Lecture 1 - advanced separations

Question 1

How does separation work?

Answer 1

The analyte which has the most affinity for the stationary phase will be retained the longest and go slower down the column. This will elute longer and its peak will be later.

Question 2

What are the different columns for GC and LC?

Answer 2

• LC uses pillars/rods.
• GC can use porous layer open tubular column, packed capillary column or wall coated open tubular column.

Question 3

What happens with a packed column?

Answer 3

• The peaks which elute last are broader than those that elute first

This is because the longer the sample is in the chromatographic system, the broader the analyte zone becomes. This causes the peaks to widen and decreases the efficiency. Packed columns have N = 1000-3000.

Question 4

What is the N value?

Answer 4

Theoretical plate value number. The higher it is the more efficient it is.

Question 5

What should a good spectrum be?

Answer 5

• Sharp narrow peaks
• Low times

Question 6

Packed vs capillary?

Answer 6

• Packed columns have lower resolution, fewer peaks and fewer plates.
• Capillary columns have small sample needed, better resolution, more peaks and faster.

Question 7

What is chromatographic efficiency calculated by?

Answer 7

The plate number, N, which is measured by;
N = x (tR/sigma)^2

s is the width of the peak, and the bigger the width the less efficient.

Question 8

How is band broadening in packed columns described?

Answer 8

• By the van Deemter equation.
• It is due to the 3 molecular diffusion processes
  H = A + B/u + Cu
  Also written as H=L/N
  L is the velocity of carrier gas

A is the Eddi diffusion
B is the longitudinal molecular diffusion
C is the mass transfer in the stationary phase

Question 9

What is the edi diffusion?

Answer 9

• Contribution of multiple paths to the height of the plate (Hp)
  Hp = 2λDp

Dp is the average diameter of the packing material
λ is a constant

Question 10

What is longitudinal diffusion?

Answer 10

• Contribution of LD to the plate height, Hd.
  Hd = 2yDm/u

Dm is the solute diffusion coeff in the MP
u is the MP velocity
y is a constant

This shows that as the solute travels more, the peak will be broader.

Question 11

What is mass transfer?

Answer 11

• Contribution of mass transfer in the SP, Hs, and mass transfer in the MP, Hm. Given by 2 equations.

Question 12

In the Van Deemter plots, how does each term change with velocity?

Answer 12

• The A term isn’t effected by velocity
• The B term decreases as velocity is increased
• The C term increases as velocity increases

Question 13

How is band broadening in capillary columns described?

Answer 13

• Using the golay equation
  H = B/u + (Cs+Cm)u

Cs term same as VD
Cm term is new and is used to describe the mass transfer of the solutes in the MP of tubular columns.
Thought as a “drag” along the walls.

Question 14

What are the differences between Golay and Van Deemter?

Answer 14

• No packing so there is no A term
• The term B/u is Golay is described and minimized, same as VD.
• The second term now has 2 contributions to describe mass transfer.