LC

Question 1

In reversed phase LC is the mobile phase polar or non-polar?

Answer 1

Polar and the stationary phase is non-polar. Binding interactions: dispersion forces, pi-pi interactions

Question 2

In normal phase liquid chromatography is the mobile phase polar or non-polar?

Answer 2

Non-polar. Stationary phase is polar and the important retention mechanism is dipol-dipol

Question 3

How do we determine which system we have (normal phase or reverse phase) in an exam?

Answer 3

Look at the stationary phase and the mobile phase

Question 4

What is a common mobile phase composition in normal phase LC?

Answer 4

Hexane 95 % and ethanol 5 %

Heptane 95 % and ethanol 5 %

to heptane 90 % and ethanol 10 %

Question 5

What is a normal mobile phase composition in reverse phase LC?

Answer 5

90 % water and 10 % acetonitrile

to 60 % water and 40 % acetonitrile

Question 6

What is a normal mobile phase composition in HILIC (non polar MP and polar SP)?

Answer 6

90 % acetonitrile and 10 % water

to 60 % acetonitrile and 40 % water

Question 7

Is logP and KD related?

Answer 7

Yes, both are about how the analyte is distributed. LogP about how it is distributed in aqueous phase and organic phase. KD about how it is distributed in MP and SP

Question 8

In normal phase LC what would happen if you change the MP composition to contain more ethanol?

Answer 8

It would bind the analytes more and therefore the retention time would decrease (as well as the retention)