Sample stacking technique

On-line sample-stacking techniques " and, more recently, the use of isotacho-phoresis have added to the potential benefits of CE by permitting the concentration of analyte in a large volume by exploiting the difference in the electric field between the dilute sample and system buffer. The electric field is much stronger in the dilute buffer-sample and hence analyte ions move faster until they reach the border with the separation buffer. At this point they slow down, causing the analyte to concentrate as a sharp sample band at the interface. 

Chen, Z.L., Lin, J.-M., Naidu, R. Separation of arsenic species by capillary electrophoresis with sample-stacking techniques. Anal. Bioanal. Chem. 375, 679-684 (2003)... 

Additionally, schemes employing isotachophoresis as a sample stacking technique at the beginning of the analysis have been reported. These techniques are used to concentrate the sample, and obtain greater sensitivity and resolution [64]. 

The objectives of each theoretical approach are not only the explanation of the experimental results or failures of practice but also the prediction of new possibilities to increase the sensitivity, separation capacity and velocity of the chromatographic procedure under investigation. Numerous theoretical reviews deal with the problems of the CE separation technique. In recent years the methods to enhance the precision in CE by the modification of operational parameters [113], the theory and methodological improvements of sample stacking of cationic and anionic solutes in CE [114-116], and the results and difficulties of the application of conductivity detection in CE technologies [117] have been reviewed. 

The separation performance and sensitivity of a capillary electrophoresis system coupled to a phosphorous-specific flame photometric detector (FPD) has been reported for the detection of alkylphosphonic acids l20 221. The liquid junction used to decouple the electric field from the FPD showed a negligible influence on the performance of the system as compared with online UV detection. The use of an on-column sample stacking preconcentration technique allowed for injection of 900 nL. With the large injection, the detection limits for the alkylphosphonic acids in water were 0.1-0.5 xgmL 1. 

Preconcentration is another commonly encountered sample pretreatment method that has been successfully integrated onto a CE chip. Ramsey and coworkers incorporated a porous membrane structure into a microfabricated injection valve, enabling electrokinetic concentration of DNA samples using homogeneous buffer conditions [5]. Sample preconcentration in nonhomogeneous buffer systems — a technique known as sample stacking —has also been achieved on-chip [6]. 

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