Sample preconcentration isotachophoresis

Whether the stacking technique is called field-amplified stacking, transient isotachophoresis, or sample self-stacking, the driving force in on-column sample preconcentration for charges species is the need to satisfy the initial KRF conditions. Under separation and sample matrix conditions where the KRF is difficult to determine, simply calculations such as flcut and a offer a rapid evaluation of conditions for optimal transient ITP. 

Isotachophoresis (ITP) was, like FASS, originally developed for CE, but it relies on zero EOF. ITP is a quantitative analytical separation technique in its own right, with a wide range of applications. It has also been successfully applied in conjunction with CE for both sample preconcentration and removal of highly abundant charged species, by coupling the FTP and CE columns. The first... 

Several strategies have been described for the preconcentration of sample components present at low concentrations. These techniques include zone sharpening,28-29 on-line packed columns,30 and transient capillary isotachophoresis (cITP).31-32 Other standard laboratory techniques are often used, including solid-phase extraction, protein precipitation, ultrafiltration, etc. Two important points to keep in mind when selecting a concentration protocol are the sample requirements of the method and the potential selectivity on relative concentrations of sample components. The latter point applies to purity and concentration analysis. 

And CITP is favorably utilized in the analysis of low-molecular-weight ionic species. A difficulty often arises with finding suitable buffer systems that provide leading and terminating ions and also form the appropriate buffer pH. One of the advantages is that the capillary can be loaded with sample up to 30-50% of its length, enabling the analysis of very dilute samples. Furthermore, the principle of predetermined solute concentrations in isotachophoresis is also used as preconcentration step for very dilute samples prior to CZE, MEKC, or CGE. 

Mohamadi et al. [96] reported online preconcentration of human serum albumin (HSA) and its immunocomplex with a monoclonal antibody on-chip coupled to isotachophoresis. The sample injection, preconcentration, and separation were carried out continuously and controlled by a sequential voltage switching program. Preconcentration was carried out with on-chip nondenaturing gel electrophoresis in methylcellulose solution. Furthermore, the authors applied this method for immunoassay of HSA. The separation of HSA and its immunocomplex was achieved in 25 seconds in 1 cm of the microchannel with induced fluorescence detection at 7.5 pM. 

Finally, it is worth noting that limits of detection of a system mentioned throughout the chapter depend on the separation performance, preconcentration steps, and mode of operation (i.e., zone electrophoresis, electrochromatography, isotachophoresis, etc.) of the separation system. Thus, one can often optimize a CE method to improve the overall system performance, given the sample stacking and other approaches that may be appropriate for a particular sample type or application. " ... 

FAS can be implemented on microchips in a very similar manner as capillary electrophoresis. However, the requirement of low ionic strength sample buffer for FAS puts limitation on its use as general preconcentration technique. Variations of the technique, such as transient isotacho-phoresis and micellar electrokinetic sweeping, have been more successfully used. Jung et al. reported (Mi-chip transient isotachophoresis... 

On-tube detection is almost universally used in CE, and the short detection path lengths provided by fused silica capillaries result in a 100- to 400-foId reduction in detection sensitivity compared with HPLC. Although various techniques are employed in CE to regain most of this sensitivity, detectivity remains a serious concern. Therefore concentration steps are often necessary in sample preparation for CE. It should be emphasized that some concentration techniques such as evaporation and lyophilization will not change the sample ion/salt ion ratio for nonvolatile salts, so no effective concentration is achieved when electromigration injection is used. The use of transient isotachophoresis has been proposed for on-line preconcentration of proteins prior to CZE separation [41,42],... 

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