Question

Explain the process of electro-osmotic flow in capillary zone electrophoresis and how electro-osmotic flow modifiers like TTAB can reduse or reverse this flow

Answer 1

Inside the separation capillary is the solution that contains the analytes or the molecules to be separated and the buffer or electrolytic medium that is in charge of conducting the current. The interior is formed by groups silanol (Si-OH), which when being deprotonated (Si-O), raise considerably pH and favor the presence of specific analytes.

As has been said, the separation is carried out according to the mass / charge ratio of the different molecules. For this to be possible it is necessary to apply a potential difference (100 to 500 V / cm) between the two ends of the capillary that will cause the molecules to move towards one end or the other of the capillary (electrophoretic mobility: cationic molecules towards the negative pole and the anionics towards the positive pole) and that they are separated from each other.

In addition, there is another phenomenon inside the capillary called electroosmotic flow that occurs because the internal surface of the capillary is charged. The electroosmotic flow is the same within the entire capillary and affects all molecules in the same way by dragging them towards one of the ends. Thus, the separation will be affected by the electroosmotic flow and by the electrophoretic mobility of each of the molecules.

In the capillaries of molten silica the direction of the electroosmotic mobility is towards the negative electrode, which causes that cationic analytes can be separated quickly, although it can be quite negative for the separation of the anionic analytes, which could require a direction of electroosmotic mobility in the opposite direction. To achieve this reversion of the flow, ammonium salts containing long alkyl chains are added to the electrolyte, the CTAB and the TTAB are the most used to change the flow direction.