Sample inlets moving injection

In the case of the electrokinetic injection (Figure 3.29), once the sample vial has replaced the inlet buffer vial, a small voltage is applied between the sample and outlet buffer vials which in effect starts a mini-electrophoretic separation. The sample ions move into the capillary in the usual order of positive ions, neutral molecules and negative ions. However, this particular type of injection means that positive ions are favoured (as they move hrst into the capillary being attracted to the cathode) and so the sample plug is not representative of the entire sample in the vial. However, if this is taken into account, this type of injection can be very useful. 

Figure 21.1 demonstrates the chromatographic process. A small volume of sample solution is injected at the column inlet (Fig. 21.1 A). The mobile solvent phase moves the sample through the column packing (Fig. 21.IB). The individual components undergo sorption and desorption on the packing, thereby slowing their motion in varying amounts depending on their affinity for the packing. Each component X is distributed between the stationary phase (s) and the mobile phase (m) as it passes down the column. According to...

The work [40] deals with the redistribution of filler particles in the process of injection molding. In this case nonuniform distribution may occur both in the cross-section of a sample and along its length. Both kinds of nonuniformity are linked together if particle moves away from the mold walls it enters the zone of high velocity flow, therefore, a deficit of particles near the walls should be accompanied with a surplus of them far from the inlet. It should be noted that all the works mentioned consider spherical particles there are no theoretical or experimental studies of the redistribution of particles of other shapes, say, fibers or bars. 

A fracture near the capillary inlet can also cause peak splitting. The break can occur if the capillary hits a vial wall or seal. The polyimide coating keeps the cracked portion intact. During injection, the sample moves into the capillary from both the open end of the tube and through the crack. The split peak is usually smaller than the main component and always has a migration time that is a little shorter. This is confirmed by examining the capillary inlet. If fractured, a small piece often detaches and the peak splitting is resolved. 

In practice, the sample is injected at the inlet to the channel. The external field is next applied across the face of the channel, as illustrated in Figure 33-16. In the presence of the field, sample components migrate toward the accumulation wall at a velocity determined by the strength of the interaction of the component with the field. Sample components rapidly reach a steady-state concentration distribution near the accumulation wall, as shown in Figure 33-17. The mean thickness of the component layer I is related to the diffusion coefficient of the molecule D and to the field-induced velocity u toward the wall. The faster the component moves in the field, the thinner the layer near the wall. The larger the diffusion coefficient, the thicker the layer. Since the sample components have different values of D and it, the mean layer thickness will vary among components. 

The actual injection method is semiautomated and consists basically of five successive steps. In the first step, the inlet of the channel is cleaned, and all liquid present in front of the chaimel is removed. In the second step, a small drop of the sample mixture is deposited in front of the channel inlet. During the third step, the moving wall is rapidly displaced over a given prescribed distance depending on the injection volume. This movement forces the sample deposited in front of the inlet into the channel. During the fourth step, the non-entered sample is aspirated and replaced by pure buffer solution. When this is... 

Figure 1 shows a schematic of a typical CE instrument. A length of fused silica capillary (typically 20-50 cm) filled with separation buffer extends between two reservoirs. To perform a separation the inlet of the separation capillary is moved to the sample for injection. The inlet is returned to the separation buffer reservoir after a volume ( 20-50nl) of sample has been introduced. An electric potential ( 10-30 kV) is applied between the two buffer reservoirs. Analytes migrate through the capillary according to their electrophoretic mobility and the electroosmotic flow (EOE). Analytes are detected either on or off column, depending on the particular detector. Separations with efficiencies of 100 000-150 000 plates and analysis times of 10-20 min are typical. 

All sample classes that can be separated by HPLC can be separated by CEC (see section on Applications ). Dipping the capillary inlet into a separate vial containing the sample in solution and applying an injection voltage for a brief time accomplish sampling. The capillary inlet is then moved back to the inlet buffer vial and elution is commenced at the (normally) higher voltage. 

Sample sizes in capillary electrophoresis are very small, 1-30 nl, which can be an advantage when sample volume is limited. The sample is introduced into the capillary by gravity, vacuum, pump, or electrokinetic injection. In all cases, the sample is introduced by momentarily moving the inlet of the capillary from the inlet buflfer vial to the sample vial, and then back. When gravity introduction is used, the sample is siphoned onto the column by raising the sample vial. The volume injected (F,) in nanoliters is calculated by... 

A minimum of two gas inlets is required, one for the inert gas used in the calibration of the dead volume and one for the reactive gas. The reference volume for gas injection might be a fixed volume type or a moving piston. In the second case it is possible to change the injection volume in addition to the loading pressure according to the sample adsorption rate and the required number of experimental points. One or two separate transducers might perform the measure of the loading pressure and the equilibrium one. In case that only one transducer is used, the pressure transducer volume must be added as dead volume in the reference volume. 

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