolyte has a higher mobility than the fastest sample component likewise the trailing electrolyte has a slower mobility than the slowest component. When an electric field is applied... 

An on-line coupling of capillary electrophoresis or reversed anionic capillary isotachophoresis (CITP) with electrospray ionization mass spectrometry has been described for the separation of monophosphate nucleosides, pyridine, and flavin dinucleotides (87). The combination with CITP gives an enhancement of sample loadability and concentration sensitivity in capillary zone electrophoresis/ mass spectrometry (CZE/MS). MS-compatible buffer systems were developed (trailing electrolyte, 10 mM caproic acid pH 3.4 leading electrolyte, 7 mM HCl/ 13 mM P-alanine pH 3.9). This technique seems to be valuable for the trace analysis of DNA and RNA, e.g., to study radiation-induced DNA damage. 

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