Topic 5: Capillary Electrophoresis

Question 1

Explain how does the separation process works in CE?

Answer 1

The main separation mechanism is based on mobility differences of the analytes in an electric field; these differences are dependent on the size and charge-to-mass ratio of the analyte ion. Analyte ions are separated into distinct zones when the mobility of one analyte differs sufficiently from the mobility of the next. Capillary zone electrophoresis (CZE) uses this as the primary separation mode.

Question 2

What is Micellar electrokinetic capillary chromoatography (MECC)

Answer 2

The separation is similar to CZE, but an additional mechanism is in effect that is based on differences in the partition coefficients of the solutes between the buffer and micelles present in the buffer.

Question 3

What is Capillary gel electrophroresis (CGE)?

Answer 3

An additional mechanism is based on solute size, as the capillary is filled with a gel or a polymer network that inhibits the passage of larger molecules.

Question 4

Label and explain how does the ion mobility detector work

Answer 4

• Ion mobility spectrometry is gas-phase electrophoresis. Under the influence of an electric field, it separates ions according to its size to charge ratio and is capable of separating isomer as well
• Similar to time-of-flight spectrometer
• Sample that is absorbed on a cotton swab is placed in the heated anvil at the left to desorb analyte vapor. Dry air doped with chemical ionization reagent (such as Cl₂ for anions and acetone or NH₃ for cations.) sweeps the vapor through a tube containing 10 millicuries of radioactive ⁶³Ni. Reagent gas ionized by beta-emission form ⁶³Ni reacts with analyte to generate analyte ions.
• Each ions travels in the same direction as the electric field
• Ions are retarded by collision with gas molecules at atmospheric pressure.
  Each ion travels at its own speed equal to KE, where E is the electric field and K is the mobility. Small ions have greater mobility than large ions of the same charge because large ions experience more drag.

Question 5

Explain how does indirect detection work.

Answer 5

A substance with a steady background signal is added to the background electrolyte. In the analyte band, analyte molecules displace the chromophoric substance, so the detector signal decreases when analyte passes by where analyte band of Cl^- and CrO₄^2- (ultraviolet-absorbing anion chromate. With less CrO₄^2-, to absorb ultraviolet radiation, a negative peak appears with Cl^- reaches the detector.

Question 6

What is electrosmosis?

Answer 6

In an electric field, cations are attracted to the cathode and anions are attracted to the anode. (see image) Excess cations diffuse towards the cathode, creating the pumping action called electroosmosis

Question 7

What is electroosmotic velocity?

Answer 7

Electroosmotic velocity is measured by adding to the sample a neutral molecule to which the detector responds

Question 8

What is electrophoretic mobility

Answer 8

the constant of proportionality between the speed of the ion and the electric field strength. Mobility is proportional to the charge of the ion and the inversely proportional to the friction coefficient.
Image below, the mobility increases with charge.

Also greater the radius, the lower the mobility

Question 9

Explain the Van Deemter equation for CE.

Answer 9

Since there is no stationary phase in CE, mulitiple flow path and mass transfer terms can be removed from the Van Deemter equation.

Question 10

Using the image below, describe how the CE apparatus works.