Lecture 3 - Ion seperation

Question 1

What is the use for anion exchange chromatography?

Answer 1

• Separates anions (negatively charged species) in a sample

Question 2

How does anion exchange chromatography work?

Answer 2

• The anions need to have affinity for the SP, so they are cations
• When the sample is injected, the negative anions will retain on the positive cations, and all other cations in the sample will flow out.

Question 3

What is the order of elution for F, Cl, Nitrite, Bromide, Sulphate, Nitrate and Phosphate?

Answer 3

F > Cl (tall peak) > Nitrite > Bromide > Sulphate and Nitrate peak overlaps but sulphate peak first > Phosphate

Question 4

What stationary phases are used?

Answer 4

• Usually poly-resins
• Some have low-capacity microporous and others have high-capacity microporous resins.
• Usually latex anion exchange particle used which has a PS/DVB bead at the core around 5-25 nm (support), Core bead is sulphonated, Small (0.1 mm) fully aminated latex particles, are electrostatically attracted to the sulfonated core beads.

Question 5

What are the advantages of using latex anion exchange particle?

Answer 5

• Has a large surface area
• Latex beads don’t swell so more stable
• Prevents anion migration

Question 6

What happens in the anion exchange process?

Answer 6

When a sample containing anions is introduced into the column, the sample ions will exchange/compete with the eluent ions.

The eluent is attached to the cationic SP then swaps with the anion.
N+-Eluent + Anion -> N+A- + Eluent

Question 7

What is the order of elution for anion exchange?

Answer 7

The speed of anions to come out of the column depends on their affinity for the cationic SP.

• Valency - monovalent (1 ion) will have a weaker interaction and move quicker.
• Divalent (2) and polyvalent (>2) will move slower.
• Smaller ions will also more through faster as they can move through the pellicular resin faster.

Question 8

What is a suppressor used for?

Answer 8

• Without a suppressor the background is high and the peak is small (low analyte signal)
• With a suppressor low background and bigger peak.

The suppressor increases sensitivity by reducing conductivity of background eluent and increases conductivity of analyte

Question 9

What is added into the eluent channel?

Answer 9

The sample with a cation and anion (NaCl)
The eluent

Question 10

What happens in the regen channel and the eluent channel?

Answer 10

The cation from the sample in the eluent channel will exchange with another cation from the regen (usually H+) through a sulfonated cation exchange membrane
The rest if the product from the regen channel and the cation which has been swapped go to waste.
The anion and new cation go to the detector along with other by products.

Question 11

What is the conductivity proportional to?

Answer 11

• The concentration of all ionic species in the solution being measured.

Question 12

Which law gives the formula for conductivity and what is it?

Answer 12

Kohlrausch’s law of independent migration states that the measured conductivity, K, depends on concentration (ci) and ionic mobility (Ii)

K = Sum of c(i) x lamda(i) /1000

Question 13

Can anionic and cationic exchange chromatography be done simultaneously?

Answer 13

Yes - the same set up can be done for cationic separation with opposite features - the SP is anionic.