Electrokinetic Sample Introduction

Sample preconcentration techniques are used with two purposes (1) to increase concentration in order to achieve detection and (2) to eliminate disturbances of the electrophoretic system during hydraulic or electrokinetic sample introduction when the conductivity of the sample is significantly different from that of the analysis buffer. It is important to keep sample manipulations and modifications to a minimum, and a rule of thumb is to prepare the sample so that its composition is at the same pH as the analysis buffer. It is also advantageous... 

Electrokinetic sample introduction is performed using the electroosmotic flow (EOF) in the microchannel as a pump and can easily be controlled by high outside voltages. 

Electrokinetic sample introduction is known to be matrix dependent therefore, hydrodynamic sample introduction is normally preferred. If electromigration injection is to be employed, matrix effects should be investigated and standard addition calibration rather than external standard calibration should be used. 

Samples were usually transferred to the microfluidic system by electrokinetic injection and pressure injection. Electrokinetic sample introduction is most commonly used for transferring samples to chips. This can be attributed to factors such as the simpUcity in achieving electroosmotic flow (EOF), i. e., no moving parts and minimal backpressure effects. The EOF in the microchan-nel acts as a pump and can easily be controlled by outside high voltages. The two commonly employed injection modes for microfluidic chips are time-based and discrete volume-based injection. In the case of the time-based (or gated injection), the amount of sample introduced into the carrier stream can be controlled by adjusting the injection time. 

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)... 

Another possible solution to this problem has been implemented by Attiya et al. [97]. The device contained a large sample introduction channel with a volume flow resistance >105-fold lower than that in the analysis microchannels. This approach enabled interfacing the large sample introduction channel with an external pump (up to 1-mL/min flow rate) for pressure-driven sample delivery without perturbing the solutions and electrokinetic manipulations within the... 

With replaceable gels, either an electrokinetic or a hydrodynamic approach to sample introduction is possible. The hydrodynamic injection is generally preferred when quantitation of the PCR product is desired. In this case, the sample, introduced as a plug into the capillary, is exactly the same as that of the sample vial from which it originated. Negative components (Cl", dNTPs,... 

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... 

Research stiU continues into the development of enhanced electrokinetic injection techniques for p-chips. Modifications made to the outlet end of the separation channel, such as its connection to a length of fused silica capillary to facilitate chip interfacing with off-chip detection systems for example, can be adopted equally well for chip inlets and sample introduction. The use of conventional capillary to interface between macroscale sample reservoirs and p-chip platforms has been successfully... 

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. 

Sample introduction is mainly performed using electrokinetic injection. The general principle with this type of injection is shown in Figure 7.2. In this schematic representation of the injection system, there are four reservoirs the sample reservoir, the sample waste, the mobile phase reservoir, and the mobile phase waste. Each of these is connected to electrodes. 

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