Capillary electrophoresis, tube coating

Fused silica capillary tubes were variously coated for capillary electrophoresis. The chemical process involving a Grignard reaction is shown in equation 30. The silanol groups on the silica surface are treated with alkali, dried, convert to chlorosUanes with thionyl chloride and vinylmagnesium bromide replaces the chlorine atoms with vinyl... 

In the fiber mode, the sorbent coated fiber is housed in a microsyringelike protective holder. With the fiber retracted inside the syringe needle, the needle is used to pierce the septum of the sample vial. The plunger is depressed to expose the sorbent-coated fiber to the sample. After equilibrium is reached or at a specified time prior to reaching equilibrium, the fiber is retracted into the protection of the microsyringe needle and the needle is withdrawn from the sample. The sorbent is then interfaced with an analytical instrument where the analyte is desorbed thermally for GC or by solvents for HPLC or capillary electrophoresis. For the in-tube mode, a sample aliquot is repeatedly aspirated and dispensed into an internally coated capillary. An organic solvent desorbs the analyte and sweeps it into the injector [68,130,133]. An SPME autosampler has been introduced by Varian, Inc., that automates the entire process for GC analyses. 

Electrophoresis in the CZE mode takes place in an open tube and in a free solution without any separation matrix in the capillary. The separation is based on the mass/charge ratio of the analytes. It is appropriate for the separation of nucleosides and nucleotides. It is not well suited for medium to large oligonucleotides, because their mass/charge ratio tend to be smaller. The use of a separation matrix becomes necessary for these species. Various capillary systems, including bare fused silica capillaries and surface-coated capillaries, have been used in CZE. 

We have already learned about the presence of an electroosmotic flow (EOF) in a silica capillary filled with an electrolytic solution and subjected to a voltage difference at the two ends (Section 6.1.5). This flow is such that the velocity profile, i/eof, is essentially fiat, and the liquid motion is toward the cathode (equation (6.1.22)). If the silica capillary tube is open (and not a packed bed of very small particles), and the surface of the capillary has not been coated in any fashion, capillary electrophoresis occurs in the presence of Veof- Therefore, the net velocity of any ionic species i averaged over the capillary cross section, is given by... 

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