Electrophoretic cell

Fig. 11-3. Mini-prep continuous elution electrophoretic cell.

Fig. 1.11 Electrospray as an electrophoretic cell. Adapted with permission from reference [28].

The unidimensional type of paper electrophoresis is an extension of free boundary electrophoresis, the method developed by Tiselius (Tl). There are several differences between the two systems. One is the presence of a substrate (supporting medium) as anticonvectional medium during the electrophoretic separation. Another important difference is the starting point. In paper electrophoresis the entire load of material due to be separated is collected on the starting line, whereas in free boundary electrophoresis the material is present in equal concentration over one leg of the electrophoretic cell. Fortunately these differences simplify the qualitative and quantitative appraisal of separation after the run on paper, and for practical work both prove to be true inherent qualities and go far to account for the success of the method (Kl, VI, Wl). 

Henning et al. (H5, H5a) improved these results by neutralizing gastric juice to pH 7.2 and concentrating it 7.5 times by 48-72 hour dialysis against Kollidon. Electrophoresis was performed at 2 mA and 110 volts in barbiturate buffer in the horizontal electrophoretic cell of... 

Some types of electrophoretic cells are stationary layer problem free , but in the other cells the electroosmotic flow can lead to erroneous results. The observed velocity of particles is a sum of the electroosmotic flow of the fluid and the velocity of particles with respect to the fluid. The latter is a function of the potential of the particles and the former is a function of the position in the cell cross section. Hydrodynamic calculations make it possible to find the stationary levels, i.e. the positions in the cell cross section where the electroosmotic flow equals zero. Certainly the position of stationary levels in commercial electrophoretic cells can be found in the user s manual, and there is no need to perform any calculations. The fastest method to determine the electrophoretic mobility is from the velocity at one stationary level, but such a procedure can lead to substantial errors. For example, when the cell position is adjusted at room temperature and then measurements taken... 

Production of heat and electrolysis during the electrophoretic experiment can cause errors for long measurement times in certain types of electrophoretic cells. The temperature affects rather the absolute value of the mobihty, but the presence of electrolysis products can result in an erroneous lEP. For example with KCl electrolyte and silver electrodes the overall reaction can be written as ... 

The pH measured just before injection of dispersion into an electrophoretic cell is not necessarily relevant to the pH in the cell during the measurement. The products of electrolysis in electrophoretic cell make the pH more basic. The effect of products of electrolysis on the pH in the cell is more significant with a short distance between the electrodes, high voltage, long measurement time, and low solid-to-liquid ratio. This problem is discussed in more detail in [220]. 

The parabola method makes it possible to measure the potential of cell walls. Usually, the cell is made of quartz, and the parabola method thus offers the possibility of determining the lEP of one material that has already been extensively studied. The potentials of macroscopic specimens of other materials can also be determined from the mobility profile [273-275] by replacement of the original cell wall of a commercial electrophoretic cell by a flat specimen of the material of interest. For example, in [276], the lEP of a basal plane of mica found from the mobility profile was different from the lEP of a mica dispersion. Only a few types of electrophoretic devices (most of which are no longer available on the market) can be used to determine potentials by means of electro-osmosis. 

When the solid phase is fixed (e.g., as a capillary, membrane, or porous plug), an electric field induces a flow of liquid termed electro-osmosis. The character of the flow depends on the construction of the apparatus. For example, in an electrophoretic cell, the liquid flows in one direction near the walls and in the opposite direction in the center of the cell, and the net flow across the cell cross-section is zero (Figure 2.2). Electro-osmosis can also be demonstrated as a difference in pressure (height of a water column) generated as a result of an electric field applied to a capillary, membrane, or porous plug. 

From the point of view of the electrophoretic cell, microelectrophoresis is a capillary electrophoresis. Experimenting in capillaries is complicated in that a streaming potential (comp. B) occurs, and the applied voltage causes the liquid in the cell to move. This movement, called electro-osmosis, occurs because the glass walls of the cell are negatively charged. There is only one particular point between the cell wall and the center of the cell, the stationary layer at a distinct distance x (x = r/y/2), where the particles move with their true velocity, which is due solely to their own charge. To get a true result, the microscope must be focused in this layer and the particles measured must be in focus. 

Besides aluminum and calcium chlorides, sodium hydroxide or hydrochloric acid was used to change the initial pH value of the Chlorella suspension, while the C-potential of the cells was measured with an electrophoretic cell under a microscope [3, S]. In spite of the less susceptibility of the C-potential to initial pH value of the suspension, the value of in an alkaline side increased to an extent of several times as much as in an acidic condition (not shown). 

However, if a feedback circuit is built in the electrophoretic cell, the velocity linked to electro-osmosis can be evaluated and then subtracted from the velocity measured to yield the electrophoretic velocity [36]. But, the operation is not easy. 

The equipment required for electrophoresis consists basically of two items, a power pack and an electrophoretic cell The power pack provides a stabilized direct current and has controls for both voltage and current output. Power packs, which have an output of 0-500 V and 0-150 mA are available and can be programmed to give either constant voltage or current. 

The electrophoretic cell contains the electrodes, buffer reservoirs, a support for paper and a transparent insulating cover. The electrodes are usually made of platinum. 

The distance / between the two electrodes (direction of the field E, along the horizontal x axis),will be supposed to be very large by respect to the other dimensions of the electrophoretic cell, respectively 26 in the vertical y direction and 2h in the other horizontal direction z, perpendicular to x. 

In fact, an electrospray ion source is more broadly capable than an electrophoretic cell to resolve multiple types of analytes since the ion source can generate gas phase ions from electrically neutral but polar compounds. Thus, when the analytes in the infused solution do not carry separable charge(s) but possess intrinsic dipoles, these compounds can be induced to interact with small cation(s) (e.g., H", Li , Na , NH4, K )or anion(s) (e.g., OH, Cl , formate, acetate) to yield adduct ions in the positive- or negative-ion mode, respectively, in a high electric field. The ionization efficiencies of these electrically neutral compounds depend on their inherent dipoles, the concentration of the small matrix ions, the affinity of the small ions for the analytes, and the electrochemical properties of the resultant adducts. ... 

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