Lecture Notes April 18th 2007
Answers to Quiz
| Q | A |
|---|---|
What does the abbreviation CZE stand for? |
capillary zone electrophoresis |
| In an electrochemical cell is the cathode positively charged or negatively charged? | negatively |
| In an electrochemical cell is the anode positively charged or negatively charged? | positively |
| Which separation technique has the higher efficiency (greater number of theoretical plates, N, per unit length): CZE or HPLC or GC? | CZE |
| What is a micelle? | An aggregation of molecules in a solvent. Typically if the solvent is water the molecules have non-polar "tails" and polar "head" groups |
Capillary zone electrophoresis
A more recently developed technique (than either HPLC or GC) based on the combination of electroosmotic flow and electrophoretic mobility in a narrow silica capillary under conditions which make the tube walls negatively charged (the surface -OH groups are ionized). The liquid in the tube contains a "buffer" i.e. a relatively high concentration of ions. When a high voltage (20-30 kV) electric field is applied. The cations move towards the negative electrode and drag the solvent with them. As they are smaller and more polarizing they drag more solvent than the anions are able to do in the opposite direction. The net effect is flow towards the negative electrode (the cathode). As cations tend to gather next to the negatively charged walls, the flow is faster near the walls that it would be in pressure induced flow and the resulting velocity profile across the tube is almost uniform. This means that there is very little broadening and so the efficiency of CE is very high, about an order of magnitude better than HPLC.
One problem is the amount of sample. If the capillary is 25 µm in diameter and the sample zone is 1.0 cm in length, then the volume injected is 4.9 nL, i.e. over three orders of magnitude smaller than can be injected into an HPLC. This causes problems with the detection of low concentrations of materials as the amount of material flowing through the detector is much smaller for CE than for HPLC. It is also difficult to make detectors that have high sensitivity for such small volumes of solution. Though in generator the same detector types as are used for LC are also applied with CE: molecular spectrometry (laser induced fluorescence is popular), electrochemistry and mass spectrometry.
Neutral species will all migrate at the same rate, but can be separated by the addition of a surfactant at a concentration above the critical micelle conentration. The micelles act as a "stationary" phase (they are moving slowly with the EOF towards the cathode), but a neutral molecule that partitions into the micelle (the non-polar interior) is slowed down compared with one which remains in the "mobile" phase. Thus of the neutral species difer in their relative affinities for the mobile phase and micelles, theya re separated by a chromatograhic mechanism. The technique is called micellar electrokinetic capillary chromatography (MEKCC)