Hydrostatic injection, capillary electrophoresis

P. Kuban, P. Kuban, V. Kuban, Flow injection-capillary electrophoresis system with contactless conductivity detection and hydrostatic pressure generated flow. Application to the quantitative analysis of inorganic anions in water samples. Electrophoresis 24 (2003) 1935—1943. 

A major advantage of capillary gel electrophoresis is that resolution is maintained with increasing field strength, owing to efficient heat dissipation. Introduction of the sample into the gel-filled capillary is typically done electrokinetically (Chapter 6), because hydrostatic injection is not possible with capillaries blocked with gel. Typical injection times between 1 and 20 sec are used at field strengths between 100 and 400 V/cm.28 Coated capillaries should be used to prevent the EOF from pumping the gel out of the capillary. 

Capillary electrophoresis separations are dependent on the relative mobilities of analytes under the influence of an electric field and do not depend on mobile phase/stationary phase interactions. A fused silica capillary is filled with a buffer and both ends submerged into two reservoirs of the buffer. A platinum electrode is immersed in each reservoir and a potential difference (5-30 kV) is applied across the electrode. An aliquot of sample of a few nanoliters is injected onto the capillary by either hydrostatic or electrokinetic injection, and the components migrate to the negative electrode. Separations of analytes arise from differences in the electrophoretic mobilities, which are dependent on the mass-to-charge ratio of the components, physical size of the analyte, and buffer/analyte interactions. An electro-osmotic flow (EOF) of the buffer occurs in the capillary and arises as a result of interactions of the buffer with dissociated functional groups on the surface of the capillary. Positive ions from the buffer solution are attracted to negative ions... 

The introduction of samples into capillaries by means of differential pressure (hydrostatic injection) has become the most popular method in capillary electrophoresis. For sample introduction, the sample vial is raised to a defined level above the detection reservoir for a specified time frequently about 5 s. To terminate the injection, the end of the capillary is removed from the sample and replaced into the vial containing the background electrolyte. 

Figure 8 Capillary zone electrophoresis. Influence of carrier electrolyte concentration on the resolution of L-ascorhic acid (L-AA) and D-ascorbic acid (D-AA) dissolved in extraction buffer. Three-second (17.7 nL) hydrostatic injections of 0.05 mg/mL solution of L-AA and D-AA dissolved in 3% MPA/1 mM EDTA. UV detection carried out at 194 nm. Carrier electrolyte was 50 mm borate, pH 9 (A) and 100 mM borate, pH 9 (B). (From Ref. 63.)...

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