Sample introduction electrokinetic method

In most cases, sample introduction on-chip is achieved using electrokinetic (EK) flow [3]. Two important EK injection modes, namely, pinched injection and gated injection, have been developed. Furthermore, some alternative injection methods are described. 

In sample introduction for CGE separation of oligonucleotides, hydro-dynamic method cannot be used, and an electrokinetic method must be used. Why (2 marks)... 

The sample can be introduced into the capillary by several methods. The simplest approach is to remove the end of the capillary from the anode buffer reservoir and place it in the sample vial that has been elevated slightly above the level of the cathode buffer container. Gravity flow for several seconds will move some of the sample in the separation capillary. Another approach is to place the anode end of the capillary into the sample vial and apply pressure to the analyte solution. The next method involves placing the anode end of the capillary in the sample vial and applying a vacuum to the cathodic side of the capillary to draw solution into the tube. The previous three means of sample introduction are referred to as hydrodynamic modes of injection. The last method involves placing the anodic end of the capillary in the sample vial and applying alow voltage for several seconds. This approach is referred to as electrokinetic injection. 

Reproducible sample introduction is a crucial factor in p-chip-based electrophoretic separations. Various p-chip sample introduction schemes are illustrated in Fig. 2. Of the many proposed injection methods, electrokinetic injection based on electro-osmotic flow (EOF) is most commonly encountered on chips, because electrically driven fluid flow is easier to generate and control than pressure-driven flow. Electrokinetic sample injection is generally... 

The third method of sample introduction is electrokinetic injection in which a potential is applied between the sample vial and the outlet end of the capillary. The sample is drawn into the capillary by the same forces that will cause it to migrate toward the detector, electrophoretic mobility and electroosmotic flow. The amount of each solute injected (Q) is determined by... 

Sample Introduction Sample can be introduced into the capillary through different ways. One way is to dip the capillary into the sample vial and then applying pressure to force the sample into the capillary. An alternative method is to dip the capillary into the sample vial along with the electrode and then to apply appropriate voltage to force the sample into the capillary. Of course, this method works only with ionized samples. The latter method of sample introduction is called the electrokinetic sampling method. 

Electromigration is the simplest injection method in CE the capillary inlet is immersed in the sample solution and high voltage is applied for a brief period (typically a few seconds). If no electroosmotic flow is present, sample ions enter the capillary by electophoretic mobility alone this mode of sample introduction is termed electrophoretic injection. If EOF is present, sample ions will be introduced by a combination of electrophoretic mobility and electroosmotic flow this mode is generally termed electrokinetic injection. 

The introduction of the samples onto the capillary column can be carried out by either displacement techniques or electrokinetic migration. Three methods of displacement or hydrostatic injection are available a) direct injection, or pressure b) gravity flow, or siphoning and c) suction. The electrokinetic injection method arose from findings that electroosmosis act like a pump (80). Both methods have advantages and disadvantages. For example, a bias has been reported in electrokinetically injected... 

An introduction to electrokinetic phenomena can be found in [240] and in handbooks of colloid chemistry. The choice of method and instrument suitable for the character of a sample is key to successful electrokinetic measurements. In principle, all techniques and all instruments should produce the same potential and the same IEP in a system of interest. A few multi-instrument studies have been published. For example, [241] reports lEPs obtained by streaming potential and by electrophoresis (using a commercial apparatus). A multi-instrument electrokinetic study of alumina in O.OIM NitNO, is reported in [242]. The IEP was also relatively consistent with different solid-to-liquid ratios. Glass capillaries with inner sides coated with spherical nanosize hematite particles showed an IEP at pH = 5, while the IEP of the original hematite obtained by electrophoresis was at pH 9.3 [243]. 

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