Hydrodynamic Injection by Pressure

Following Poiseuille s law, the amount of sample loaded will increase with an increase in pressure the sample concentration and the injection time and will decrease with increases in solution viscosity and the length of the capillary. 

2 Electroklnetlc Injection In electrokinetic injection, the sample is introduced in the capillary by applying a voltage (in general, lower than that used for the separation), while the injection end is dipped in the sample (Fig. 3.6). Under these conditions, the analytes contained in the sample are injected by electromigration as well as by electroosmotic flow. The amount of sample loaded increases with the electrophoretic mobility of analyte, the electroosmotic flow mobility, the inner radius, the voltage, the sample concentration, and the injection time. The amount loaded will decrease with the capillary length. 

Because the amount of analytes injected depends on both the ion mobility and the electroosmotic flow, variables that are difficult to control, electrokinetic injection is adopted only when hydrodynamic injection is not applicable even though it is theoretically superior in terms of selectivity. A potentially 

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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. 

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