Electromigration injection

With hydrostatic injection mechanisms, injection reproducibility can be better than 1-2% RSD. The volume of sample loaded is a function of the capillary dimensions, the viscosity of the buffer, the applied pressure, and the time, and it can be calculated using 

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Electrokinetic (also called electromigration) injection is performed by placing the inlet of the capillary and an electrode in the sample vial. Following this a voltage is applied during a defined period of time. The sample constituents are actively carried into the capillary, and when present, the EOF also passively carries them into the capillary. For this reason, neutral compounds are also injected. The active migration is due to the effective electrophoretic mobilities of the constituents. The amount (B), in units of concentration injected into the capillary is expressed by [2,38]... 

Electromigration injection is performed by replacing the source electrolyte vial with the sample vial and applying voltage, as illustrated in Figure 6.3. The advantage of this approach over the others is that more sample... 

Figure 6.4 Comparison of sensitivities for different sample injection techniques using a 50 mg/ml peptide mixture (a) hydrostatic injection, (b) electromigration injection where the sample is dissolved in buffer, and (c) electromigration injection where the sample is dissolved in water. (Adapted from Ref. 1 with permission.)...

Sample is introduced into the capillary by hydrostatic injection (gravity, pressure, or vacuum) or by electromigration injection (voltage). 

The sample is introduced into the separation channel by electromigration injection. Thus far, detection has been solely by laser-induced fluorescence, using an argon ion laser or a helium/neon laser as the excitation source and a photomultiplier tube (PMT) or charge-coupled device (CCD)... 

Figure 10.2. Peak identity and concentrations (ppm) for a 30-anion eicctropherogram displayed in an 89-s electropherographic segment. Electromigration injection at 1 kV for 15 s. Peaks 1 = thiosulfate...

Injection volumes are in the nanoliter range to avoid system overloading, since the total volume of the capillary is in the /rl range. Direct injection techniques have been developed to ensure efficient and reproducible injection. Techniques employed are electrokinetic injection (i.e., electromigration injection), hydrodynamic injection by pressure or vacuum, and hydrostatic injection by gravity. Organic acids are almost exclusively detected with indirect UV, whereas other analytes have been measured by direct or conductivity detection. 

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. 

Fig. 6 Comparison of pressure injection vs. electroinjection. Propranolol (P, 10 mg/L) and quinine (Q, 10 mg/L) in serum deproteinated with 2 volumes of acetonitrile. Top Pressure injection 30 sec bottom Electromigration injection at 2 kV for 30 sec. Electrophoresis buffer 90 mmol/L phosphate buffer, pH 6.2.

On-tube detection is almost universally used in CE, and the short detection path lengths provided by fused silica capillaries result in a 100- to 400-foId reduction in detection sensitivity compared with HPLC. Although various techniques are employed in CE to regain most of this sensitivity, detectivity remains a serious concern. Therefore concentration steps are often necessary in sample preparation for CE. It should be emphasized that some concentration techniques such as evaporation and lyophilization will not change the sample ion/salt ion ratio for nonvolatile salts, so no effective concentration is achieved when electromigration injection is used. The use of transient isotachophoresis has been proposed for on-line preconcentration of proteins prior to CZE separation [41,42],... 

Sample injection in CE requires the introduction of very small amounts of analyte at the capillary inlet with high precision. All commercial instruments offer electromigration injection and at least one type of displacement injection. 

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