Capillary electrophoresis Flashcards
Is CE chromatography?
No, even though it is about separation, chromatography is about separation based on distribution between two phases. In CE no stationary phase and no partitioning
What is electrophoresis?
Separation of charged analytes in an applied electric field
Why do ions move at different speed?
It depends on their charge and hydrodynamic radius (size)
What is a challenge in CE?
Joule heating
Heat released from the current flowing through the tube, proportional to diameter of the tube and applied electric field. It causes a loss of resolution
How can we minimise Joule heating?
Narrower tubes (but detectability goes down)
Less conducting buffer solutions
Cold temperatures (higher density of water in cold temp means less convection)
What are the components of CE?
Empty fused silica capillaries
High voltage power supply
(cheap equipment)
What are advantages of CE over HPLC?
High efficiency
Wide pH range
Low solvent consumption
Ability to study equilibria in solution
What are disadvantages of CE over HPLC?
Stability issues
High detection limits
Limited selectivity
What is the most commonly used capillary material?
Fused silica
What is BGE?
Background electrolyte
What is EOF?
Electroosmotic flow
What is the wall/surface of the fused silica capillary like?
It has OH groups, when pH is higher than pKa the H+ leaves so negatively charged O- on wall and H+ in BGE
Is the surface/wall negatively charged when pH is larger than pKa?
Yes
Where is the EOF moving?
Towards the cathode
What does the cross-sectional flow profile look like?
Flat unlike HPLC
How can you control the EOF?
Temperature, electric field, buffer pH, ionic strength of buffer
What are the ions movements based on?
Charge and size
Will analytes move more or less if the charge is more?
Move more
Will analytes move more or less if the size is large?
Move less
Is there an A, B or C term in the Van Deemter curve for CE?
Only B term. Only longitudinal diffusion. No resistance to mass transfer and no multiple flow paths.
How can we improve separation efficiency in CE?
Alter pH to increase analyte mobility
Reduce temperature to decrease diffusion
Increase separation voltage
How do we minimise Joule heating?
Low voltage
Narrow capillary
Thermostating
What type of detector is used in CE?
UV-Vis
What does electrodispersion lead to?
Fronting and tailing, caused by differences in conductivity and mobility in the sample zone and in the buffer
Separation in CE is primarily driven by _____ instead of selectivity like chromatography
Efficiency (the term containing N in Rs = efficiency * selectivity * retentivity)
Is there any selectivity and retentivity in CE?
No. There is no distribution between phases
Which different injection modes exist?
Hydrodynamic and electrokinetic (remember lab)
Where is the detector located in CE?
On-column detection
What is tM?
Migration time
What is teo?
Time of electroosmotic flow
What is the “diagram” called?
Electropherogram
What is CE about in short?
Separation of ionic compounds in an electric field inside a narrow capillary
What is the most important parameter to control?
pH
What makes the EOF move?
The charged capillary wall surface
Can you do CZE (capillary zone electrophoresis) both in positive and negative mode?
Yes
How can you do chiral separation using CE?
Using cyclodextrins the enantiomers will bind differently which allows them to be separated
What is CZE?
Capillary zone electrophoresis (the simplest form)
What is MEKC?
Capillary electrokinetic chromatography
(Micelles, separation based on hydrophobic effect)
How does MEKC work?
Analytes partition between BGE and micelles based on hydrophobic effects and ion pairing
What do you use MEKC for?
Amino acids, nucleotides, pharmaceuticals, vitamin
What are the surfactants used for in MEKC?
To create a pseudo-stationary phase (micelles)
What is CEC?
Capillary electrochromatography
How much does Rs increase when you increase the voltage?
Increases with the square root of the applied voltage
