Advanced Separation Science

Question 1

What is the equation for Resolution?

Answer 1

Rs = 2(tᵣ₂ - tᵣ₁) / W₂ + W₁

Question 2

What is the equation for k’?

Answer 2

k’ = tₛ / tₘ = (tᵣ - ₘ) / tₘ

Question 3

What is k’?

Answer 3

k’ is the capacity factor

It is an experimental factor that is used to compare migration rates of solutes in columns.

The longer a component is retained by a column, the greater the capacity factor is.

Question 4

Why is it useful to use k’?

Answer 4

It is useful for monitoring performance.

It is useful for working out how many column volumes of MP are needed to elute the analyte.

Question 5

What is the selectivity factor α?

Answer 5

For a separation to occur, analytes must have different capacity factors.

The selectivity factor is the ratio of the capacity factors.

Question 6

How can we calculate α?

Answer 6

α = k’₂ / k’₁

α = tₛ₂ / tₛ₁

Question 7

What is another equation of the capacity factor k’?

Answer 7

k’ = nₛ / nₘ = CₛVₛ / CₘVₘ

Question 8

What is the equation for the partition coefficient K?

Answer 8

k’ = K x (Vₛ / Vₘ)

As the ratio of phase volumes is constant for a given column and mobile phase, the capacity factor for any analyte is directly proportional to its partition equilibrium constant.

Question 9

What is σ?

Answer 9

The standard deviation in length units

Question 10

What is tau?

Answer 10

Standard deviation in time units

Tau = W/4

Question 11

What is the equation for σ² (variance)?

Answer 11

σ² = 2Dt

D = diffusion coefficient

Question 12

When does fronting occur?

Answer 12

From overloading the solute

Question 13

What does tailing occur?

Answer 13

Small quantities of solute are retained more strongly than large quantities.

We can correct by masking strong adsorption sites on SP.

Question 14

What is plate height?

Answer 14

The constant of proportionality between the variance (σ²) of the band, and the distance it has travelled.

Question 15

What is the equation for plate height involving σ²?

Answer 15

H = σ² / x and H = σ² / L

x = distance solute has travelled

It is the measure of variance per unit length

Question 16

What is efficiency, N?

Answer 16

For a solute emerging from a column of length L, the number of plates in the entire column, N, is the length divided by the plate height.

Question 17

What is the equation for efficiency N?

Answer 17

N = L/H = Lx / σ² = L² / σ²

Question 18

What is the van Deemter equation?

Answer 18

H = A + B/u + Cu

A = multiple flow paths
B/u = longitudinial diffusion
Cu = mass transfer/equilbiration time

Question 19

How can the contribution of multiple flow paths be reduced?

Answer 19

Smaller stationary phase particles.

Multiple flow paths is absent in open tubular (capillary) columns

Question 20

How can longitudinal diffusion be reduced?

Answer 20

Faster linear flow which reduces the amount of time spent on the column

Question 21

What is mass transfer/equilibration time?

Answer 21

Finite time for analyte to equilibrate between the mobile and stationary phases

Question 22

How can the mass transfer/equilibration time be reduced?

Answer 22

Decreasing the stationary phase thickness/column radius

Increase the temperature

Question 23

What flow rate do we want to run a column at?

Answer 23

The flow rate which gives us the lowest value of H.

Question 24

How does ln(k’) and k’ change with boiling point T for a homologous series of analytes?

Answer 24

ln(k’) scales with 1/T

k’ increases as the boiling point T increases

Question 25

How does ln(k’) and k’ change for all analytes?

Answer 25

ln(k’) scales with 1/T

k’ decreases as T increases

Question 26

What are two types of column?

Answer 26

Packed (with zeolite or alumina)

Open tubular (capillary)

Question 27

What is the flow resistance in a packed column?

Answer 27

High

Question 28

What is the flow resistance in an open tubular column?

Answer 28

Low

Question 29

What is the length of a packed column?

Answer 29

2-3 metres

Question 30

What is the length of an open tubular column?

Answer 30

15-100 metres

Question 31

What is the phase ratio (vₛ / vₘ) for a packed column?

Answer 31

High

Question 32

What is the phase ratio (vₛ / vₘ) for an open tubular column?

Answer 32

Low

Question 33

What is the efficiency of a packed column?

Answer 33

Low

Due to high ‘A’ term (van Deemter equation) and short column

Question 34

What is the efficiency of an open tubular column?

Answer 34

High

Question 35

Can you overload a packed column?

Answer 35

No

Question 36

Can you overload an open tubular column?

Answer 36

Yes, if too much analyte is injected.

Question 37

What is eluent strength εᵒ?

Answer 37

The solvent adsorption energy, defined as 0 for pentane on bare silica

Question 38

What happens has εᵒ increases?

Answer 38

Solutes elute more rapidly

In RP, less polar solvents have a higher εᵒ.

Question 39

How does log k’ decrease with the volume fraction of the organic modifier (OM)?

Answer 39

It decreases linearly with volume fraction of organic modifier

Question 40

What is the benefit of having non-porous silica?

Answer 40

The phase ratio (Vₛ/Vₘ) is much lower, and k’ decreases by a factor of 10-100.

Question 41

Why does non-porous silica decrease k’?

Answer 41

The bonding can only occur on the external surface, instead of internal pores.

Question 42

How fast is transverse diffusion in GC?

Answer 42

It is fast

Question 43

How fast is transverse diffusion in LC?

Answer 43

It is slow

Question 44

How does having a slow transverse diffusion rate impact H?

Answer 44

It decreases H

Question 45

How does particle size impact plate height?

Answer 45

Large particles increase A (less uniform flow through the column)

Large particles increase C (takes longer to diffuse through the inner pores)